Coefficient of Restitution & Ball Selection
A second number that shapes energy transfer at impact, and how it works alongside compression instead of measuring the same thing twice.
Coefficient of restitution, or COR, describes how efficiently energy transfers between two colliding objects — in golf, between clubface and ball. A COR of 1.0 would mean a perfectly elastic collision with zero energy lost; real driver faces and golf balls fall well short of that, but modern combinations get remarkably close under the right conditions.
COR and Compression Are Related, Not Identical
It's easy to assume compression and COR measure the same thing, since both describe energy behavior at impact. They don't. Compression measures how much the ball's core deforms under a fixed load; COR measures how efficiently that stored deformation energy returns as ball speed. A ball can have relatively low compression and still return energy efficiently if its core material is well-suited to that deformation range — which is exactly why two low-compression balls from different manufacturers can produce noticeably different ball speeds off the same swing.
Where the Clubface Comes In
COR isn't purely a ball property, either — the clubface itself has a COR rating, capped by the USGA at 0.83 for conforming drivers. A driver face already sitting near that legal limit interacts differently with a stiff, high-compression ball than it does with a softer one, which is part of why the same driver head can test noticeably differently depending on which ball is loaded into the fitting session.
Fitting a driver head without also fitting the ball is testing half of a two-part collision and assuming the other half doesn't matter.
What This Means for Ball Selection
In practice, we don't ask clients to think about COR directly — it's baked into the ball speed and smash factor numbers we already capture on the bay. But it's the reason we test actual ball models against your actual driver rather than trusting a compression number alone: two balls in the same compression band can still produce a measurable smash factor gap once face and ball interaction is accounted for.
The Takeaway
Compression tells you roughly which balls are worth testing. COR — measured indirectly through smash factor on the bay — tells you which one actually performs best with your specific driver face. Both matter, and neither replaces the other.
See Smash Factor Across Multiple Ball Models
Every driver fitting compares smash factor across the ball models inside your compression band.