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Yoda
01-17-2007, 06:54 AM
Can a cue ball have more top spin than actual rolling speed? That is, can the cue ball be spinning faster than it is rolling forward?

randyg
01-17-2007, 08:07 AM
YES....SPF=randyg

dr_dave
01-17-2007, 08:57 AM
<blockquote><font class="small">Quote Yoda:</font><hr> Can a cue ball have more top spin than actual rolling speed? That is, can the cue ball be spinning faster than it is rolling forward? <hr /></blockquote>
Yes, about 25% more. For illustrations and more information, see my October '05 instructional article (http://www.engr.colostate.edu/~dga/pool/bd_articles/2005/oct05.pdf).

Regards,
Dave

Fran Crimi
01-17-2007, 11:42 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Yoda:</font><hr> Can a cue ball have more top spin than actual rolling speed? That is, can the cue ball be spinning faster than it is rolling forward? <hr /></blockquote>
Yes, about 25% more. For illustrations and more information, see my October '05 instructional article (http://www.engr.colostate.edu/~dga/pool/bd_articles/2005/oct05.pdf).

Regards,
Dave <hr /></blockquote>

Maybe it's just a semantics issue for me but how can the ball be both rolling and spinning (with top spin) at the same time? Wouldn't it be sliding and spinning instead?

Fran

dr_dave
01-17-2007, 11:56 AM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Yoda:</font><hr> Can a cue ball have more top spin than actual rolling speed? That is, can the cue ball be spinning faster than it is rolling forward? <hr /></blockquote>
Yes, about 25% more. For illustrations and more information, see my October '05 instructional article (http://www.engr.colostate.edu/~dga/pool/bd_articles/2005/oct05.pdf).

Regards,
Dave <hr /></blockquote>

Maybe it's just a semantics issue for me but how can the ball be both rolling and spinning (with top spin) at the same time? Wouldn't it be sliding and spinning instead?<hr /></blockquote>
"rolling" = "natural follow" = spinning (topspin) and moving forward with no sliding whatsoever
"stun" = "pure sliding" = no topspin or bottom spin whatsoever
"extra topspin" = "power follow" = topspin faster than rolling rate, which causes back sliding
"English" = "side" = side spin

I have a good illustration of all of the cases in Diagram 5 of my April '04 instructional article (http://www.engr.colostate.edu/~dga/pool/bd_articles/2004/april04.pdf).

Regards,
Dave

Fran Crimi
01-17-2007, 12:37 PM
Sorry, I didn't understand your response.

I'll rephrase the question. When you refer to topspin faster than rolling rate, is the cue ball sliding and spinning or rolling and spinning or a combination of both?

Thanks,
Fran

dr_dave
01-17-2007, 01:10 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> Sorry, I didn't understand your response.

I'll rephrase the question. When you refer to topspin faster than rolling rate, is the cue ball sliding and spinning or rolling and spinning or a combination of both?<hr /></blockquote>Sorry my answer wasn't very direct. I'll try again.

If the rate of topspin is faster than the rolling rate, there is sliding. So the answer to your question is: the ball is both spinning (with topspin) and sliding, but not rolling (until the sliding wears off, which occurs naturally due to the sliding). Diagram 4 in my April '04 instructional article (http://www.engr.colostate.edu/~dga/pool/bd_articles/2004/april04.pdf) illustrates all of this very clearly. I know you don't need the illustration, but some people might benefit.

Regards,
Dave

Fran Crimi
01-17-2007, 01:28 PM
<blockquote><font class="small">Quote dr_dave:</font><hr>
If the rate of topspin is faster than the rolling rate, there is sliding. So the answer to your question is: the ball is both spinning (with topspin) and sliding, but not rolling (until the sliding wears off, which occurs naturally due to the sliding).
<hr /></blockquote>

Okay, thanks for clarifying that. That's what I thought. Therefore the answer to Yoda's question would be no, since the cb can't spin (topspin) faster than it's rolling speed at the same time that it's rolling. Correct?

It can spin faster than rolling speed but that would mean that it's sliding and not rolling as Yoda asked, right?

See, I think the problem with Yoda's question is that his/her clarification contradicts the initial question a bit. If you answer according to the clarification, then the answer must be no. Know what I mean?

Fran

dr_dave
01-17-2007, 01:47 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr>
If the rate of topspin is faster than the rolling rate, there is sliding. So the answer to your question is: the ball is both spinning (with topspin) and sliding, but not rolling (until the sliding wears off, which occurs naturally due to the sliding).
<hr /></blockquote>

Okay, thanks for clarifying that. That's what I thought. Therefore the answer to Yoda's question would be no, since the cb can't spin (topspin) faster than it's rolling at the same time that it's rolling. Correct?

It can spin faster than rolling speed but that would mean that it's sliding and not rolling as Yoda asked, right?<hr /></blockquote>
Now I think you're getting into a level of semantics not intended by Yoda. To quote his first question:<blockquote><font class="small">Quote Yoda:</font><hr>Can a cue ball have more top spin than actual rolling speed?<hr /></blockquote>The answer is still: yes, 25% more.

I agree that his restatement of the question had semantics issues, but I think his intent was clear.

Regards,
Dave

Fran Crimi
01-17-2007, 01:57 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr>
If the rate of topspin is faster than the rolling rate, there is sliding. So the answer to your question is: the ball is both spinning (with topspin) and sliding, but not rolling (until the sliding wears off, which occurs naturally due to the sliding).
<hr /></blockquote>

Okay, thanks for clarifying that. That's what I thought. Therefore the answer to Yoda's question would be no, since the cb can't spin (topspin) faster than it's rolling at the same time that it's rolling. Correct?

It can spin faster than rolling speed but that would mean that it's sliding and not rolling as Yoda asked, right?<hr /></blockquote>
Now I think you're getting into a level of semantics not intended by Yoda. To quote his first question:<blockquote><font class="small">Quote Yoda:</font><hr>Can a cue ball have more top spin than actual rolling speed?<hr /></blockquote>The answer is still: yes, 25% more.

I agree that his restatement of the question had semantics issues, but I think his intent was clear.

Regards,
Dave

<hr /></blockquote>

You may be absolutely right, Dave, and Yoda may have incorrectly stated the words "rolling forward". I just figured since he was familiar with the term 'roll' with his use of it in his first sentence, that by the words "rolling forward" he meant 'rolling forward'.

/ccboard/images/graemlins/grin.gif Silly me. /ccboard/images/graemlins/grin.gif

Best,
Fran

dr_dave
01-17-2007, 02:19 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr>You may be absolutely right, Dave, and Yoda may have incorrectly stated the words "rolling forward". I just figured since he was familiar with the term 'roll' with his use of it in his first sentence, that by the words "rolling forward" he meant 'rolling forward'.

/ccboard/images/graemlins/grin.gif Silly me. /ccboard/images/graemlins/grin.gif<hr /></blockquote>

Agreed ... silly you. /ccboard/images/graemlins/wink.gif

Dave

Fran Crimi
01-17-2007, 02:20 PM
Dave, what's the significance of comparing the spin rate while a cue ball is sliding to a rolling rate where the cue ball is rolling? How is it helpful to a pool player to know this information?

