Bicycle Wheel Standards Explained: Sizes, Axles, Freehubs & Brake Interfaces

Also called: 700c, 29er, 28in, 622

622mm bead seat diameter

The most common adult wheel size in the world. Road, gravel, hybrid, and 29er mountain bikes all use the same 622mm bead seat diameter - '700c' and '29 inch' are marketing names for the identical rim size. A 29er MTB rim is simply a wider, stronger 700c rim.

Why it exists: 700c descends from an old French sizing system (a 700mm outer diameter with a class-c tire). When mountain bikers wanted faster-rolling big wheels in the 2000s, they reused the existing 622 rim standard and branded it '29er'.

Typically found on: Nearly all road and gravel bikes, most hybrids and e-bikes, most modern trail/XC mountain bikes

Also called: 650b, 27.5, 584

584mm bead seat diameter

A mid-size wheel between 26in and 29in. '27.5 inch' (mountain) and '650b' (road/gravel) are the same 584mm rim. On gravel bikes, 650b wheels are paired with extra-wide tires to keep the overall wheel diameter close to a 700c setup.

Why it exists: Originally a French touring size from the 1930s-60s, it was revived around 2013 as a nimbler alternative to 29in for mountain bikes and later adopted by gravel riders wanting to fit very wide tires in road-ish frames.

Typically found on: Mountain bikes ~2014-2020 (still common on small sizes and long-travel bikes), gravel bikes with wide-tire setups, many e-bikes

Also called: 24 inch, 20 inch, BMX, 406, 451, 507

507mm (24in), 406mm (most 20in), 451mm (20in BMX race/recumbent)

Kids bikes and junior MTBs typically use 24in (ETRTO 507); BMX, folding bikes, and many cargo bikes use 20in. Beware: most 20in bikes use ETRTO 406, but some race BMX and folding bikes use the incompatible 451 size - check the number molded into the tire.

Why it exists: Smaller riders and compact bikes need proportionally smaller wheels; cargo bikes use them for a lower load deck and stronger wheels.

Typically found on: Kids bikes (24in), BMX, folding bikes (Dahon, Tern), front wheels of cargo/longtail e-bikes (20in)

Also called: 12x100, 15x100, road thru-axle

100mm hub spacing; 12mm (road/gravel) or 15mm (MTB/some gravel) axle

A thru-axle is a stiff rod that slides through closed fork holes and the hub, then threads into the far fork leg - the wheel cannot come out unless the axle is removed. 12x100 is the standard on disc road and gravel bikes; 15x100 ('non-Boost') was the MTB front standard from about 2008 to 2016 and appears on some early disc road/gravel forks.

Why it exists: Disc brakes put strong twisting loads on the front hub. Thru-axles locate the wheel precisely (keeping the rotor centered in the caliper) and greatly stiffen the fork-wheel connection.

Typically found on: 12x100: virtually all modern disc road and gravel bikes. 15x100: MTBs ~2008-2016, some early gravel bikes

Also called: Boost 110, 15x110

110mm hub spacing, 15mm thru-axle

The current mountain bike front standard, 10mm wider than 15x100. The wider hub lets the spoke flanges sit farther apart, building a laterally stiffer wheel, and improves tire clearance. Not interchangeable with 15x100, though some hubs convert with different end caps.

Why it exists: Introduced by SRAM/Trek around 2015 so bigger 29in and 27.5+ wheels could be built as stiff as old 26in wheels, and to make room for wider tires.

Typically found on: Nearly all mountain bikes and most e-MTBs from ~2016 on

Also called: 20x110, DH axle, Boost 20x110 / Super Boost front

110mm hub spacing, 20mm thru-axle

A burly 20mm thru-axle used on downhill and freeride forks since the early 2000s. The original predates Boost; a newer 20x110 Boost variant uses wider hub flanges - check your fork model to know which you have.

Why it exists: Downhill forks needed the stiffest possible wheel attachment long before thru-axles reached trail bikes.

