Fork axle-to-crown length: what it is and how to measure it
Fork axle-to-crown length is the distance from the crown race seat (the face the lower headset bearing sits on) to the centre of the wheel axle. It is given in two ways: AC, in a straight line, and L, parallel to the head tube. The two are linked by the fork offset R: L = √(AC² − R²).
What axle-to-crown length means
It is the dimension that sets the height of the bike's front end. There are two ways to state it, and it pays to know which one a manufacturer or a program uses:
- AC (direct axle-to-crown): the straight-line distance from the crown race seat to the axle centre.
- L (length parallel to the steerer): the distance from the crown race seat to the axle centre measured along the steering axis, leaving out the forward shift that the offset adds.
Wikipedia's article on bicycle geometry defines fork length the second way: parallel to the steer tube, from the lower fork crown bearing to the axle centre.
How to measure fork axle-to-crown
The start point is always the crown race seat face, where the lower bearing sits. The end point is the axle centre in the dropouts.
- Locate the axle centre in the dropouts.
- Measure in a straight line from the crown race seat face to that centre: that is AC.
- If you need L, do not measure it diagonally with a tape: calculate it from AC and the offset, or measure it parallel to the steering axis on the drawing or the jig.
L versus AC: the difference
The axle sits a distance R (the offset) away from the steering axis. L, R and AC form a right triangle with AC as the hypotenuse, so AC is always longer than L when there is offset:
L = √(AC² − R²) and AC = √(L² + R²)
The difference is a few millimetres, but it is not zero. Example with L = 395 mm:
| Offset R (mm) | L (mm) | AC (mm) | AC − L (mm) |
|---|---|---|---|
| 40 | 395 | 397.0 | 2.0 |
| 45 | 395 | 397.6 | 2.6 |
| 50 | 395 | 398.2 | 3.2 |
| 55 | 395 | 398.8 | 3.8 |
Values calculated with the formula above; L = 395 mm is only an example. Taking one dimension for the other leaves the fork off by that difference. To convert your own numbers, use the fork trail and offset calculator, which also converts between L and AC.
How BikeCAD handles fork length
BikeCAD lets you give the rigid fork length either as L (parallel to the head tube) or as AC (direct axle-to-crown), with R as the offset. If you pass a BikeCAD design to another program or to the workshop, check first which of the two it uses. What each letter in the fork dialog measures is covered in BikeCAD and the rigid fork.
Why changing axle-to-crown changes frame geometry
A frame is designed around a fork length. According to Wikipedia, a longer fork raises the front of the bike and decreases its head angle, making it slacker. A shorter fork does the opposite.
Head angle is part of the trail formula: with the same offset and wheel, a slacker head angle gives more trail, as the same source states. So changing axle-to-crown does not only move the front end up or down; it also changes trail. You can see the effect in the trail calculator.
Replacing a suspension fork with a rigid fork
If the frame was designed for a suspension fork, start from the axle-to-crown that the suspension fork's manufacturer publishes. Check whether it is given in a straight line or parallel to the head tube before converting it to L or AC with the formula above. How much to subtract for sag is covered in suspension-corrected rigid fork.
Axle-to-crown in ForkCAD
In the "Length and offset" group, the "Measure length as" option switches between "Length parallel to steerer" (L) and "Axle-to-crown (direct)" (AC); "Offset" completes the triangle. With a rim caliper brake, ForkCAD checks that the brake hole is not too close to the crown race seat or the crown's arch (or the bottom of the steerer on unicrown and segmented forks); if it is tight, it marks AC or L in red and tells you which length fixes it. This is covered in crowned fork for caliper brakes.
FAQ
Are axle-to-crown and fork length the same?
They describe the same distance in two ways. AC is the direct distance from the crown race seat to the axle centre; L is that distance measured parallel to the head tube. With offset, AC is always slightly longer than L.
Where is fork axle-to-crown measured from?
From the crown race seat face, where the lower headset bearing sits, to the centre of the wheel axle.
How do I convert AC to L?
With the offset: L = √(AC² − R²). For example, AC = 397.6 mm with R = 45 mm gives L = 395 mm.
What happens if I make the fork longer?
The front end rises and the head angle gets slacker; with the same offset and wheel, trail increases.
Sources
- Wikipedia: Bicycle and motorcycle geometry (fork length measured parallel to the steer tube, effect of a longer fork on head angle, head angle and trail).