Thanks,
Fran

dr_dave
01-17-2007, 02:39 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> Dave, what's the significance of comparing the spin rate while a cue ball is sliding to a rolling rate where the cue ball is rolling? How is it helpful to a pool player to know this information?<hr /></blockquote>
Honestly, I don't think it is that useful knowing that you can apply more than a rolling amount of spin with a power follow shot. Although, somebody asked, so I answered. For most follow shots, the cue ball will be rolling at object ball impact.

However, I do think it is critical for a pool player to understand (through knowledge and/or intuition) how a cue ball develops and changes between stun, slide, and roll during different types of shots (e.g., see HSV 3.1 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV3-1.htm)). I think a lack of understanding of and "feel" for these concepts prevent many people from correctly:
- executing stop and stun shots at various speeds and distances.
- applying the 90 degree rule
- applying the 30 degree rule

Regards,
Dave

Fran Crimi
01-17-2007, 03:01 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Fran Crimi:</font><hr> Dave, what's the significance of comparing the spin rate while a cue ball is sliding to a rolling rate where the cue ball is rolling? How is it helpful to a pool player to know this information?<hr /></blockquote>
Honestly, I don't think it is that useful knowing that you can apply more than a rolling amount of spin with a power follow shot. Although, somebody asked, so I answered. For most follow shots, the cue ball will be rolling at object ball impact.

However, I do think it is critical for a pool player to understand (through knowledge and/or intuition) how a cue ball develops and changes between stun, slide, and roll during different types of shots (e.g., see HSV 3.1 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV3-1.htm)). I think a lack of understanding of and "feel" for these concepts prevent many people from correctly:
- executing stop and stun shots at various speeds and distances.
- applying the 90 degree rule
- applying the 30 degree rule

Regards,
Dave <hr /></blockquote>

Thanks. I agree with everything you wrote. Rolling rate applied to a sliding ball is really just an abstract concept, though, isn't it? The only true rolling rate is a the rate of the cue ball rolling, and that's a variable rate. You can take the speed of a cue ball sliding and then compare it to a cue ball rolling at the same speed and then attach a number of revolutions to it, strictly by comparison, but in reality, the sliding cue ball is still sliding. Revolutions on a sliding ball and revolutions while rolling are mutually exclusive. Probably not even worth comparing, at least I think not.

Fran

Jal
01-17-2007, 04:25 PM
<blockquote><font class="small">Quote dr_dave:</font><hr>Yes, about 25% more. For illustrations and more information, see my October '05 instructional article (http://www.engr.colostate.edu/~dga/pool/bd_articles/2005/oct05.pdf).<hr /></blockquote>Dr. Dave,

I know you know, but just a side note.

You don't get exactly what you paid for. There's only about an 11% increase in spin rate over hitting at the offset which produces immediate natural roll. You end up taking away more cueball speed than added spin. (This is from a rough calculation based on an elastic collision and ignoring squirt.)

Jim

SpiderMan
01-17-2007, 04:48 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> Dave, what's the significance of comparing the spin rate while a cue ball is sliding to a rolling rate where the cue ball is rolling? How is it helpful to a pool player to know this information?

Thanks,
Fran <hr /></blockquote>

Knowing that "overspin" is possible tells the player that he may be able to achieve a variation in result by hitting above the point on the CB which produces immediate natural roll.

This could occur on a follow shot where the CB is close to the OB, so that some of the "overspin" still remains at contact. At table's length, the higher cueing would produce no difference because of the rapid decay of the overspin.

SpiderMan

dr_dave
01-17-2007, 04:56 PM
<blockquote><font class="small">Quote Jal:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr>Yes, about 25% more. For illustrations and more information, see my October '05 instructional article (http://www.engr.colostate.edu/~dga/pool/bd_articles/2005/oct05.pdf).<hr /></blockquote>Dr. Dave,

I know you know, but just a side note.

You don't get exactly what you paid for. There's only about an 11% increase in spin rate over hitting at the offset which produces immediate natural roll. You end up taking away more cueball speed than added spin. (This is from a rough calculation based on an elastic collision and ignoring squirt.)<hr /></blockquote>Jim,

You make a good point. I haven't verified your 11% number, but it would make sense if you are comparing an immediate-roll shot to a maximum-offset shot, assuming the initial cue stick speed is the same for both shots. In TP A.12 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/new/TP_A-12.pdf), I show that the 25% number is still valid, given the proper context.

Thank you for pointing this out. I haven't thought about this in detail before.

Regards,
Dave

Jal
01-17-2007, 05:20 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> ... In TP A.12 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/new/TP_A-12.pdf), I show that the 25% number is still valid, given the proper context.<hr /></blockquote>Yes, of course. I was just about to edit my post to say much the same. Sorry if it seemed to contradict yours. (And I also know you know it didn't, technically speaking, but... /ccboard/images/graemlins/confused.gif)

Sooo, in case anyone is getting confused, you will see a 25% increase in the spin/speed ratio, which means it actually is spinning 25% more than if it were rolling. It's just that the cueball's speed will be less along with the spin being increased at this greater tip offset.

Jim

Jal
01-17-2007, 05:54 PM
<blockquote><font class="small">Quote SpiderMan:</font><hr>Knowing that "overspin" is possible tells the player that he may be able to achieve a variation in result by hitting above the point on the CB which produces immediate natural roll.

This could occur on a follow shot where the CB is close to the OB, so that some of the "overspin" still remains at contact.<hr /></blockquote>Absolutely.

<blockquote><font class="small">Quote SpiderMan:</font><hr>At table's length, the higher cueing would produce no difference because of the rapid decay of the overspin.<hr /></blockquote>There would be a little bit of a difference. If, for instance, the cueball reaches natural roll before colliding with something, it will be going slower than if hit to produce immediate natural roll, given the same stick speed.

Ron Shepard derives a tip offset of just under (1/4)R above center as that which yields the greatest speed at natural roll. Since it's rolling, it also has the greatest topspin rate. Any other offset results in less speed and spin (at rolling).

Jim

SpiderMan
01-17-2007, 06:41 PM
<blockquote><font class="small">Quote Jal:</font><hr> <blockquote><font class="small">Quote SpiderMan:</font><hr>Knowing that "overspin" is possible tells the player that he may be able to achieve a variation in result by hitting above the point on the CB which produces immediate natural roll.

This could occur on a follow shot where the CB is close to the OB, so that some of the "overspin" still remains at contact.<hr /></blockquote>Absolutely.

<blockquote><font class="small">Quote SpiderMan:</font><hr>At table's length, the higher cueing would produce no difference because of the rapid decay of the overspin.<hr /></blockquote>There would be a little bit of a difference. If, for instance, the cueball reaches natural roll before colliding with something, it will be going slower than if hit to produce immediate natural roll, given the same stick speed.

Ron Shepard derives a tip offset of just under (1/4)R above center as that which yields the greatest speed at natural roll. Since it's rolling, it also has the greatest topspin rate. Any other offset results in less speed and spin (at rolling).

