Bike Hacks: How I Made My Custom 85 mm Suspension Fork


Bike Hacks: How I Made My Custom 85 mm Suspension Fork

Travis‘s Otso Fenrir shows up a lot in his reviews. So does the 85mm-travel Fox Step-Cast 34 he customized for it. We’ve gotten some questions about how he finagled this hack. He’s here to give answers, but not to recommend you try it.

First, an important disclaimer: Though this story could function as a tutorial, that’s not its main purpose. Mostly because it won’t work on every fork brand. I only know it works on Fox and Marzocchi, and possibly not even every Fox and Marzocchi. But I brag enough about my custom 85 mm fork that I figured it would be interesting to talk about how I actually did it.

So, I’d suggest thinking of this like one of those Youtube videos where someone builds a jungle hideaway out of mud and bamboo. Maybe you’ll never do it yourself, but it’s kinda cool to watch. Regardless, there is real risk of injury if the mods aren’t done perfectly, and further risk if the fork is not re-assembled properly. Plus, even if everything’s dialed, this will probably void your fork warranty. So, read at your own risk.

When I was building up my Otso Fenrir frame, I knew I wanted front suspension. I just didn’t want a lot of it. Even at sag, a recommended 100 mm fork would raise the bottom bracket and slacken the head angle beyond what I thought would be optimal for my purposes. I crunched some geo numbers, and I decided the perfect resting travel would be 85 mm. But of course, there are no off-the-shelf 85 mm forks. And anyway, I already had a 120 mm FOX 34 Step-Cast that would work perfectly if I could only shorten it by 35 mm.

Shortening fork travel used to be relatively simple. Assuming the fork was air-sprung, you could add a spacer below the top-out bumper, which would pull the lowers up and shorten the travel. But most forks now have self-equalizing positive and negative air pressure. There’s a little dimple somewhere on the inner surface of the air-spring tube that lets air bleed from the positive chamber into the negative chamber. Installing spacers will move the piston above that dimple, which would be bad. So instead, most traditional forks can be fitted with air-spring assemblies of various lengths that will net you any travel (in 10 mm increments) within that fork’s approved range.

Unfortunately, I had something more unique in mind, but I remembered that a friend who worked at a major suspension brand once told me it was possible to shorten an air spring yourself. So, I called him up to determine if this process would work on my fork, and to get a walkthrough. That’s a luxury you probably won’t have, so again, please heed the disclaimer at the top of this story.

The part of the air spring I had to modify is pretty simple. It’s an aluminum shaft with a fixed piston on the end that contains the positive air, a sliding piston below it that contains the negative air, and a top-out bumper in between. The length of that aluminum shaft is what determines where within a given fork’s compatible travel you’ll land. A longer shaft will “lower” the fork lowers, exposing more stanchion and increasing the travel. A shorter shaft will raise the lowers, exposing less stanchion and decreasing the travel.

Of course, there’s a limit to how long you can go. The stanchions need to rest a minimum depth into the lowers. And the damper assembly can not extend indefinitely. But as long as the fork can move enough for the piston to pass over that negative-air transfer port (which is usually less than 20 mm), there technically isn’t a limit to how short you can go. Dampers are very rarely position-sensitive. And shortening a fork’s travel technically eases the loads on the chassis, so there are no durability concerns. The main issue is that the air spring volume starts to get pretty large relative to the travel, which is not ideal. But that’s what volume spacers are for. With five spacers in my 85 mm Fox 34, I got it feeling just like I wanted.

I started by pressing the cotter pin, or “spring pin” out of the main piston. I had to make a cradle for the piston so I wouldn’t damage it. It takes a lot of force to get that pin through. But once it was out, it didn’t seem to have stretched or deformed the hole in the shaft. It would again have taken a lot of force to get it back in, so I figured it was safe to just use a bit that matched its diameter. But first, I had to decide how much to cut it

It was as simple as lopping off exactly as much shaft as I wanted to shorten the travel by. A pipe cutter works great for this. It’s straight and clean. What’s tricky is calculating where to drill the hole. It had to be accurate, but with the old tip to measure from, it just took some planning. That, and a friend’s enormous fifty-year-old drill press.

This is where precision is most important. If the hole is crooked, the piston will be crooked. There are jigs made specifically for holding cylindrical objects perpendicular to a drill bit, but they’re expensive. So, I made one out of wood. Quick note about these photos: This is a dramatization. I did my Fenrir fork mods a couple years ago, and that jig was even more janky-looking than this one. I had to stack some business cards under one of the “clamps” to get the shaft perfectly horizontal. Somehow, I nailed it first try on this one.

When actually drilling, though, I wasn’t going for first try. I used a small bit for a pilot hole to make sure the final plunge wouldn’t get run off center. But once I did, it went through quite nicely. I smoothed out some rough edges, double-checked the diameter, used my custom cradle to pound the pin back in, cleaned the whole thing up, and re-assembled.

Like most of the setup choices on my Fenrir, this hack isn’t for everyone. For the most part, frames are designed around forks that, ya’ know, actually exist. Going too far off script can create unforeseen problems with things like geometry, fitment, and clearance. But as lines between categories blur and disappear, I think it’s cool to know hacks like these are possible. Still, though. Be careful.