|03-16-2015, 09:44 AM||#1|
Joined: Oct 2012
DR650 Mojave Shock Rebuild
A while back I was tripping around Australia on my DR650 when my back end went soft and bouncy. I'd put in a new Cogent Dynamics Mojave shock prior to leaving, but I'd covered 30,000km (mostly over corrugated rough terrain), so it wasn't too surprising. But what was a bit frustrating was I couldn't find anyone online who'd tackled a rebuild themselves. So I dropped Rick at Cogent an email and he set me on the right track - he sells rebuild kits containing all the seals & o-rings you need, and there's three levels of seal kits to choose from. Sweet deal. He also helped me out with a bunch of technical questions, and so I thought I'd pay it forward and put up a couple of pics and some info of the rebuild. (Note that the CD Mojave shock is essentially the same as an Ohlins shock - if you've ever pulled apart an Ohlins monotube floating piston shock it'll be exactly the same.)
First up is removing the spring. Measure the length of the spring prior to removing it and record the value - you'll want to set this back to the same position afterwards. To remove the spring simply rotate the upper spring nut. This backs the pre-load off the spring and eventually the spring will be loose.
With the spring loose the bottom collar can be removed, and the spring and thrust washer slid over the shaft and taken off.
Next wind the rebound adjustment out (anti-clockwise). This is the knob at the bottom of the shock shaft - see red knob in pic above. Count the number of clicks as you back this off and record the value. You'll want to reset this to the same position during the re-assembly.
Now onto the good stuff. The shock by default comes with a sealed type nitrogen port - basically a plug with a hole drilled in it and filled with rubber. The proper way to vent / charge the nitrogen is using a needle. However to vent it you can back the plug out slowly until gas starts to weep around the threads (there's not much gas in there). Note that a small screw must be removed from hex head of the plug to allow the plug to be backed out.
With the gas removed we move on to removing the seal head. (The "seal head" is basically a removable end plug in the shock body. The shock shaft passes through the seal head as it strokes in and out. The protective cap over the seal head is simply an interference fit. Put a screwdriver (or something softer if you're worried about scratches) under the lip and give it a few SOFT taps. Work on opposite sides if it doesn't come out evenly.
Below the protective cap is a circlip. Use a pick of similar to remove the clip. Slide both up the shaft out of the way. Once the circlip is removed a nice even pull on the shock shaft will pull the seal head and valve stack out of the shock body. Note that when the seal head exits the shock body the shock oil is no longer contained - keep the shock facing upward unless you want a mess ;p
Now you've got two main parts - we'll call them the "shock body", and "shock shaft". Drain the oil from the shock body. You'll notice an aluminium piston sitting in the body - time to remove it. There's plenty of ways to do this, but I used my air compressor with a rubber tipped air gun. First off hold a rag over the open end of the shock body - the piston is about to be ejected out this hole and you want to catch it while keeping mess to a minimum! Then put your rubber tipped gun against the nitrogen port and give it a quick tap of air. The piston should shoot out.
Next up is something not common to all shocks, but we're going to remove the remaining end cap on the shock body. (This is the end cap with the built in shock mount.) This will allow us to easily bleed the shock and accurately set the oil level on re-assembly.
The end cap is screwed onto the body, and sealed with both an o-ring and hydraulic thread sealant (Loctite 542). You need to clamp the shock body a vice - however use caution here. If you just clamp it in a vice you stand a better-than-average change of slightly crushing the body out of circle and wrecking it. The trick here is to use a shock clamp - a square bit of metal in two halves with a round hole through the middle to fit the shock body. Basically it clamps with an even force around the body, stopping out-of-round distortion. I made my own out of a giant chunk of steel, but I've heard of guys using wood and a holesaw to make their own too.
When the shock body is clamped in the vice use a torch to heat up the end cap - the hotter the better (my stickers all melted off). This melts the hydraulic thread sealant which is the main force holding the end cap on. (Note you'll most likely stuff the internal o-ring with the heat, so you really need to replace it on re-assembly. Mine looked fine once I got it all apart, but for a 50cent o-ring why take the chance.) Once it's nice and hot put a spanner on the mount end and rotate - it should be easy to undo. If it's tight, keep heating. I barely had to clamp the shock body in the vice to remove the end cap.
So the shock body is all disassembled, now we can move onto the shock shaft. First up we clamp the shaft in the vice by the mount end. (It should be common sense, but NEVER CLAMP ON THE SHAFT!) Use a spanner to remove the nut on the top of the valve stack.
