STT Lab
Work out your gearing, chain length and chain wear before you buy. Free tools from the people who cut the sprockets.
Gear ratio & speed
See what a different sprocket size does before you change it. Enter your speed and engine revs if you want the new figures too.
- Fewer front teeth or more rear teeth: quicker acceleration, lower top speed, higher revs.
- More front teeth or fewer rear teeth: lower revs when cruising, slower pick-up.
- One tooth at the front changes the ratio about as much as three teeth at the rear.
Chain length
Changing sprocket sizes often needs a longer or shorter chain. Enter your chain as it is now and the sizes you want; we work out the links you need.
- Count the links on your chain now, or read it from the box (e.g. 428 × 104L).
- The answer is rounded up to an even number, because a chain joins inner link to outer link.
- Check the axle still sits inside the adjuster range after fitting.
Chain wear
A worn chain stretches and eats sprockets. Measure it in two minutes with a ruler or vernier.
- Put the bike on its stand and pull the bottom run of the chain tight.
- Measure from the centre of one pin to the centre of the pin 20 links along (21 pins).
- Measure in two or three places and use the longest.
Is my sprocket worn?
Look at the teeth from the side. Replace the sprockets and chain together: a new chain on worn sprockets wears out fast.
- Teeth leaning or hooked in one direction, like a wave.
- Tips worn to sharp points, or teeth chipped or missing.
- You can pull the chain away from the back of the rear sprocket and see more than half a tooth.
- The chain jumps or clunks under hard acceleration.
Found your size?
Check which STT sprockets and chains fit your bike, or buy from a dealer near you.
Results are calculations to guide you. Always check fitment and chain adjustment on the bike.