Hub spacing: this is how your fork comes alive…
Getting your bushings square and opened up properly is a good first step in allowing your fork to feel buttery and active. But once you tighten down your fork to the hub, things can get a little squirrely. A modern Boost 15mm hub measures 110mm wide, give or take a hundredth of a millimeter. If you have a matching Boost fork, the bottom of the lowers are spaced to 110mm also. Given that you have two sets of bushings that are dictating where the stanchions can and cannot go smoothly, and also a hub that will only allow the lower legs of the fork to sit at one given width, there can be some issues here. If the bushings and hub don’t agree with each other, you will end up with stiction in the fork’s stroke. In other words, the fork will feel sticky and bound up. This can be barely perceptible on the minimal end and very noticeable on the extreme end. While there are other factors that can introduce stiction in a fork also, if your hub and bushings don’t agree, the fork will never feel good.

In order to get the bushings to play well, there are a few things you need to do first before measuring and either shimming or shaving the hub itself. The damper, spring, and wiper seals need to be removed. You only want the uppers and the lowers in play here. The bushings should be sized to .03-.10mm depending on your taste, making sure each bushing is not tighter than the other. You want as much consistency in all four bushings as possible. Then inserting the uppers into the lowers is where you begin the testing phase to find out where the fork legs really want to sit.

This is a tool (made by Oliver at Blue Liquid Labs) that helps you dial in very minute amounts of width into the legs. Sometimes the difference of only a fraction of a millimeter has a dramatic effect on how well a fork cycles with or without stiction. This tool acts as a hub, allowing the axle to be installed and tightened, replicating the fork being installed in the wheel. The tool width is able to be changed in tiny amounts by turning the end, introducing either less or more width into the fork legs.

Once you find the sweet spot, you can remove the spacing tool and measure the width of it to get the spacing your hub needs to be at. The hub then needs to either be shaved at the hub caps ends or shims added inside. Splitting the difference to each side…

Why do some Fox shocks become squelchy and noisy so easily?
One thing that is pretty commonly known is that a few of Fox’s shocks tend to become noisy and lose damping ability pretty quickly. Or at least over a shorter period of time than you would think is normal. Some Fox X2 shocks have been known to be aerated (oil with air in it – in the case of shocks) out of the box, brand new.

The issue usually is the use of a PU (Polyurethane) inside the sealhead. PU seals are known to fail rapidly in this type of application. You can see in this photo the light brown o-ring just inside the sealhead. The main shaft of the shock slides inside that o-ring and any imperfections that develop will let air and oil mix, creating aeration and a noisy, slurpy sound. From studies done on PU seals, it has been found that some main reasons for failure come from a weakening hydrogen bond and the wearing away of the the seal material from dynamic application (repeated shaft movement at various velocities).

A solution to this problem is to just replace the offending seal(s) with a standard NBR seal and call it a day. These work for sure. But an even more helpful seal is one that is made of Viton or FKM. These have lower permeability levels than NBR which helps with longevity of keeping air and oil separate.

Viton or FKM seals are a dark brown color most of the time, but can also come in black and some other colors. They look just like any other o-ring but will do a better job at sealing the sealhead, especially with higher temps. Everyone just wants their shock to work right when they go out to ride. The most reliable shocks I have seen usually have simple internals with seals that are on the larger side and fewer of them. The larger the seal, the less potential permeability over time there is. The first thought would be that larger seals would create lots more seal drag. But the reality of it is that you don’t even notice it, especially if the shock just works right every time.

I only use FKM for dynamic seals in Fox shocks. If I come across a PU seal, it goes right in the garbage where it belongs. They may work in other applications just fine, but not in shock sealhead!