Custom Fox 40's

Adapting Fox 40 Forks for Trail Use: A Custom Machining Project

This project started with a simple question — what if I could run a set of Fox 40 downhill forks on a trail bike? The goal was to combine the stiffness and precision of a dual crown fork with a more versatile travel range suitable for trail riding. Of course, making that work required some serious re-engineering — from custom air spring internals to a bespoke thru axle

The Concept

Fox 40s are built for pure downhill abuse, with 200mm of travel and a chassis designed for big hits. For trail use, that much travel and the tall axle-to-crown height make them impractical without modification as it would slacken the head tube angle from 66 degrees down to 63 making it too slack for my liking. I wanted to retain the fork’s structural integrity as I like how stiff and stable the forks feel especially in big drops and through rock gardens. To achieve this I modified and made custom internals on the lathe that allow me to adjust the travel in 20mm increments between 200 and 140mm of travel allowing me to tune how much travel I want for the terrain I am riding on.

Custom Air Spring Internals

The heart of the project was a fully custom air spring assembly designed to fit within the Fox 40 stanchions. I machined new internal components on the lathe that allow for the adjustability via removable spacers, maintaining proper negative spring volume and compression ratios at each travel setting. The adjustable design means the fork can be set to 140, 160, 180, or 200mm travel simply by altering spacer configuration — all while retaining a consistent feel and proper progression. On top of this I did play around with the chamber volumes and top out spring package to further adjust the fork to my liking.

Headset Adapter

To mate the dual crown fork with a modern trail frame, I machined a custom headset adapter. This small but critical piece allows the smaller Fox 40 steerer to seat cleanly in the frame’s tapered head tube. The adapter was turned from aluminum, with precision tolerances to ensure a perfect press-fit on the fork's lower crown to ensure that it does not move and correct bearing interface angle, in this case 36 degrees, along with retaining compatibility with the original headset bearing the bike came with from the factory .

Hub Machining and Custom Thru Axle

Fox 40 lowers use a unique axle interface, specifically 20x110 non boost so adapting them to a trail wheelset required machining a custom 2 part thru axle to allow for the 15mm axle train wheel to be utilised. The axle is made in 2 main parts, the removable axle and a threaded boss that sits in the opposite side of the fork and is held in place by the pinch bolts that the fox 40 uses. To save weight the axle is hollow which also allows for a 5mm hex to be broached into it allowing for standard tools to be used to install and remove the wheel. One problem that occurred is that the bike used a boost hub (15x110) and the for used its 20x110 interface which I thought would have the same rotor offset but this isn't the case as boost hubs have the rotor spaced 5mm further out from the center. Admittedly this could have been fixed by using the next revision of the forks but since I didn't have them I disassembled the hub and re machined the rotor mounting face on the CNC mill which lead to a perfect fit.

image of the comparison of the stock Maxle, Fox Thru Axle, and the finished two-piece axle.

Assembly and Testing

After assembly and a bit of testing I then hit the trails and the forks felt absolutely amazing compared to the bikes old ones as they were notably stiffer which made them super confidence inspiring and lead to much more feel for what the front wheel was doing leading to more precise braking.

Reflections

This build was so rewarding and satisfying as it combined a combination of design, machining and problem solving as every component had to be custom designed for this project to ensure compatibility and also to be strong enough to not break unexpectedly. As a bonus as well the bike ended up being 0.2Kg lighter than when it was running the old forks and somehow it also improved pedaling performance which I wasn't expecting as this modification was focused on giving better performance when descending!