Workshop Stories: Fixing MCDs

[Johan Bornman]

Workshop Stories: Motion Control Dampers

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A week or two ago someone here on The Hub asked some questions about his Rock Shox Reba?s Motion Control Damper. Let?s refer to this red damper as the MCD.

 

The MCD in all its glory. Simple, but effective compression damping.

 

The MDC is a basic compression damper that controls lockout and, well?.compression damping on Rox? short-travel forks. It can be connected to a so-called Pop Lock which is a remote lockout lever mounted on the handlebar. With or without the Pop Lock, the remote and non-remote dampers are very similar and suffer from the same maladies ? dirt and abrasion which hampers the turning motion required to turn it to lock-out position. Faulty dampers are either so stiff that they cannot be turned to the lockout position or, they stay there and refuse to return.

 

The problem stems from a poor design that allows dirt to enter the top cap ? that?s the black aluminium cap you see on top ? and abrade the aluminium to a point where it develops burrs and grips the plastic insert so that it can?t turn. The only solution is to remove the damper, disassemble it, modify it slightly and re-assemble it again.

 

It?s a relatively simple task you can do with a few tools.

 

Here?s how:

 

Tools required:

 

24mm socket and ratchet.

1.3mm allen key

Flat screwdrivers

Shaft clamp

Vice

Emery cloth

12mm flat spanner

Soapy water.

5wt oil if you intend doing an oil change as well.

Lots of cloths.

 

Step 1: Remove the damper from the fork. Clean the fork and get rid of stuff that can fall into open orifices. You can leave the fork in the bike but clamp the bike in a stable stand of sorts so that it stands on its two wheels. Remove the Poplock cable by removing the cable from its pulley by releasing the grub screw that holds the cable. Use a 24mm socket and ratchet to carefully screw the damper out. Slowly pull it from the fork crown, allowing the oil to drip back into the fork.

 

Step 2: Inspect the damper. Now that you have the damper on a clean surface that doesn?t mind oil, have a good look at it. Try and turn the lock-out pulley, either by using a 12mm spanner to turn the shaft (you?ll need some leverage on remote lock-out models since it has a strong spring inside) or in the case of non-remote models, turning the knob. The motion should be smooth and the knob should turn freely. Note that a sticky cable or Poplock lever could also hamper the damper?s movement.

 

 

Turn the hexagon section just beneath the knurled knob to feel for smoothness of movement.

 

Usually a sticky MCD also has a faulty cable and Poplock lever. Don?t fit crappy cable to the Poplock, use proper gear cable inner and outer.  If the cable has a kink, replace it.

 

Remove the circlip that holds the blue pulley and pull the pulley off. Remove the knurled knob that turns the floodgate with a small (1.3mm, probably an imperial size of sorts) allen key.

 

Step 3: Disassemble the MCD.  The MCD comes apart quite easily. The black aluminium top cap is screwed onto the red plastic orifice damper by way of a left hand thread. In other words, righty loosey. Clamp the red orifice shaft in a suitable shaft clamp (I make them from Nylon blocks I buy at Maizey?s Plastics, drill a suitable hole and saw the block in half) and unscrew the top cap using a 24mm socket. With a firm pull, remove the top cap. Remove the shaft with its spring (remote models only). Now turn the MCD orifice damper around, clamp it arse-end upright and with two screwdrivers, pry off the bottom black cap.

 

 

The blow-off inertia valve as seen from the inside. It should rotate freely and move easily against its retaining spring.

 

Inspect the valve for smooth operation and see if the blow-off valve (the silver round thing at the bottom) moves freely on its spring and can be lifted with a fingernail against its retaining spring.

 

 

 

Make yourself a shaft clamp from a nylon block. They're handy for all sorts of delicate bicycle work.

 

 

The top cap is screwed on quite tight. You'll need to hold the red orifice damper cyclinder tightly with a suitable tool in order to screw it off.

 

Step 4: Fix the Top Cap  The top cap is the problem. Dirt entered the hole where the shaft rotates and damaged the aluminium. You?ll notice a series of silver lines or abrasion. You?ll also notice that the plastic insert that moves in the hole is not damaged.

 

 

 

Those silver marks inside the hole are the abrasions that prevent free movement of the shaft. They must come off.

 

 

Most of the friction will come from here.

 

Strangely enough, the dirt only damages the aluminium.  Roll a piece of emery cloth like a zol and insert it in the hole. Rotate it so that the hole is reamed smooth. Blow the gritty dust off, rinse the cap and test it against the insert. Carry on sanding it until the two mating surfaces move freely against each other. Rinse it once more to get the grit out.

 

 

Some emery cloth and a bit of elbow grease is all that's needed to get the hole back to pristine condition.

 

Step 5: Assemble the MCD. Insert the silver shaft into its position and allow the spring?s long end to slip into its hole in the red orifice damper.

 

 

Slip the spring's end through the hole in the red damper body.

 

Now rotate the shaft one full turn against the tension of the spring and allow the plastic shaft top assembly to ?hook? into the recess in the red damper. Hold it there and carefully fit the aluminium cap. You?ll figure out a way of keeping the spring in its place and slip the cap over after one or two tries.

 

 

 

 

If wound up correctly, the spring will look more or less like this.

 

Clamp the damper and screw the cap back on. Remember, lefty tighty.

 

 

 

Take note of the two mating parts that hold the spring from shooting back. This is the correct end position.

 

 

 

Now turn the damper upside down and take note of the blow-off valve?s position.

 

 

Take note of the one large and one small clip and their corresponding positions in the red MCD body.

 

The bottom cap can only fit in one position since the two clips are not of equal size. However, the valve can be fitted incorrectly. Make sure that the valve is open with the spring in its most relaxed position or, with the Lockout not turned on.

 

 

 

Note the vale should be in the open position like here, when fitted properly.

 

Clip the black cap and valve in place.  Test the Lockout movement. It should move freely and snap back to position (on the remote version only). Shake it and see if inertia makes the bottom valve jump open. Listen for the telltale click.

 

Replace the O-rings if you have any spare. If not, hold thumbs.

 

Step 6: Replace everything. Put the white plastic washer on top of the alu top-cap, with its bowl side facing upwards. Put the blue pulley on top of that and secure it with the circlip. Now replace the knurled knob and secure it?s grub screw using an allen key.

 

Put some grease on the threads in the crown and slowly push the damper back into the stanchion, making sure that the threads don?t damage the O-rings. Push slowly, otherwise oil shoots out. Once in, tighten it with the 24mm socket. Replace the cables and Poplock.

 

Step 7: Test. Since you only worked on the damper side of the fork, don?t associate spring-related problems with your latest operation. Stick to damper issues. Test the lockout by turning the blue pulley or lockout know all the way clockwise. Note that lockout only work if the floodgate isn?t all the way open.

 

See if the Poplock releases nice and snappy. See if you get lockout.

 

Go and ride.

 

 

 

 

 

Johan Bornman2009-07-08 14:26:22

[Goodbadugly]

Thanks. Couldn't figure out how to get the top part off. Ahhh. The lefty thread... (I even tried to remove the little roller pin). Will fix this little problem over the weekend.