Fork Length & Trail Calculator

"If It Ain't Long It's Wrong"...Sugar Bear

We're going to make it easy for you to do the calculations for fork length and trail when you go to order your expensive billet front end to put on that equally expensive custom frame. Harley-Davidson screwed the pooch with their butt ugly Rocker fat-tired bike. Roll your own, become a "Master Builder"...Keep the economy grinding along by spending all your hard earned coin on these ego-centric toys. Hey, they build them in one hour on TV!!!

We know you were day dreaming about sex when the triangles were floating about the blackboard, or for those of you who were already warped, you were doodling motorcycles while the teacher droned on about sines, cosines, tangents. Now all these years later, when your brain cells are half-pissed away, all you have to do is make a few simple measurements and we'll help you figure out the fork length and trail.

If you are clever you can use the calculator to help you figure out what the other variables should be if you have a specific front end in mind and are building a frame.

 Definitions:

Your front suspension geometry is defined by the following six variables which are defined as:

    • Offset-- Centerline of top of steering neck to centerline of top of fork tubes.
    • Rake-- The angle in degrees of the steering neck from the vertical.
    • Fork Length-- The distance between the top of the fork tubes to the centerline of the axle.
    • Diameter of the front tire.
    • Trail: Distance defined by vertical line from axle to ground and intersect of centerline of steering neck and ground.
    • Raked Triple Trees-- In order to bring trail figures back into line, triple trees with raked steering stems can be used. Expressed in degrees. Usually adjustable in 3, 5, and 7 degrees of rake.
    • Calculations Tire Diameter: See calculator

Harley Fork Length and Trail Calculator

Front Tire inches Diameter of front tire. 21" front typically 27.60". 19" front typically 26.10". 16" front typically 25.30"
Rake degrees Steering Neck Rake: Softail 32 deg. FXRS models 29 deg. FXLR 31 deg. Evo Dyna FXD/DX/DXT 28 deg. Evo Dyna FXDL/DWG 32 deg. Evo FLH 26 deg. Sportster 29.6 deg. Choppers 36 deg to 45 deg. FLH series 26 deg. V-Rod 34 deg.
Triple Clamp Offset inches Distance from center of top of steering neck pivot to centerline of top of fork tubes. Typically in the range of 2.4". V-Rod 2.0".
Neck Height inches Length from the center of the steering neck at top of triple clamp perpendicular to the ground. If you only have the frame, measure from center of top of steering neck perpendicular to the ground and add 1" to this measurement. Place frame on floor and block lower frame rails at the height you want the bike to sit. Softail 39.67".
Triple Tree Rake degrees For standard non-raked triple trees enter 0 degrees: The FXDWG uses 3 degree trees; V-Rod uses 4 degree trees. For chassis that have been raked to 40 degrees or more, and too much trail is present, Raked Triple Trees are necessary. Enter the degrees of rake, typically 3, 5, or 7 degrees.


Note: Trail figures need to be in the range of 4 to 6 inches. FXR's are about 4.22" and Softails about 5.8"
Fork Length in Inches
Trail in inches