Jim
<hr /></blockquote>

Right - I was referring only to the fact that, at table's length, both would have natural roll (no slide or overspin), and assuming that the shooter would use whatever stick speed necessary to achieve the desired CB speed.

SpiderMan

Jal
01-17-2007, 06:55 PM
Sorry about that Spiderman. I didn't get your meaning.

Jim

SpiderMan
01-17-2007, 06:58 PM
<blockquote><font class="small">Quote Jal:</font><hr> Sorry about that Spiderman. I didn't get your meaning.

Jim <hr /></blockquote>

I see now how I might have said it more clearly. Probably I should make more use of the "preview" feature.

SpiderMan

Fran Crimi
01-17-2007, 11:11 PM
[ QUOTE ]
Knowing that "overspin" is possible... <hr /></blockquote>

Correct me if I'm wrong here, but isn't knowing that the CB can be sliding and spinning at the same time a fundamental piece of knowledge of the game --- pool 101 stuff?

We're not talking ground-breaking stuff here, right?

Fran

Billy_Bob
01-18-2007, 07:09 AM
I think both of the following shots are cases where the cue ball is spinning forward faster than it is moving forward. I can never "see" this happening even though I am using a measles ball, but I assume this is what is going on. I can only see the effects of the shot and what the cue ball does after it contacts the cue ball.

Note that both of the following shots are very difficult and may be impossible when using a house cue with a cruddy tip. (I can't make these shots when using a house cue which has a slick tip.) The tip needs to grip the cue ball extremely well.

Hook shot - This shot is high center, closed bridge, medium speed and tons of follow through. Repeat MEDIUM speed. Fast will not work. (For me at least.) Aim slightly to the right side of the object ball. The placement of the cue ball is critical. Note its placement in relation to the rack spot. (BTW I have a 7 ft. table if that matters.) A few inches up or down table in either direction and the shot is not possible. You are basically shooting a slight left cut into the side pocket. There is forward roll on the cue ball and it wants to continue toward the long rail after contacting the object ball. But after the cue ball hits the long rail, the friction on the rail re-orients the cue ball so the forward roll is re-directed toward the short rail. Thus the cue ball has the hook effect as shown.

[ Wei Table http://cuetable.com/WeiTable ]

START(
%AZ1Z7%PN0P4%WX7Y7%XN2P7%eB4`2%_j9X9%`Z5Y1%aX8[3

)END


Make two balls with one shot. To make this shot, you need a tip which has been recently scuffed (good tip in good condition) and chalked well. Use a closed bridge, fast speed, and TONS of follow through. I tell people to use so much follow through it seems they are trying to send the tip of their cue through the far wall after contacting the cue ball. Aim to the left side of the object ball.

START(
%AD3D3%CD2Z7%Ph0F7%WE9E1%Xg2F6%eA8`4%bD9X8%cF4O7%d C6F0

)END

SpiderMan
01-18-2007, 08:05 AM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> &lt;/font&gt;&lt;blockquote&gt;&lt;font class="small"&gt;Quote:&lt;/font&gt;&lt;hr /&gt;
Knowing that "overspin" is possible... <hr /></blockquote>

Correct me if I'm wrong here, but isn't knowing that the CB can be sliding and spinning at the same time a fundamental piece of knowledge of the game --- pool 101 stuff?

We're not talking ground-breaking stuff here, right?

Fran <hr /></blockquote>

You asked "How is it helpful to a pool player to know this information?", so I provided one reason and an (obvious) example. Perhaps it was just a rhetorical question on your part?

SpiderMan

Bob_Jewett
01-18-2007, 12:18 PM
<blockquote><font class="small">Quote Yoda:</font><hr> Can a cue ball have more top spin than actual rolling speed? That is, can the cue ball be spinning faster than it is rolling forward? <hr /></blockquote>
You hit on an important concept that seems to be largely overlooked (or underappreciated). That is the "quality" of the spin on the ball. I was introduced to that term by a European balkline and Artistic Billiards player, and the usage seems to be common there. What is meant by "quality" is the spin/speed ratio. Suppose you have enough draw on the cue ball to bring it back two feet. If this is for a very soft shot, you have a very high quality of spin. But suppose you have that much draw with break-shot speed on the cue ball. Then the quality of spin is low.

It is possible to assign a number to the quality of the spin. For a cue ball that is rolling smoothly without slipping on the cloth, call the quality of the follow "unity" or 1.00 or 100%. Note that this does not refer to the speed, since what we are interested in is the spin/speed ratio. In the case of unity quality, the bottom of the ball is stationary.

(This last statement takes a little while to grasp for most. Here is a related brain teaser: On any train locomotive that is moving forward even at 100 MPH, there is a part of the locomotive that is moving backwards at all times. What is that part?)

Similarly for a draw shot, if it has "100% quality," the top of the ball is stationary. And for a shot with only side spin, it is possible to have the side of the cue ball opposite to the tip side stationary as the ball moves down the table.

So, in this context your question translates to "is it possible to have follow with more than 100% quality?" That is, can we have the bottom of the cue ball actually moving backwards on the table as the cue ball itself is moving forward? As others have pointed out, the answer is yes. Under most conditions, it's not useful to do that, though. Robert Byrne in his "Advanced Technique" book gives a simple test you can try to see if you can get "overspin" on the cue ball.

Of course, if the cue ball strikes another ball, the speed is reduced (sometimes to 0) and the spin remains, so you might say that the quality of the spin is very large in such a case, but the term normally seems to be used only prior to the cue ball hitting any other ball. One common way to obtain very high quality of spin is to play a masse shot, in which the bed of the table is in effect removing the speed from the cue ball and at the same time increasing the spin by holding the ball against the tip.

Fran Crimi
01-18-2007, 12:45 PM
No, I wasn't being anything but truthful, and no, you didn't answer my question. My question had to do with comparing spin and roll rates. Forget it. It's not important.

Fran

Fran Crimi
01-18-2007, 12:49 PM
[ QUOTE ]
Suppose you have enough draw on the cue ball to bring it back two feet. If this is for a very soft shot, you have a very high quality of spin. But suppose you have that much draw with break-shot speed on the cue ball. Then the quality of spin is low.
<hr /></blockquote>

I can't believe what's going on here. How can you possibly say this has been overlooked? How can anyone play a decent game of pool without at least addressing this, maybe not so consciously or calculatively---but I'd like to see anyone with decent speed control who has never addressed this in their game.

Guys...what is the big deal here? I don't get it. I don't mean to sound pretentious but my students get this information very very early on.

Fran

Bob_Jewett
01-18-2007, 01:26 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> ... I can't believe what's going on here. How can you possibly say this has been overlooked? ... <hr /></blockquote>
Fran, I did not say that "Fran Crimi has criminally overlooked this aspect of the game." I said that it was "largely overlooked." My experience is that many players don't understand this and do not have the conscious concept of the quality of spin. For example, many players don't understand that draw can increase the quality of the side spin on the ball.

Again, I was not attacking Fran Crimi.

