Bicycle Fork Standards Explained: Steerers, Axles, Travel & Brake Mounts

Also called: Tapered, 1.5 taper, 28.6/38.1 mm

28.6 mm at the top (stem clamp) widening to 38.1 mm at the base; crown race seat typically 39.8 mm (some 40.0 mm)

The current default. The top keeps the normal 1-1/8" stem interface while the base swells to 1.5", so the fork crown junction — where stress is highest — is much stiffer and stronger without extra weight up top.

Why it exists: A compromise after the full 1.5" straight steerer proved heavier than needed: taper delivers most of the crown stiffness while keeping standard 1-1/8" stems and light upper headset bearings.

Typically found on: Nearly all suspension MTBs since ~2012 and nearly all road, gravel, and e-bikes since ~2014

Also called: OverDrive 2, 1-1/4" tapered, 1.8" e-bike taper

Either 31.8 mm top / 38.1 mm base (Giant OverDrive 2 style), or 28.6 mm top / 45 mm base (1.8" e-MTB steerers)

Two less-common variants. Some brands (notably Giant, and some 1990s Fisher 'Evolution' bikes with straight 1-1/4") used a 1-1/4" upper for extra stiffness, which requires a matching stem or shim. A newer 1.8" lower taper appears on some heavy-duty e-MTBs.

Why it exists: Brand-specific pushes for more front-end stiffness — especially for heavy, powerful e-bikes that stress the crown area harder.

Typically found on: Giant/Liv road and MTB (OverDrive 2 era), some current e-MTBs from Giant, Mondraker, Husqvarna, and others

Also called: Boost 110, 15x110

15 mm axle diameter, 110 mm hub spacing (10 mm wider than standard)

The current mountain bike front standard. Widening the hub 10 mm lets the wheel be built with wider-set flanges and better spoke angles, making 29" wheels as stiff as older 26" wheels, and adds tire and fork-arch clearance.

Why it exists: Introduced by SRAM and partners in 2015 to fix flexy 29er wheels and to make room for plus-size tires; the industry adopted it almost universally within two seasons.

Typically found on: Almost every quality MTB and e-MTB from ~2017 on, across all brands

Also called: 20 mm DH axle, 20x110 Boost

20 mm axle, 110 mm spacing. Old (pre-Boost) 20x110 and newer '20x110 Boost' use the same numbers but different hub flange/rotor positions — they are not interchangeable

The heavy-duty front axle for downhill and long-travel forks. The original version dates to early-2000s freeride; modern dual-crown forks (Fox 40, RockShox Boxxer, Zeb DH) use the Boost-offset version.

Why it exists: Maximum stiffness for downhill impacts and 200 mm+ rotors, where even 15 mm axles flex.

Typically found on: Downhill and freeride bikes, dual-crown forks, some single-crown enduro forks of the 2000s

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

12 mm axle diameter, 100 mm hub spacing; thread pitch varies by brand (commonly M12x1.5 or M12x1.0)

The standard front axle on disc-brake road, gravel, and cyclocross bikes, and increasingly on XC race MTB forks. Same 100 mm spacing as QR, so many hubs convert with end caps, but the fork must be built for it.

Why it exists: Road disc brakes needed thru-axle security and stiffness, but 15 mm was heavier than necessary for drop-bar loads; 12 mm became the settled compromise around 2015–2017.

Typically found on: Nearly all disc-brake road and gravel bikes since ~2017; many commuter and e-commuter forks

Also called: fat fork spacing, 150 mm front

15 mm thru-axle at 150 mm spacing (suspension and modern rigid fat forks) or 9 mm QR at 135 mm (early/entry fat bikes)

Fat bikes space the front hub far wider so 4–5" tires and their rims can be laced with reasonable spoke angles. Early fat bikes reused 135 mm rear-style QR hubs up front; suspension fat forks (RockShox Bluto) standardized 15x150.

Why it exists: A 100/110 mm hub cannot support a 100 mm-wide rim without terrible spoke geometry; wider flanges fix wheel strength and tire clearance.

Typically found on: Fat bikes (Surly, Salsa, Trek Farley, Specialized Fatboy and similar)

Also called: PM, PM160, PM180, 74 mm post mount

Two threaded posts facing the caliper, 74 mm center-to-center; the native size (PM160, PM180, PM200) is the smallest rotor that fits without an adapter

The dominant MTB standard: the caliper bolts directly onto threaded posts aligned with the rotor. Simple +20/+40 mm adapters step up to larger rotors. A fork's 'native' post size matters — you can go bigger with adapters but never smaller.

Why it exists: Direct caliper mounting is lighter, easier to align, and stronger than IS tabs plus a bracket; fork makers adopted it en masse in the mid-2000s.

Typically found on: Virtually all suspension forks since ~2008; most MTBs, many e-bikes and early road discs. Modern enduro/DH forks are often native PM180 or PM200

Also called: FM, flat-mount road

Caliper sits flush on a flat face; front caliper bolts through an adapter plate into the fork, natively sized for 140 or 160 mm rotors

Shimano's compact, low-profile standard for drop-bar and city bikes. On the front fork, a thin plate between fork and caliper sets rotor size — flipping or swapping the plate switches between 140 and 160 mm. Rotors larger than 180 mm generally are not supported.

Why it exists: Post Mount looked and packaged poorly on slim road forks; Flat Mount (introduced 2014) tucks the caliper against the fork leg with less weight and cleaner lines.

Typically found on: Nearly all disc road, gravel, cyclocross, and many commuter/e-commuter bikes since ~2016

By bike type and year

Start from what the bike is. Disc-brake road/gravel bike from 2017+: almost certainly tapered steerer, 12x100 thru-axle, Flat Mount. Quality MTB from 2017+: tapered steerer, Boost 15x110, Post Mount. MTB from ~2009–2016: likely 15x100 or QR, straight 1-1/8" or tapered steerer. Any rim-brake bike or MTB before ~2005: QR 9x100. Road bike before ~1998 or a cruiser/commuter with a stem that plunges into the fork: 1" threaded. Look up your bike's year/model spec sheet — manufacturer archives and sites like BikePedia or 99Spokes usually list fork, axle, and steerer.

By measuring

Axle: if the wheel is held by a cam lever in open slots it's QR 9x100 (measure inside-to-inside between fork tips: 100 mm). If a rod threads through closed holes, measure its diameter (12, 15, or 20 mm) and the hub spacing (100, 110, or 150 mm). Steerer: remove the stem or look below it — calipers on the exposed steerer read 25.4 mm (1"), 28.6 mm (1-1/8"), or 31.8 mm (1-1/4"); if the head tube is visibly wider at the bottom than the top, it's tapered. Travel: with the fork topped out, measure the exposed shiny upper tube (stanchion) from dust seal to crown and subtract about 10–20 mm, or push the rubber O-ring down, compress fully, and measure the O-ring's travel. Axle-to-crown: straight-line distance from axle center to the top of the crown.

By stamped markings and stickers

Suspension forks carry the answer on them. Check the back of the crown and the inside of the legs for a model/serial sticker — RockShox and Fox print model, year, travel, offset, and wheel size there, and the serial can be decoded on SRAM's or Fox's support site. Leg graphics usually state the model (e.g. 'FOX 34', 'RockShox Pike'); the number in Fox names (32/34/36/38/40) is the stanchion diameter in mm. Max rotor size is often printed near the brake mount. Rigid forks may have the size cast into the crown or steerer (e.g. '1-1/8').