Aim Compensation in Pool and Billiards

... how to compensate for squirt, swerve, and throw when using sidespin.

Dr. Dave's answers to frequently-asked questions (FAQs), mostly from the AZB discussion forum

for more information, see Section 4.04 in The Illustrated Principles of Pool and Billiards,
Disc I of How to Aim Pool Shots (HAPS),
and Disc II of the Video Encyclopedia of Pool Shots

aim compensation when using sidespin

Is there a style of play (e.g., using aim-and-pivot aiming systems) that can compensate for all of the effects of squirt, swerve, and throw when using sidespin?

Background information on important related topics can be found here:

• aiming with english handout
• back-hand english (BHE) and front-hand english (FHE)
• cue pivot length
• squirt, swerve, and throw effects

There is no sliver-bullet "style of play" that can magically solve all of the "challenges" associate with compensating one's aim for squirt (CB deflection), swerve, and throw when using sidespin. The BHE and FHE aim-and-pivot methods can be used to adjust for squirt (and in some cases the combination of squirt, swerve and/or throw, using an "effective pivot length" for each shot), but swerve and throw vary too much with shot speed, shot distance, cue elevation, type and amount of english, amount of forward roll, ball and cloth conditions, etc. The best you can do is be knowledgeable of all of the effects (e.g., see "Squirt - Part VIII: squerve effects" - BD, March, 2008 summarizing squirt and swerve effects, and "Throw - Part XI: everything you ever wanted to know about throw" - BD, June, 2007 summarizing throw effects) and/or have solid intuition based on many years of successful practice and play. There is no magical "style of play" that can solve all of the world's problems ... playing pool at a high level is simply not easy.

See also:

and here are some other more-detailed videos on the topic:

• NV A.19 - Colin Colenso's back-hand-english saga (for squirt compensation)
• NV B.24 - Joe Tucker's squirt, swerve, and BHE/FHE combo (part 1, part 2, part 3)

The BHE method can be used to adjust for both squirt and swerve if you adjust your bridge length based on the distance and speed of the shot. Basically, at slower speeds and greater distances between the CB and the OB, a longer bridge length is required because the "effective squirt" (or squerve) is reduced due to swerve. This will only work with a near level cue, because with an elevated cue, swerve becomes a larger factor and can make "effective squirt" negative. Also, with a follow shot, swerve happens sooner than with a draw shot (of the same cue elevation), giving less effective squirt (squerve), so again a longer pivot length would be necessary. Colin Colenso has come up with some experiments and formulas to help methodically determine the required effective pivot length (i.e., required bridge length) for any shot (see below).

An alternative to using BHE with a variable bridge length is to use a fixed bridge length and appropriate combinations of BHE and FHE as described briefly on the BHE/FHE resource page and in detail on Disc I of How to Aim Pool Shots (HAPS).

What squirt (cue ball deflection), swerve, and throw effects do I need to be aware of?

A complete summary of all squirt (cue ball deflection), swerve, and throw effects and rules of thumb can be found below in the numbered list beneath the videos and illustrations.

As background, here are some pertinent video demonstrations from the Video Encyclopedia of Pool Shots (VEPS):

The following diagram from Jeremiah Gage (courtesy of BullseyeBilliards) nicely illustrates the numerous interactions that occur in the execution of a shot, especially when english (sidespin) is used.

The numbered list below is a quick summary of important squirt (cue ball deflection), swerve, and throw effects, along with links to supporting resources. Let's start with a short glossary of definitions and an illustration of some of the terminology used in the effects list. More definitions can be found in the online glossary, and additional info and examples can be found in the linked resources.

Squirt (cue ball deflection), Swerve, and Throw Effects
(everything you ever wanted to know about squirt, swerve, and throw)

Squirt:

1. Squirt increases with the amount of sidespin.
2. Squirt does not depend on shot speed (although, squerve does; see squirt speed effects for more info).
3. Squirt increases with the amount of shaft endmass (e.g., a low-squirt cue has less endmass and results in less squirt).
4. Squirt is slightly less with a heavier CB and slightly more with a lighter CB (see CB weight effects).

Swerve:

5. Swerve increases with cue elevation and the amount of sidespin.
6. Swerve occurs with practically all sidespin shots because the cue must be elevated to clear the rails.
7. Swerve is delayed with faster shot speed.
8. Swerve occurs only while the CB is sliding; once rolling begins, the CB heads in a straight line.
9. Swerve occurs earlier with sticky cloth and later on slick cloth.
10. Swerve occurs earlier with a follow shot than with a draw shot (see "Squirt - Part VIII: squerve effects" - BD, March, 2008).
11. Swerve angle is larger with a draw shot than with a follow shot (see "Squirt - Part VIII: squerve effects" - BD, March, 2008).
12. Swerve angle can be predicted and visualized using the Coriolis massé-shot aiming system.

