Dr. Dave's answers to frequently-asked questions (FAQs), mostly from the AZB discussion forum
for more information, see the strategy resource page and Chapter 5 in The Illustrated Principles of Pool and Billiards, Disc I and Disc II of the Video Encyclopedia of Pool Shots (VEPS), Disc I and Disc II of the Video Encyclopedia of Pool Practice (VEPP), and Disc II of the Video Encyclopedia of Eight Ball (VEEB)
30-degree and 90-degree rules
What are the 30-degree and 90-degree rules, and how can they be used for cue ball control?
See 30-degree and 90-degree rules.
45-degree rule for center-table position and routes
What is the 45-degree rule, and how is it used to position the CB at or through the center of the table?
It states that if the CB rolls into the short rail at close to a 45-degree angle, it will head off two rails fairly close to the center of the table. For more info, see the following demonstration from Disc II of the Video Encyclopedia of Pool Shots (VEPS):
See "VEPS GEMS - Part III: English and Position Control" (BD, March, 2010) for more information.
Here's a good drill from Disc II of the Video Encyclopedia of Pool Practice (VEPP) for practicing getting to the center of the table off pocket hangers:
See "VEPP – Part V: Hanger Table-Center Drills," (BD, August, 2012) for more information.
How does the cue-ball-control clock system work?
Buddy Hall does a good job of explaining and demonstrating the clock system in the following video: Buddy Hall's clock system.
This system is a good reference for known shots like this. For other shots at different angles and distances to the rails, things will vary some. Cloth and cushion conditions also affect how well the system applies.
drills for practicing cue ball control and position play
What are some useful drills for getting better at cue ball control and position play?
The following drills are excellent for learning and improving cue ball control and position play:
Drills for working on other skills can be found on the drills resource page.
leaving an angle on a shot and coming into the line of a shot
How do I do this?
See the following demonstrations from Disc III of the Video Encyclopedia of Eight Ball (VEEB) and from Disc II of the Video Encyclopedia of Pool Shots:
Sometimes, it is important to play for a precise point or line, rather to a general area. Here's an example from Disc III of the Video Encyclopedia of Pool Shots:
nearly straight-in shots
How can I get position on the next shot if the current shot is nearly straight in?
See : NV B.30 - Cue ball position control with nearly straight-in shots.
And here are some examples from Disc III of the Video Encyclopedia of Eight Ball (VEEB):
For more information, see: "VEEB - Part VI: Straight-In Shot Options" (BD, April, 2016).
rail cut-shot CB control
How can I predict and control the final cue ball direction with rail cut shots?
For rolling-cue-ball rail cut shots, you can use the 30-degree rule, as illustrated in the diagram below. The peace sign can be used to visualize the expected angle off the OB if the rail were not there, and the mirror of this angle gives you the final cue ball direction. An interesting fact is that the sum of the angle of the initial CB line relative to the rail and the mirror angle relative to the rail normal always adds to 60 degrees (see the quote from mohrt below). This is because 90 - 30 = 60.
Using stun or draw or less than full forward roll can also be used to alter the path.
The mirroring in the diagram also works well with the trisect system for predicting CB direction off the rail with draw shots. For more information, and to see the effects of speed on all rail cut shots, see: "Rail Cut Shot CB Control" (BD, February, 2017).
See the rail cut shot resource page for lots of examples of how english can be used to alter the path.
If you come into the rail at 30 degrees, the path will be 30 degrees out. If you come in at 15 degrees, it will be 45 degrees out. If you come in at 25 degrees, it will be 35 degrees out. Notice it always adds up to 60 degrees.
How do I avoid a scratch in different situations?
See the following video demonstrations:
What effect does shot speed have on the 90-degree and 30-degree rules?
Shot speed has no effect on the 90-degree rule. With a stun shot, the CB heads straight down the tangent line, regardless of speed.
With follow and draw shots, the CB persists along the tangent line longer before curving to the final direction, as demonstrated in this video:
Follow and draw are delayed even more if the CB hops into the OB. For more information and demonstrations, see the ball hop effects resource page.
When using the 30-degree rule, this diagram shows how you need to shift the peace-sign down the tangent line with faster speeds, to predict the final CB direction:
For more information, see "90° and 30° Rule Follow-up - Part V: the final chapter" (June, 2005).
With draw shots, a similar shift with speed occurs, based on this diagram:
For more information, see "Draw Shot Primer - Part I: physics" (January, 2006).
The condition of the cloth also has an effect. With a slicker cloth, the CB persists along the tangent line longer before curving to the final direction. For more info, see cloth effects.
useful reference lines
How can I know where the cue ball will go on every type of shot?
