squirt robot results

[dr_dave]

Joe, remember your question about how far from the tip added mass makes a difference? Take a look at Dave's Diagram #4 - looks like the effect disappears at about 7 inches.

Dave, does this 7-inch distance correlate well with the speed of transverse waves and the ~1ms tip/ball contact time (I don't know the math).

I haven't done any measurements or calculations for the transverse wave speed, but I would be surprised if it didn't correlate.

I just browsed through Ron Shepard's famouns squirt document and didn't see anything about the transverse wave there either.

Regards,
Dave

 

[dr_dave]

squirt stiffness experiments

Dr. Dave,

I have a question, comment, and two potential experiments on this.

Question: Did the Meucci shaft actually exhibit stiffness variance at different orientations?

I did do a crude stiffness test by hanging a static weight from the end with the cue clamped to a table with a fixed length hanging off. Surprisingly, there was little if any variation in stiffness.

Comment: Your Diagram 3 and relevant experiment are premised on the fact that the Meucci shaft actually has stiffness variance. However, your results are also consistent with a shaft that has no stiffness variance (notwithstanding Meucci's marketing to the contrary). Your data may actually show that Meucci shafts have no stiffness variance based on orientation rather than conclusively showing that stiffness has no significant effect on squirt. In particular, your data illustrates that Meucci's construction results in a much stiffer shaft in all orientations, than a standard Player's shaft. Put another way, your experiment may simply show that Meucci is marketing an effect that doesn't exists.

Excellent points. Per the static tests I did on this one Meucci shaft, it appears that stiffness is fairly constant at different orientations.

I'm not sure how the Meucci stiffness compares to the Players. I haven't done that test. The Players cue and Meucci-shaft cue did have similar squirt values, which indicates to me they have similar "endmass."

My concern is that your final conclusion is premised on the existence of stiffness variance - but the shaft may actually have none (due to construction, glues, material properties, etc). Although results of other experiments may indeed support your final conclusion, I do not believe that relying on the Meucci shaft experiment (alone) is "conclusive" on this point.

The "end-mass" might vary slightly with orientation due to slight stiffness variation (see TP A.31), but the effect is obviously not significant in the data."​

Good points. I tried to be careful in how I wrote the paragraph from which you quoted the sentence (see the bottom paragraph on page 3 of the artilce), but I probably should have been even more careful. You are correct. My data prove nothing about the relationship between squirt and stiffness.

Potential experiment 1: use weights on the end of the meucci shaft (at the same orientations as your squir experiment), to illustrate the Meucci variance in stiffness. Perhaps putting a fulcrum at various distances for dramatic effect.

I've done some crude tests like this and saw little or no stiffness variation.

Potential experiment 2 (I think it would be interesting to run this anyway, just to see the data): since you have data with the meucci shaft now. Would it be possible to modify the shaft, and/or run tests with new shafts, to create a situation that may reflect stiffness better? My thought was to repeat the experiments using shafts as shown below.

Excellent idea. I actually have something similar planned with my students. They are running many more tests over the next few weeks. I will try to post the results (and/or include them in a future article) as soon as possible.

I also want to do radial consistency tests (i.e., squirt vs. twist angle) for several different cues. I can't imagine stiffness could vary enough in a typical cue to cause significant changes in squirt, but I am still curious to see some data.

Thank you for your excellent comments and questions,
Dave

 

[dr_dave]

Myth Destroyer limitations

IMO, it's the bridge length *and* the materials used by the Myth Destroyer that makes its result unrealistic. The material (bridge and grip) is too stiff, much stiffer than a hand could ever be.

If the bridge length is longer than about 7-8" (which I think is the case with the Myth Destroyer), the bridge material should have no effect on squirt (that I am aware of, anyway).

However, the squirt readings from the Myth Destroyer are suspect for other reasons. Because the cue is elevated, the CB swerves, and this swerve depends on shot speed and ball/cloth conditions. Also, because an OB is used, throw also comes into play, and throw varies with ball conditions, shot speed, and squirt angle. Therefore, the reported "squervo" values (combination of squirt, swerve, and throw) involve much more than the cue's squirt characteristics. Therefore, it doesn't make much sense comparing numerical values to tests by others. Also, there is a lot more that can go wrong with the tests (e.g., dirt, humidity, wear, speed, etc. can affect the results).