Squerve:

13. Squerve (net effect of squirt and swerve = net CB deflection) can be zero with certain speeds and cue elevations for a given shot distance, amount of sidespin, and cue.
14. Squerve is less for follow vs. draw shots (see squirt tip-contact-height effects).
15. Squirt or squerve can be canceled using back-hand english (BHE) and/or front-hand english (FHE) aim-and-pivot methods.

Throw:

16. For small cut angle shots (i.e., fuller hits), the amount of CIT does not vary with shot speed, but increases with cut angle (see throw speed effects).
17. For larger cut angle shots (i.e., thinner hits), the amount of CIT is significantly larger for slower speed shots as compared to faster speed shots (see throw speed effects).
18. The amount of CIT decreases some with larger cut angles, but not by much (especially for slower speed shots) (see "Throw - Part II: results" - BD, September, 2006).
19. Maximum CIT occurs at close to a half-ball hit (30-degree cut angle) (see "Throw - Part II: results" - BD, September, 2006).
20. In general, throw is larger at slower speeds, and for stun shots (see throw speed effects and throw draw/follow effects).
21. Maximum throw, under typical conditions, is about 1 inch per foot of OB travel, or 1/2 a ball per diamond on a 9' table, which is about 5 degrees.
22. Both follow and draw reduce throw, and they do so by the same amount (see throw draw/follow effects).
23. Effective throw (combined effect of throw and OB swerve) can be slightly larger with follow vs. draw.
24. The largest discrepancy between throw values for stun and follow/draw shots occurs close to a half-ball hit (30-degree cut angle) (see throw draw/follow effects).
25. The difference between the throw of stun and follow/draw shots is not as great at larger cut angles (see throw draw/follow effects).
26. More sidespin gives you more SIT only up to a point. Additional sidespin beyond that point actually reduces the amount of SIT (i.e., more sidespin doesn't always give you more throw) (see throw speed effects and maximum throw).
27. SIT is largest for a slow stun shot with about 50% of maximum sidespin (see throw speed effects and maximum throw).
28. The amount of throw can increase significantly as a small amount of sidespin is added, especially for a stun shot (see "Throw - Part IV: spin-induced throw" - BD, November, 2006).
29. SIT is independent of speed (i.e., the throw is the same at all speeds) for small amounts of sidespin (see throw speed effects).
30. "Gearing" OE results in absolutely no throw. The amount of sidespin required for "gearing" increases with cut angle. At a half-ball hit, the amount of sidespin required is about 50% (see gearing outside english).
31. At very small cut angles, IE and OE create similar amounts of throw (although, in opposite directions) (see "Throw - Part VII: CIT/SIT combo" - BD, February, 2007).
32. For large cut angles, a small amount of OE can result in more throw than shots with no sidespin (see see "Throw - Part VII: CIT/SIT combo" - BD, February, 2007).
33. For large cut angles, IE results in less throw than shots with no sidespin (see see "Throw - Part VII: CIT/SIT combo" - BD, February, 2007).
34. IE increases throw at small cut angles, but actually reduces the amount of throw at larger cut angles (see see "Throw - Part VII: CIT/SIT combo" - BD, February, 2007).
35. OE can cause throw in either direction depending on the amount of sidespin and the cut angle (see "Throw - Part VI: inside/outside english" - BD, January, 2007).
36. Even for large cut angle shots (thin hits), excess OE (more than the "gearing" amount) can be applied to throw the OB in the SIT direction (see gearing outside english).
37. The amount of throw with IE can be much more consistent than with OE if the amount of sidespin varies a little. In other words, the amount of throw varies more with tip placement for OE vs. IE. This might explain why some people prefer using IE on cut shots ... because they can better anticipate and adjust for the amount of throw.
38. The least amount of throw, and the most throw consistency, occurs with fast IE shots.
39. With a small-gap combo with a gap size close to 3/8" (9.5mm), the 2nd ball heads very straight (i.e., the throw effect cancels the cut effect) over a fairly wide range of 1st-ball angles, regardless of ball conditions (see small-gap combos).

In support of the last two inside english (IE) items above, see the 2nd plot on page 3 of TP A.28, which corresponds to 30-degree cut-angle stun shots at different speeds and varying amount of both outside english (positive in the plots) and inside english (negative in the plots). In the plot, notice how consistent and small the amount of throw is for fast-speed inside-english shots over a wide range of sidespin amounts (see the left side of the green curve, which is very low and level).

