Physics and Swerve

Dan White · Sep 21, 2018

I know this has been discussed ad nauseum but I need a little help conceptually on this. I understand that swerve is caused by rotation about the axis that goes from the front to the back of the ball, as if the cue stick were penetrating though the ball. I made the following diagram way back:

Now if I hit with a downward angle with, say 1:30 english, there is a vertical vector component that causes rotation around that axis, I call it the "swerve axis" or "3rd axis" in the drawing. If I hit the same shot with a perfectly level cue there is no noticeable swerve. Here's my question: If I hit at 1:30 and a level cue, the tip is striking the ball at 7:30 on the tip. If the "lower left" part of the tip is hitting the "upper right" part of the ball, this causes the ball to squirt left. However, the tip is not just pushing the ball to the left. It is also pushing down on the ball, even with a level cue. It seems to me that a level cue DOES impart rotation around the swerve axis. If the cue shaft were like an I beam it would impart more downward force than a low deflection shaft, and if I hit hard it would push down more than soft.

Am I correct to say that rotation about the swerve axis does occur with a level cue, but the magnitude is small enough not to be noticeable? Has anyone compared the size of this force to that of a cue that is angled downward?

garczar · Sep 21, 2018

Dan White said:
I know this has been discussed ad nauseum but I need a little help conceptually on this. I understand that swerve is caused by rotation about the axis that goes from the front to the back of the ball, as if the cue stick were penetrating though the ball. I made the following diagram way back:

View attachment 501058

Now if I hit with a downward angle with, say 1:30 english, there is a vertical vector component that causes rotation around that axis, I call it the "swerve axis" or "3rd axis" in the drawing. If I hit the same shot with a perfectly level cue there is no noticeable swerve. Here's my question: If I hit at 1:30 and a level cue, the tip is striking the ball at 7:30 on the tip. If the "lower left" part of the tip is hitting the "upper right" part of the ball, this causes the ball to squirt left. However, the tip is not just pushing the ball to the left. It is also pushing down on the ball, even with a level cue. It seems to me that a level cue DOES impart rotation around the swerve axis. If the cue shaft were like an I beam it would impart more downward force than a low deflection shaft, and if I hit hard it would push down more than soft.

Am I correct to say that rotation about the swerve axis does occur with a level cue, but the magnitude is small enough not to be noticeable? Has anyone compared the size of this force to that of a cue that is angled downward?

Squirt and swerve are both rotations around the verticle axis of the cue ball. Follow and draw rotate on the horiz. axis. Level cue swerve is easy to see if you spin it enough at a slow enough speed. Ball starts out on the squirt line and curves back over distance. If hit slow enough at that blue arrow it will squirt left and then swerve back IF hit slowly enough. The effect is greater if you hit it on the equator.

iusedtoberich · Sep 21, 2018

I personally think based on my own play, that high outside spin with a level stroke causes a TON of swerve. Even more than low outside at times. And if you have the CB frozen to the rail and are forced to cue even higher, the swerve is out of this world high. My perception my be different than yours:grin-square:

dr_dave · Sep 21, 2018

Dan White said:
I know this has been discussed ad nauseum but I need a little help conceptually on this. I understand that swerve is caused by rotation about the axis that goes from the front to the back of the ball, as if the cue stick were penetrating though the ball. I made the following diagram way back:

View attachment 501058

Now if I hit with a downward angle with, say 1:30 english, there is a vertical vector component that causes rotation around that axis, I call it the "swerve axis" or "3rd axis" in the drawing. If I hit the same shot with a perfectly level cue there is no noticeable swerve.

With a follow shot, the swerve happens sooner; and at slower speed, the swerve can happen practically immediately. For more info, see:

squirt cue elevation effects and immediate swerve

Dan White said:
Here's my question: If I hit at 1:30 and a level cue, the tip is striking the ball at 7:30 on the tip. If the "lower left" part of the tip is hitting the "upper right" part of the ball, this causes the ball to squirt left. However, the tip is not just pushing the ball to the left. It is also pushing down on the ball, even with a level cue. It seems to me that a level cue DOES impart rotation around the swerve axis.

You are correct. With an above center hit, the CB squirts down into the table some causing the CB to hop even more than it would otherwise (due to typical elevation necessary to clear the rails). For demonstrations, see:

follow shot CB hop

Dan White said:
Am I correct to say that rotation about the swerve axis does occur with a level cue

... not if the hit is on the horizontal centerline (at 3 or 9 o’clock). There is absolutely no swerve with a perfectly level cue hitting on the horizontal equator.

Dan White said:
... but the magnitude is small enough not to be noticeable?

For a slower shot, the swerve is less noticeable because it happens almost immediately.

Dan White said:
Has anyone compared the size of this force to that of a cue that is angled downward?