Fran Crimi
01-18-2007, 06:08 PM
Oh, c'mon, Bob. I'm not assuming you're attacking me. I think that you are grossly underestimating what pool players naturally figure out as they progress in their games. They may not use the same terminology as you do to describe it but the knowledge is the same. Any decent 9 Ball player didn't get to where they were by overlooking the relationship between spin and slide. In fact, I start my beginners early with this stuff because it's so fundamental.

You're not giving players enough credit, and this has nothing to do with me.

Fran

cushioncrawler
01-19-2007, 01:15 AM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote Yoda:</font><hr> Can a cue ball have more top spin than actual rolling speed? That is, can the cue ball be spinning faster than it is rolling forward? <hr /></blockquote>
You hit on an important concept that seems to be largely overlooked (or underappreciated). That is the "quality" of the spin on the ball. I was introduced to that term by a European balkline and Artistic Billiards player, and the usage seems to be common there. What is meant by "quality" is the spin/speed ratio. Suppose you have enough draw on the cue ball to bring it back two feet. If this is for a very soft shot, you have a very high quality of spin. But suppose you have that much draw with break-shot speed on the cue ball. Then the quality of spin is low.

It is possible to assign a number to the quality of the spin. For a cue ball that is rolling smoothly without slipping on the cloth, call the quality of the follow "unity" or 1.00 or 100%. Note that this does not refer to the speed, since what we are interested in is the spin/speed ratio. In the case of unity quality, the bottom of the ball is stationary.

(This last statement takes a little while to grasp for most. Here is a related brain teaser: On any train locomotive that is moving forward even at 100 MPH, there is a part of the locomotive that is moving backwards at all times. What is that part?)

Similarly for a draw shot, if it has "100% quality," the top of the ball is stationary. And for a shot with only side spin, it is possible to have the side of the cue ball opposite to the tip side stationary as the ball moves down the table.

So, in this context your question translates to "is it possible to have follow with more than 100% quality?" That is, can we have the bottom of the cue ball actually moving backwards on the table as the cue ball itself is moving forward? As others have pointed out, the answer is yes. Under most conditions, it's not useful to do that, though. Robert Byrne in his "Advanced Technique" book gives a simple test you can try to see if you can get "overspin" on the cue ball.

Of course, if the cue ball strikes another ball, the speed is reduced (sometimes to 0) and the spin remains, so you might say that the quality of the spin is very large in such a case, but the term normally seems to be used only prior to the cue ball hitting any other ball. One common way to obtain very high quality of spin is to play a masse shot, in which the bed of the table is in effect removing the speed from the cue ball and at the same time increasing the spin by holding the ball against the tip. <hr /></blockquote>
Bob.
1.... This reminds me that i spent weeks (months actually) looking into (ie imagining and thinking about) the bottom of the ball. In the end, by memory, i came to a few conclusions.

For a hi-speed shot, the (rolling) qball is throwing dirt backwards initially -- this iz koz the air rezistance iz paramount. Then, after the qball slows down, it is throwing dirt forwards -- this iz koz rolling rezistance (cloth rezistance) iz paramount.

2.... The train question -- yes, the bits of the wheels below the top of rail are going backwards. Which remindz me, one of my billiards mates used to drive steam trains, and on one occasion he had a bad headwind and the train ran out of coal a long way short of the station, here the bottom bits of the wheels werent doing anything at all.

3.... Re topspin, my main shot in english billiards iz the 3/4 ball runthroo in-off from in hand in the Dee to get a middle-pocket in-off, bringing the object-ball (the red) back for another middle-pocket in-off. When the separation between the qball and red is say 2 or 3 balls, the in-off is still very gettable, but, i havta allow for a slightly smaller run-throo angle, koz of overspin (i allwayz hit the qball very very high, and i uze a whippy cue, or i use an upwards flourish if uzing a stiffy).

4.... One more thing. I often see everyone talking about getting natural roll by hitting the qball 2/7ths above center, or some such thing. This is incorrect. If u hit the qball above center, or below center, or anywhere at all, u will get zero topspin and zero bottomspin. U will only get topspin or bottomspin if the cue bends during impact (ie a whippy cue), or if the cue is allowed to moov up or down during impact (ie if u uze a facile action). If u uze a very stiff cue (like i sometimes do), u will not be able to get natural rolling at short range to the object-ball, no matter how high u hit the qball. At short range, and at medium range, i am happy to hit the qball lowish, but, i make a big effort to lift the qtip throo the qball, to make sure that i dont get any stun-effect. madMac.

Yoda
01-19-2007, 07:27 AM
Wow. I never expected so much response to my post. I rarely post, just read to get information.

The reason I asked the question was that I had a master instructor tell me that you cannot put more top spin on a cueball than its natural roll. I thought he was wrong and thats why I asked.

I think its useful to know this. Dr. Dave's 30 degree rule has proven very helpful to me in planning position. Thanks Dr. Dave! However, I think it is an estimate (which is certainly better than a wild guess), but sometimes you have to know when you need to adjust.

My feeling is that if you hit an object ball close to the cueball, say a foot away, rather hard with a lot of topspin, the cueball can be sliding and spinning at the same time. It may be FLOATING and spinning for all I know. At any rate I think its spinning faster than you would expect just due to natural roll. In that case, you might want to adjust the 30 degree rule and assume a few degree less.

Am I correct or way off base?

Fran Crimi
01-19-2007, 08:24 AM
<blockquote><font class="small">Quote Yoda:</font><hr> Wow. I never expected so much response to my post. I rarely post, just read to get information.

The reason I asked the question was that I had a master instructor tell me that you cannot put more top spin on a cueball than its natural roll. I thought he was wrong and thats why I asked.

I think its useful to know this. Dr. Dave's 30 degree rule has proven very helpful to me in planning position. Thanks Dr. Dave! However, I think it is an estimate (which is certainly better than a wild guess), but sometimes you have to know when you need to adjust.

My feeling is that if you hit an object ball close to the cueball, say a foot away, rather hard with a lot of topspin, the cueball can be sliding and spinning at the same time. It may be FLOATING and spinning for all I know. At any rate I think its spinning faster than you would expect just due to natural roll. In that case, you might want to adjust the 30 degree rule and assume a few degree less.

Am I correct or way off base? <hr /></blockquote>


There you go...someone who just naturally figured out that the cue ball can spin and slide at the same time. Good for you, Yoda. That means you're spending some time working on your game.

As for what the Master Instructor told you, I guess it's possible that someone can make a blunder like that but I'd rather hope that there was some kind of miscommunication where the instructor was trying to say that the ball can't spin any faster if it's already rolling as opposed to when it's sliding. Maybe you could contact the instructor for a clarification and further discussion?

Fran

dr_dave
01-19-2007, 09:14 AM
<blockquote><font class="small">Quote Yoda:</font><hr>Dr. Dave's 30 degree rule has proven very helpful to me in planning position. Thanks Dr. Dave! However, I think it is an estimate (which is certainly better than a wild guess), but sometimes you have to know when you need to adjust.<hr /></blockquote>Good point. As your levels of sophistication and ability improve, there are subtle adjustments you can make. The links under "30 degree rule" in the thread summary portion of my website (http://www.engr.colostate.edu/~dga/pool/threads.html) point to resources that explain the types of adjustments required in different situations. Also, I have 12 articles in the instructional articles section of my website (http://www.engr.colostate.edu/~dga/pool/bd_articles/index.html) dedicated to the 90 and 30 degree rules. They provide illustrations and examples covering all of the details.

