Gearing outside spin is the exact amount of outside spin required to have absolutely no throw to send the OB exactly in the expected “line-of-centers” direction. This can be useful in play to guarantee you will get a “good hit” on the ball, especially when cling/skid/kick is a concern (e.g., under dirty and/or chalk-smudged ball conditions) or if you have trouble judging and compensating for throw, or to prevent cut-induced effects with bank shots. Here’s a good illustration and video demonstrating the effects of different amounts of spin:
The “gearing” outside spin handout and the following video, from Vol. I of How to Aim Pool Shots (HAPS), demonstrates how to use the 40% rule (see below) to aim shots with “gearing” outside spin to eliminate cut-induced throw:
For more information, see “HAPS – Part III: Gearing Outside English,” (BD, January, 2015). It can be difficult to have a feel for the exact amount of OE to use for different shots, even using the 40% rule; although, this feel can probably be developed fairly easily over time and experience. As long as one is just a little off with the amount of OE, the amount of throw can still be small enough to be a non-factor.
Diagram 2 in “Throw – Part VI: inside/outside english” (BD, January, 2007) shows how the amount of sidespin required for “gearing” OE varies with cut angle. Gearing OE is possible even with a 90° cut. And it is even possbile to throw the OB with spin-induced throw (SIT) at cut angles close to 90° (see impossible cut shots for examples).
Below is an illustration of an easy way to visualize how much tip offset is required to create a gearing amount of outside spin (see TP A.26 for the derivation). The technique is called the 40% rule. Here’s how it works: If you imagine or use your cue to visualize the line through the OB to the pocket, that gives the “line of centers.” If you parallel-shift this to the CB (the red line in the diagram), that defines the “line-of-centers” point on the CB (the red dot in the diagram). The tip contact point must be a little less than half (2/5 or 40% to be precise) of the distance from center ball to the “line-of-centers” point to create “gearing” outside spin.
With long, slow, follow shots, you must apply extra sidespin since some will be lost on the way to the OB. And on shots with significant drag action (e.g., draw or stun shots), where most or all of the backspin wears off on the way to the OB, less sidespin is required since the drag action increases the effect of the sidespin. For more info, see drag shot effects.
An alternative way to determine the gearing amount of outside spin is to take 80% of 1 minus the ball-hit fraction. For example, for a 1/2 ball hit (0.5 or 50% ball-hit fraction), the gearing amount of spin is 0.8 (1 – 0.5) = 0.4 [alternatively, 80% of (100% – 50%) = 40%]. This works for all ball-hit fractions. For more info, see TP A.26.
The 40% or 2/5 rule for gearing spin also applies to the amount of spin required to create perfect natural running spin on kick shots. Here’s an illustration from Patrick Johnson (from AZB post) showing how this works:
Should I use outside spin to reduce the effects of throw (i.e., “spin the ball in”)?
See: Should You Spin or Throw Balls In?
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