Typically found on: Downhill and freeride bikes with dual-crown forks (RockShox Boxxer, Fox 40)

Also called: 142x12, 12x142

142mm hub spacing, 12mm thru-axle

The first widely adopted rear thru-axle standard, now universal on disc road and gravel bikes and used on MTBs from roughly 2011-2016. The hub internals match old 135mm QR spacing - the extra 7mm is end caps that seat in the frame - so many hubs convert between 135 QR and 12x142 with a cap swap.

Why it exists: Created to give disc-brake frames a stiffer, self-centering rear wheel attachment while reusing existing 135mm hub internals.

Typically found on: All modern disc road and gravel bikes, most e-road/gravel bikes, MTBs ~2011-2016

Also called: Boost 148, 12x148

148mm hub spacing, 12mm thru-axle

The current mountain bike rear standard, 6mm wider than 12x142. The added width moves the drive-side spoke flange outward for a stronger, stiffer wheel and shifts the chainline outboard for tire clearance. Not compatible with 142 frames or hubs without conversion parts.

Why it exists: Same 2015 Boost initiative as the 15x110 front: build stiffer big wheels and clear wider tires without lengthening chainstays.

Typically found on: Nearly all mountain bikes from ~2016 on, most e-MTBs, some drop-bar MTB-adjacent bikes

Also called: Super Boost Plus, 157 Trail, 12x157

157mm hub spacing, 12mm thru-axle

An even wider rear standard borrowed from downhill bikes (which used 150/157mm for years) and applied to some trail and enduro bikes. Requires a Super Boost-specific hub; Boost 148 wheels will not fit.

Why it exists: Pushes wheel stiffness and tire clearance further than Boost for long-travel bikes; championed by Pivot, Knolly, and some Devinci and Norco models.

Typically found on: Pivot trail/enduro bikes, Knolly, some Devinci/Norco, downhill bikes

Also called: HG spline, Shimano/SRAM 8-11 speed

Splined steel/alloy body; road 11-speed version is 1.85mm longer than MTB 8/9/10-speed body

The dominant freehub design since the late 1980s: a splined cylinder the cassette slides onto, secured by a lockring. Fits Shimano and SRAM cassettes from 8 to 11 speed (plus Shimano 12-speed road on the 11-speed road body). Smallest possible cog is 11 teeth. MTB bodies are shorter; road 11-speed bodies take MTB cassettes with a 1.85mm spacer.

Why it exists: Shimano's Hyperglide system replaced screw-on freewheels, moving the ratchet into the hub for stronger axles and easier cassette swaps.

Typically found on: The vast majority of 8-11 speed bikes: road, MTB, hybrid, e-bike

Also called: MS, Micro Spline 12-speed

Shorter body with 23 small splines; allows 10-tooth smallest cog

Shimano's freehub for its 12-speed MTB groups (Deore through XTR, and some gravel setups). The finer spline pattern lets the cassette start at a 10-tooth cog. Only Micro Spline cassettes fit; HG and SRAM XD cassettes do not.

Why it exists: Introduced in 2019 because HG's 11-tooth minimum cog limited gear range; a 10-tooth cog needed a new, smaller-diameter interface.

Typically found on: Shimano 12-speed MTB builds (Deore/SLX/XT/XTR) and many e-MTBs from 2019 on

Also called: XD driver, XDR (road)

Threaded driver body; XDR is 1.85mm longer than XD

SRAM's freehub for cassettes with a 10-tooth cog. XD is the MTB version (11/12-speed Eagle); XDR is the road/gravel version, 1.85mm longer, used by 12-speed AXS road groups. An XD cassette fits an XDR body with a 1.85mm spacer, but XDR cassettes never fit XD bodies.

Why it exists: SRAM launched XD in 2012 to enable its 10-42 and later 10-50/10-52 wide-range cassettes, which HG's 11-tooth limit made impossible.

Typically found on: SRAM Eagle 12-speed MTBs (XD), SRAM AXS road/gravel bikes (XDR)

Also called: Campy freehub, N3W

Campagnolo-specific splines; N3W body is 4.4mm shorter than classic Campagnolo

Campagnolo cassettes use their own spline pattern, incompatible with Shimano/SRAM. The classic body covers 9-12 speed; the newer N3W standard (2020) is shorter, supports Ekar 13-speed with a 9-tooth cog, and takes classic Campagnolo cassettes with an adapter.

