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:
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.
and here are some other more-detailed videos on the topic:
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.
|BHE: back-hand english
CB: cue ball
CIT: cut-induced throw
FHE: front-hand english
gearing OE: the amount of outside english that results in no throw
|IE: inside english
OB: object ball
OE: outside english
SIT: spin-induced throw
squerve: combination of squirt and swerve
Squirt (cue ball deflection), Swerve, and Throw Effects
(everything you ever wanted to know about squirt, swerve, and throw)
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.
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