Dr. Dave's answers to frequently-asked questions (FAQs), mostly from the AZB discussion forum
for more information, see Section 4.06 in The Illustrated Principles of Pool and Billiards,
Disc II of the Video Encyclopedia of Pool Shots,
Disc II of How to Aim Pool Shots (HAPS),
and Disc V of the Video Encyclopedia of Nine-ball and Ten-ball (VENT)
How do I aim and get position off rail cut shot?
For general advice on how to aim and pocket rail cut shots, see Section 4.06 in The Illustrated Principles of Pool and Billiards. Also see the rail cut shot handout. People sometimes give the advice that one should hit the cushion and OB at the same time with rail cut shots. This will work if gearing outside english is used; but with no english or with inside inside english, cut-induced throw (CIT) will push the OB into the cushion (causing it to rebound away from the cushion) which will cause the OB to rattle out of the pocket (unless the OB is fairly close to the pocket and/or if the pockets are not very tight). The best general advice concerning rail cut shots is to ignore the cushion and just aim the shot normally (unless CB control is a major factor; or if the cut angle is very large, in which cas you can use a cushion-first hit with running english to help pocket the ball). The following clip from Disc V of the Video Encyclopedia of Nine-ball and Ten-ball (VENT) demonstrates these concepts well:
For more info, see: "VENT – Part IX: Rail Cut Shot Effects" (BD, June, 2018).
Is it possible to overcut an OB frozen to a rail?
Yes. For an example, see:
HSV A.137 - rail cut-shot hitting the rail first, hitting the ball while compressing, fast, with draw and natural (running) english
from Patrick Johnson:
the CB contacts the rail a short distance away from the OB and moves toward the OB while in contact with the rail. Maybe it will help to show a "time lapse series" of drawings showing how the CB approaches the OB while in contact with the rail:
1. Initial contact with the rail. The CB's "equivalent straight line path" is the same as it's initial path into the rail.
2. Closer to the OB but not yet in contact with it. The CB's "equivalent straight line path" has changed, but the CB is still moving into and compressing the rail.
3. Contact with the OB. The CB's "equivalent straight line path" has changed more, but the CB is still moving into and compressing the rail.
from Bob Jewett (in AZB post):
Here's a shot that helps to understand how compressible the cushions are. The 1 and 9 are a chalk apart. The shot is to shoot directly at the 1 ball. The 9 goes straight in. Shoot firmly.
How do I get the desired cue ball position off a rail cut shot?
See the rail cut shot handout, "VEPS GEMS - Part III: English and Position Control" (BD, March, 2010), and "HAPS - Part VI: Rail Cut Shots" (BD, March, 2015). The following video from Disc II of How to Aim Pool Shots (HAPS) covers the important basics:
Concerning CB direction with frozen rail cut shots, the following super-slow-motion video clips are listed in order based on how far down-table the CB travels. The OB is pocketed in all of the clips from two diamonds up from the corner pocket.
All of these cases are also presented with graphics and narration on the HSV DVD.
Here are some demonstrations from Disc II of the Video Encyclopedia of Pool Shots that shows some examples:
Here's another good example from Disc V of the Video Encyclopedia of Nine-ball and Ten-ball (VENT):
And here's a related example (by Carlo Biado) where some sidespin can be retained with a ball-first hit at an angle into the cushion and slick conditions (new cloth and/or clean, non-humid conditions):
For many examples and explanations of how to judge fouls with rail cut shots, see:
non-frozen CB direction
How can I predict the direction the cue ball will head with non-frozen 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 rail cut shot position control for lots of examples of how english can be used to alter the path of the CB.
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.
straight frozen-cushion rail shots
Why is it difficult to hit the OB straight when both the CB and OB are frozen to a rail, like this (image from ktrepal85 in AZB post)?
First of all, to have the OB hug the rail, a very accurate hit is required. Also, the cushion must be true, with no bumps or waves. Also, the faster you hit the shot, the more the CB will bounce due to the cue elevation required for the cue to clear the head rail. When the CB bounces off the table, it will deflect off the nose of the cushion some away from the rail since part of the CB is actually under the nose of the cushion when it is frozen to the rail. For demonstrations, see the video below.
One option to improve accuracy on a shot like this is to hit it as softly as possible to minimize how much the CB hops. Another option is to use a touch of inside english (with the right cue elevation for the given shot speed) to squirt the CB away from the cushion and have it swerve back toward the rail. And if the CB doesn't quite swerve back enough, spin-induced throw helps straighten the OB. All options and approaches, along with example game-situation shots are demonstrated in the following videos that include a clip from Disc V of the Video Encyclopedia of Nine-ball and Ten-ball (VENT):
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