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    Jan 2005

    AVALANCHE: FOX DHX 4.0/5.0 SSD modifications and RACE tuning

    Craig at avalanche downhill racing is now doing FOX DHX shocks, the modifications and tuning swap stuff out and custom parts put in making it Speed Sensative damping.

    Talked to him the other day after seeing it on the site I have my dhx 5.0 from my Sunday being done.

    Fox DHX 5.0Speed sensitive Damper Adjuster Kit

    Fox DHX 5.0/4.0 and VanRC Speed Sensitive DamperAdjuster Kit

    Replaces ProPedal Adjuster with a low speed compression adjuster and custom valved high speed piston system in the existing Fox piggyback housing. This conversion complements the revalved main piston to completely convert the DHX/VanRC to a true speed sensitive damper.

    Technical Discussion Here:

    Includes revalve and setup for rider, frame and conditions.

    Also includes complete rebuild service, new improved seals, oil, nitrogen charge and typical wear parts* included in price

    *(Typical wear parts: shaft seal, dust scraper, shaft bushing, reducer DU bushings, and o-rings)


    Shape factor bottom-out bumper (Available now)

    Reservoir Volume Expansion Kit, standard length or extended length (Available Soon).

    Not available as a self install kit.

    Available soon, DHX 3.0 optimized SSD internal replacement Valve, research being done currently on viability of DHX Air shock conversions to SSD technology

    Why change out Propedal valve in the DHX?....The design of the Propedal creates a great position sensitive shock. As the pressure in the shock goes up the propedal valve gets harder to open, this creates more compression damping. The pressure can be increased 3 ways, first my adding more air pressure to the schrader valve, second as the shaft compresses, the displaced oil compresses the reservoir floating piston down, which drives up the pressure in the reservoir, and lastly the chamber compensation bottom out can be dialed in to decrease the reservoir volume and thus cause the pressure to be driven up even faster as the shaft compresses. The Propedal adjuster allows the rider to compensate for the pressure on the valve by applying opposite spring pressure to the valve reducing the force required to overcome this internal pressure and allowing the valve to blow-off more easily.

    So this all sounds great but in a real world where courses and terrain are not perfectly smooth and void of square edge holes, this is where the Propedal system fails to allow the valve to blow-off. When the shock hits square edge bumps deep in the stroke the valve becomes harder to blow-off and is actually getting firmer the deeper it gets in the travel (Position sensitive damper).

    This is why the Speed Sensitive Damper (SSD) is preferred, as the shaft speed needs to increase the compression adjustment valve needs to blow-off when the bumps become square edged or what we call high speed hits. This creates the need for the conversion and replacement of the propedal system to a shimmed ported valve. Since the shim stiffness is not affected by the build up of pressure in the reservoir, the shims can create damping at low and medium speed compression hits and be valved to blow-off when higher shaft speeds occur thus allowing the damper to smoothly resist the square edged hits. Because the main piston was valved for a very progressive position sensitive Propedal valve we can now revalve the main piston to contribute to the compression circuit and due to the decreased ramp up in the overall spring rate of the damper, the rebound stack can be revalved to be more linear . The main piston can now control it's 70% share of the compression damping giving us many more tuning options for rider set-ups. Rebound can be tuned to be more reactive on small bumps and provide lift where needed while preventing that pogo stick feel.


    16 mm Speed Sensitive flow controlled 4 port Compression Valve 15 Clicks of Low Speed Compression, uses exiting Propedal knob as new low speed adjuster Revalvable 6 mm ID shims High speed adjustment - Internal Shim Stack Nitrogen charged to 160 psi, fixed (i.e. no need for adjustment now) Retains bottom out system to aid with pressure ramp up needed for regressive linkage frames Installs directly into the DHX housing...which preserves same top housing and adjuster knob for fit compatability of existing frame applications Low friction main shaft Seal Shape Factor Bottom-out Bumper, optional Increased volume Reservoir cap, optional Extended Reservoir cap for maximum volume, optional
    Last edited by bullcrew; 12-17-2011 at 10:24 PM.
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