Tip hardness and the amount of spin. Is the physics really settled?

[ineedaspot]

The direction of the force is the direction of the stick. Yes there is a subtle sideways component; that is the force that causes squirt.

One key point to keep in mind is that it is the SAME FORCE that propels the ball and spins the ball. You can' t propose the direction of one change without the direction of the other changing.

The squirt force, as small as it is, reduces the amount of spin on the ball. A squirty stick striking a ball off center is equivalent to a hypothetical squirtless stick striking in a slightly different direction with LESS offset.

Actually, when it comes to top and bottom, this isn't true, because the ball can't move downwards (and it takes quite a bit of force to get the cue ball to pop up noticeably into the air). As for side-spin, I agree, the direction the ball moves is also the direction of the net impulse, the only thing that can really change is the point on the ball where the force is applied.

But this does leave open the possibility of something strange going on with draw. Is it possible that the amount of extra draw you get from having a softer tip depends on the cue angle? I.e. maybe if you have a level cue, soft and hard tips draw about the same, but with a slightly elevated cue there is a difference. Or vice versa.

We actually HAVE discussed the effect of a changing force throughout the duration of tip-ball contact. A soft tip stays on the ball longer, and when you strike with a soft tip at an offset of, say 10mm, the tip stays on the ball while the ball begins to rotate--so that by the time the tip leaves the ball the offset might be at 12mm. If so, then the effective offset for the shot might be 11mm. And for a harder tip the effective offset might be slightly smaller.

But as far as the details of the compression providing a complex force that leads to more or less spin, it can only do that by also changing the direction the cueball moves. And we know from experience that doesn't happen,

OK. I was under the impression that, based on measurements of the contact duration, it was implausible for a soft tip to impart noticeably more english, due to the fact that the duration was very short in either case, and the difference between a hard and soft tip wasn't that much.

This is at odds with what I've observed, so I was wondering if maybe the model was wrong and there was something else going on. But if this model is consistent with more spin (or, more precisely, greater spin-to-speed ratio) for softer tips, then it would be adequate.

 

[ineedaspot]

The difference is simply one of force - a harder tip transfers more force to the CB. More force doesn't matter with side spin because it adds to both RPMs and to CB speed without changing the spin-to-speed ratio - so more RPMs doesn't = more spin effect.

Draw is different because the CB is stopped by the OB, removing CB speed from the equation - so more RPMs = more draw.

In other words, you'd get the same result with a softer tip if you just hit a little harder.

pj
chgo

But what I'm saying is the opposite -- I get more spin (i.e. more draw) with a softer tip, hitting the ball equally hard.

 

[dr_dave]

I see what you are saying, but I think it is dangerous to talk about things other that whether [XXX] produces a greater spin-to-speed ratio.

Your harder tip gets more draw in the same way a heavier cue gets more draw for a given cue speed.

The danger I think of saying a
--harder tip gets more draw or a
--heavier cue gets more draw or that
--eating Wheaties for breakfast leads to more draw
is that some will think you're talking about spin-to-speed ratio

I get your drift.

Damn ... I should have eaten my Wheaties this morning! :grin-square:

Best regards,
Dave

 

[dr_dave]

Yes, that would be the prediction of the "instantaneous impulse" model, which essentially assumes that the contact time is short enough that we can treat it as instantaneous, with the force vector pointing in the same direction the entire time.

Is this backed by experiment? The reason I ask is that this prediction is at odds with what I actually experience when changing from a harder to a softer tip -- I get noticeably more draw. And other people seem to have the same experience.

So it seems to me that there may be something else going on with the tip-ball contact that is not captured by the instantaneous impulse model.

Actually, many people claim they can get more draw with a hard tip. The only way to truly answer your question (if the physics reasoning is not enough) is to do an extremely careful experiment to accurately measure any differences between a hard tip and soft tip on the same cue and shaft stroked the exact same way and with the exact same tip offset each time. This is not easy to do without a well-designed machine.

FYI, experiments have been done to carefully measure tip contact times for various tip hardnesses and shot speeds. For more info, see the tip contact time resource page. Also, lots of super slow motion footage is available showing how the tip deforms (and how the effective offset increases slightly during contact) with off-center hits. To view these, go to the tip deformation resource page.

The only important assumptions in the impulse model is that the tip contact time is very short and that the direction of the force doesn't change much during contact. These assumptions might not be perfect, but I think they are certainly appropriate. If one wanted to be more precise, one could use an "effective tip offset" which is the tip offset in the middle of the contact time, but it won't make much difference with the extremely short contact times.

Regards,
Dave

 

[dr_dave]

But what I'm saying is the opposite -- I get more spin (i.e. more draw) with a softer tip, hitting the ball equally hard.

But other people claim they get more draw with a harder tip. Who are we to believe? That's the problem with "anecdotal" claims.

