Crank length
Cranks are measured from the centre of the axle to the centre of the pedal hole. The length is usually stamped on the back of the crank arm.
| Length | Usually suits |
|---|---|
| 165mm | Smaller riders, and riders who want more ground clearance or a faster cadence |
| 170mm | Small to medium frames |
| 172.5mm | Medium to large frames. The most common road size |
| 175mm | Taller riders and many MTBs |
If your current length feels fine, stick with it.
Chainring sizes
Bigger rings give harder, faster gears. Smaller rings make climbing easier.
| Setup | Rings | Good for |
|---|---|---|
| Standard | 53/39 | Racing and fast, flat riding |
| Semi-compact | 52/36 | Fast riding with some hills |
| Compact | 50/34 | Most road riders and hilly routes |
| Gravel (Shimano GRX) | 46/30 or 48/31 | Loaded riding, steep off-road climbs |
| 1x (single ring) | Usually 30–44T | MTB and gravel. Simpler, with a narrow-wide ring to hold the chain |
How chainrings fit
- Bolted rings use a bolt circle diameter (BCD), such as 110mm or 130mm. A new ring must match the BCD and bolt pattern.
- Direct mount rings fit straight onto the crank with no spider. They must match the crank's mounting standard (for example SRAM 8-bolt or Shimano's own design).
- Narrow-wide rings have alternating tooth widths to grip the chain on 1x setups. Use one for any single-ring conversion.
Rings wear out like chains and cassettes. Hooked or shark-fin shaped teeth mean it's time to replace them.
Spindles and bottom brackets
The spindle is the axle joining the two cranks. It must match your bottom bracket.
| Spindle | Diameter | Used by |
|---|---|---|
| Shimano Hollowtech II | 24mm | Most Shimano road and MTB chainsets |
| SRAM DUB | 28.99mm | Most current SRAM chainsets |
| SRAM GXP | 24/22mm stepped | Older SRAM chainsets |
| 30mm | 30mm | Race Face, Rotor and others |
FAQs
Can I put smaller rings on my road chainset?
Often yes, as long as they match the BCD and your front mech and chain length can handle the change.
Will a new chainset fit my bike?
Check the speeds, the spindle against your bottom bracket, and the chainline for MTB (boost or non-boost).
Do I need a new chain with new rings?
If your chain is worn past 0.5%, yes.