What the inputs mean
- Rake: the angle of the steering head from vertical, in degrees. Motorcycle makers publish it as caster angle or rake.
- Fork offset: the perpendicular distance from the steering axis to the front axle. It is the triple clamp offset plus any offset built into the fork legs at the axle.
- Front tire radius: from the tire size, half the overall diameter. 120/70ZR17 is 17 in + 2 × 84 mm, so the radius is 11.81 in (299.9 mm).
The formula
trail = (tire radius × sin(rake) − offset) ÷ cos(rake)Trail is the distance along the ground from the tire contact patch back to where the steering axis meets the ground, which is ahead of the contact patch on a normal bike. Normal trail, used in some engineering texts, is trail × cos(rake): the same distance measured at right angles to the steering axis.
Worked example
Rake 24°, offset 1.20 in (30.5 mm), 120/70ZR17 front tire. sin(24°) = 0.4067 and cos(24°) = 0.9135. Trail = (11.81 × 0.4067 − 1.20) ÷ 0.9135 = 3.94 in (100.2 mm). With triple clamps that cut the offset to 1.00 in, the trail grows to 4.16 in (105.7 mm): 0.20 in less offset gives 0.22 in more trail.
Comparing two setups
Enter your current geometry as setup A and the planned change as setup B. The table shows the trail, the normal trail and every input side by side, and the drawing overlays both: A solid, B dashed. Lowering or raising one end changes the rake, so measure or look up the new steering head angle rather than guessing it.
What changes trail
- More rake (raising the front or lowering the rear) gives more trail. 25° instead of 24° in the example gives 4.18 in.
- Less offset gives more trail.
- A taller front tire gives more trail, a little.
More trail generally means more straight line stability and slower steering; less trail means quicker steering. How a given bike feels depends on its whole chassis, so change one thing at a time and follow the bike maker's or a chassis builder's guidance.
Questions
How do you calculate motorcycle trail?
Trail = (tire radius × sin(rake) − fork offset) ÷ cos(rake). With a 24° rake, 1.20 in offset and a 120/70ZR17 tire (11.81 in radius), trail is 3.94 in, about 100 mm.
Does more rake mean more trail?
Yes, with the same offset and tire. In the example above, going from 24° to 25° of rake raises trail from 3.94 in to 4.18 in.
What does fork offset do to trail?
More offset reduces trail, less offset increases it. Cutting offset by 0.20 in in the example adds 0.22 in of trail, because the change is divided by cos(rake).
What is the difference between trail and normal trail?
Trail is measured along the ground. Normal trail is measured at right angles to the steering axis and equals trail × cos(rake): 3.94 in of trail at 24° is 3.60 in of normal trail.