What Is Gear Rollout and Why Does It Matter?
Gear rollout is the single number that determines whether your child can race. It measures how far a bike travels in one complete pedal revolution — and if it's too high, you're going home. Here's how it works.
The Simple Explanation
Imagine turning the pedals exactly one full revolution. The distance the bike moves forward is the rollout. It depends on three things:
- The chainring size (the front cog attached to the pedals) — measured in teeth
- The sprocket size (the rear cog on the wheel) — measured in teeth
- The wheel and tyre size — which determines the circumference of the rear wheel
The Formula
Rollout is calculated as:
Rollout = (Chainring teeth / Sprocket teeth) × Wheel circumference
The gear ratio (chainring divided by sprocket) tells you how many times the rear wheel turns per pedal revolution. Multiply that by how far the wheel travels in one rotation (its circumference), and you get the total distance — the rollout.
A Worked Example
Example: 48T chainring, 16T sprocket, 700c wheel with 25mm tyre
- Gear ratio: 48 / 16 = 3.0
- Wheel circumference (700c + 25mm tyre): approximately 2.105 metres
- Rollout: 3.0 × 2.105 = 6.315 metres
At a 6.05m rollout limit (under-16), this combination would be illegal by about 26.5cm. You'd need either a smaller chainring or a larger sprocket.
Why Are There Limits?
British Cycling and the UCI set rollout limits for each age category to keep young riders in appropriate gears. The limits encourage higher cadence (pedalling speed) and better technique development. It also levels the playing field — races are won on fitness and skill, not who has the biggest chainring.
UK Rollout Limits by Category
Current British Cycling limits
- Under-12: 5.40m maximum rollout
- Under-14: 6.05m maximum rollout
- Under-16: 6.05m maximum rollout
- Junior (under-18): 7.93m maximum rollout
Note: European events may use slightly different limits. Always check the specific event regulations.
What Changes the Rollout?
Each component affects rollout differently:
- Bigger chainring = higher rollout (more distance per revolution). Going from 46T to 48T increases rollout noticeably.
- Smaller sprocket = higher rollout. An 11T sprocket gives much higher rollout than a 16T. This is why the smallest cog on your cassette matters most.
- Wider tyres = slightly higher rollout. A 28mm tyre has a larger circumference than a 23mm, so rollout increases. The difference is small but can push a borderline setup over the limit.
How to Check Your Rollout
The easiest way is to use our gear rollout calculator. Enter your chainring size, sprocket size, and tyre width, and it instantly shows whether you're within the limit for each age category.
For the advanced calculator, you can see every possible chainring/sprocket combination and which ones are legal — useful when you're deciding what parts to buy.
On race day, commissaires typically check rollout by measuring the distance the bike travels when the cranks make one full revolution with the chain on the biggest chainring and smallest sprocket. Make sure you've checked your gearing before you get to the start line.