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View Full Version : Vernon Elliott cross-side bank shot



dr_dave
05-30-2007, 05:01 PM
This shot was discussed in a previous thread (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&Board=ccb&Number=247480&page =&view=&sb=&o=). Also, video demonstrations can be found at: video demo 1 (http://www.youtube.com/watch?v=ql-vXWEA_TU) and video demo 2 (http://www.youtube.com/watch?v=vEyCQSN8qU0). Bob Jewett also has a great article in the June '07 issue of BD that describes the physics behind the shot and how to execute it.

FYI, I just filmed and posted HSV A.142 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/new/HSVA-142.htm), a high-speed video clip that shows the shot in super-slow-motion. I cheated a little by adding chalk to the object ball surface. (You can see the big chalk smudge on the ball in the video). The chalk greatly increases throw and spin transfer (i.e., it causes "cling"), and makes the shot much, much easier (you hustler's out there should remember this).

Some people in the past wondered if the shot requires a double kiss to work. The video shows that that balls are far from a double kiss.

Happy viewing. Isn't physics fun?
Dave

Bob_Jewett
05-31-2007, 12:41 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> ... Some people in the past wondered if the shot requires a double kiss to work. The video shows that that balls are far from a double kiss.... <hr /></blockquote>
It is interesting to see that even with a fairly soft shot, the object ball goes quite far into the cushion. The cue ball has plenty of time to move on before any chance of a kiss.

dr_dave
05-31-2007, 03:38 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> ... Some people in the past wondered if the shot requires a double kiss to work. The video shows that that balls are far from a double kiss.... <hr /></blockquote>
It is interesting to see that even with a fairly soft shot, the object ball goes quite far into the cushion. The cue ball has plenty of time to move on before any chance of a kiss. <hr /></blockquote>Bob,

I was actually a little surprised by how much clearance there was ... not even close to a double-kiss.

Even more impressive to me was the magnitude of the change in rebound angle due to throw and spin transfer effects. I think the chalk on the object ball had a big effect here. I took some measurements (see below). The approach angle (relative to the rail normal) was about 20 degrees, and the rebound angle was about -11 degrees. Therefore, the throw and spin transfer (and rail throwback) effects account for a total change in direction from the theoretical (angle of incidence = angle of reflection) rebound line of about 30 degrees!!! I was shocked when I first saw this on the screen. Isn't high-speed video fun?

Regards,
Dave

http://www.engr.colostate.edu/~dga/pool/bd_articles/Vern_Elliot_cross-side_bank.gif

dr_dave
06-01-2007, 10:06 AM
This is from a different thread. I just thought I would include it here so all of the discussion would be in the same place.

<blockquote><font class="small">Quote cushioncrawler:</font><hr> Dr Dave -- The Vernon Elliot cutshot slo-mo appears to proov what i woz theorizing a few months ago. The qball haz clockwize spin. The OB comes off the cushion with clockwize spin. This is what i predicted (what a guy -- FIGJAM). The OB got (tried to get) a good dose of Anti-Clockwize spin during ball'to'ball impact, and this rezulted (initially) in retrograde XX cushion force, and later just before the ball left the cushion the cushion had enuff XX deformation remaining enabling it to continue with this retrograde XX force.<hr /></blockquote>Good summary. I was glad the video showed this effect also. BTW, the Vernon Elliot video is presented and discussed in a different thread (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&amp;Board=ccb&amp;Number=252128&amp;page =0&amp;view=collapsed&amp;sb=5&amp;o=). Check it out. I recently posted some angle measurements there.

<blockquote><font class="small">Quote cushioncrawler:</font><hr>Clearly too, the YY cushion deformation (in the slo-mo) is huge. I had never been convinced of Bob's (and other's) theory that the cushion sort of bunches up ahead of an attacking ball, and in effect changes (squares-up) the effektiv angle of attack. I allways reckoned that this could be simply explained by simple hysterisis. But, seeing such a big deformation at what is really a softish impact, i am starting to agree that this "squaring-up" is a big factor here (whether mainly hysterisis on not).<hr /></blockquote>If you want to see some really big rail deformation and how it affects rebound angle and ball spin, see HSV 6.1 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV6-1.htm).

