BOS Fox Racing Cartridge Kit review

Impressive upgrade – at a price

Our rating 
3.8 out of 5 star rating 3.8
GBP £340.00 RRP

Our review

An impressive upgrade from the suspension wizards at BOS Engineering – at a price…
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This replacement damping cartridge for Fox 40 forks isn’t cheap, but it’s a superb upgrade, especially for pre-2008 forks.


BOS Engineering are no strangers to mountain biking. Their suspension expertise was used in conjunction with Nico Vouilloz – 10-times downhill World Champion and winner of 16 World Cup rounds.

Returning to mountain biking after a stint with rally cars and motorbikes, BOS produce cartridge kits for Marzocchi, RockShox and now Fox, as well as their recent shock and fork creations.

This unit replaces the entire internals of the damping leg in a Fox 40, and is made up of a hard anodized aluminium cartridge with a rebound dial at the top, and a compression dial at the bottom. 

Initially, the fork feels a little under-damped with the cartridge fitted, and the rebound range wasn’t quite as wide as we’d expected – even with full rebound wound on, the action is fairly lively. 

Unlike the 2007 Fox 40 damping unit that we’d been running, this BOS cartridge feels quite progressive throughout the stroke, but with an action that feels highly speed sensitive. It feels more linear at low speed, but more progressive at speed.

We almost expected a slight damping spike with an action like this, but the compression feels very smooth and very controlled. Again, on the ‘shop floor’, the compression adjuster doesn’t appear to alter a huge amount, but on the trail it affects the ride a lot. With it mostly wound off, the fork sits and performs admirably, but it’s when you start adding compression that the technology starts making sense. 

The fork sits up higher in the travel than before (to schieve that with the stock damper we’d have had to wind on a load of low-speed compression which compromises sensitivity) and resists blowing through the travel on big G-outs. This is really good for fast downhill riding when you G-out somewhere that’s rough under wheel, because the fork continues to pull you forwards over the rough stuff.

Despite our previous comments about the rebound range, we found it more than adequate when combined with the fork’s performance. 

The Fox 40 is a great downhill fork in its own right, especially in the ’08 guise, but earlier units without the adjustable compression damping can really benefit from this cartridge. Your best bet would be to buy an old 40R or RC2 second-hand, and have this unit fitted.


We feel that this kit has so much to offer that we are going to continue to run it on a long-term bike and throw it down as many courses as we can. We’re particularly interested in its Alpine performance, so look out for continued in-depth testing over the coming months.

Product Specifications


Name Fox 40 Cartridge Kit
Brand BOS

Material Anodised Aluminium
Damping Adjustment Compression Rebound