Regards,
Dave

Jal
01-19-2007, 12:17 PM
<blockquote><font class="small">Quote Yoda:</font><hr>...My feeling is that if you hit an object ball close to the cueball, say a foot away, rather hard with a lot of topspin, the cueball can be sliding and spinning at the same time.<hr /></blockquote>Yes. But it will do this after impact even if you hit it slow.

<blockquote><font class="small">Quote Yoda:</font><hr>It may be FLOATING and spinning for all I know.<hr /></blockquote>Yes.

<blockquote><font class="small">Quote Yoda:</font><hr>At any rate I think its spinning faster than you would expect just due to natural roll.<hr /></blockquote>Yes.

<blockquote><font class="small">Quote Yoda:</font><hr>In that case, you might want to adjust the 30 degree rule and assume a few degree less.<hr /></blockquote>For a change of pace, no. No, because the 30 degree rule is based on just this happening (spinning faster than if it were rolling after impact). However, the 30 degree rule isn't exactly 30 degrees anyway (see Dr. Dave's article).

Jim

Bob_Jewett
01-19-2007, 01:28 PM
<blockquote><font class="small">Quote cushioncrawler:</font><hr> ... 4.... One more thing. I often see everyone talking about getting natural roll by hitting the qball 2/7ths above center, or some such thing. This is incorrect. If u hit the qball above center, or below center, or anywhere at all, u will get zero topspin and zero bottomspin. U will only get topspin or bottomspin if the cue bends during impact (ie a whippy cue), or if the cue is allowed to moov up or down during impact (ie if u uze a facile action). ... <hr /></blockquote>

In my experience, the above paragraph is absolutely false. It is easy to get a lot of spin on the cue ball with a stiff cue as many Artistic Billiards players frequently demonstrate, even with firm bridges.

But perhaps you were playing a word trick with the quoted paragraph. All cue sticks bend during the collision when striking the cue ball off center. All cue sticks move away from the center of the cue ball when striking the cue ball off center (or more precisely, the front part of all cue sticks do so). So in some sense what you say is correct, since your supposed requirements for putting spin on the cue ball are true for all cue sticks.

cushioncrawler
01-19-2007, 03:40 PM
<blockquote><font class="small">Quote Yoda:</font><hr> ....I think its useful to know this. Dr. Dave's 30 degree rule has proven very helpful to me in planning position. Thanks Dr. Dave! However, I think it is an estimate (which is certainly better than a wild guess), but sometimes you have to know when you need to adjust.

My feeling is that if you hit an object ball close to the cueball, say a foot away, rather hard with a lot of topspin, the cueball can be sliding and spinning at the same time. It may be FLOATING and spinning for all I know. At any rate I think its spinning faster than you would expect just due to natural roll. In that case, you might want to adjust the 30 degree rule and assume a few degree less. Am I correct or way off base? <hr /></blockquote>
Yoda. Yes, i agree with u and all, "overspin", ie topspin surplus to "natural roll", can happen. Like i said earlyr, with the qball and objectball only say one or two balls apart, i havta play my favorit 3/4 ball runthroo in-off narrower -- here what i do is i contact the objectball 3/4 ball, but place my qball (which is in-hand) for a slightly narrower angle than the uzual 30dg.

Here above notice that i mention that the 3/4 ball deflexion angle for me is 30dg, which is the angle that i get for my tests for my medium pace long-loozerz on a 12' table uzing my 2-1/16" krapamiths.

But az per uzual the actual angle depends on how u define it -- for a 1/2 ball loozer, my measured angle was 35dg -- 1/4 ball was 28dg. Just thort it worthwhile mentioning this sort of thing, koz i am sure that some players will be doing tests and measurements one day. Anyhow, there are at least 3 different ways of defining the deflexion angle -- hence there are at least 3 possible answers (angles), yet there is only one possible result (at one particular pace anyhow) -- this is starting to sound like the billiards version of the catholic Trinity (but our angles are their angels). madMac.

cushioncrawler
01-19-2007, 04:04 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote cushioncrawler:</font><hr> ... 4.... One more thing. I often see everyone talking about getting natural roll by hitting the qball 2/7ths above center, or some such thing. This is incorrect. If u hit the qball above center, or below center, or anywhere at all, u will get zero topspin and zero bottomspin. U will only get topspin or bottomspin if the cue bends during impact (ie a whippy cue), or if the cue is allowed to moov up or down during impact (ie if u uze a facile action).... <hr /></blockquote> In my experience, the above paragraph is absolutely false. It is easy to get a lot of spin on the cue ball with a stiff cue as many Artistic Billiards players frequently demonstrate, even with firm bridges. But perhaps you were playing a word trick with the quoted paragraph. All cue sticks bend during the collision when striking the cue ball off center. All cue sticks move away from the center of the cue ball when striking the cue ball off center (or more precisely, the front part of all cue sticks do so). So in some sense what you say is correct, since your supposed requirements for putting spin on the cue ball are true for all cue sticks. <hr /></blockquote> Hi Bob, What i meant was that we all talk about needing to hit the qball 2/7ths above center to get good rolling, but whenever i see this i allways think to myself that "its not that simple". This holy 2/7ths only exists when other factors help. For instance, if the cue were a very solid bar of very heavy steel, ie zero give, ie lots of inertia, the qball would be stunned (zero topspin) no matter how high u contacted the qball above the center.

But just thinking about what i said, i woz wrong regarding screw shots, there would be a little bit more than zero screw on the qball when hit below center with our solid steel bar. Likewize, there would be a little bit more than zero sidespin if hit with (attempted) english.

If people (and players) dont realise this sort of thing, then they have a false picture of what actually happens. And, it duz affect the answers to the original question on this thread. Where this sort of thinking duz become a big deal is when we are talking about cushions and rebound theory, koz here the holy 2/7ths duz come completely off the rails (pun intended). All sorts of people (schoolkidz etc) say all sorts of silly things, koz they havent got the slightest clue about the real factors in play, and they dont realize that the 2/7ths is meaningless and not holy in any way whatsoever. madMac.

Jal
01-19-2007, 05:39 PM
<blockquote><font class="small">Quote cushioncrawler:</font><hr>...But just thinking about what i said, i woz wrong regarding screw shots, there would be a little bit more than zero screw on the qball when hit below center with our solid steel bar. Likewize, there would be a little bit more than zero sidespin if hit with (attempted) english.<hr /></blockquote>Hi Mac,

Take a perfectly rigid cue of infinite mass...okay, have it the weight of a freight train to make stroking easier. Naturally, you'll see a lot of squirt, but the squirt itself should allow you to get spin on the ball. It'll do this by assuming a value such that the "point" of contact will be moving in the direction of the stick as the cueball picks up speed and spin. Namely, if the tip offset angle is "rho", ie, rho = 30 deg at offset .5R, then the tangent of the squirt angle "gamma" will be:

tan(gamma) = (5/2)tan(rho)/[(5/2) + sec(rho)^2]

If hit at (2/5)R from center (rho = 23.6 deg), the squirt angle would be 16.5 degrees. This is the worst squirt you should ever expect to see in a cue at this offset. The spin/speed ratio (RW/V) would then be 0.31 instead of 1. But you would get spin, I believe.

