Frame fit and part compatibility
Will that part actually fit?
Which frame size, headset, bottom bracket, axle, tire, tube, cassette or rotor fits your bike, worked out from the manufacturer's own published numbers before you order.
Every question on this site
13 guidesEach answer comes from the maker's or the standards body's own published documents, listed at the bottom of its page with the date we read them.
Frame and bearings
Frame size, and the two bearing standards to identify before you order: the headset and the bottom bracket.
- Medium or large?
Why two bikes both called "medium" can differ by more than a size, and how to use published stack and reach to order the right one — including how to compare a candidate against a bike you already ride.
- Which headset code fits my head tube?
Turn head tube bores, cup type and fork diameters into the S.H.I.S. code headset makers list, with bore ranges, a worked 44/56mm example and the traps that give a wrong code.
- Which bottom bracket fits my frame and crank?
Turn a bare-shell bore and width into a shell standard, then match a 28.99mm DUB spindle to SRAM's part number and spacer layout.
Wheels, tires and axles
Which tire, tube and axle fit, read from the numbers printed on the parts and in the manufacturer's documents.
- Will this tire fit my rim?
Bead seat diameter is what governs, not the 700C or 29er label. How to check diameter, match tire width to internal rim width, and find the pressure ceiling that applies if your rim is hookless.
- Which inner tube and valve do I need?
Read the ISO size on your tire, check a tube's listed range for your bead seat, match Presta or Schrader to the rim hole, and measure for valve length.
- I lost my thru axle. What do I order?
Find the hub width, thread pitch and length your frame needs when the old axle is gone, and see why the axle's brand or name doesn't settle the pitch.
- Which tires fit your hookless rim, and at what pressure?
Real published minimum tire widths and maximum pressures for hookless rims from ENVE, Zipp, and CADEX, plus how each brand decides which tires it approves.
- Is your tubeless sealant still good, and how much to add?
Dose by tire width and how long sealant lasts, using the figures Schwalbe, Stan's and Muc-Off publish for their own sealant, plus how to check the level without breaking the tire bead.
Drivetrain and brakes
Cassettes and freehub bodies, and how a brake rotor mounts to the hub.
- Why won't the cassette slide on?
The freehub body decides which cassette fits, and matching the speed count proves nothing. How to identify the body on your wheel, plus the two follow-on checks for chain and derailleur capacity.
- Six-bolt rotor, splined hub: will it fit?
Tell Center Lock from six-bolt, check which rotors each adapter maker names, and fit it with the published lockring torque, bolt limits and arrow direction.
- Will Your Rear Derailleur Take a Bigger Cassette?
Check your derailleur's published total capacity and max low sprocket against the cassette you want — both have to clear, and they fail differently.
- Is It Just the Chain, or the Cassette Too?
Match your chain wear percentage to your cassette's speed count, then run the one test that tells you whether the cassette needs replacing too.
- Are Your Pads Worn, Your Rotor Worn, Or Both?
Measure pad and rotor thickness against the minimums your brake's own manufacturer publishes, so you buy exactly what's worn instead of guessing or replacing both.
Five numbers worth knowing
Labels mislead and size letters are not measurements. These come from published geometry, standards designations and manufacturer compatibility documents.
A tire fits a rim only if the bead seat diameter matches. That number, not the marketing label, is what governs. 700C, 29-inch and most modern '28 inch' are all 622mm. 27.5-inch and 650B are both 584mm. And Schwalbe states that three different diameters — 559mm (mountain), 571mm (triathlon) and 590mm (Dutch touring) — have all been sold as '26 inch'.
A frame size letter is not a measurement, and it is not consistent even within one brand. Canyon's own published geometry gives the Endurace CF 7 Disc in size M as stack 590mm / reach 378mm, and the Ultimate CF SL 8.0 in size M as stack 567mm / reach 391mm — 23mm apart in stack and 13mm in reach, same manufacturer, same letter.
On a hookless (straight-side) rim there is a pressure ceiling that overrides the number on the tire sidewall. ENVE states the ETRTO/ISO maximum for hookless is 5 bar / 72.5 psi, and 4.5 bar / 65 psi for 30-34mm tires; Zipp states its TSS rims must never be inflated above 5 bar / 72 psi. Schwalbe states hookless rims may only be used with tubeless-ready (TLE/TLR) tires.
A cassette fits the freehub body it was designed for and nothing else. Shimano's compatibility document C-731 lists five spline patterns with different tooth counts and lengths: MICRO SPLINE (23 splines, 26mm), HG spline L2 (18 splines, 37.65mm, road 12-speed only), HG spline L (9 splines, 36.75mm), HG spline M (9 splines, 34.9mm) and HG spline S (9 splines, 31mm). SRAM states its XDR body is 1.85mm longer than XD.
Matching the speed count does not establish that a part fits. KMC's product page for its X12 12-speed chain states it is not compatible with SRAM T-Type, eTap or Flattop — all of which are also 12-speed. The governing fact is always the manufacturer's stated compatibility, not the number of gears.
Who this is for
People who own or have chosen a bicycle and need one specific part to fit: a frame size, a headset, a bottom bracket, a replacement axle, a tire or tube, a cassette or a brake rotor. It is for the person at a checkout page or holding a part that will not go on, not for someone browsing for a new hobby.