RG (radius of gyration) describes how the weight is distributed in the core: low RG balls rev up sooner, high RG balls store energy and skid farther before turning. Differential describes how much the core can flare: higher differential means more track flare and more potential hook. USBC limits RG to between 2.430 and 2.800 inches and differential to 0.060 inches. Use them together with the coverstock to space out an arsenal.
Every bowling ball spec sheet lists RG and differential. Here is what they mean.
Radius of gyration (RG)
RG measures how far the core’s mass sits from the center of the ball, in inches.
| RG | What it does | Lane conditions |
|---|---|---|
| Low (about 2.46 to 2.50) | Revs up early, rolls sooner | Heavier and longer oil |
| Medium (about 2.51 to 2.54) | Balanced | Typical house shots |
| High (about 2.55 and up) | Stores energy, skids farther | Lighter oil, dry backends |
USBC specifications allow RG between 2.430 and 2.800 inches.
Differential
Differential is the gap between the ball’s highest and lowest RG. It shows how much the ball’s track can flare as it rolls. More flare exposes fresh cover to the lane and adds hook potential. USBC caps differential at 0.060 inches.
Symmetric and asymmetric cores
Core symmetry is a separate design label from RG and differential. See symmetric vs asymmetric cores for the PSA and layout details. Neither label alone predicts how much a ball will hook.
Using specs to plan an arsenal
An expanded four-ball setup can give each ball a different job:
- A strong solid reactive with low RG and high differential for heavier oil.
- A medium ball for the typical house shot.
- A pearl or urethane with higher RG for lighter oil and transition.
- A plastic spare ball.
For a smaller three-ball bag, see how to build a complementary setup.
The layout your pro shop drills is one part of the ball choice, and so is your ball surface. Read specs as a guide to the ball’s potential, then ask the driller how pin-to-PAP layout measurements apply to your release.
RG and differential describe the ball; lane oil is a separate part of the decision. Compare house, Challenge and sport oil patterns and ask for the actual lane sheet before changing equipment.
Browse arsenal planning and drilling and pro shop.
Common questions
Is low RG or high RG better?
Neither. Low RG suits heavier oil and bowlers who need the ball to rev early; high RG suits lighter oil and bowlers who want length.
What is a high differential bowling ball?
Generally one around 0.050 inches or more, close to the USBC maximum of 0.060. It can flare more and create a bigger hook shape.
What is mass bias?
On asymmetric balls, the intermediate differential (often called mass bias) describes how lopsided the core is. Higher values let the ball respond more sharply to friction.
Do RG numbers matter for beginners?
Less than coverstock and fit. They become useful once you own two or more balls and want each to do something different.