The formula predicts that you will never see squirt so bad that the resultant force points directly at the center of the cueball. The smallest value that sec(rho) can assume is 1, so the squirt angle gamma will always be less than the offset angle rho.

<blockquote><font class="small">Quote cushioncrawler:</font><hr>...If people (and players) dont realise this sort of thing, then they have a false picture of what actually happens. And, it duz affect the answers to the original question on this thread. <hr /></blockquote>I don't know Mac, (2/5)R still does it for me. I don't see anything except a super squirty cue, much more than is normally encountered, changing that in a significant way.

Nevertheless, along the same lines, I've been wondering about another thing. When you apply spin, the separation speed between the stick and the ball indicates that the force vs time curve must become unsymmetric about the midpoint (T/2), the more offset, the more unsymmetric. It must become unsymmetric in such a way that the peak force happens earlier during the contact period, and severely so at large offsets.

The tip/stick/ball apparently accomplishes this as the tip gets pushed aside: the geometry of the surfaces helps to unload them. But how well does it do this? Apparently quite well given the spins that are achieved. But there might be some particular tip curvature and thickness that works better than others, if max spin is the goal.

<blockquote><font class="small">Quote cushioncrawler:</font><hr>Where this sort of thinking duz become a big deal is when we are talking about cushions and rebound theory, koz here the holy 2/7ths duz come completely off the rails (pun intended). All sorts of people (schoolkidz etc) say all sorts of silly things, koz they havent got the slightest clue about the real factors in play, and they dont realize that the 2/7ths is meaningless and not holy in any way whatsoever.<hr /></blockquote>Well, I used it to predict that the spin transfer on a full hit perpendicular into a cushion should cause the object ball to come off at the throw angle. But this seems to be way off from viewing Dr. Dave's spin transfer video. But much the same reasoning seems to predict fairly accurately the shortened angle of a ball stunned into a cushion (no spin).

I'd like to know what you're talking about in more detail if possible. It might explain the discrepancy.

Jim

slim
01-19-2007, 07:36 PM
Yes..............

cushioncrawler
01-19-2007, 08:05 PM
<blockquote><font class="small">Quote Jal:</font><hr>.... I don't know Mac, (2/5)R still does it for me. I don't see anything except a super squirty cue, much more than is normally encountered, changing that in a significant way.

Nevertheless, along the same lines, I've been wondering about another thing. When you apply spin, the separation speed between the stick and the ball indicates that the force vs time curve must become unsymmetric about the midpoint (T/2), the more offset, the more unsymmetric. It must become unsymmetric in such a way that the peak force happens earlier during the contact period, and severely so at large offsets.

The tip/stick/ball apparently accomplishes this as the tip gets pushed aside: the geometry of the surfaces helps to unload them. But how well does it do this? Apparently quite well given the spins that are achieved. But there might be some particular tip curvature and thickness that works better than others, if max spin is the goal.

.... Well, I used it to predict that the spin transfer on a full hit perpendicular into a cushion should cause the object ball to come off at the throw angle. But this seems to be way off from viewing Dr. Dave's spin transfer video. But much the same reasoning seems to predict fairly accurately the shortened angle of a ball stunned into a cushion (no spin).

I'd like to know what you're talking about in more detail if possible. It might explain the discrepancy. Jim <hr /></blockquote>Hi Jim. Dont forget that when a qball is hit above center by our steelrod, there is no (ordinary) squirt, koz, the qball is trapped by the bed. Now, if the steelrod to qball friction is a lot less than the bedtoball friction, the qball will come out (bobble out) sort of rolling. But, if the bedtoball friction is much weaker than the rodtoball friction, then the qball karnt have any topspin. How can it?? As a cue leaves a qball, if there is no misscue, both are locked together, the speed of the contact point is mutual. Now, if the steel rod haznt got any vertical velocity, then the contact point on the ball karnt have any vertical velocity, and the ball karnt have any topspin. But if u said that the qball must squirt out of its predicament (ie go forwards and upwards a bit), then i wouldnt argue too much, but here i would point out that the qball would then have to have bottomspin anyhow, not the topspin that we were looking for.

Regarding pool table cushions, every close up slow-mo that i have ever seen proovs again and again that everything that everyone has ever said about the 9/14ths ht of the cushion and perfict rebound etc is all complete bullkrap (a bit of an exaggeration, but it will do just fine as a starting point for my arguement).

Did u see the Austrian slow-mo where the qball iz flying (slightly off the bed) square into a cushion, with no topspin, it sinks into the cushion, the contact being well above center of ball, and the ball comes back with zero topspin, whereas it shood have had ooodles of top. Once again, as per our steel cue, the point of contact on the ball can only do what the cushion was doing when contact ended (unless there was slippage, which is often likely).

If the cushion had been a square-faced english cushion the Austrians' ball would have had a fair amount of topspin after rebound, koz, the english cushions allow lots of vertical moovment. In the above impact, the cushion would have mooved with the ball, ie if the ball wants to have topspin then the cushion would not do very much to stop it, ie the cushion would be mooving upwards "with" the ball (but its actually all much more complicated than that i reckon). Pool cushions dont like to moov up or down i reckon. madMac.

dr_dave
01-20-2007, 08:56 AM
<blockquote><font class="small">Quote cushioncrawler:</font><hr>What i meant was that we all talk about needing to hit the qball 2/7ths above center to get good rolling, but whenever i see this i allways think to myself that "its not that simple". This holy 2/7ths only exists when other factors help. For instance, if the cue were a very solid bar of very heavy steel, ie zero give, ie lots of inertia, the qball would be stunned (zero topspin) no matter how high u contacted the qball above the center.<hr /></blockquote>The only way an off-center hit could result is absoultely no spin (i.e., stun) is if there were absolutely no friction between the tip and the ball (see TP 2.1 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/TP_2-1.pdf), TP 4.2 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/TP_4-2.pdf) and TP A.12 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/new/TP_A-12.pdf)). In other words, your infinitely stiff and infinitely heavy stick would also need to have a perfectly-lubricated, mirror-smooth, hard-surface tip.