<blockquote><font class="small">Quote cushioncrawler:</font><hr>Allso, notice that the ball'to'ball impakt angle is in fact not very thin at all. It might be about 1/32nd ball, not 1/128 or something else very thin.<hr /></blockquote>You are right. See my related posting in the Vern Elliott thread (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&amp;Board=ccb&amp;Number=252191&amp;page =0&amp;view=collapsed&amp;sb=5&amp;o=&amp;vc=1).

Regards,
Dave

cushioncrawler
06-02-2007, 08:05 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote cushioncrawler:</font><hr>..Clearly too, the YY cushion deformation (in the slo-mo) is huge. I had never been convinced of Bob's (and other's) theory that the cushion sort of bunches up ahead of an attacking ball, and in effect changes (squares-up) the effektiv angle of attack. I allways reckoned that this could be simply explained by simple hysterisis. But, seeing such a big deformation at what is really a softish impact, i am starting to agree that this "squaring-up" is a big factor here (whether mainly hysterisis on not).<hr /></blockquote>If you want to see some really big rail deformation and how it affects rebound angle and ball spin, see HSV 6.1 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV6-1.htm)..... <hr /></blockquote>Yes -- That stunned ball really goes deep into the cushion, and under. I couldnt see (in freeze-frame) any wrinkles in the cushion-cloth (near the leading edge of contact) that might confirm Bob's and Koehler's and other's theory about that being the cause of squaring-up of rebound angle. But, one can see a lot of daylite in the contact in the trailing edge (ie hysteresis). The Vernon cutback is a slower pace shot, but the natural squaring-up (hysteresis) effect must be huge anyhow.

I notice that the OB has very little topspin after it leaves the cushion, despite the fact that it had stun at impact (ie rolling before impact can be expected to rezult in less topspin on the OB after), and despite the fact that the qball buried down under the cushion (ie a higher effectiv cushion ht should too give more topspin on rebound). In fact, i reckon that the piddly little amount of topspin here is all mostly due to bedcloth friction while the ball is being "spat out" by the cushion, not due to direct cushion action at all.

I would say that if u remooved the cushion cloth and played this shot again the qball would come off (the rubber cushion) with tons of topspin, and would not have buried under, and would have jumped up off the table on leeving. Bare rubber cushions are funny, uzing sidespin u can get the ball to come straight back to u even when attacking at 45dg (uzing a cue) or even a bit more than 45dg (with fingerspin). madMac.

dr_dave
06-04-2007, 08:51 AM
<blockquote><font class="small">Quote cushioncrawler:</font><hr>I notice that the OB has very little topspin after it leaves the cushion, despite the fact that it had stun at impact (ie rolling before impact can be expected to rezult in less topspin on the OB after), and despite the fact that the qball buried down under the cushion (ie a higher effectiv cushion ht should too give more topspin on rebound). In fact, i reckon that the piddly little amount of topspin here is all mostly due to bedcloth friction while the ball is being "spat out" by the cushion, not due to direct cushion action at all.<hr /></blockquote>I thought the amount of rebound topspin looked about right for the speed of the shot. In case you didn't know, I have lots of good high-speed clips showing a kicked ball with follow, draw, and stun rebounding off a rail (see HSV 7.13-7.20 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/index.html)). HSV 7.18 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV7-18.htm) very nicely illustrates rebound of a stunned ball that picks up topspin and hop (from being driven down by the cushion).

Regards,
Dave

Bob_Jewett
06-04-2007, 11:49 AM
<blockquote><font class="small">Quote cushioncrawler:</font><hr> ... Yes -- That stunned ball really goes deep into the cushion, and under. I couldnt see (in freeze-frame) any wrinkles in the cushion-cloth (near the leading edge of contact) that might confirm Bob's and Koehler's and other's theory about that being the cause of squaring-up of rebound angle. ... <hr /></blockquote>
If I'm the Bob you're referring to, I hope I never claimed any such thing. As shown in one of my articles, the speed of the ball into the cushion has almost no effect on the rebound angle on a pool table. (Please, if anyone wants to dispute that, read the article and do the experiment first.) I also found that on a snooker table, a higher speed will cause a ball to bank slightly longer (BCE, 12x6, steel rails).