Once removed put a screwdriver or similar on top of the shaft and slide the spacers, shims, valve head, and more shims off onto the screwdriver. It is imperative these are kept in the correct order!! Take a note of the orientation of the setup so you don't put it back on upside down during re-assembly! Put it to one side.
Use an Allen key to reach inside the top of the shock shaft and undo the small screw with a hole in it. This is where the rebound needle works (that red knob at the bottom of the shock shaft). Remember how we wound the rebound adjuster right out prior to dis-assembly? That prevents damage to the rebound needle in this step.
Before we progress with removing the rebound adjustment rod, we'll take the valve head shaft off the top of the shock shaft. Grab a spanner (30mm or 32mm from memory) and use it to undo the large hex at the top of the shock shaft.
Poke a pick or similar down the valve head shaft and the rebound needle will pop out. While the needle itself will pop out OK, it has an o-ring on it that I found pretty hard to remove without causing damage - if you don't want to replace the o-ring, don't try to remove it!
Now the seal head assembly can be removed from the shock shaft. Just slide it off. When the seal head is off note the seals inside the head...you want to put the new seals in in the correct place and the right way around! There's also a bush pressed into the seal head. All the seals can be removed without removing the bushing in the seal head.
As a word of caution (I say this because I was an idiot and wrecked my seal head doing this), if you plan on removing the bushing do so with care!! I thought the internal seals were captive behind the bushing, so I proceeded to remove the bushing (for lack of better tools, with a screwdriver). The idea and implementation were OK...except there's a small lip between the internal seals and the bushing...and I didn't notice it. So when I went to pound on the bushing out I actually put a screwdriver into an aluminium lip...against which a seal rests! Not good. I ended up ordering a new seal head from Rick.
Finally we'll remove the rebound adjuster rod. This is a mechanical component only (i.e. does not come in contact with the oil and is not pressure retaining. It sits inside the shock shaft. It should be coated in grease, so might be a bit sticky, but a bit of prying and shaking should get it loose.
Cleaning & seal replacement
Cleaning is pretty simple. Use copious amounts of contact cleaner and or a parts washer. I removed all my seals (where possible) prior to using contact cleaner, but do what you feel is right. It's very important to get everything clean prior to re-assembly, as any dirt / dust can cause seal failure.
For the valve head and shim stacks (the stuff we put onto the screwdriver) use a jar with the bottom filled with contact cleaner and dunk it all in (keep it in order - I transferred mine a an old bit of wire).
Now is also the time to replace any seals you want to replace (some of the pics here were taken out of sequence, so ignore something that appears assembled when it shouldn't be).
Coat the rebound adjuster rod in a light coat of grease and push it back down inside the shock shaft.
Next up is re-installing the seal head assembly. The seal head is an EXTREMELY tight fit over the shaft, and you don't want to nick a seal ring! Lube the shaft and seal head up real good with seal lube (or failing that a good helping of fork oil). In an ideal world we'd use a seal bullet over the shaft end. (This is basically a hard plastic condom for the shaft with a tapered tip to guide the seals over the shaft's sharp top edge.) In my non-ideal world (i.e. not near enough to the US to get one cheaply) I used the corner cut off a zip-lock bag. This basically helps to "soften" the top edge of the shock shaft for when the seals are installed. (I also took a file to the lip and VERY SLIGHTLY beveled the sharp edge - probably "not recommended" but I cant see how that would have any effect on the shock whatsoever.)
Once the seal head is on we can install the valve head shaft. I used a dab of blue Loctite (Loctite 242) on the threads. Be careful when using Loctite around the valve head and rebound systems!! You don't want it getting anywhere other than the threads. I don't know the "proper torques", but if you've done wrenching before you tend to know a "tight but not too tight" feel. Remember it's got Loctite on it and its a big nut on a small thread - it wont need much force.
Next put some seal lube / fork oil on the rebound needle and pop it back into the shock shaft. Make sure it is up the right way!! (Long end facing up, short end facing down.) Make sure he seats in nicely then re-install the small bolt at the top of the shaft. Again I used a small dab of blue Loctite, but was EXTREMELY careful to only get it on the threads!! (You don't need much torque on this.)
Now we can put the valve head and shims back on. By now they're spotless and clean. (As a side note, if you wanted to adjust the damping, the shim stack can be altered prior to re-assembly to achieve this. However since the shock is aftermarket and built for the DR there's probably no need - but the option is there.) Grab the stack-up of parts on the screwdriver and slide them back onto the shaft. Make sure they are sitting down properly. In some cases (I think it was on my forks actually, not here) there is a cupped washer that can sit funny, so just have a quick look and check it's all looking right. Once the valve head and shims are re-installed it's simply a case of putting the nut on top of the shaft. (Use a drop of red loctite [Loctite 271] on this nut.) Toque will be in the vicinity of 35Nm, although that's just a reasonable guess based on some Racetech video's I've watched.
