Simple Squirt Test

[pt109]

Since I have never seen object ball swerve, or at least never recognized it, I made the same assumption as PJ: the above is a typo. If not, I still have an offer open of $200 to anyone who can show me how to make an object ball curve significantly with a legal shot. (The details are posted elsewhere.)

I feel your $200 is safe.
However,I think I can make an object ball curve,by driving it into another ball.
Lotta draw,higher than medium speed,fullish contact on second object
ball.I've used it to curve around an intervening ball and into a pocket.
Same effect but to a lesser degree as making an in-off

 

[Patrick Johnson]

Warping a bank in is the result of follow/draw--- not left/right.

Dave

You can't transfer significant follow or draw to the OB for most banks, but you can hit it slow enough to have forward roll (follow), and you can hit it hard enough to "warp" off the rail like it has some draw. I've always assumed the draw effect is the result of the cushion nose being a little higher than the ball's equator and putting lots of cross-table topspin on, which is "draw" compared with the angle of rebound. Maybe Bob knows the details.

pj
chgo

 

[Jal]

...

6. When the shot goes in and the CB spins in place, the distance between the balls (face to face) + 1 1/8" is your shaft's Pivot Length.

Patrick,

Three points, if I may:

1) Using Dr. Dave's derivation of pivot distance as applied to your method:

http://billiards.colostate.edu/technical_proofs/new/TP_B-1.pdf

P = d(r+R)/R - R

where P is the pivot distance from the tip, d is the distance between ball centers in your diagram, R is a ball radius (1.125"), and r is the tip's radius of curvature. So, approximating a dime radius as 11/32", and a nickel radius as 13/32", the factor (r+R)/R, to be applied to d, is 1.31 for a dime radius, and 1.36 for a nickel. Then subtract one ball radius as per the formula. This is a significant difference.

It yields the pivot point as measured from the very tippy-tip-tip of the tip. It's in fact where you'd want to pivot after lining up at centerball if the tip were touching the cueball. Subtract whatever gap, G, you have between the tip and cueball from P to get the adjusted distance:

P' = d(r+R)/R - R - G

2) Saliva should be applied to the contact point of the object ball. Throw will mess things up even worse than the spit.

3) Judge a full hit exclusively by the direction of the OB. The 9-ball will swerve sideways after a full hit for several inches on a hard shot. It still has the spin to accomplish this after colliding with the OB, since it didn't swerve to any significant extent prior to the collision. The 9-ball shouldn't actually spin in place.

At least that's my take.

Jim

 

[Patrick Johnson]

..........

 

[iralee]

Nope. The test involves making an object ball go around an obstacle without hitting a cushion. It does not allow the well-known hook off a slippery rail or the less well-known reverse hook for some high-speed banks.

I believe one can make a fast rolling object ball curve around an obstacle after caroming into another OB. I assume this also does not count...

-Ira

 

[Patrick Johnson]

..........

 

[Patrick Johnson]

I believe one can make a fast rolling object ball curve around an obstacle after caroming into another OB. I assume this also does not count...

-Ira

This would be the same (well known) effect as a fast rolling CB curving forward after caroming off an OB - we don't usually think of it in connection with OBs, but of course you're right about it.

And to answer for Bob (sorry, Bob), no it doesn't count. His test is intended to show that the CB by itself can't transfer masse spin to an OB (although I don't know if he'd say it that way).

pj
chgo

 

[dr_dave]

PJ,

I'm sure you know this, but others might not. I have a similar procedure and video demonstration of how to measure a cue's "natural pivot length" here:

http://billiards.colostate.edu/threads/cue.html#pivot​

Regards,
Dave

 

[AtLarge]

Good point. The stripe on Brunswick Centennial balls is exactly .5 diameter wide, so the measurement is perfect on them, but maybe approximate on other balls. ...

I have some Centennials that are probably no more than 5 years old (not sure since I'm not the first owner) and the stripe is approximately 1 1/16" wide rather than 1 1/8" wide. On my Aramith Super Pro's and Aramith Tournaments, the stripes are about 1 1/4" wide.

 

[dr_dave]

... may result in significant OB swerve.

Since I have never seen object ball swerve, or at least never recognized it, I made the same assumption as PJ: the above is a typo. If not, I still have an offer open of $200 to anyone who can show me how to make an object ball curve significantly with a legal shot. (The details are posted elsewhere.)

He obviously meant OB throw, not OB swerve.

FYI, info and videos regarding OB turn and swerve can be found here:

http://billiards.colostate.edu/threads/swerve.html#turn​

Regards,
Dave

 

[dr_dave]

nice write-up PJ. I remember seeing something very similiar to this in one of Dr. Dave's videos.

Here it is:

NV B.71 - Back-hand English (BHE), from VEPS II
​

Regards,
Dave

 

[dr_dave]

And to answer for Bob ... His test is intended to show that the CB by itself can't transfer masse spin to an OB

Actually, it can; but the effect is much too small to be of practical importance (based on the analysis at the end of TP A.24).