Most people are confused by the effect of follow on the banked ball. That's a different matter.

cushioncrawler
06-04-2007, 03:46 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote cushioncrawler:</font><hr> ... Yes -- That stunned ball really goes deep into the cushion, and under. I couldnt see (in freeze-frame) any wrinkles in the cushion-cloth (near the leading edge of contact) that might confirm Bob's and Koehler's and other's theory about that being the cause of squaring-up of rebound angle.... <hr /></blockquote>If I'm the Bob you're referring to, I hope I never claimed any such thing. As shown in one of my articles, the speed of the ball into the cushion has almost no effect on the rebound angle on a pool table. (Please, if anyone wants to dispute that, read the article and do the experiment first.) I also found that on a snooker table, a higher speed will cause a ball to bank slightly longer (BCE, 12x6, steel rails). Most people are confused by the effect of follow on the banked ball. That's a different matter.<hr /></blockquote>Hi Bob -- Perhaps it woz Bob Byrne, sorry, and definitely Koehler. But B &amp; K might be correct, at least for hi-speed attacks at closer to say 40dg (ie 40dg off the normal), if slo-mo shows a little wrinkle near the leading edge.

I agree that with the L profile (flatfaced) cushions on a 12' snooker table the rebound angle seems to lengthen as speed inkreeces. But, with K?? profiles, something peculiar happens. I think its like this....

"e" for rebound for a 00dg attack angle is less than 1.00 due partly to hysteresis. Hencely, one could expect a say 50dg rebound for a 40dg attack. But, for attack angles other than 00dg, hysteresis allso in effect (possibly) changes the effectiv attack angle (ie the effective angle prezented by the cushion), due largely to the huge cushion deformation with K?? profiles. Wrinkling at the leading edge, if any, would help the hysteresis here, as allready mentioned. So, for K?? cushions, the rebound angle is the rezult of a little war between hysteresis and hysteresis. This duznt seem to happen (to the same extent) with L cushions. madMac.

cushioncrawler
06-04-2007, 04:11 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote cushioncrawler:</font><hr>I notice that the OB has very little topspin after it leaves the cushion, despite the fact that it had stun at impact (ie rolling before impact can be expected to rezult in less topspin on the OB after), and despite the fact that the qball buried down under the cushion (ie a higher effectiv cushion ht should too give more topspin on rebound). In fact, i reckon that the piddly little amount of topspin here is all mostly due to bedcloth friction while the ball is being "spat out" by the cushion, not due to direct cushion action at all.<hr /></blockquote>I thought the amount of rebound topspin looked about right for the speed of the shot. In case you didn't know, I have lots of good high-speed clips showing a kicked ball with follow, draw, and stun rebounding off a rail (see HSV 7.13-7.20 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/index.html)). HSV 7.18 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV7-18.htm) very nicely illustrates rebound of a stunned ball that picks up topspin and hop (from being driven down by the cushion)....<hr /></blockquote>Dr Dave -- Had a look, thanks. Boy, i am still amazed at the awfull action (rebound) from a K?? profile. Unknown to most, very fast'n'lively'n'responsive cushions are bad news for top-level play in English billiards (and i think snooker) -- but the K?? profiles appear to be too much the other way, too "dead". Slippery new cushion cloths are a menace on any table, they "deaden" any cushion, perhaps Dr Dave's cushions were "too new" in the videos.

Certainly new cushion cloths would magnify the way a ball sinks "under" the cushion -- this "trapping" of the ball is huge in the videos. And of course a large amount of trapping can allways be expected if the ball hits with stun, and even more-so if with back-spin.

Even so, the lack of nice "natural" rolling (topspin) on the ball as it leaves the cushion is amazing. The balls seems to be happy to come back with zero topspin, and in one or two cases with actual backspin. I feel sure that the same sorts of videos etc on a 12' table would show much different behaviour. But even on a 12' table there is little chance of meeting that giddy ideal of a rolling ball coming back off with natural roll, ie the 0.7D ideal (but it might if we uzed nude rubber cushions, likewize for the K?? cushions).