Now we can put the shock shaft and shock body back together. Use seal grease / fork oil and generously lube up all seals. Slide the valve head into the shock body, then the seal head. Once the seal head is installed we can re-install the circlip behind the seal head. Now stroke the shock as far out as possible. This will pull the seal head back against the circlip.
Now we're ready to fill with oil and bleed the air out. (Rick told me the shock uses 5w - 7.5w shock oil - depending on your application. I opted to use 5w.) Grab your shock and clamp it in the vice. I clamped the bottom mount on the shock shaft rather than the shock body, but either way works as long as the top of the body (i.e. the open end) is facing upwards. With the shaft fully extended fill the shock body with oil. This is where it got messy for me, and I wont pretend to be an expert, but fill it as full as you can. Grab the piston and remove the little screw in the middle - this is the bleed screw. (You'll notice the piston has 2 small holes in the top. These are for a tool to grab the piston to stop it rotating while you tighten / loosen the bleed screw. In lieu of the proper tool circlip pliers work well ;p) Remove the bleed screw and, if you haven't already (and want to), replace the o-ring behind the screw head. (This seals the bleed screw to stop shock oil leaking through to the nitrogen side.)
Lube the piston seals and push the piston down into the shock body. Once its sitting down about 10mm from the body lip (i.e. most of the air is drained and fluid is coming through the bleed hole) replace the bleed screw. Now we need to cycle the shock to get the remaining air trapped (around the valve head) out. FOR THIS NEXT STEP MAKE SURE YOU PUSH DOWN ON THE PISTON THROUGHOUT THE RETRACTION CYCLE...unless you like being covered in oil that is!! (Also, if you dont push down during the retraction stroke the seal head will stroke with the valve head, meaning nothing gets bled!) Push down on the piston with your fingers and retract the shock. Then extend the shock, and repeat the cycle. You should feel a steady force over the whole stroke(in both directions). If it's moving nicely then it suddenly jumps at the end, that's the air we're trying to remove - keep going. If it feels rock solid and wont stroke then your valve stack hasn't opened - sometimes it takes a bit of force to open it up for the first time, after which it should stroke easily! After a couple of strokes open the bleed port on the piston and push all the air out. Repeat until there's a steady restriction over the full extension and retraction strokes (and no more air comes out the piston bleed hole). If you find the piston is about to pop out the top of the shock when you're retracting the shock, you've got too much oil. Open the bleed port and bleed a small amount out and then continue. The key is to keep the shock as full as possible to make setting the oil level easy.
Setting the oil level - fully EXTEND the shock. Once fully extended we want to measure the depth of the piston relative to the lip on the shock body. This should be 18mm. If it's more you'll have to pop the piston out and add more fluid. (Tip - to do this retract the shock without pushing down on the piston...just make sure you don't push too hard or fast, and you have a nice big rag over the shock end to contain the mess. After more fluid is added you'll have to go through the bleed procedure again) If the piston depth is less than 18mm then life is easy! Remove the bleed screw and push the piston down until it reaches a depth of 18mm then replace the bleed screw. (When setting the oil level ensure the seal head is right at the end of the shock!! - i.e. press down on the piston prior to setting the level to confirm.)
Clean the shock oil out from the top of the piston (nitrogen chamber).
Now we have to re-install the top of the shock - ensure the end cap o-ring has been replaced with a new one (remember the heat we applied to remove the end cap?). Clamp the shock body in the vice using the shock clamps. Put Loctite 542 (hydraulic thread sealant) on the shock body threads and screw the end cap on. Again, you won't need a stupid amount of torque on this as the hydraulic sealant locks it all together nicely.
Finally you can look at charging the shock with nitrogen. Replace the nitrogen plug and use a nitrogen needle to charge the shock to 150psi. Again, it should be common sense, but DO NOT USE AIR! Replace the small screw into the nitrogen plug internal hex.
The final steps are to replace the shock spring (reverse steps of removing it), setting the spring preload (rotate the nut until the spring length is the same as when it was removed), and setting the rebound adjuster (the red knob at the bottom of the shock shaft) back to it's original (recorded) settings.
And you're done!
|03-16-2015, 04:43 PM||#2|
Joined: Sep 2011
Wow man, thanks for taking time to post all of this!
I'm just glad to know how to take the spring off, thinking about powder coating it a different color :)
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