Why it exists: Campagnolo has always maintained its own drivetrain interfaces; N3W was created for the Ekar gravel group's tiny 9-tooth cog.

Typically found on: Road bikes with Campagnolo groupsets; Ekar-equipped gravel bikes (N3W)

Also called: ISO 6-bolt, IS 6-bolt

Six M5 bolts on a 44mm bolt circle

The original disc rotor mount: the rotor bolts to the hub with six small Torx or hex bolts (typical torque 4-6 Nm). Universal across brands and common on MTB and budget disc bikes. A 6-bolt rotor can be fitted to a Centerlock hub with an adapter, but not the reverse.

Why it exists: Adopted as an open international standard when disc brakes arrived on mountain bikes in the late 1990s.

Typically found on: Mountain bikes, budget and mid-range disc bikes, most e-MTBs

Also called: Center Lock, CL

Splined interface secured by a lockring (external-notch or internal-spline type)

Shimano's rotor mount: the rotor slides onto hub splines and a single lockring holds it, making rotor swaps fast with a cassette lockring tool. Now the norm on disc road and gravel bikes and increasingly on MTB wheels. Rotor braking performance is identical to 6-bolt; only the mounting differs.

Why it exists: Shimano introduced it (~2003) to speed rotor installation and improve rotor-to-hub stiffness versus six small bolts.

Typically found on: Most disc road/gravel bikes, Shimano-hub wheels, many higher-end MTB wheels

Also called: TLR, Tubeless compatible, Hookless, TSS

Hookless pressure limit ~72.5 psi / 5 bar; ETRTO defines rim-tire width pairings

Tubeless-ready rims have a sealed (or tape-sealed) bed and a bead shelf that locks the tire, letting you run sealant instead of a tube for fewer flats and lower pressures. Hooked rims keep the traditional bead hook and accept both tubed and tubeless tires. Hookless rims drop the hook for a straight sidewall - lighter and cheaper to make, but they require approved tubeless-specific tires and cap pressure at ~72.5 psi.

Why it exists: Tubeless came from MTB (Mavic/Michelin UST, 1999, then Stan's NoTubes conversions) to eliminate pinch flats; hookless emerged in the 2010s as carbon molding made straight sidewalls attractive.

Typically found on: Nearly all modern MTB and gravel wheels (tubeless-ready); hookless on many carbon wheels (Zipp, ENVE, Giant)

By markings

Look for the ETRTO number: tires are molded with a size like 25-622 or 57-584 (width-bead seat diameter) on the sidewall, and most rims print the same format, e.g. 622x21 (diameter x internal width). Hub end caps and shells are often etched with the axle spec (e.g. 12x142) and hub model, which you can look up. For freehub type, check the cassette: an 11T smallest cog means HG (or Campagnolo); a 10T smallest cog means XD/XDR or Micro Spline.

By measuring

Remove the wheel and measure the hub width face-to-face: about 100mm front/135mm rear with a thin 5mm skewer means quick-release; 100/110mm front and 142/148/157mm rear with a large removable rod means thru-axle (measure the rod diameter: 12, 15, or 20mm). For internal rim width, remove the tire and measure between the inner sidewalls with calipers. Measuring across the rim at the bead seats gives roughly the ETRTO diameter (622mm for 700c/29).

By counting

Count spokes (common: 24, 28, 32) - it narrows down the wheel model. Check the rotor mount: six small bolts around the hub center means 6-bolt; a single large lockring means Centerlock. Count cassette cogs and read the smallest cog's tooth count: 11T smallest is HG (or Campagnolo); 10T smallest means XD, XDR, or Micro Spline; 9T means Campagnolo N3W.

By your bike's spec sheet

If the wheels are original, search your bike's exact model and year (e.g. '2019 Trek Fuel EX 8 specs') - the listing gives wheel size, axle standards, and freehub type. Bikes from 2016 on with disc brakes are very likely 12x100/12x142 (road/gravel) or 15x110/12x148 Boost (MTB).