The downward squirt effect is equivalent to the effect of cue elevation, so if your cue squirts 2 degrees, the downward force is equivalent to elevating the cue 2 degrees. FYI, for example data, see published squirt data.

Check out the following video (starting at the 2:36 point) where I use a short cue and reverse cue elevation to cancel the effects of downward squirt:

NV J.5 - Straight Rail Shot Effects, and Shooting Past a Side Pocket

Regards,
Dave

garczar · Sep 22, 2018

dr_dave said:
With a follow shot, the swerve happens sooner; and at slower speed, the swerve can happen practically immediately. For more info, see:

squirt cue elevation effects and immediate swerve

You are correct. With an above center hit, the CB squirts down into the table some causing the CB to hop even more than it would otherwise (due to typical elevation necessary to clear the rails). For demonstrations, see:

follow shot CB hop

... not if the hit is on the horizontal centerline (at 3 or 9 o’clock). There is absolutely no swerve with a perfectly level cue hitting on the horizontal equator.

For a slower shot, the swerve is less noticeable because it happens almost immediately.

The downward squirt effect is equivalent to the effect of cue elevation, so if your cue squirts 2 degrees, the downward force is equivalent to elevating the cue 2 degrees. FYI, for example data, see published squirt data.

Check out the following video (starting at the 2:36 point) where I use a short cue and reverse cue elevation to cancel the effects of downward squirt:

NV J.5 - Straight Rail Shot Effects, and Shooting Past a Side Pocket

Regards,
Dave

Good stuff Dave. As usual. One thing on the video of the ".01 hop", you were smokin' it pretty good. In most follow shots you'll never have that kind of hop will you? I rarely hit a follow shot with anywhere near the force used in that video.

Bob Jewett · Sep 22, 2018

garczar said:
Good stuff Dave. As usual. One thing on the video of the ".01 hop", you were smokin' it pretty good. In most follow shots you'll never have that kind of hop will you? I rarely hit a follow shot with anywhere near the force used in that video.

The standard demo for cue ball hop is to put a dime a short distance in front of the cue ball and try shots at fast speeds at first and slow down until you hit the dime.

dr_dave · Sep 22, 2018

garczar said:
Good stuff Dave. As usual. One thing on the video of the ".01 hop", you were smokin' it pretty good. In most follow shots you'll never have that kind of hop will you? I rarely hit a follow shot with anywhere near the force used in that video.

A you decrease the speed, the hop will be shorter and closer, but it doesn't take much speed to create slight hop. On force follow shots, the hop can be significant and can easily cause an overcut if the shot is not straight.

Regards,
Dave

parogen · Sep 22, 2018

dr_dave said:
... not if the hit is on the horizontal centerline (at 3 or 9 o’clock). There is absolutely no swerve with a perfectly level cue hitting on the horizontal equator.

Won't eventual natural follow after initial contact develop some swerve?

bbb · Sep 23, 2018

dr_dave said:
... not if the hit is on the horizontal centerline (at 3 or 9 o’clock). There is absolutely no swerve with a perfectly level cue hitting on the horizontal equator.

Regards,
Dave

2 questions dr. dave
are you saying no swerve only squirt with a level cue and 3 or 9 oclock english?
when a cue ball is frozen to the rail and you apply english i believe i hqve read you get swerve but no squirt
is this true?
if so
why?
thanks for your reply

Dan White · Sep 23, 2018

dr_dave said:
With a follow shot, the swerve happens sooner; and at slower speed, the swerve can happen practically immediately. For more info, see:

squirt cue elevation effects and immediate swerve

You are correct. With an above center hit, the CB squirts down into the table some causing the CB to hop even more than it would otherwise (due to typical elevation necessary to clear the rails). For demonstrations, see:

follow shot CB hop

... not if the hit is on the horizontal centerline (at 3 or 9 o’clock). There is absolutely no swerve with a perfectly level cue hitting on the horizontal equator.

For a slower shot, the swerve is less noticeable because it happens almost immediately.

The downward squirt effect is equivalent to the effect of cue elevation, so if your cue squirts 2 degrees, the downward force is equivalent to elevating the cue 2 degrees. FYI, for example data, see published squirt data.

Check out the following video (starting at the 2:36 point) where I use a short cue and reverse cue elevation to cancel the effects of downward squirt:

NV J.5 - Straight Rail Shot Effects, and Shooting Past a Side Pocket

Regards,
Dave

Thanks, Dave. I was interested really in the force generated by the cue in a downward direction for follow when the cue is level. It sounds like this can create a small amount of swerve just after contact. It is less intuitive to understand what is happening with a draw shot at say 5:00 and a LEVEL cue. I would expect swerve in the opposite direction (to the left) in this case as the tip is pushing up on the ball. However, the ball is also spinning backwards and rotating and probably being lifted off the cloth a bit, so do you think it is possible to get that counter clockwise movement on the ball in this scenario? I'm thinking of things like a gyroscopic effect with all that draw possibly negating a ccw motion.

