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Shop Talk | Fox DPX2 Squishy Sound | 2021 October
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Offered services (plus sales tax if applicable)
- $60 100 hour Air Sleeve Service + Shipping
- $122 Light Damper Rebuild (detailed below) + Shipping
- $144 Light Damper Rebuild + Fox Air Seals + Shipping Recommended
- $180 Factory Level Overhaul + Shipping
Many bikes came with Fox's DPX2 piggy-back rear shock in the latest batch of all-mountain and enduro bike options over the past few years. Unfortunately, what all of us found is that it seems to need damper service more often than any other rear shock Fox has ever produced.
What is a Damper?
The damper is what controls the rate of progression on a compression and/or rebound cycle. If allowed to just bounce up and down, uncontrolled, the air spring on this shock would eventually buck you off the bike. The damper controls this by forcing oil through small ports and shims which the air spring must work against. Once air (of any sort) gets into that oil - something called cavitation - the performance and continuity of your damper becomes erratic and inconsistent.
The DPX2 comes with an adjustment knob for both the rebound and compression circuits in the shock. The rebound is red and the compression is blue. The blue knob is preset to 3 different positions, basically equalling soft, medium, and firm.
These adjustments give you control over how the shock responds in trail riding, whether to high speed, rocky riding, or big drops. You can control how "fast" the fork compresses or returns. Particularly, the DPX2 does this very well for a mass-produced shock. Until the oil gets air into it.
Cavitation
When a rear shock is assembled, the damper oil is compressed using and internal air spring. In engineering speak, this is called a "hydraulic accumulator." The shock's damper might work ok with out it but the damper really needs this to function properly.
The internal air spring in a Fox damper is filled with Nitrogen gas. The chamber is charged with 150 psi and backs a small puck with a seal on it inside the piggy-back housing. This small puck is the IFP (internal floating piston). The seal eventually goes bad on all dampers with an accumulator. On the DPX2, it just seems to go bad quicker.
When a mechanic or suspension tech talks about static seals versus dynamic seals, what they're really referring to is a seal that separates one environment from another while it moves or slides inside a housing (dynamic) or only separates one environment from another with near minimal movement (static). To help understand which seals wear the quickest, we target dynamic seals. The cheif dynamic seals on a rear shock are the air seals. The standard for most riders is to replace them every 100 hours or so even though manufacturers might recommend more or less often.
In the DPX2, other than the air seals, the IFP seal is the most heavily used dynamic seal in the shock and wears even quicker. While there may be some characteristic in the piggy-back design that is exacerbating the IFP seal wear, it doesn't change the fact that in order to keep this rear shock working correctly we need to have it serviced more often than others.
As the IFP seal wears, the high-pressure nitrogen escapes past the seal and into the oil, inversely pressurizing the wrong side of the IFP and mixing oil and air (nitrogen). Air passes through the damper circuits (ports and shims) infinitely faster than oil. This is why it sounds slurpy and rides like there is no support in the firm position.
Remedy
This forces us to service this shock at increased intervals and therefore keeps the oil fresh rather than allowing it to degrade and ruin other internal parts over time. Replacing the IFP seal and oil restores the damper to its original ride quality. Because of the nature of rear shock service, air seals are also replaced during this service.
The shock is good to go for at least another 70 hours. If the IFP holds beyond that then the 100 hour mark is when you should replace your air seals. If you want to stay ahead of the IFP wear, I recommend doing the IFP and air seals at the same time. Over time, the shock will need new seals throughout the damper, which ought to be done around every 200 hours of riding. All food for thought.
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For Fox's detail page on this shock, visit: Ridefox.com.