Having said all of that, as long as the experiments are done carefully and consistently, the relative comparisons between cues should be useful info.

Regards,
Dave

 

[Cornerman]

3a) In order for the bridge not to matter, it would have to be (a) greater than 7" away from the tip, or (b) result in no change in endmass at all. I have tried to contemplate different ways for (b) to occur, and the best I could come up with was an open bridge.

I think we have to make sure that you're describing significant effects or not. I think the theory of skin pliability coupled with the .001 sec contact time makes bridge hand mass insignificant specifically because there is no coupling with the bridge hand and the transverse wave.

Bridge length past 7" is irrelevant (as Dr. Dave showed). The "rigidness" of the bridge (i.e, too stiff) results in something like Mike's vice grip experiment. And although the results are clearly an extreme (super tight grip and a questionable distance), they illustrate that a bridge does in fact add end mass.

-td

Again, I don't see how you conclude this. What it shows is that adding end mass past 7" is irrelevant in this setup. It doesn't say anything about a human bridge since it cannot be justified as a similar model.

I believe you've gone to a non-sequitur since the vice grip experiment is specifically adding a mass to the cuestick in a rigid fashion, whereas any bridge hand mass isn't rigidly coupled. That rigid coupling is a huge part of the reasoning behind squirt and the false readings from the Myth Buster.

Fred

 

[Cornerman]

If the bridge length is longer than about 7-8" (which I think is the case with the Myth Destroyer), the bridge material should have no effect on squirt (that I am aware of, anyway).

However, the squirt readings from the Myth Destroyer are suspect for other reasons.

I think I've explained this before, but I'll do it again. The Myth Destroyer has a more rigid coupling to the grip (at least it did when I saw it action several times). It also is not spring loaded. So, when he drops the arm, the contact time between the tip and ball is most likely longer than it ever could be compared to a human interaction. The v-groove block he has is a solid piece as well. So, the increased contact time in the Myth Destroyer setup will add more mass to the equation simply from the lengthened contact time and the tranverse wave travel length during contact.

If the transverse wave is sufficiently falsely increased to reach the Myth Destroyer bridge length, it absolutely will affect the squirt measurement.

Fred

 

[td873]

So, when he drops the arm, the contact time between the tip and ball is most likely longer than it ever could be compared to a human interaction.

I'm not following how grip "most likely" affects contat time between the tip and cue ball. (I assume grip means on the butt??) Perhaps you elaborate a bit more? I'm trying contribute, but to be honest, I don't think I ever understood this. Doh!

Bridge and endmass

I'm going to withhold any long-winded discussion on bridges and endmass, so as to not detract from Dr. Dave's valuable contribution in this thread. These types of technical matters typically result in long posts fraught with math, theory, and application of both. Eww. And my efforts are usually filled with misguieded beliefs and ignorance


Dr. Dave, do you currently have any thoughts on whether a closed bridge (within the 1-7" range) would affect endmass, and ultimately squirt? Are there any experiments that would provide some insight here. I find it quite interesting.

-td

 

[dr_dave]

grip pressure and weight

I think I've explained this before, but I'll do it again. The Myth Destroyer has a more rigid coupling to the grip (at least it did when I saw it action several times). It also is not spring loaded. So, when he drops the arm, the contact time between the tip and ball is most likely longer than it ever could be compared to a human interaction. The v-groove block he has is a solid piece as well. So, the increased contact time in the Myth Destroyer setup will add more mass to the equation simply from the lengthened contact time and the tranverse wave travel length during contact.

If the transverse wave is sufficiently falsely increased to reach the Myth Destroyer bridge length, it absolutely will affect the squirt measurement.

Thank you for clarifying. I agree a non-human grip pressure might affect the results. An even worse consequence of a tight grip and heavy machine "forearm" is the fact that double hits might result. We have seen this with our machine at larger tip offsets. To the naked eye, the shot looks fine, but in super-slow-motion, you can see the tip double hit the CB. This obviously creates larger squirts than expected.