Well that got wordy fast. Simply put, does a level cue draw shot cause the ball to rotate to the left around the swerve axis due to an upward force from the tip?

Bob Jewett · Sep 23, 2018

parogen said:
Won't eventual natural follow after initial contact develop some swerve?

No.

There is a different kind of curve that happens on tables with cloth that has directional nap (single fibers that stick up out of the cloth like a lawn, and are brushed to all lie down in a single direction). On such tables a ball with side spin will curve because of interaction with the nap. And depending on the direction, it can curve opposite to the direction you would expect for normal swerve. (But see below.)

The standard for pool and carom is napless (worsted) cloth.

You can do an experiment yourself: Set a ball spinning in place, either with your hand or by a stop shot with extreme side. Then tap it with a second ball to send it slowly across the cloth to roll maybe a diamond or two and stop while it is still spinning. Do that with left and right and in various directions (since your table might not be flat). See how much curve the side spin causes.

(I have never seen a convincing demo of nap curve, but a lot of people talk about it so it might be real.)

dr_dave · Sep 23, 2018

parogen said:
dr_dave said:

... not if the hit is on the horizontal centerline (at 3 or 9 o’clock). There is absolutely no swerve with a perfectly level cue hitting on the horizontal equator.

Click to expand...

Won't eventual natural follow after initial contact develop some swerve?

Nope. With a perfectly level cue and equator hit, the CB will slide and then roll perfectly straight, assuming the table is level.

Try it. I have. I also did the math/physics.

Regards,
Dave

dr_dave · Sep 23, 2018

bbb said:
2 questions dr. dave
are you saying no swerve only squirt with a level cue and 3 or 9 oclock english?

That’s correct. The CB will squirt but will head along a perfectly straight line in the squirt direction.

bbb said:
when a cue ball is frozen to the rail and you apply english i believe i hqve read you get swerve but no squirt
is this true?

Anytime you hit the CB off center, there is squirt. Now, with a frozen-ball follow shot away from the rail, the cue is usually elevated more than normal. This causes more swerve than normal, and the swerve happens almost immediately at slower speeds, so it looks like there is no squirt and only swerve.

Regards,
Dave

dr_dave · Sep 23, 2018

Dan White said:
Thanks, Dave. I was interested really in the force generated by the cue in a downward direction for follow when the cue is level.

The force and squirt downward with a follow shot is the same as the force an squirt sideways with a pure sidespin shot.

Dan White said:
It sounds like this can create a small amount of swerve just after contact.

That’s correct, if there is side with the top.

Dan White said:
It is less intuitive to understand what is happening with a draw shot at say 5:00 and a LEVEL cue ... does a level cue draw shot cause the ball to rotate to the left around the swerve axis due to an upward force from the tip?

Yes. Even more interesting and less intuitive is the reverse swerve you get by hitting a follow shot (with side) with the cue on an upward angle (for example, by hitting up at a CB sitting on top of a cube of chalk on the rail).

Try it out.

Regards,
Dave

dr_dave · Sep 23, 2018

Bob Jewett said:
No.

There is a different kind of curve that happens on tables with cloth that has directional nap (single fibers that stick up out of the cloth like a lawn, and are brushed to all lie down in a single direction). On such tables a ball with side spin will curve because of interaction with the nap. And depending on the direction, it can curve opposite to the direction you would expect for normal swerve. (But see below.)

The standard for pool and carom is napless (worsted) cloth.

You can do an experiment yourself: Set a ball spinning in place, either with your hand or by a stop shot with extreme side. Then tap it with a second ball to send it slowly across the cloth to roll maybe a diamond or two and stop while it is still spinning. Do that with left and right and in various directions (since your table might not be flat). See how much curve the side spin causes.

(I have never seen a convincing demo of nap curve, but a lot of people talk about it so it might be real.)

For those interested, I have demonstrations of the lack of curve with a spinning ball rolling straight here:

OB "swerve" and "turn" resource page

Enjoy,
Dave

bbb · Sep 23, 2018

dr_dave said:
That’s correct. The CB will squirt but will head along a perfectly straight line in the squirt direction.

Anytime you hit the CB off center, there is squirt. Now, with a frozen-ball follow shot away from the rail, the cue is usually elevated more than normal. This causes more swerve than normal, and the swerve happens almost immediately at slower speeds, so it looks like there is no squirt and only swerve.

Regards,
Dave

thank you very much

Physics and Swerve

Dan White

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garczar

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iusedtoberich

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dr_dave

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garczar

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Bob Jewett

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parogen

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Dan White

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