• 12-28-2012
    Matty F
    FOX CTD High/Low Speed Circuits
    Looking at the thread on the O/C CTD service got me wondering whether or not the O/C compression damper is shimmed or orifice damped, as none of the photos nor Fox diagrams seem to reveal anything other than a check shim.

    This also got me thinking about the Fox Fit Trail Adjust CTD service procedures I pre-studied a few days back in anticipation of when I have to dig in to one of these nightmares (be prepared to buy more expensive proprietary tools). Now obviously this design has a shim stack in it, but it doesn't seem to me that the design has isolated high/low speed compression circuits. It looks to be that though the oil always passes through the shim stack, there is no blowoff to bypass the low speed orifice valves that are controlled by the CTD/Trail adjuster knobs.

    Can anyone confirm or deny this? Like I said, I have only checked out photos on the Fox service site and not actually had one of these apart in my hands. I would hate to see Fox's high end damper not have separately adjustable high and low speed damping. Even without external high speed damping adjustment on an RLC cartridge, at least you can alter the shim stack inside independently.

    For those of you wishing to investigate:

    CTD Trail Adj. Compression Assembly:

    CTD Trail Adj. Service Procedure:
    FIT CTD with Trail Adjust Cartridge Service
  • 12-28-2012
    Thanks for those links. I am still waiting on my new Covert with 34 CTD fork to come it, so I haven't had time to play around with it yet. And damn that looks like a tricky rebuild with all of those small parts...I am generally one who likes to keep things simple.

    I just looked briefly at the rebuild instructions, but they discuss a flow divider, low speed needle valve, and shim stack in the Reassembly section. To me this indicates that there are separate circuits for high/low speed compression. But most likely it would a set-from-the-factory high speed shim stack and an adjustable low speed needle valve. This is how almost all compression adjust forks are unless they specify independent HSC adjust (which is usually just a spring that adds preload to the shim stack).

    Or is CTD supposed to adjust both HSC and LSC at the same time? The Fox website states that the Trail mode has 3 LSC settings on the Trail Adjust models, but doesn't say if there are HSC differences between C, T, or D modes.
  • 12-28-2012
    Matty F
    I saw that, but it looks to me like the flow divider adjusts the orifice size, but there is no blowoff bypass the low speed orifice at all, except in Climb mode, which I believe is how low/high speed circuits have been separated on the RLC FIT dampers. It looks like the flow adjuster on the CTD just limits how much oil gets to the shims no matter what the shaft speed.
  • 02-01-2013
    Interesting diagram


    Looks like the slow speed circuit flows down one of the 3 outside channels on the compression piston

    3 channels, 1 for each of the modes, with the rotating adjuster directing the low speed flow down the relevant channel for C, T & D modes.