SIT and cut angle

[dr_dave]

Everything seems intuitive to me except for the above (shown graphically below). Why is it that you actually need less side spin for cut angles greater than 60 degrees to maximize throw? I would have thought the curves would asymptotically approach 100% spin as the cut angle increases, but they don't. The curves actually turn around at some point.

View attachment 617586

That's a tough one to explain. I think the collapse is due to the amount of spin getting closer to the gearing amount, where throw is less. If you are interested to learn more, compare the graph at the top of the last page of TP B.25 – Percentage Sidespin Required for Maximum SIT at Any Cut Angle to the graph in TP A.26 – The amount of sidespin required for “gearing” outside english.
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[dr_dave]

I assume it is because for a thinner cut shot the contact between the balls is lighter and that will in general get more throw. (Assumes the cue ball speed is fixed.)

I think a better explanation is: The retrograde motion of the outside spin causes the relative speed between the ball surfaces to be less, which results in more friction and throw. For an explanation for why friction and throw vary with relative surface speed, see throw speed effects.

 

[Patrick Johnson]

Really? I thought the opposite: heavier contact = more friction/throw.

In general hard shots -- shots at high speed with a lot of force at the contact point -- have less throw than the same shot at slower speed.

Doh! I forgot that more force = more speed.

Would a heavier CB (more force) at the same speed affect the amount of throw?

pj
chgo

 

[Bob Jewett]

Doh! I forgot that more force = more speed.

Would a heavier CB (more force) at the same speed affect the amount of throw?

pj
chgo

Probably not significantly for anything you would call a pool ball. Speeds vary by more than a factor of 10. Even 20% on the weight of a cue ball would change other things a lot more.

 

[dr_dave]

I assume it is because for a thinner cut shot the contact between the balls is lighter and that will in general get more throw. (Assumes the cue ball speed is fixed.)

The fixed CB speed in the analysis is one thing partly leading to the non-intuitive results. For larger cut angles, the OB speed is less and the outside spin resulting in maximum SIT results in less relative speed between the CB and OB surfaces which results in more friction and throw.

 

[Imac007]

As the cut angle increases, you need to hit the CB with more and more side spin to achieve the maximum SIT:

You mentioned that for straight shots max throw is found at 50% english, which is 25% of the radius.
This applies on to straight shots.
On a cut shot, there is a gearing english reference, found at 40% of the offset, applied as outside english.
At that rotation the cb “walks” along the surface neither “pushing” or “throwing“ the ob.
Each shot angle has a different gearing benchmark.
More English than the 40% causes the ob to be thrown, analogically, like a tire spitting gravel.
Max english on a thin cut is at the miscue limit.
The gearing english offset maximizes on thin cuts and is at nearly 40% of the radius, which is around 28mm.
There is zero throw at that amount of applied english.
The miscue limit is at about 50% of the radius.
There is only positive throw beyond the gearing english 40% and the miscue limit of 50% of the radius.

Dr. Dave has a series of graphs, all shots at slow speed and with 25, 50 and 100% english, across angles from straight through around 60°, quite thin fractionally.
He said a soft half ball center ball stun shot had max CIT.
Outside english, reveals the gearing english benchmark for each angle.
Gearing is true regardless of draw, stun or follow.

https://billiards.colostate.edu/tutorial/throw/
https://billiards.colostate.edu/bd_articles/2007/feb07.pdf

Addendum:
The use of percentages above can be confusing.
When 100% english is used, the miscue limit is considered 100%.
General knowledge is that that occurs at about half the radius.
Instead of saying ½ of the radius I chose the contextually poorly chosen 50% of radius as description.
Both description are equivalent.

 

[dr_dave]

You mentioned that for straight shots max throw is 50%.
On a cut shot, there is a gearing english reference, found at 40% of the offset, applied as outside english.
At that rotation the cb “walks” along the surface neither “pushing” or “throwing“ the ob.
Each shot angle has a different gearing benchmark.

For those interested, this stuff is described and demonstrated here:

Gearing Outside Spin​

More English than the 40% causes the ob to be thrown, analogically, like a tire spitting gravel.
Max english on a thin cut is at the miscue limit.
The gearing english offset maximizes on thin cuts and is at nearly 40% of the radius, which is around 28mm.
The miscue limit is at about 50% of the radius.

For those interested, the 40% rule is also described and demonstrated on the gearing outside spin resource page. And a graph showing the amount of spin required at every cut angle for gearing outside spin (and no throw) can be found on page 2 of TP A.26 – The amount of sidespin required for “gearing” outside english. Even for a really thin cut, only 80% of maximum spin (40% of the ball radius) is required to get gearing and spin-induced throw (SIT) can still be used to throw the ball. That's what makes some of the "impossible cut shots" possible.

Dr. Dave has a series of graphs, all shots at slow speed and with 25, 50 and 100% english, across angles from straight through around 60°, quite thin fractionally.
He said a soft half ball stun shot had max CIT.
Outside english, reveals the gearing english benchmark for each angle.

https://billiards.colostate.edu/tutorial/throw/
https://billiards.colostate.edu/bd_articles/2007/feb07.pdf

If you want to see a lot more throw graphs, see TP A.28 – Throw plots for all types of shots and TP B.3 – Throw calibration and contour plots for various cut angles, speeds, English, and roll.

Enjoy!