Regards,
Dave

 

[Patrick Johnson]

But what I'm saying is the opposite -- I get more spin (i.e. more draw) with a softer tip, hitting the ball equally hard.

Then you must be hitting farther from center. Somebody above suggested this could be because you're less afraid of miscueing with a softer tip. That makes some sense to me.

I suppose the harder tip could also be glazed a little, resulting in some slippage (less than a full miscue) on the CB - but I don't believe a well groomed hard tip grabs any worse than a soft tip.

pj
chgo

 

[qbilder]

IMO, more than hardness, I believe quality matters most in how much spin can be induced. Dave is obviously spot on that harder tips are more efficient with energy transfer, so given the same quality, a harder tip should spin more. However, most folks trying this out likely aren't using equal quality hard & soft tips. By quality I mean the tip's ability to hold chalk, which is the abrasive component that allows for the friction needed to spin the ball. Some tips simply outperform other tips, regardless of hardness. At what point is a miscue noticeable as a miscue? Whether it slips a tiny bit or it slips a lot & is an obvious miscue, it is losing traction. Tips that do not lose traction will induce the most spin with least effort. The difference between hard & soft should only be the power required to push the ball at a certain speed. Sensibly, a softer tip requires more effort because the compression makes for a less efficient power transfer. Spin is a different animal, IMO, and has much less to do with hardness and more to do with the tip's traction ability.

 

[Patrick Johnson]

Dave is obviously spot on that harder tips are more efficient with energy transfer, so given the same quality, a harder tip should spin more.

It should produce more RPMs, but as said before RPMs alone only mean more spin effect with "vertical spin" (follow and draw) - not with side spin.

Just picking a nit...

pj
chgo

 

[ineedaspot]

But other people claim they get more draw with a harder tip. Who are we to believe? That's the problem with "anecdotal" claims.

Regards,
Dave

Yes, that's a good point. It's definitely possible that both are right -- in some cases harder tip means more draw, in others less. This would point to some other variable that's not being taken into account. I speculated above that the cue angle for draw shots could be this variable. Or maybe something about the tip consistency other than hardness, maybe elasticity (in the sense of low coefficient of restitution -- so a super-ball would be soft but elastic).

It's also possible that both are wrong. It is all anecdotal. I'm generally skeptical of anecdotal claims, a lot of stuff can be psychological. But this one seems real to me, real enough that, given that there's no experimental evidence, I wonder if maybe there's something that the basic model is missing here.

 

[ineedaspot]

Then you must be hitting farther from center. Somebody above suggested this could be because you're less afraid of miscueing with a softer tip. That makes some sense to me.

I suppose the harder tip could also be glazed a little, resulting in some slippage (less than a full miscue) on the CB - but I don't believe a well groomed hard tip grabs any worse than a soft tip.

pj
chgo

I understand that possibility, and I definitely haven't performed any scientifically controlled experiment, but I don't believe that to be the case.

Skepticism of anecdotal claims is always justified, but in this case there isn't any experimental evidence, and it's at least possibly in my mind that the simplified model of ball-tip contact is missing something. Particularly when it comes to draw, because there can't really be much deflection in the vertical axis, and the cue usually isn't perfectly level, it seems that it leaves a lot of leeway for the force vector to point in different directions without violating anything obvious.

 

[Patrick Johnson]

...given that there's no experimental evidence, I wonder if maybe there's something that the basic model is missing here.

It's easy to do some controlled experimenting yourself. Just use a marked cue ball (a striped object ball works) and check chalk marks after each shot to be sure you're hitting the same spot with both tips.

pj
chgo

P.S. Are both tips the same size?

 

[dr_dave]

I wonder if maybe there's something that the basic model is missing here.

I trust the model until carefully-derived experimental evidence clearly shows that it is faulty. All of the evidence I have seen suggests that the model is very good. For example, all of the conclusions derived from the model concerning draw shots seem consistent with the experience of top players.

Regards,
Dave

 

[dr_dave]

... Particularly when it comes to draw, because there can't really be much deflection in the vertical axis, and the cue usually isn't perfectly level, it seems that it leaves a lot of leeway for the force vector to point in different directions without violating anything obvious.

Cue elevation definitely affects the action of draw shots, but this is very well understood (both physically and at the table) and predicted by the "model." For more info, see draw shot cue elevation effects.

Regards,
Dave

 

[qbilder]

It should produce more RPMs, but as said before RPMs alone only mean more spin effect with "vertical spin" (follow and draw) - not with side spin.

Just picking a nit...

pj
chgo

Please explain RPM, and perhaps what you mean by nit? I am not seeing what vertical spin has anything to do with it. Efficiency is efficiency, whether it's vertical or horizontal. I understand RPM as revolution per minute. Is that what you mean? Or are you using RPM as a reference to power?