Regards,
Dave

Jal
01-20-2007, 01:24 PM
<blockquote><font class="small">Quote cushioncrawler:</font><hr>Hi Jim. Dont forget that when a qball is hit above center by our steelrod, there is no (ordinary) squirt, koz, the qball is trapped by the bed. Now, if the steelrod to qball friction is a lot less than the bedtoball friction, the qball will come out (bobble out) sort of rolling. But, if the bedtoball friction is much weaker than the rodtoball friction, then the qball karnt have any topspin. How can it?? As a cue leaves a qball, if there is no misscue, both are locked together, the speed of the contact point is mutual. Now, if the steel rod haznt got any vertical velocity, then the contact point on the ball karnt have any vertical velocity, and the ball karnt have any topspin. But if u said that the qball must squirt out of its predicament (ie go forwards and upwards a bit), then i wouldnt argue too much, but here i would point out that the qball would then have to have bottomspin anyhow, not the topspin that we were looking for.<hr /></blockquote>Mac, with regard to applying topspin with the heavy cue, I can't see how what you're saying could be wrong. In order to balance the torques from the stick and cloth so that no spin occurs, the "squirt" angle would have to be larger than the offset angle. Namely,

tan(gamma) = tan(rho)/[1 - u/cos(rho)]

where u is the coefficient of ball/cloth friction. You wouldn't actually get squirt, of course, except for some initial settling in, but the stick force would have to be pointing down at this angle to cancel the torque from the cloth. So I do believe you're right, given the premises concerning the cue.

With a normal one, I doubt that the (2/5)R offset for producing immediate natural roll is affected very much, except at abnormal elevations. With a cueball speed of 10 mph, which is very fast for this much offset, contact time of 2 msec, and a total downward angle of 6 degrees from cue elevation plus squirt (say 3+3 respectively), the ball should penetrate the cloth about an additional 1/40 inch during impact. I suppose this might be beginning to get significant, but since the ball is trying to roll, it's not obvious that the cloth/bed would prevent this (or require that (2/5)R be something much different). But maybe you see it otherwise.

<blockquote><font class="small">Quote cushioncrawler:</font><hr>...Did u see the Austrian slow-mo where the qball iz flying (slightly off the bed) square into a cushion, with no topspin, it sinks into the cushion, the contact being well above center of ball, and the ball comes back with zero topspin, whereas it shood have had ooodles of top. Once again, as per our steel cue, the point of contact on the ball can only do what the cushion was doing when contact ended (unless there was slippage, which is often likely).<hr /></blockquote>I think you might be refering to the ending sequence? In the opening one, the ball looks to be a little higher than the nose and does develop some backspin. The ending one does appear to be analogous to the rigid cue/topspin situation, with presumably static friction holding between the cushion and ball. Interesting observation.

Jim

Fran Crimi
01-20-2007, 02:18 PM
[ QUOTE ]
...If people (and players) dont realise this sort of thing, then they have a false picture of what actually happens. And, it duz affect the answers to the original question on this thread.... <hr /></blockquote>

Hey Mac, philosophically, if someone has a false picture of of what actually happens with spinning and sliding balls, but they still perform beautifully in spite of that false picture, say, like Efren Reyes, then how significant would it be for them to have the correct picture?

Fran

cushioncrawler
01-20-2007, 03:32 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> &lt;/font&gt;&lt;blockquote&gt;&lt;font class="small"&gt;Quote:&lt;/font&gt;&lt;hr /&gt;
...If people (and players) dont realise this sort of thing, then they have a false picture of what actually happens. And, it duz affect the answers to the original question on this thread.... <hr /></blockquote> Hey Mac, philosophically, if someone has a false picture of what actually happens with spinning and sliding balls, but they still perform beautifully in spite of that false picture, say, like Efren Reyes, then how significant would it be for them to have the correct picture? Fran <hr /></blockquote> Hi Fran -- I guess that a knowledge of science rarely helps (or harms) a billiards player (or golf or tennis). But i reckon that it occasionally helps guys like me. For instance whenever i gave my very stiff cue a trial, i used to find that when i hit the qball high i got too much stun effect, even when the objectball was a long way away. Nowadays, i know that with a stiffy (especially a heavy one) i havta uze a facile flourish to help the cue get the needed topspin or screw or english. For me, knowing the cause has helped me to understand and believe and allow for the problem. Its like i allwayz say...
.................. Believing iz Seeing.

Fran Crimi
01-20-2007, 04:47 PM
Thanks. I appreciate your honest answer. I think the problem I as well as others have in reading these threads is that they very rarely get translated into something practical that we can put our fingers on. I can probably say with certainty that I will never be able to measure 2/7ths of a cue ball by eyeballing it, which is what I would have to do during a game, yet you present it as being important.

See, the interesting thing is that good players know how to get the maximum roll from a cue ball by using words like "slightly above center". They can relate to that, not because they're stupid, but because that's all you can see when you're eyeballing a shot during a game. And truthfully, Mac, I figured that out 20 years ago about striking the cue ball "slightly above center."

As a teacher, I feel for my students and I try to keep things as simple and as straight forward for them as possible. If you guys are going to make something basic and simple look complicated, then unfortunately, I'm going to feel like I have to butt in (pardon the pun)---simply because I feel that it's not fair to present it to the public in such a complicated way.


Fran

cushioncrawler
01-20-2007, 05:16 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote cushioncrawler:</font><hr>What i meant was that we all talk about needing to hit the qball 2/7ths above center to get good rolling, but whenever i see this i allways think to myself that "its not that simple". This holy 2/7ths only exists when other factors help. For instance, if the cue were a very solid bar of very heavy steel, ie zero give, ie lots of inertia, the qball would be stunned (zero topspin) no matter how high u contacted the qball above the center.<hr /></blockquote>The only way an off-center hit could result is absoultely no spin (i.e., stun) is if there were absolutely no friction between the tip and the ball (see TP 2.1 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/TP_2-1.pdf), TP 4.2 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/TP_4-2.pdf) and TP A.12 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/new/TP_A-12.pdf)). In other words, your infinitely stiff and infinitely heavy stick would also need to have a perfectly-lubricated, mirror-smooth, hard-surface tip. Regards, Dave <hr /></blockquote>Hi Dr Dave -- That 2/7ths figure of mine shood have been 2/5ths, i think i might commit suicide (whilst listening to "Nights In White Satin"). Just had (another) look at TP2.1 and TP4.2 and TPA.12, all good and correct and usefull.

But, a "horizontal impulse" can only exist in MathLand. In RealLand, the contact point on a ball can only go where the cue or cushion lets it go. A contact point is allways mutual, unless there is slippage. If a ball is sent off with (say) spin, then 100% of this spin exists at the end of qtip-ball contact, ie the qtip (or cushion) are az one at this instant. This is where cues (a bit) and cushions (a lot) start departing from simple theory. Not saying that simple theory iznt any good, its all we have sometimes.

I agree that there will allways be spin if a qball is hit off-center, unless (as u say) there is zero friction. But, not (i reckon) for above-center (but central) hits -- koz here the qball's uzual squirt is blocked by the bed. In my posting to JAL i said that there might yet be squirt if the cue were very (infinitely) stiff and very (infinitely) heavy , but that this squirt would involve the qball going upwards (somewhat) and would (i reckon) rezult in backspin (not forwardspin). In fact, this sort of thing happens in a way with an English cushion when the cushion cloth is new and slippery -- a ball hitting the cushion at hi-pace slips "under" the cushion, and is then "spat out" (and sometimes upish a bit) with allmost zero topspin. madMac.

cushioncrawler
01-20-2007, 05:50 PM
<blockquote><font class="small">Quote Jal:</font><hr> ..... since the ball is trying to roll, it's not obvious that the cloth/bed would prevent this (or require that (2/5)R be something much different). But maybe you see it otherwise.<hr /></blockquote>Jim, what i am saying iz that the qtip trys to prevent rolling. But, here, if u used a bent cue with the bend upwards, then the cue's natural bending during impact might actually help rolling.