Regards,
Dave

 

[Bob Jewett]

I'm not following how grip "most likely" affects contact time between the tip and cue ball. ...

As I recall my Hooke's Law stuff, the contact time is proportional to the square root of effective mass divided by stiffness of the spring. If you increase the mass of the cue stick to (stick + robot), then the effective mass will increase some, but not in the same ratio. The effective mass is found by the "parallel" masses formula: M = 1/(1/m1 + 1/m2) which varies only slowly for a change in the larger mass of a mismatched pair.

Far more important for stick/ball studies in general is that the stiffly held stick does not slow down as much as it should during contact. Instead of going to 50% of its starting speed when it hits the ball, the stick might only slow to 75% of its starting speed, depending on how much the stiffly-coupled robot arm weighs.

 

[td873]

As I recall my Hooke's Law stuff, the contact time is proportional to the square root of effective mass divided by stiffness of the spring. If you increase the mass of the cue stick to (stick + robot), then the effective mass will increase some, but not in the same ratio. The effective mass is found by the "parallel" masses formula: M = 1/(1/m1 + 1/m2) which varies only slowly for a change in the larger mass of a mismatched pair.

Far more important for stick/ball studies in general is that the stiffly held stick does not slow down as much as it should during contact. Instead of going to 50% of its starting speed when it hits the ball, the stick might only slow to 75% of its starting speed, depending on how much the stiffly-coupled robot arm weighs.

Ah. Thanks. I see said the blind man.

But just to make sure I'm not falling asleep: less slow down means more energy is transferred to the cue ball at impact (viz a viz a human shooting). This in turn means that there is a longer contact time, which ultimately effects squirt.

Am I misinterpreting this by concluding that a heavier butt (perhaps significicantly heavier), on the same shaft, will affect squirt to some degree? That, is, the "heavier" cue won't slow down as much as the "light" one. Would this be an effective end mass change? Or is it something different

-td <- curious george today since we have all the brainiacs online

 

[Bob Jewett]

... But just to make sure I'm not falling asleep: less slow down means more energy is transferred to the cue ball at impact (viz a viz a human shooting). This in turn means that there is a longer contact time, which ultimately effects squirt. ...

It's more complicated than that. In fact, because the contact is governed by Hertz' Law (which applies to round objects in bouncing contact) it turns out that the contact time decreases for a faster shot, slightly.

It is a common mistake to think that contact time is really important or should be prolonged for some reason.

As Dave mentioned, if the stick does not slow down as much as normal, you have a chance for a second contact which could seriously affect squirt.

 

[junksecret]

Thanks to everybody in this and other threads for the positive feedback and supportive words. I appreciate it.

Regards,
Dave

PS: I see your smirk above, but if you want to see an interesting example of a poll graph, see my article dealing with low-squirt cues. I wish I had posted the poll on AZB instead. I might have gotten a lot more data. (smirk back at you)

Doc!!!!

Well I gotta tell ya, I find this stuff interesting and entertaining and the videos downright captivating but since my math skills end somewhere around solving x+1=2, I'm having a tough time of it as the thread evolves and my head hurts, BUT...

I think you should post that EXACT poll here. I'm convinced you'd get a lot more than 39 replies and a lot more than 39 opinions....(operating under one of the laws of physics I *am* aware of... "opinions are like a$$holes, everybody's got one")

Anyway, I think ya should...thanks and keep up the GREAT work!

 

[CueAndMe]

I'm very interested in pivot points at the moment, and it was disappointing to see that a pivot point is only useful for a single tip offset. So the pivot point doesn't move significantly for the speed of shot, but it does move significantly for varying amounts of english? Can you estimate by how much?

Also, how was the weight added to the shaft in your experiment? Was it even all the way around? If so, perhaps lopsided weights should be tried in different orientations.

Fun stuff here. Thanks.

