What are “two-times” and “three-times” across bank shots, and how do they work?


Here’s an example by Efren Reyes using the same effect across the long side of the table:

from Patrick Johnson:

It’s a series of spin events:
1. The CB colliding with the OB puts some “holdup” spin on the OB, which shortens the angle off the first rail.
2. The OB colliding with the first rail at an angle reverses the spin that was put on by the CB (just like any ball picks up “running” spin when it hits the rail).
3. The reversed spin throws the OB toward the side pocket off the second rail.

three-times cross-side bank shot

Why does the spin direction reverse when the ball hits the cushion?

The spin reverses since there is a sideways friction force opposing the spin direction, which slows the spin, and a sideways rebound force (since the cushion is compressed sideways in addition to being compressed perpendicular) opposing the sideways motion. The second effect is the main cause for the spin reversal.

Dr. Dave keeps this site commercial free, with no ads. If you appreciate the free resources, please consider making a one-time or monthly donation to show your support: