How to work on Cartridge Forks, 101
Thanks for stopping by this page. Chances are that you're one of two groups of people. You're either looking to work on your own forks and install a Race Tech gold valve, or you're curious as to what's been done in your forks already/what you're sending off to have done. I'm going to try and write this with as much specific information as possible for all right-side up cartridge forks though I won't have pictures to illustrate some of the tasks. The forks pictured are from a 1991 VFR and are non-externally adjustable (bleh, damn you Honda!!), notes relating to other forks will be included where needed. The pictures are all thumbnailed for the bandwidth impaired, but the pictures are quite large. I also want to apologize in advanced for the slightly blurry pictures ~ I was trying to hold the camera with 1 hand, and that hand was well oiled so things kept sliding around on me. As for me, I'm a Motorcycle Mechanics Institute grad who was top of his class, certified in Harley, Suzuki and Yamaha ~ though I'd pretty much only ever owned Honda till 2003. Upon graduation I went to work for Max McAllister at Traxxion Dynamics here in Georgia and was put into the lead-tech position within a few months. I've worked on hundreds of forks from most every bike out there on the road and tuned suspension for some upper level AMA teams. I hope I know what the heck I'm talking about ~ but take everything here with a grain of salt anyway and check with your preferred expert should you want a second opinion.
This HowTo is a constant work in progress. I highly recommend you read all the way through this write up before attempting any part.
Its upkeep is done by Brian McCoy ~ please email corrections, updates or comments to me. Last Updated: 11/24/2003.
1.) First things first, you need to record ALL your settings. The number of clicks out on both rebound (fork cap), compression (by the calipers), turns out for spring preload and distance from the fork cap to the clip-on/top triple. You need to be able to return everything on the forks back to the way they are now when fully assembled. Write down all of your baseline settings and keep them for your records. At this point back everything all the way out (I had nothing to adjust... more complaints to Honda). Next you'll want to remove the calipers (NOTE: do NOT just leave them hanging, you want to relieve the stress on the brake line by holding them up with a zip-tie or bungee), remove the front wheel and pull the fender. Before you loosen the fork clamps on the triples, you want to loosen the fork cap. If this means pulling the handle bars/clip-ons then do so. You only want to free the cap, not completely unscrew it. This will make your life Much easier in a few minutes.
1a.) If you're like me and working without a vise, you need to also loosen the allen that holds the cartridge in place at this time. If you're working WITH a vise, skip down to removing the forks (2.) I hadn't thought of taking pictures of this (the thought to take pictures came later), but if you run the axle through the forks so they're tied together (like it would be normally, only no wheel), you can get the leverage needed to loosen the bolt. Run your 6mm allen socket up from the bottom of the fork and into the screw:
Obviously to do this, the axel cannot go all the way through whatever fork leg you're working on. Also, if you're working without a socket, a normal allen wrench using a breaker bar, or the closed end of a wrench as an extension will get you through this part. You just want to break the bolt loose and leave removing it till later.
2.) Once everything's loose you can finish pulling the forks from the triples. With them free you'll want to unscrew the fork cap all the way (don't worry, it's attached and nothing will shoot across the room.)
Now, depending on your fork, you'll likely see 2 lock nuts where I only have 1 in the picture. Sometimes they're the same, sometimes they're different. The end result is always the same - you want to loosen them so you can unscrew the cap from the damper rod. This is a good time to remove the nuts from the end of the rod and set with the cap.
3.) Time to dump everything out into your oil drain. I usually pull the spacers and washers from the forks first (they normally don't have any oil on them - at least if you keep your forks upright). For those of you with a vise, go ahead and clamp the fork into the vise by the brake hangers or axle clamp area ONLY. There is NO other safe way to clamp forks in a vise. You'll want to remove the cartridge at this point by loosening the 6mm screw all the way. Mine had never been worked on before and came apart Very easily, this isn't always the case (some people work on forks and use loctite, you'll probably have to drill the screw head to get the forks apart if this is the case).
This is what should come out of your forks. Some tricks to get the cartridge out if it's spinning in the fork are to pull down on the tube as hard as possible while unscrewing. This compresses the 'top out' spring (the small spring 3/4 the way up the fork cartridge) and provides friction. You can also try pulling down directly on the damper rod, though I don't feel that works as well. If all else fails, a couple bursts from an air impact will usually loosen everything up. You can also try reassembling the forks dry (with the spring and spacers), then compressing the forks while attempting to unscrew the allen ~ this again creates needed friction
4.) Time to disassemble the cartridge now. There are 2 major fork makers out there, KYB and Showa. Both have a different way of assembling their cartridges. I have a Showa so that's what the pictures are from, but I'll describe the differences in KYB.
For Showa forks, the compression stack/holder is on the inside of the cartridge and it pushes in as shown. Sometimes there are 3 peenings around the outside base of the cartridge, just drill those out till you can push the stack in as shown. The picture is terrible and doesn't show the retaining ring just inside the lip, but it'll be obvious in person. Just use a small pick or screwdriver to remove that.
5.) With the retaining ring removed (or the compression holder removed on KYB forks), just use the rebound rod to tap out the compression holder and completely disassemble the cartridge.
This is a good point to stop working and do some Major cleaning. Get ALL the oil off everything. Cleanliness is your friend... little bits of crud, metal flakes and anything else can really cause chaos in your forks. This would also be a great point to pull the tubes apart if you want to change your fork seals.
6a.) With everything apart and clean, we'll start on the final disassembly of the shim stacks. Both Showa and KYB peen over the nut on the rebound rod:
This is where the bench grinder comes in handy. Hold the nut to the wheel at about a 45Deg angle and grind as little nut/rod as possible while removing the peening. The nut should screw off easily with the peening gone. You'll probably need to hold the rebound rod to do that though. For this you NEED custom made soft jaws for your vice. As I mentioned above for the KYB guys, this is best done with a piece of 2x4. Drill a hole the same or just slightly smaller than size of the rod. Then cut the wood in half through the hole and place the rod in the hole and clamp it in the vise. You want Equal pressure from all sides and you do NOT want to mar the surface in any way. This thing slides past a bushing and any nicks/dings will quickly ruin the bushing and cause the forks to work Very poorly. Basically, if you ding/mark/crush the rebound tube, it's trash. My example of a 91 VFR was no worry since it's solid and since I have no vise, I used some vise grips with padded jaws to hold the rod.
6b.) For the compression stack, Showa uses an allen screw to hold things together while KYB uses a base with a stud and a nut. KYB bases need the peening on the nut ground off in the exact same way as the rebound rod.
Again, Clean, Clean, Clean... get all those metal shavings away from your work area. Clean up any burrs left on the rebound rod, or in the cartridge body if you had to drill out peenings there. You should be here now:
7.) Okay, we can Finally get to the actual shim stacks. I've laid out both the rebound and the compression here. Rebound is the top row, compression is on the bottom:
From left to right, the pieces are:
The first pieces back onto the rod/allen (last for the KYB compression stack) are the hat and spring, then the check valve and then the piston. Note that the piston is directional. There's an inset on one side that the hat rests in:
These pistons are what get replaced by the Race Tech Gold Valve or the Traxxion Axxion valve. Showa pistons actually flow pretty well, but KYB pistons often have Very small orifices that meter the oil instead of letting shims do the work. Being a cheap SOB, I just ran my pistons over some 800grit emery paper on a surface plate (okay, so I used a glass panel from the front door, heh). The shim stacks are now the 'magical' equation... and you're going to hear LOTS of different theories on why one way's better than another. I have my own opinions and I like what I learned at Traxxion best, which is preloading the shims (just like you preload the fork spring). You need either an Axxion valve, or a shim add-on kit that Traxxion sells to do this (I suppose you could try to make your own, but that'd be beyond my ability to explain.) Below are 2 pictures showing the Traxxion shim kit and difference in thickness:
This causes the shim stack to deflect .05mm right off the bat and firms up the slow-speed bump section... This is the 'feel' part of the forks. With the preload in there, then you use a Very light stack that deflects easily under a sharp load (sharp bump, the stuff you want soaked up) and you get why so many people are talking about the 'superior' Traxxion forks. The compression stack with these shims is:
As you can see, this is a Very short and simple stack. Stock on my VFR forks was 5 17mm x.10mm shims and it was WAY too soft. Race Tech will have you build a tiered stack, but what that usually gives is something very soft in the feel dept. and to hard in the absorbsion... or you can make the stack thicker for better feel and even Harsher bump absorbsion and have an LE stack.
The standard rebound stack is:
You'll probably have to work with the shims in the stock stacks and they won't always be .15mm thick. Most times they'll be .10mm thick, in that case you can use 3 .10 to replace 2 .15 shims (it's not exact, but it's close). When finishing your stack and screwing everything together you want to watch and make sure that the piston is fully seated on the check hat and the valve moves freely. If that valve gets caught between the hat and the piston, it'll bend and your forks will NOT work at all. It's a simple thing to overlook, but important to be aware of and watch. You can use a light loctite here, just make DANG sure you get it ONLY on the threads. I skip the loctite in forks because of the possible complications on down the road.
8.) If you've made it this far, you're basically home free. The rebound rod should be together, the compression bases should be together and you should be just doing simple assembly now. This is the reverse of disassembly - insert the rebound rod into the cartridge body, and then push in the compression base. Put the retaining ring back in place and tap the compression base into place with the rebound rod. If the rebound rod seems to Stick to the compression base, you'll need to take things back apart and flatten out the nut on the rebound rod (it's getting stuck in the head of the allen screw). With the cartridge back together, place the bottoming cup on the bottom (note that this may not come out of all forks, sometimes it's trapped between the leg and the tube) and put the cartridge up inside of the inverted fork. With your finger on the end of the rebound rod, you should be able to wiggle it enough that the allen screw will pop up a little. Just tighten it down, it should suck itself up and provide all the resistance you need to apply proper torque.
9.) Before adding oil, I like to put the nut on the rebound rod (sometimes I do this before putting the cartridge back in the fork leg). In most cases, there will be a directional nut like the one I have pictured. It only has threads through half the body and the other half will slip over the non-threaded rod. Make sure you check this, and run the nut all the way to the end of the threads.
10.) Time to add oil and set the level. I usually pour in half the bottle to each fork and let one sit as I work with the other. With the fork fully compressed, place your hand over the top and create a seal. Now extend the fork, this creates a vacuum in the fork and helps draw oil into the cartridge. Do this a couple of times, then grab the end of the rod and work it in and out as completely as possible, making sure there's no skip in its function. Skips mean that there's still air in the system and you need to work all of that out. With the cartridges primed, measure your oil level. I do this with a length of clear fuel line attached to a horse syringe, the hose zip-tied to a tape measure and sunk into the fork the proper depth. It works well, was cheap and gives me consistent results. As for what oil levels do, they control the air spring in the fork. The air spring controls how easy it is to bottom out the forks. Since we're looking to eek the most performance out of our bikes, it makes sense to eek the most performance out of our forks and you need to use ALL the travel to do so. For those that use zip-ties to determine fork travel, that's fine and dandy - but do you know where you Actually bottom out the fork? Probably not - this is why it would be a great idea to mark that spot while your forks are disassembled. Fully compress one fork leg, now you know where full bottom is - I tend to mark my forks with a center punch. It dings the tube and provides an indelible mark that's hard to miss. Since the fork is fully compressed it'll never go past a fork seal (unless you're installing new, in which case just put it on from the other side with a plastic bag over the end). More oil makes it harder to compress completely, less makes it easier... adjust as necessary.
11.) With the fork oil measured, it's time to reassemble the fork completely. Pull the damper rod up as far as possible, drop in the spring, washer, spacer, washer and finally the dished washer that sits under the rebound shaft nut. If you have a fork with a rebound meeting rod/needle setup, those go in next. When installing the cap, screw it onto the shaft until it seats. Do NOT try to torque it on, especially on caps that have a rebound adjustment. Bring the locking nut(s) up to the cap and snug everything up. If you have a rebound cap, check now to see that you have 3 full turns. You do NOT need more than 3 full turns as it will not give you 'more' rebound adjustment... it'll just give you more dead adjustment. Finally it's time to screw the cap back onto the fork tube. This can be a tricky issue, a custom soft-jaw creation (like you made earlier for the rebound rod), will help a LOT or you can use the 'cheap-mans' way. Just run the fork leg back up into the triple clamps, and clamp it down that way.
Make sure you do NOT cross-thread the cap as it's quite easy to do. Drop the fork out of the triples and give it a quick push. It should move freely and easily with all adjustments set full out. If it's hard to compress, or does move easily in and out - then there's something wrong inside. Most times it'll 'stick' or pause on the way back up and that's due to an out of round rebound rod/cartridge body (I warned you up above!!) If you've been careful though, everything should work just fine. Reinstall your forks and button up the rest of your front end. Set your adjustments back to where they were when you marked them earlier and go for a cautious test ride. You'll probably have to adjust the settings a bit if you changed any of the shim stack, but it should be worth the experimentation.
I didn't really cover fork springs or preload above so I wanted to touch on the here. Most places have a spring reference chart that's pretty accurate, but be cautious with your spring purchases. Race Tech gets their springs from 2 different manufacturers in the US and they do NOT rate springs the same. One set of springs will have the rate engraved in the end, and these are tested in the way the 'world' generally accepts as the defacto standard. This shop also supplies 100% of the Traxxion springs. The other shop does NOT mark their springs and these are consistentaly 1 rate shy of what they're marketed as due to an uncommon method of testing. The springs are fine; RaceTech just mis-markets/mislabels them (and generally lies about checking each and every spring that comes into their shop.)