Dr. Dave's answers to frequently-asked questions (FAQs), mostly from the AZB discussion forum
What's the best way to hit a lag shot?
Obviously, the goal is to leave the CB as close to the head rail as possible as often as possible. For the best and most consistent speed control, it is better to hit above center. If you hit at or below center, you need to judge the speed loss due to drag on the cloth, and this will vary with cloth and ball conditions.
To help achieve the best speed and distance consistency, it is best to hit the CB at about 20% of the radius above the center (0.2R), which is about 60% of the total ball height. For illustrations and explanations, see "The Lag Shot" (BD, October, 2011). For the detailed math and physics, see TP B.12 - Optimal tip height for speed/distance control. Hitting above center can also result in a more accuracy with CB direction (see follow accuracy).
When the CB rebound off the rail, it will lose speed due to rail inefficiency. Also, roll into the rail typically converts to stun on rebound (e.g., see HSV B.15), which slows the ball (due to skidding) even more. That's why it is generally better to try to hit the head rail with your lag shot. This will generally result in the best average lag results (i.e., proximity to the rail). If you are a little short, you will be a little short. If you are a little long, the CB will lose speed off the rebound, and end up very close to the rail ... closer than if you are a little short.
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