Regards,
Dave

 

[mikepage]

[... re: saliva on the balls...]

Nice idea. I think the high speed of the shot and maximum sidespin also serve to minimize throw, but this would ensure no contamination.

that really depends on whose saliva it is, imo.

 

[MOJOE]

Dr. Dave, thanks as always.. I have found so much valuable info from the VEPS dvd's. They are my most treasured billiards resource and I have a ton of material!

 

[dr_dave]

Dr. Dave, thanks as always.. I have found so much valuable info from the VEPS dvd's. They are my most treasured billiards resource and I have a ton of material!

You're welcome ... and thank you!

I aim to squerve,
Dave

 

[Dead Crab]

Since I have never seen object ball swerve, or at least never recognized it, I made the same assumption as PJ: the above is a typo. If not, I still have an offer open of $200 to anyone who can show me how to make an object ball curve significantly with a legal shot. (The details are posted elsewhere.)

My error. I retract it.

 

[Jal]

Thanks for the input, Jim, but I think that's too much math for a test that's supposed to be simple for an average pool player to do. Also, my test specifies to hit the edge of the stripe, so it measures the pivot point on the long axis of the cue which passes through the actual tip/CB contact point (see the drawing), making it independent of the tip's curvature and (I think) making the pivot length discrepancy insignificant (only as much as is produced by the sideways offset to the cue's center axis).

I remember that when we discussed this (has it been three years!), you corrected some fuzzy-wuzzy thinking on my part. Actually, I'm pretty sure mine was outright incorrect. Having somewhat of a background in math doesn't always help (some of us).

Whatever exactly that was about, having looked at this afresh, I'm afraid I have to say that your arithmetic would be produce a pivot distance roughly 1/3'rd less than the actual distance. I'm assuming the pivot point is to be used in the standard way for backhand english, i.e., line up at centerball and pivot to your heart's content. It would take a little math to show why this is true, but for now, suffice it say that while tip curvature probably only has a very small physical effect on the amount of squirt, it is integral to the geometry of locating the pivot point, as per Dr. Dave's derivation. For typical curvatures then, the approximate (but very close) formula should be:

P = 1.33d -R

where that 1.33 is an approximate average of (R+r)/R, with r being the radius of the tip. Sorry Patrick, I know this adds a little complication, but I'm not sure I'd want to ignore that .33 part.

... I think the high speed of the shot and maximum sidespin also serve to minimize throw, but this would ensure no contamination.

I may have measured up to 3X the amount of throw predicted by theory on a high-speed full-hit shot off of one of Dr. Dave's videos. I really have to get around to re-doing this in case I blundered doing the measurements (they did conform with theory nicely at slow speeds). At any rate, I wouldn't take the gross reduction in throw entirely for granted.

Jim

 

[Patrick Johnson]

..........

 

[Patrick Johnson]

Me:
The stripe on Brunswick Centennial balls is exactly .5 diameter wide, so the measurement is perfect on them, but maybe approximate on other balls.

AtLarge:
I have some Centennials that are probably no more than 5 years old (not sure since I'm not the first owner) and the stripe is approximately 1 1/16" wide rather than 1 1/8" wide.

Hmm... you're right. I checked tonight by freezing four balls together this way:



If the stripes were exactly 1/2 diameter wide all their edges would line up with each other. And it looks to me like 1/16" less than 1/2D is about right judging by the gaps.

Since other balls have even narrower stripes, it makes sense to check them by freezing balls together like this and seeing how much gap there is between the edges of the stripes. To be sure you're hitting the "CB" at 1/2R from center, hit outside the stripe 1/2 the amount of each gap shown by this comparison.

Thanks for catching that. I think the difference is practically negligible for Centennials (the edge of the stripe is only 1/32" less than 1/2R), but could be meaningful for other balls.

pj
chgo

P.S. I don't think the age/wear of the balls matters, as long as they're all the same size.

 

[dr_dave]

... I think the high speed of the shot and maximum sidespin also serve to minimize throw, but this would ensure no contamination.

I may have measured up to 3X the amount of throw predicted by theory on a high-speed full-hit shot off of one of Dr. Dave's videos. I really have to get around to re-doing this in case I blundered doing the measurements (they did conform with theory nicely at slow speeds). At any rate, I wouldn't take the gross reduction in throw entirely for granted.

Jim,

I'm not sure which video you are referring to, but cling may have been involved. In a few throw and spin-transfer videos, I sometimes added a chalk smudge at the CB-OB contact point to illustrate the effect of cling. If I did, it would be indicated in the video title or narration. Cling could certainly affect the results of PJ's experiment; but with enough trials, it should be clear if cling occurs.

Concerning the theory of SIT speed and English-amount effects, this article has some good illustrations:

Throw - Part V: SIT speed effects (BD, December, 2006)​

And shot #'s 577 and 578 in the following video seem to agree with the theory fairly well:

NV B.86 - Cut-induced throw (CIT) and spin-induced throw (SIT), from VEPS IV​

Regards,
Dave