 

[ineedaspot]

It's easy to do some controlled experimenting yourself. Just use a marked cue ball (a striped object ball works) and check chalk marks after each shot to be sure you're hitting the same spot with both tips.

pj
chgo

P.S. Are both tips the same size?

Yeah, maybe I'll try that one of these times.

As for your question, yes, for the most part I'm talking about the same size and shape tip. A while back I used to try different tips on my same cue and noticed that softer tips were easier to draw. Recently I changed my tip, which had gotten pretty hard, and again I noticed it was easier to draw. And when I've played with other people's cues, harder tips generally seemed to correlate to less spin, particularly less draw.

Also, a few times, I've picked up house cues with really soft tips, and gotten a lot of draw out of them. Of course, there's a benign explanation for all this. For example, maybe I was afraid of the house cues being horrible so I overcompensated by hitting further out. And anytime you pick up a new cue, it makes a difference psychologically. So I don't have anything resembling scientific data, just on-and-off observation, and I've talked to several people who play more and better than I do who agreed.

 

[Patrick Johnson]

Please explain RPM, and perhaps what you mean by nit? I am not seeing what vertical spin has anything to do with it. Efficiency is efficiency, whether it's vertical or horizontal. I understand RPM as revolution per minute. Is that what you mean? Or are you using RPM as a reference to power?

Yes, RPMs means revolutions per minute. And "picking a nit" means I'm making a minor side point.

Here's my earlier post about that:

More force doesn't matter with side spin because it adds to both RPMs and to CB speed without changing the spin-to-speed ratio - so more RPMs doesn't = more spin effect.

Draw is different because the CB is stopped by the OB, removing CB speed from the equation - so more RPMs = more draw.

In other words, you'd get the same result with a softer tip if you just hit a little harder.

pj
chgo

 

[qbilder]

Yes, RPMs means revolutions per minute. And "picking a nit" means I'm making a minor side point.

Here's my earlier post about that:

Gotcha. Makes sense to me.

 

[The Renfro]

Basically I don't think the physics will be settled until more testing has been performed...

At high speed I will agree that all tips will get the same amount of spin because you are compressing them to the max and they are all acting like solids and not springs.... And the contact times will be similar... As will the amount of english and deflection...

At slow speeds now the soft is acting like a spring and the hard may still be acting like a solid... COR or energy transfer will be higher with the harder tip but that doesn't have to go hand and hand with more spin... If you have tried to soft spin a super hard tip you likely didn't like your results... too much trave not enough turn...

Or maybe it is just a function of the LD tendencies that need to be applied to the soft tip since it will stay in contact long enough to carry the cueball further down the cueing path before release.....

Most of the cuemakers I have talked to contend that a soft tip will get more english at slow and medium speeds and will be lower deflection at these speeds... Maybe it is the tip riding around the ball... Maybe it is a superior contact created by the extended contact window... Maybe this superior contact trumps superior COR when dealing with tip offsets... Even chalk will not extend the contact times of a hard tip....

This is one of the things that has me willing to kick money into the slomo camera fund...

 

[Bob Jewett]

If anyone wants to try a test, here is a possible setup.

Put the object ball on the head spot. Put the cue ball about a foot away and towards the head rail. The balls are pointed to the foot spot. Shoot straight at the object ball to send it over the foot spot twice and return. Try to draw the cue ball back to the head cushion without the object ball hitting the head cushion.

Try it with two tips and see whether one tip can do the shot and one can't. You need to take more than one try with each cue and you have to be at the miscue limit (probably) so there will be some number of miscues.

If the shot is too easy, move the object ball farther from the cushion. (and vice-versa). If yonder bumper is dead, the shot will be easy.

This test tells you about the quality of your draw -- that is, the spin you get for a given speed. I believe that's what the OP is really interested in.

 

[SilverCue]

If anyone wants to try a test, here is a possible setup.

Put the object ball on the head spot. Put the cue ball about a foot away and towards the head rail. The balls are pointed to the foot spot. Shoot straight at the object ball to send it over the foot spot twice and return. Try to draw the cue ball back to the head cushion without the object ball hitting the head cushion.

Try it with two tips and see whether one tip can do the shot and one can't. You need to take more than one try with each cue and you have to be at the miscue limit (probably) so there will be some number of miscues.

If the shot is too easy, move the object ball farther from the cushion. (and vice-versa). If yonder bumper is dead, the shot will be easy.

This test tells you about the quality of your draw -- that is, the spin you get for a given speed. I believe that's what the OP is really interested in.

I've done this before but only with one stick and I don't remember what configuration.
I remember it took me about 3 tries.
I did it 1st try using the trick.
I'll test it again next time I'm on the table.

BTW, I've switched to playing with a Tiger Jump Break tip.
More control and power strokes are no problem.