Gearing and Roll-out

One of the most important aspects of bike setup is gearing. Gearing for BMX racing is a compromise, on the one hand you need lower gearing for a great snap from the gate which can give you that important hole-shot into turn 1, versus higher gearing for maximum top speed.

Gearing that’s too low will give you a great snap out of the gate, but will cause your cranks to spin-out, which can limit your top speed.

Gearing that’s too high means you could be slow out of the gate, but once you wind up you may have a greater top speed.

Somewhere between these extremes will give your best lap time.

The best way to work out the gearing that suits you is to time yourself. Perform sprint runs over about 60-80m distance – from a standing start or gate drop, time yourself at the 20m mark (first jump) and at the 60-80m mark (first turn) in the same sprint run. The objective would be to find the best times from a range of gear ratios, you can decide whether a slightly better 20m time (hole-shot) is more desirable than the 60-80m time. If you’re a good blocker out in front then go for the hole-shot time.

So, what is roll-out?

A good starting point for gear selection and for comparison purposes is to calculate your roll-out. Roll-out is the distance the bike travels in one full revolution of the crank.

You should keep track of your roll-out when changing tyres for example or when comparing with your mates who have similar physical traits and abilities. Also, as you improve in speed and strength then you should be able to gradually increase your roll-out to give you a blistering top speed.

Roll-out = (Number of Chain Ring Teeth) / (Number of Sprocket Teeth) X (Outside Wheel Diameter in Inches)

The formula above isn’t quite correct because the result must be multiplied by PI (3.142) to give actual inches of roll-out (called “Gear Inches” by track cyclists). But PI is usually dropped to keep things simpler.

So in summary, low gearing will result in a shorter roll-out, and high gearing will result in a longer roll-out distance.

So what is the correct roll-out for any given rider?

The answer is “it depends”. There are a number of factors to consider:

  • Rider’s size (sprockets need much lower gearing than 15 yo’s)
  • Rider’s strength (strong legs can handle higher gearing)
  • Rider’s speed (ability to spin the cranks fast – FASTER is BETTER, so train for fast crank speed)
  • Rider’s endurance (if you’re not fit then lower gearing may help you keep some speed up in the final straight)
  • Crank length (shorter cranks can spin easier, longer cranks give you more leverage to handle higher gearing but don’t spin as fast – it’s all a big compromise)

So, an 8yo may have an ideal roll-out of 48 and a Pro 54 – but remember roll-out is just a guide, the best gearing is what gives you the fastest lap times – so time yourself and train hard!