With K?? cushions it is allmost a lie to call it "rebound", the ball is "spat" back. The cushion "effective e" must be murder. But, new cushion cloths might be the main culprit here. madMac.

dr_dave
06-05-2007, 03:37 PM
<blockquote><font class="small">Quote cushioncrawler:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr>I have lots of good high-speed clips showing a kicked ball with follow, draw, and stun rebounding off a rail (see HSV 7.13-7.20 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/index.html)). HSV 7.18 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV7-18.htm) very nicely illustrates rebound of a stunned ball that picks up topspin and hop (from being driven down by the cushion)....<hr /></blockquote>Dr Dave -- Had a look, thanks. Boy, i am still amazed at the awfull action (rebound) from a K?? profile. Unknown to most, very fast'n'lively'n'responsive cushions are bad news for top-level play in English billiards (and i think snooker) -- but the K?? profiles appear to be too much the other way, too "dead". Slippery new cushion cloths are a menace on any table, they "deaden" any cushion, perhaps Dr Dave's cushions were "too new" in the videos.<hr /></blockquote>I think my cloth is pretty average for a pool table. It certainly isn't "too new." I would describe it as "medium" (speed and "stickiness") relative to typical pool conditions. I live in Colorado, where the humidity is very low, so you might expect the cloth to play a little less "sticky," but I still think the conditions in the videos are fairly average compared to the various pool table conditions I've seen.

Regards,
Dave

Jal
06-06-2007, 01:51 PM
<blockquote><font class="small">Quote cushioncrawler:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote cushioncrawler:</font><hr>I notice that the OB has very little topspin after it leaves the cushion, despite the fact that it had stun at impact (ie rolling before impact can be expected to rezult in less topspin on the OB after), and despite the fact that the qball buried down under the cushion (ie a higher effectiv cushion ht should too give more topspin on rebound). In fact, i reckon that the piddly little amount of topspin here is all mostly due to bedcloth friction while the ball is being "spat out" by the cushion, not due to direct cushion action at all.<hr /></blockquote>I thought the amount of rebound topspin looked about right for the speed of the shot. In case you didn't know, I have lots of good high-speed clips showing a kicked ball with follow, draw, and stun rebounding off a rail (see HSV 7.13-7.20 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/index.html)). HSV 7.18 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV7-18.htm) very nicely illustrates rebound of a stunned ball that picks up topspin and hop (from being driven down by the cushion)....<hr /></blockquote>Dr Dave -- Had a look, thanks. Boy, i am still amazed at the awfull action (rebound) from a K?? profile. Unknown to most, very fast'n'lively'n'responsive cushions are bad news for top-level play in English billiards (and i think snooker) -- but the K?? profiles appear to be too much the other way, too "dead". Slippery new cushion cloths are a menace on any table, they "deaden" any cushion, perhaps Dr Dave's cushions were "too new" in the videos.

Certainly new cushion cloths would magnify the way a ball sinks "under" the cushion -- this "trapping" of the ball is huge in the videos. And of course a large amount of trapping can allways be expected if the ball hits with stun, and even more-so if with back-spin.

Even so, the lack of nice "natural" rolling (topspin) on the ball as it leaves the cushion is amazing. The balls seems to be happy to come back with zero topspin, and in one or two cases with actual backspin. I feel sure that the same sorts of videos etc on a 12' table would show much different behaviour. But even on a 12' table there is little chance of meeting that giddy ideal of a rolling ball coming back off with natural roll, ie the 0.7D ideal (but it might if we uzed nude rubber cushions, likewize for the K?? cushions).

With K?? cushions it is allmost a lie to call it "rebound", the ball is "spat" back. The cushion "effective e" must be murder. But, new cushion cloths might be the main culprit here. madMac. <hr /></blockquote>Mac, in order for a stunned ball to rebound with the amount of topspin you might expect for that much "tip offset" (about (1/4)R), the cushion would have to roll off the ball in the vertical direction, just like a cue's tip does. Instead, it does the opposite and flaps back down (to the extent that it flaps up in the first place).

Another way of looking at it is that the force vs time must be very different when putting spin on the ball, as indicated by this:

http://ww2.netnitco.net/users/gtech/F_vs_t_vs_offset.jpg

The above curves are for elastic collisions and only a schematic. (I have no idea what the actual curves look like, other than that they become very unsymmetric with tip offset.) Energy losses also distort the curves, in much the same way I think, but this is a different issue.

If you imagine a cushion compressing and then decompressing in a symmetric fashion about T/2, it just couldn't put any spin on the ball. The usual real world energy loss makes it unsymmetric too, but this doesn't help with spin.

As far as the "natural" shortening of the rebound angle, Bob Jewett describes a mirror system where, for a stunned ball with no sidespin, the virtual target pocket is located at about 70% of the width of the mirrored table. In other words, the parallel velocity component of the ball is reduced to 70% of the post-impact perpendicular component, which is itself reduced to about 70% of its pre-impact value. Sliding friction with the cushion and bed, along with subsequent masse action, seem to be able to explain virtually all of the reduction of the parallel component.