<blockquote><font class="small">Quote cushioncrawler:</font><hr>...Did u see the Austrian slow-mo where the qball iz flying (slightly off the bed) square into a cushion, with no topspin, it sinks into the cushion, the contact being well above center of ball, and the ball comes back with zero topspin, whereas it shood have had ooodles of top. Once again, as per our steel cue, the point of contact on the ball can only do what the cushion was doing when contact ended (unless there was slippage, which is often likely).<hr /></blockquote>I think you might be refering to the ending sequence? In the opening one, the ball looks to be a little higher than the nose and does develop some backspin. The ending one does appear to be analogous to the rigid cue/topspin situation, with presumably static friction holding between the cushion and ball. Interesting observation. <hr /></blockquote> Jim -- Here iz one of my theoryz -- I reckon that a cushion's ability to make the ball rebound and the ability to put top or bottom on the ball, are two seperate effects.

A hypothetical. Imagine a ball with very little rolling hitting a flat vertical cushion squarely at hi-speed, with zero slippage. The ball's topspin goes to zero well before half-impact. The ball then starts to get backspin, even tho it is still sinking deeper into the cushion. The backspin reaches a peak and then drops to zero, even tho the ball is still sinking deeper into the cushion. The ball then gains topspin, which reaches a max and then drops to zero, etc etc. Here, the ball's topspin is swinging "back and forth" during impact.

Will the ball leave the cushion with top or bottom?? This depends on what phase it was in when impact ended. See????? This sort of effect, and others i wont try to describe, must come into at least some cushion impacts. But this is getting away from the original thread re topspin. madMac.

dr_dave
01-20-2007, 06:18 PM
madMac,

Your points about squirt are well taken. The "vertical squirt" effect does reduce the effectiveness of a follow shot. A cue with more shaft-end weight and stiffness creates more "downward squirt" with an above-center hit, and this does result in less horizontal impulse and less topspin. However, for a typical low-squirt cue, this effect is very small.

Regards,
Dave

Fran Crimi
01-21-2007, 09:29 AM
[ QUOTE ]
That 2/7ths figure of mine shood have been 2/5ths, <hr /></blockquote>

Wow. That's a huge mistake. And now I see that the famed 30 degree rule is really a variable 30 degree rule, give or take a few degrees. What other corrections are coming down the pike, guys?

Not for anything, Mac, but you sure made your 2/7ths rule sound very important in your posts, almost critical. What about all the posters who read your post and told their friends and haven't read your correction? Oh no! This could change the face of pool! (j/k of course)

Fran

cushioncrawler
01-21-2007, 01:49 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> &lt;/font&gt;&lt;blockquote&gt;&lt;font class="small"&gt;Quote:&lt;/font&gt;&lt;hr /&gt;
That 2/7ths figure of mine shood have been 2/5ths, <hr /></blockquote> Wow. That's a huge mistake. And now I see that the famed 30 degree rule is really a variable 30 degree rule, give or take a few degrees. What other corrections are coming down the pike, guys? Not for anything, Mac, but you sure made your 2/7ths rule sound very important in your posts, almost critical. What about all the posters who read your post and told their friends and haven't read your correction? Oh no! This could change the face of pool! (j/k of course) Fran <hr /></blockquote> Hi Fran -- Take it eezy on me or i will go out and buy a "Nights In White Satin" record and play it to myself every night, and then one night i might write a letter, never meaning to send. Yes, i play (old) records every night, this is a CD-free zone. Ive got Fats Waller, Duke, Satchmo, Dean Martin (my favorit), Perry Como, Bing (dont like bing), InkSpots, lots of 78s.

Anyhow, the gist of my threads was that the holy 2/5ths figure was much over-rated, and was theory only, and was in fact impossible theory only. Thusly any sort of mistake like this 2/7ths bizness only helps my arguements.

But this sort of booboo duznt allwayz affect play. For instance, John Roberts Jnr (1847 - 1920?) wrote in hiz books that the natural angle was 45dg, yet it was really 33dg the whole time (ivorys=33dg, kraps=35dg), yet he was unbeatable at English Billiards. But i feel sorry for all of the skoolkidz that have uzed my 2/7ths and have now failed their assignments. madMac.

SpiderMan
01-21-2007, 02:07 PM
Most players understand this intuitively, even if not analytically. But if you want to be able to quantify the effects of spin vs roll rates for a collision per my example, see Dr Dave's Tutorial (http://www.engr.colostate.edu/~dga/pool/technical_proofs/new/TP_A-4.pdf) .

Dave has explained this far better than I'd ever hope to duplicate, and with more clarity than any source I've seen. It's not rocket science, but does require a basic calculus background to follow. Or you can skip directly to the graphs - they can be interpreted without the anaylsis.

SpiderMan

dr_dave
01-21-2007, 04:18 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr>And now I see that the famed 30 degree rule is really a variable 30 degree rule, give or take a few degrees.<hr /></blockquote>Fran,

I think you were just kidding here, but I don't want people to misunderstand. The 30 degree rule states that the cue ball's direction changes by very close to 30 degrees for a natural roll shot for all cut shots between a 1/4-ball hit and a 3/4 ball hit. The "give or take a few degrees" is actually implied by the rule. The valuable knowledge is that the angle is very close to 30 degrees over this huge range of shots, and the peace-sign technique makes this knowledge very useful at the table. The details of how and when to applly the rule, as accurately as you desire, can be found in the articles (http://www.engr.colostate.edu/~dga/pool/bd_articles/index.html) and in my book.

Regards,
Dave

dr_dave
01-22-2007, 01:22 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Yoda:</font><hr>Dr. Dave's 30 degree rule has proven very helpful to me in planning position. Thanks Dr. Dave! However, I think it is an estimate (which is certainly better than a wild guess), but sometimes you have to know when you need to adjust.<hr /></blockquote>Good point. As your levels of sophistication and ability improve, there are subtle adjustments you can make. The links under "30 degree rule" in the thread summary portion of my website (http://www.engr.colostate.edu/~dga/pool/threads.html) point to resources that explain the types of adjustments required in different situations. Also, I have 12 articles in the instructional articles section of my website (http://www.engr.colostate.edu/~dga/pool/bd_articles/index.html) dedicated to the 90 and 30 degree rules. They provide illustrations and examples covering all of the details.<hr /></blockquote>Fran,

FYI, if you want a 1-page summary of the 30-degree rule, with lots of good visual resources, feel free to use my 1-page summary (http://www.engr.colostate.edu/~dga/pool/resources/30_degree_rule_summary.pdf). You and others are welcome to use this for instruction (e.g., as a handout for your students). I have several resources like this available in the instructor and student resources section of my website (http://www.engr.colostate.edu/~dga/pool/resources/index.html).

Happy teaching,
Dave