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Rear Shock Oil Change

Tools / Items / Time needed:

Racing Suspension Fluid or Shock Oil (not fork oil); weights recommended range from 5, 7.5, 10 and as an extreme, 15W. "Pick" and chose, as the wrong weight means buying more fluid. You’ll need ~285 ml / 9.5 ounces. I’m told ATF is 7.5W, but for the cost, and the trouble to change (and as rarely as we do), I’m sure the "specific purpose" stuff would be better.   [ See also Rear Shock Oil for more discussion on oil ]

  • Total time; about an hour
Tool Required:
  • #2 Phillips screwdriver (side panel removal)
  • 17 and 19mm socket, two ratchet handles, extension bars
  • 10 and 17mm wrench
  • Small-tipped funnel (rec’d having a small "stirring-straw" like you use at McD’s for coffee) or a syringe
  • Air pump (low volume)
  • Small 2x4 board

Nice To Have

  • Safety goggles
  • Torque Wrench (ft-lbs)
  • Grease gun

Supplies

  • Grease
  • Rags
  • Cleaner

Remove side panels on each side. Place the 2x4 under the rear tire. It  doesn't need to be "jammed" under, but set to hold the tire/swing arm, to remove weight from the shock. (this will allow attaching the shock back on very easy, saving you needing someone to raise the rear wheel).  Remove the "bolt, suspension arm" (zerk-bolt) holding the bottom of the shock to the uni-trak linkage.

Unbolt the centermost zerk-bolt from the uni-trak. Swing the center piece  ("rocker arm") of the uni-trak forward, and the two tie-rods (side arms) of the uni-trak rearward.
 

CAUTION! BLEED ALL AIR from the Schrader valve while the shock is upright,  to prevent fluid from spraying out. Unbolt the air hose from the frame (use  a wrench/socket on each side of the frame), and unscrew the damper  adjustment knob.

Holding the shock from below, remove the bolt on top of the shock; the shock will now lower downward and can be angled to remove from the bike.

Once you're sure you bled all air, use the 10mm wrench to unbolt the air hose from the shock.
 

Invert the shock. You should get approximately 285ml / 9.5 ounces of fluid out. As you leave it "inverted" to facilitate draining, you can clean up the linkage, grease the rollers in the linkage, verify the zerk bolts are clean, lubed, that grease flows from the zerk to the hole, etc (will need a grease gun). if grease does not flow, removing the zerk fitting and cleaning any dried grease, as well as "pricking" the ball-fitting (pushing it in to free it up) typically fixes that problem.

Measure out the fluid you drained from the shock (it should be around 285ml, which is 9.5 ounces).  Now, measure out the same amount of new fluid. If you use the funnel, insert the stirring straw. This allows air to flow, allowing the fluid to flow; otherwise, it liquid/vapor locks. (You can get the fluid flowing again by "plungering" a toothpick or other long, narrow device into and out of the fluid). If you use the syringe, don't press hard; pressure builds up, and if you unseat the tip the fluid will squirt everywhere. This is where the safety goggles can save you an ER visit.

Here's a tip from Andy Donohue on refilling the shock:

" To fill the shock with oil I used a clean pump type oil can ($2-$3 at the hardware store). I put a little oil in there and pumped it until no more would come out. Then I added the measured amount 285ml. This assured that I would get the right amount in the shock. The nozzle fit in the hole for the air hose and it only took a few minutes to fill (no mess either) - Listserv #3263
 

And since it's so popular, here's another a tip from Greg Lane (COG Digest 4068) on refilling the shock:

I tried the funnel and straw method without much success.  I then went to the local farm store and bought a syringe for 65 cents that they use to inoculate horses, cows, sheep, etc. It held 100cc at a crack and within 5 minutes I had the shock refilled with no mess.

And finally, yet another tip from Peter Lindquist on refilling the shock:

My solution? The small funnel matched up almost perfectly with the stub for the air line (previously removed for the draining process), a couple bits of duct tape attached the two firmly. Laying the shock horizontal, pour in enough suspension fluid to back up into the funnel itself. Now comes the secret ingredient, a hair-dryer. Lay it down pointing at the 'top' of the shock and brace so it won't vibrate away. Turn on the dryer, and you'll almost immediately see bubbles coming from the shock as the air heats and expands inside the shock body. With every bubble, air exits, oil enters.

Grease the o-ring on the end of the air hose. Reassemble in reverse order. Be sure the air hose sits "untwisted" when bolting/tightening the schrader assembly back to the frame bracket, as twists in the hose may cause the air hose to fail.

Torques; Top shock mounting bolt; 29ft-lbs Lower linkage bolts (to uni-trak); 43 ft-lbs

Go ahead and grease-gun the zerk fittings since you're there. Add you choice of air pressure with a low-volume pump. Be sure to set the damper-knob to your preference. I know you will enjoy the difference!

 

Article By: Mark Cipriano

 Updated January 2005

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