V-brake boss position on a fork: cantilever boss height for 700C
On a 700C fork (ISO 622 rim), Shimano puts the V-brake and cantilever bosses 283 ±1 mm from the wheel axle. According to Shimano’s drawing, that height is measured along the blade to the boss axis, not in a straight line from the axle. For V-brakes, the bosses are 80 mm apart centre to centre (77 to 85 mm depending on rim width).
Below are the boss heights for each rim size, how to measure them, what happens if they are measured in a straight line, and the dimensions of the boss itself, all from Shimano’s technical document.
Cantilever boss height by rim size
Shimano gives the height H from the centre of the axle to the centre of the boss:
| Rim | Height H (mm) |
|---|---|
| ISO 559 (26") | 253.5 ±1 |
| ISO 622 (700C) | 283 ±1 |
| ISO 630 (27") | 286 ±1 |
| ISO 584 (650B) | Shimano does not give it |
How boss height is measured: along the blade
The Shimano drawing shows two dimensions with right-angle marks:
- H is measured parallel to the blade, along its section through the bosses, from the wheel axle to the boss axis.
- A is the perpendicular distance from the wheel axle to the blade line through the bosses.
In other words, the axle does not sit on the blade line: it is offset from it by A, and H runs along that line. The straight-line distance from axle to boss is the hypotenuse: √(H² + A²).
What happens if you measure 283 mm in a straight line
If you mark 283 mm in a straight line from the axle, the boss ends up low. With a 622 rim and A = 45 mm:
- The straight-line measurement is 283 mm.
- Its component along the blade is √(283² − 45²) ≈ 279.4 mm.
- The boss sits about 3.6 mm lower than the standard calls for.
The right way is the other way around: 283 mm along the blade, which with A = 45 gives about 286.6 mm in a straight line.
Axle-to-blade distance A
Shimano allows −8 ≤ A ≤ 70 mm. With the V-brake mounted on the other side of the fork, the range is −70 ≤ A ≤ 8 mm.
In ForkCAD you enter the boss height and the program gives you the distance A and the boss spacing, and flags them in red when they fall outside what Shimano allows.
V-brake and cantilever boss spacing
| Brake | Centre-to-centre spacing (mm) |
|---|---|
| V-brake | 80 (77 to 85 for rims 19 to 33 mm wide) |
| Cantilever (BR-R550) | 65 to 85 |
With the bosses centred on the blade, the spacing is the distance between the blade centerlines at boss height. That same width between the blades is what limits tyre and fender clearance.
Top-view position
In the top view, Shimano’s positioning drawing (p. 57) uses C and D for how far each boss sits ahead of or behind the centreline:
- Cantilever: C = D.
- V-brake: −2 ≤ C = D ≤ 6 mm.
Brake boss dimensions
| Item | Value |
|---|---|
| Thread | M6×1 |
| Shank (unpainted) | Ø7.98 (0/−0.04) × 16.4 (+0.2/0) mm long |
| Neck next to the base | Ø10 (+0.1/−0.06) × 5.5 (+0.2/0) mm long |
| Brazed-on base | Ø23 mm maximum |
| Spring hole | Ø2.4 (+0.4/0) mm, 8.2–8.5 mm from the boss axis; V-brakes use the centre one |
Cable stop on the fork
The fork cable stop goes at least 135 mm above the bosses, or at least 15 mm (BR-R550) or 20 mm (other models) above a new brake shoe.
For disc brakes, the dimensions are those of IS, post mount and flat mount fork mounts. For rim caliper brakes, the hole goes through the crown: see crowned fork with caliper brake.
FAQ
What is the V-brake boss height on a 700C fork?
283 ±1 mm from the axle, measured along the blade to the boss axis (Shimano). For 26" it is 253.5 ±1 mm and for 27", 286 ±1 mm.
What about 650B?
Shimano does not give a boss height for ISO 584 rims. In ForkCAD you enter it yourself.
How far apart are V-brake bosses?
80 mm centre to centre, 77 to 85 mm depending on rim width (19 to 33 mm). For BR-R550 cantilevers, 65 to 85 mm.