What is spin/speed ratio, and what effect does it have on a shot?

Spin/speed ratio (AKA spin-rate factor or SRF) is the amount of spin (side, top or bottom) the CB has in proportion to its forward speed. The spin/speed ratio is a measure of the intensity of the spin; and with sidespin, it is directly related to the amount of OB throw and rebound-angle change off a cushion. Immediately off the cue tip, the sidespin spin/speed ratio depends only on the horizontal tip offset from center. The vertical tip height (and resulting top or bottom spin) has no immediate effect on the sidespin spin/speed ratio. For a technical proof, notice that the analysis in TP A.12 – The relationship between cue ball spin and cue tip offset is not affected by tip height.

However, tip height does affect the amount of horizontal-axis spin (top/bottom), and that does affect the spin/speed ratio during the shot due to drag action. With more drag (as with draw shots), the spin/speed ratio increases (effectively intensifying the effect of the sidespin) as the CB approaches the OB or a cushion.

Also, when the CB hits an OB, its spin/speed ratio changes since the CB loses speed, but not significant spin, during the ball collision. There is also further spin-intensifying drag action as the CB slides across the cloth and develops forward roll. This spin intensifying effect is greater with a smaller cut angle (i.e., a fuller hit), where more of the CB speed is lost. So for a nearly straight follow shot where the CB heads nearly straight into a cushion, the spin/speed ratio and resulting rebound angle change off the cushion is much larger than with a shot with a bigger cut angle. Here’s an example:

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