Here's an example from Disc I of The Video Encyclopedia of Pool Shots where the reference lines can be useful to plan cluster break-out shots:
For more info, see:
where the CB goes for different types of shots
Where will the cue ball go after it hits an object ball?
For a stun shot, most people know the right answer: in the tangent line direction, perpendicular to the OB direction. This is the 90-degree rule (see "The 90° rule: Part I - the basics" - BD, January, 2004). If you want a more precise answer that accounts for various effects (e.g., friction and english), see the following instructional articles:
"90° and 30° Rule Follow-up - Part II: speed effects" (BD, March, 2005).
"90° and 30° Rule Follow-up - Part III: inelasticity and friction effects" (BD, April, 2005).
"90° and 30° Rule Follow-up - Part IV: english effects" (BD, May, 2005).
"90° and 30° Rule Follow-up - Part V: the final chapter" (BD, June, 2005).
FYI, here is a convenient one-page summary of the 90-degree rule.
For a rolling CB, the cue ball changes direction by about 30 degrees for a wide range of cut shots (1/4 to 3/4 ball hit). This is the 30 degree rule (see "The 30° rule: Part I - the basics" - BD, April, 2004). If you want to be more precise, the angle is a little more (about 34 degrees) closer to a 1/2-ball hit and a little less (about 27 degrees) closer to a 1/4-ball or 3/4-ball hit (see TP B.13 for even more details and precision). If you want to know how to account for speed effects, see "90° and 30° Rule Follow-up - Part V: the final chapter" (BD, June, 2005). If you want an easy way to use your hand to accurately visualize the cue ball direction, use the Dr. Dave peace-sign technique. FYI, here is a convenient one-page summary of the 30-degree rule.
For a draw shot with good draw action, and for cut angles smaller than about 40 degrees (i.e., ball-hit fraction greater than about 3/8), the trisect system is your answer (see "Draw Shot Primer - Part III: using the trisect system" - BD, March, 2006). You can use a modified version of the Dr. Dave peace-sign technique to predict the cue ball direction (see the article, NV B.43, and NV B.67 for illustrations and examples).
For shots "in between" all of these different cases, the cue ball will go somewhere in between the indicated directions. The only way to get a feel for how much "in between" the cue ball will go is to practice ... a lot! However, knowing the reference directions and how they correspond to different tip positions can help you judge the tip positions required for different CB directions, as demonstrated in: NV B.68 - Tweener cluster breaks, from VEPS I.
To see how speed affects CB trajectories, see speed effects.
For more info, see Disc I and Disc II of the Video Encyclopedia of Pool Shots and:
What if the cut is very thin or hit very full?
For roll shots, there are good approximations for the CB deflection angles.
For a fairly full hit, with a ball-hit-fraction greater than 3/4, the CB will deflect about 3-times the cut angle.
For a fairly thin hit, with a ball-hit-fraction less than 1/4, the CB will deflect about 70% of the angle between the aiming line and the tangent line.
See "Rolling Cue Ball Deflection Angle Approximations" (BD, November, 2011) for illustrations, examples, and more information.
There are similar rules for draw shots. For more information, see "Draw Shot Cue Ball Directions" (BD, December, 2011).
As with the 30 degree rule and trisect system, the full-hit and thin-hit rules apply to the final direction of the CB. The actual final path of the CB is shifted down the tangent line with higher speed.
Video demonstrations of these types of shots can be found in Disc I of the Video Encyclopedia of Pool Shots.
from Jal (from AZB post, which contains additional information):
When the balls are close enough to each other and/or you're hitting hard enough such that the cueball doesn't lose any significant backspin on the way to the object ball (or gain more topspin), there is a method of determining the cueball's direction once it reaches natural roll after the collision. I call it the Bottom-Center-Arrow method, or B-C-A for short, in that it's easy to remember.
Imagine a circle centered on the ghostball with the bottom of the circle running through the center of the cueball. This circle represents the face of the cueball from the shooter's perspective. To determine the CB's roll direction after the collision for any vertical offset (no sidespin applied), draw a line from the center of the real cueball parallel to the line of centers between the ghostball and the object ball. This will intersect the tangent line at 90 degrees, call it point A. Thus, we have a triangle with the CB at vertex B (bottom of the circle), the ghostball at C (center of the circle) and point A from which we'll draw an arrow such that it intersects the vertical axis of the large circle. This yields the CB's direction once roll sets in, given that vertical tip offset on the face of cueball. Here's a diagram:
... friction, amongst other things, has an effect on this idealized geometry.
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