 

[Patrick Johnson]

I'm very interested in pivot points at the moment, and it was disappointing to see that a pivot point is only useful for a single tip offset. So the pivot point doesn't move significantly for the speed of shot, but it does move significantly for varying amounts of english? Can you estimate by how much?

A cue's pivot point is the same for all tip offsets with a level cue (i.e., without swerve). However, in real life, where there are no level cues, swerve changes the end result for different tip offsets, shot distances/speeds, butt elevations and even ball/table conditions.

So the amount of aim compensation can be different for different shots, even with the same tip offset (this is what I call the "effective pivot point"), but the cue's "natural" pivot point (as Dave calls it) doesn't change.

pj
chgo

 

[conetip]

squirt etc

The cue always remains straight along the aiming line. The squirt angle is measured relative to this line. The amount of English is varied with the machine by shifting the CB position sideways. We are still careful to measure the squirt angle relative to the cue line and CB direction. If one is using aim-and-pivot English (see my Nov '07 article for illustrations), the cue line is changed but the squirt angle definition is still the same. Does that make sense?

Regards,
Dave

I was wondering how you can calculate endmass for a cue. And if so ,what was the difference in tip endmass on a low squirt cue compared to a normal maple shaft cue.?
How much length is measured for the endmass?
Is it taken over 7 inches or just the last 2 inches being more critical?
Thanks Neil

 

[dr_dave]

pivot point and squirt

I'm very interested in pivot points at the moment, and it was disappointing to see that a pivot point is only useful for a single tip offset.

The natural pivot point for a cue can be used to adjust for squirt at all tip offsets. Unfortunately, adjusting for squirt alone is not enough on most shots with English. One must also adjust for swerve and throw, which vary with pretty much everything. See my November '07 BD article and the answers to FAQs on this topic for more info.

So the pivot point doesn't move significantly for the speed of shot, but it does move significantly for varying amounts of english? Can you estimate by how much?

The amount of squirt doesn't vary with speed, but both throw and swerve do. The natural pivot point, which adjusts for squirt at any tip offset, is fixed.

Also, how was the weight added to the shaft in your experiment? Was it even all the way around? If so, perhaps lopsided weights should be tried in different orientations.

See my February '08 BD article for more info. Binder clips were used, and they were attached in the vertical position for every test, for consistency. I don't think the orientation should have much (if any) effect.

Regards,
Dave

 

[Patrick Johnson]

I was wondering how you can calculate endmass for a cue.

When you measure the pivot point you're measuring "endmass", but you're expressing it in inches (shorter pivot point = more endmass; longer pivot point = less endmass). I don't think "effective endmass" can be expressed by the amount of total mass in the first 7 inches of the shaft because it would most likely vary with the distribution of the mass too (a heavier ferrule on lighter wood would squirt more than a lighter ferrule on heavier wood even if they both have the same total mass in their first 7 inches). This responds to your last question below.

And if so ,what was the difference in tip endmass on a low squirt cue compared to a normal maple shaft cue.?

I think it must be proportional to their pivot lengths.

How much length is measured for the endmass?
Is it taken over 7 inches or just the last 2 inches being more critical?

I think the answer to that must be yes: it's measured over 7 inches (or whatever for the particular cue) but each gram has more effect as it's nearer the tip.

pj
chgo

 

[conetip]

Thankyou for the reply and explanation. Neil

 

[AuntyDan]

...bridging closer than 7" can affect squirt in remarkable ways

I have heard people ascribing bridge length to squirt, usually referring to long bridges as reducing squirt.

It would seem simple enough to adjust this experiment to change the bridge length with the same shaft and chart the results.

 

[StevenPWaldon]

Dr Dave-

I find this information fascinating, especially debunking the myth of speed/deflection. However, I was disappointed that there wasn't information regarding the data; methods used or how you managed to account for swerve. I wanted some metadata in the report.

If you argue that, at lower speeds, perceived deflection is lower because of swerve, then how did the data control for swerve? At what point did your data manage to discern between the two?

 

[Patrick Johnson]

how did the data control for swerve?

His "squirt robot" uses a level cue so there is no swerve.

You can see the robot here (on the second page).

pj
chgo