Before replacing anything, identify what your bike already uses — most of it is printed on the part itself. Match the standard, not just the size. When in doubt, take the old part (or the whole bike) to a shop.
Wheel & tyre size
Ignore the old inch labels where you can — the ETRTO bead-seat diameter (a number like 622) is the one that guarantees a fit. It's printed on the tyre sidewall, e.g. "700 × 35C (37-622)".
| Common name | ETRTO bead seat | Typical use |
|---|---|---|
| 700C / 29" | 622 mm | Road, gravel, hybrid, 29er MTB |
| 650B / 27.5" | 584 mm | Gravel, smaller road, trail MTB |
| 26" | 559 mm | Older & kids' MTB, folders |
| Not interchangeable | — | Match the ETRTO number exactly, not the inch label |
Tyre width & clearance
- Tyre width is the middle number: "700 × 32C" = 32 mm.
- A frame and fork only fit up to a maximum tyre width — leave a few mm clearance for mud.
- Rim internal width suits a range of tyre widths; very wide tyres need wider rims.
Gears: cassette & chain
Drivetrains are sold by number of rear sprockets ("speeds") — 8, 9, 10, 11 or 12. The chain, cassette and shifter must generally match the same speed, and usually the same brand family.
| Part | Must match | Notes |
|---|---|---|
| Cassette | Freehub body + speed | Shimano/SRAM 8–11 share a freehub; SRAM XD and Shimano Micro Spline are different |
| Chain | Speed (width) | An 11-speed chain for an 11-speed cassette, and so on |
| Shifter & derailleur | Speed + brand pull | Shimano and SRAM cable pull differ — keep a family together |
Brakes: rotors & mounts
- Disc rotors come in sizes (commonly 160 mm and 180 mm) — match what the frame, fork and caliper allow.
- Rotor fixing is 6-bolt or Centerlock — adapters exist, but check.
- Pads are specific to the caliper model — take the old ones in to match.
Axles: quick-release vs thru-axle
| Standard | Where | Width × pitch |
|---|---|---|
| Quick-release (QR) | Older rim-brake bikes | 100 mm front / 130–135 mm rear |
| Thru-axle (road/gravel) | Most disc bikes | 12×100 front / 12×142 rear |
| Boost thru-axle | Modern MTB | 15×110 front / 12×148 rear |
- Tyre sidewall — ETRTO size and pressure range.
- Count the rear cogs — that's your "speed".
- Look at the axle — a flip-lever skewer (QR) or a bolt/lever that unthreads fully (thru-axle).
- Read the parts — brand and model are printed on the derailleur, brake and rotor.
Standards change often and adapters don't always exist. If you're unsure, take the old part — or the whole bike — to a shop before buying. It's cheaper than buying the wrong thing twice.