A good technical analysis and presentation of squirt effects can be found in Ron Shepard's "Everything you Always Wanted to Know About Cue Ball Squirt, but Were Afraid to Ask." Here's a good summary of squirt effects from that document.

from Colin Colenso:

It's little wonder that pros are adept at, and usually rely almost solely upon simply estimating how to align to any shot they want to make using english.

There really has been no other way to learn how to execute english for a wide range of shots. So they learn a huge range of shots by repetition and this gives them the intuitive feel to make, or get close to almost any shot they attempt with varying degrees of cut angle, speed and tip offset.

When I first learned about BHE a few years ago I thought it was some instant quick fix. But I soon learned that there were variables that affected the success on many shots quite significantly. These are:

1. The effective pivot point changes according to speed and distance traveled. (Swerve is the culprit).

2. The actual contact point required to make shots varies considerably with CB speed, cut angle and type and rate of spin on the CB.

So without knowing how much to adjust for all of these variables, BHE is only useful for a limited range of shots.

[Here is a method, with formulas, that can be used to select a bridge length to compensate for both squirt and swerve, assuming you have already adjusted your aim for any throw effects.]

PPe = PPi + DVK

PPe = required effective pivot point for any shot based on distance and shot speed.
D = Distance from CB to OB (or target) in feet.
V = Velocity Factor where 0 is maximum speed and 4 is slow, or one table length roll including bouncing off one rail (see below).
K = correction factor to account for cloth slickness given by:

K = (PPe* - PPi) / 15

PPi = The Intrinsic Pivot Point. Estimated by finding the effective pivot point for a shot over 5 feet hitting at maximum velocity, such that swerve has insignificant influence on the shot. My cue's PPi is 9.5 inches. Low squirt cues are 12 to 14 inches.

PPe* = the pivot point required for a 5 foot shot at speed factor 3, which is medium slow, enough to bounce 2 rails back to the original position. This figure will be different for each cue on each table. It brings the slickness variable into the formula.

My preferred cue on my table has PPe* = 13.7 inches. (This could change with humidity changes). It's PPi is 9.5 inches, so my K value for my cue on my table at the moment is (13.7 - 9.5)/15 = approx 0.28. 0.28 is the adjustment needed at distance 1 foot and speed factor 1. The number 15 is derived from the PPe* being at 5 foot at speed factor 3. 5x3 = 15. PPe* could use any shot as a basis with a different numerator, but 5 foot is a good number because it is about the length of the cue, it can be played with little elevation and it is long and slow enough to provide a decent difference with PPi, hence giving it a reasonable margin of error. PPe* can vary by around 2 inches depending on cloth slickness. It is a number that can be derived pretty accurately within half a dozen hits on a new table.

So for any shot my PPe = 9.5 + D x V x 0.28

So if I have a shot at speed factor 2 over 4 feet my PPe = 9.5 + 2.24 = approx 11.7 inches.

Below is a chart with PPe's for the full range of speeds and distances for my cue. You should be able to plug data into this formula and get PPe's that correspond to those in the chart. Note: The key to making this formula simple was creating the methodology of the speed factor. In the chart below, the speeds are divided into 6 markings, rather than the 5 for speed factors 0-4.

General Speed Factor (V) Rules are:

0 = Max speed, would bounce about 5 rails.
1 = Firm speed, would bounce 4 rails and back to starting position.
2 = Medium speed, would bounce 3 rails and back to starting position.
3 = Slow-Medium speed, would bounce 2 rails and back to starting position.
4 = Slow speed, would bounce 1 rail and back to starting position at center table.

[To compensate for how squerve changes for draw vs. follow shots (for more info, see "Squirt - Part VIII: squerve effects" - BD, March, 2008), we can also make an adjustment to the pivot point based on the height of the cue:]

Based on the above observation I was able, via some testing to establish a pivot point adjustment key depending on the height the cue ball is struck with english.

Strangely enough it seems to work pretty well independent of distance, speed or cue offset. Though this hasn't been tested comprehensively.

So the total PPe Effective Pivot Point formula, as it has been refined is:

PPe = PPi + DVK + H

H = Height of hit on CB using the numbers given below in the diagram.
(Note: In most practical cases we don't need to hit higher than the 1 range. Most draw shots fit into the -1 range.

For a low squirt cue, such as a Predator Z, I would increase the numbers in the below chart by about 33%. So they might range from 4 to -1.3. Current information suggests the Predator Z has a natural or intrinsic pivot point of between 12 and 13 inches, compared to a normal shaft which is in the 9 to 9.5 inch range.

top of page      |     main FAQ index     |     website home page