Frequently asked questions and information about BMX gearing and BMX gear ratios.
What is a gear ratio?
A gear ratio is simply the number of teeth on the front sprocket, divided by the number of teeth on the rear wheel driver. This ratio tells you how many times the rear wheel turns for every full rotation of the cranks when you pedal.
How is the ratio deteremined?
As mentioned above, the gear ratio is determined by dividing the number of teeth on the front sprocket by the number of teeth on the rear driver. For example, using a 25T sprocket and a 9T driver gives you a 2.78 gear ratio, meaning that for every full rotation of your cranks, your rear wheel turns 2.78 times.
Why is the 2.78 gear ratio considered standard?
In BMX, the gear ratio dates back to the early ’80s in BMX racing and became standardized throughout the industry. At the time, the go-to gear ratio was 2.75, using a 44T sprocket and a 16T freewheel, before smaller sprockets and drivers were developed for cassette hubs in freestyle. As drivetrains got smaller, the industry settled on 25T sprockets and 9T drivers, as the 2.78 ratio closely matched the original 2.75.
Why would I want a non-standard gear ratio?
Riders seeking a higher top speed might choose a higher gearing, such as a 28T sprocket with a 9T driver. Conversely, younger riders might prefer a lower gearing, like a 23T sprocket with a 9T driver, which makes it easier to start pedaling.
What is the best gear ratio?
There is no single ‘best’ ratio, but a 25T sprocket and 9T driver is by far the most popular. Higher gearing, such as a 28T to 9T ratio, is also gaining in popularity.
What do low and high gear ratios mean?
A low gear ratio makes it easier to start pedaling because your wheel turns slower and provides more torque. However, this results in a lower top speed. On the other hand, a high gear ratio makes the wheel turn faster, giving a higher top speed, but provides less torque, making it harder to get started.
What is rollout?
Rollout refers to the distance the bike travels with one full rotation of the cranks. To calculate this, the diameter of the tire is also needed. For the purpose of this website, we’ve used a 21″ outer diameter of an inflated tire for comparison. Rollout is more often a factor in BMX racing than in freestyle BMX, but it’s still a useful data point when comparing gear ratios.
What about Gear Inches?
Gear inches are another way of comparing gear ratios, but they are not commonly used in freestyle BMX, so they are not covered here for simplicity’s sake.
What about tire size?
Tire sizes can affect the gear ratio and rollout, as wider tires can have a larger diameter. However, in BMX freestyle, this effect is typically not significant enough to factor into gear ratios, and we use a 21″ inflated diameter size for our rollout data.
What about crank arm length?
While crank arm length doesn’t change the actual gear ratio, it does affect how that ratio feels. However, in freestyle BMX, crank arm length is more commonly chosen based on foot positioning rather than its impact on pedaling.