Whether this is just a comedy of errors and a happy coincidence (on my part) remains to be seen. But if other things are also shortening the angle, such as cushion throwback (as Dr. Dave's video clearly demonstrates is occurring with the Eliot shot), or significant rolling resistance along the cushion, then I think you have to throw away some of the friction to make room for them. But where does it go?

Jim

cushioncrawler
06-06-2007, 05:20 PM
<blockquote><font class="small">Quote Jal:</font><hr> ....Mac, in order for a stunned ball to rebound with the amount of topspin you might expect for that much "tip offset" (about (1/4)R), the cushion would have to roll off the ball in the vertical direction, just like a cue's tip does. Instead, it does the opposite and flaps back down (to the extent that it flaps up in the first place).<hr /></blockquote>Jim -- Exactly. U would expect that any rolling of the ball coming off the cushion shood be matched by an identical speed of the bit of the cushion at the ball'to'cushion contact, in the vertical plane here. And -- Yes -- The silly looking K?? cushion in the video is allmost allways doing the exact opposite (in the vertical plane), ie the cushion is going down, ie putting (trying to put) backspin on the ball. It looks like there are now 2 people in the world (me'n'u) who understand some of this stuff.

<blockquote><font class="small">Quote Jal:</font><hr>Another way of looking at it is that the force vs time must be very different when putting spin on the ball, as indicated by this.....

The above curves are for elastic collisions and only a schematic. (I have no idea what the actual curves look like, other than that they become very unsymmetric with tip offset.) Energy losses also distort the curves, in much the same way I think, but this is a different issue.<hr /></blockquote>Jim -- The L-profile cushions bend down during the first half of impact, then bend back up during the second half -- they are made/designed thinnish, to more eezyly bend up'n'down, koz the manufacturers back between WW1 and WW2 were either very smart re cushion theory, or they did some very good trial'n'error. The scene now switches to the idiots designing the K?? cushions -- here they adopt a profile that can do no other than bend up during the first half of impact -- idiots.

<blockquote><font class="small">Quote Jal:</font><hr>If you imagine a cushion compressing and then decompressing in a symmetric fashion about T/2, it just couldn't put any spin on the ball. The usual real world energy loss makes it unsymmetric too, but this doesn't help with spin.<hr /></blockquote>Jim -- Bending "down then up" can do the trick.

<blockquote><font class="small">Quote Jal:</font><hr>As far as the "natural" shortening of the rebound angle, Bob Jewett describes a mirror system where, for a stunned ball with no sidespin, the virtual target pocket is located at about 70% of the width of the mirrored table. In other words, the parallel velocity component of the ball is reduced to 70% of the post-impact perpendicular component, which is itself reduced to about 70% of its pre-impact value. Sliding friction with the cushion and bed, along with subsequent masse action, seem to be able to explain virtually all of the reduction of the parallel component.<hr /></blockquote>I think that the first serious attempt at ball physics in an English language book woz in Captain Crawley's book in about 1960. His Cambridge friend calculated that Vxx can be reduced by about 2/7ths due to XX-stun becoming XX-rolling, off (along) the cushion, and that if the cushion's "e" is 5/7ths then this happy wedding can produce a mirror image rebound. This analysis woz very flawed, but it woz a good start anyhow.

<blockquote><font class="small">Quote Jal:</font><hr>Whether this is just a comedy of errors and a happy coincidence (on my part) remains to be seen. But if other things are also shortening the angle, such as cushion throwback (as Dr. Dave's video clearly demonstrates is occurring with the Eliot shot), or significant rolling resistance along the cushion, then I think you have to throw away some of the friction to make room for them. But where does it go?... <hr /></blockquote>Jim -- I agree. If hysteresis or something rezults in an "effektiv cushion" acting as if the cushion is squarer, ie as if the ball is attacking the cushion squarer, then now we will have to hide some of the friction otherwize the angles wont "add up" properly, ie the silly looking ball will be (shood be) squaring up so much that it comes back to u. In fact, that woz the case with the "strip rubber" cushions used at about WW1 -- a hard-hit ball would square up and sometimes start coming back to the end it came from. With the K?? profiles, there is so much ball'to'cushion slippage going on that the XX friction forces are only whats left over after the YY and ZZ friction forces have done their dirty work. madMac.