This article describes a complete fork overhaul on a 1978 R100/7. I performed this work on my personal bike with the tools I had in my garage. I’d had the bike about 8 months before deciding it was time to do this, but it couldn’t be put off any longer. Stiction was a big problem and there was very little damping left in the forks.
With the forks extended, more than slight braking was needed to get them to compress, and when they did, the drop was sudden and deep. They would then stick at the bottom until lots of throttle was applied to the bike. There was no motion over small bumps, the font end just bounced up and down. The forks had not been worked on for at least 3 years, 1.5 of which was spent sitting outside, the bike never ridden.
To do this job, you’ll need the following parts:
- 1 Bottle of 5w or 7.5w fork oil (280cc for each fork).
- 2 Fork seals.
- 2 Lower cap seals.
- 2 Damping rod crush washers.
- 6 Damping rod rings.
- 2 Extension bumpers (don’t buy these up front).
- 2 Compression bumpers (don’t buy these up front).
I scanned over the fork removal section of my manual. It was right after the section on tearing apart the steering head bearings, so they went on to talk about removing all sorts of things that I really didn’t feel like tearing apart. It may be the techincally correct way for a shop, but in this case we’re sitting in my garage in our bare feet with parts and tools scattered about the floor.
Start with the obvious; remove the axle, fendor bolts, wheel, and the brake caliper. I don’t need to tell you to have the bike on the center stand because if you didn’t, you realized this error when you pulled the axle out (which also means you’re fairly muscular as well).
Take a moment to get the gator clamps nice and loose, if you’ve got gators. There’s a pin (it’s actually a vent) that holds the gators straight on the lower triple clamp, so you may have to pry on it to get it loose.
Not much comes off up top, I pulled the four nuts that secure the handlebar mounts to the upper triple clamp, removed the mounts, and let the bars swing down and rest on the tank. You can’t tell the new scratches from the old on my tank, but you might want to toss a rag there if you don’t want to add to the battle scars. That’s the extent of non-fork dissasembly.
For the forks themselves, pull the nuts out of the bottom and push the stud up and drain the forks. At this point if you’re lacking a vise, slip the axle back in the forks and break the large caps loose on the bottom of the forks. If you can’t get it loose, don’t worry about it. There’s another way that I’ll talk about later on. Pull the sliders off, push the damping rods up the standing tubes and pull them out the top. The damping rings will bind on the threads, so work them out carefully. I found that I could “unscrew” the damping rod while pulling on it and use the threads to my advantage. Use the large screwdriver to gently pry open the lower triple clamp and pull out the standing tubes. Save yourself some trouble here and loosen the pinch bolt first. All the headlight and signal gear stayed nicely put for me.
There is a small rubber ring sitting at the bottom of each standing tube. I poked at mine through the hole in the bottom with an allen wrench and bumped it out the top of the tube. If your rubbers are still soft, and undamaged, then you can reuse them. Your call. I reused mine.
At the bottom of the standing tubes is a floating disk that the damping rod slides through held in place by a couple of screw in caps and a circlip. You’ll need a special pin wrench to get the caps off, and I’m sure BMW would love to sell you one. My disk was moving around a little and I was comfortable that I could clean above it well enough, so I left the bottom plugs in place. You may be forced to pull them to get the rubber bumper out if yours is real snug, or you can just leave it all there as is and just clean it all up real well.
I flushed the standing tubes out with solvent and ran a rag through them to clean them up. A last quick flush and then I stood them up against the bike to dry out a little. Watch out for rust! The inside of one of my forks had a light coating of rust. I scrubbed what I could, and then rolled a little fork oil around inside the tube when done cleaning it.
Everything was badly gunked up with gooey sludge, especially the springs on the damping rods. It’s no wonder these things weren’t working very well. You’ll need to remove both of the bolts from the ends of the damping rod to really clean out the damping valves. A vise with wood blocks works well to hold the rod, but since I didn’t have one I had to improvise. A wrench on each end and some serious grunt got the things loose for me. The lower end contains a small spring and a check ball. the upper end has a larger spring and a small aluminum ring. Scrub everything down real good and slip it back together. Both end bolts should be torqued to 12ft/lbs. Finally, replace the damping rings on the large end of the damping rod, there’s three of them on each rod. These rings will seal against the fork tube just like piston rings, and so I treated them as such, aligning the gaps in the rings at 120 degree intervals. I have no idea if this is the BMW approved method or not, but that’s what I did.
Set the damping rods aside and go after the sliders next. In my case, I was unable to get the bottom caps off the sliders. I just didn’t have the right socket for them. I took the sliders down to my dealer and had them do it. This works out well for everyone concerned though, as it’s at this point that you might need a parts run anyway and it’s always good to put a little green in your dealers pocket.
I had them pull the bottom caps off and clean and inspect the compression bumpers and associated parts. My forks had the anti-dive springs in them, which use a different compression bumper (which is why I said not to buy it up front). My bumpers were in good shape and were reused. While there I had them install new slider seals and bottom cap seals and clean everything up. True to form my dealer had me out the door in 30 minutes and under $25 for the whole job. Give them fair warning, call first and make sure they can slip you in.
I reinstalled the damping rods in the standing tubes while they were out and easily handled. Slip the rubber extension bumper on the damping tube and let the damper guide it down. Use lots of care when installing the damping rod to avoid damaging the rings on the rod. With lots of patient wiggling and turning with the threads the damping rod cleared the threads and could then be pushed down the rest of the standing tube with a long dowel. My extension bumper was pretty snug, so it took some pushing to get it down. Put the standing tubes back in the triple clamps and sinch them up. Make sure the holes in the standing tubes face each other. This keeps the flow holes from being on a load bearing surface when the forks are in action putting a smile on your face.
With the damping rods hanging all of the way out of the standing tubes, slip the gators on the standing tubes (noting which side is up) and then slide the lowers up on the standing tubes. You’ll have to watch, and guide the damping rod stud through the hole in the bottom of the slider. Once it’s through, slip the washer & nut back on and tighten them up. At this point you’re ready to fill up the tubes with fork oil.
Measure out 280cc of fork oil and add it to a fork (280cc each). Work the oil into the damping rods by sliding the lowers up and down the tubes. It’ll take about 5-10 strokes before you start to feel much damping action. I spent a fair amount of time, about 20-30 strokes to make sure the air was worked out. I could already tell at this point what a world of difference there was in the forks. Moving the sliders while tearing things apart was very difficult and jerky. The sliders were moving smoothly up and down the tubes, and it was very easy to tell the difference between compression and rebound damping. When the oil is worked into each fork, get the gators lined up and clamped down. Make sure the air vent tube goes into the hole in the top of the gator. Re-install the springs and top nuts with the spacers.
Putting the wheel back on and fumbling with the brake caliper takes about 3 hands, but it can be done if you’ve only got two like me, it just takes a little longer. After the requisite cleaning up, put the caliper back on the bike and slip the eccentric rod all the way in. I added some standard BMW Red Lube #10 to the rod to keep it from siezing up, and pushed the pad piston all the way back into the caliper to give me some working room. Slide the wheel in place, and start the axle on its way through. Don’t forget the spacer that goes on the brake side of the wheel. With the wheel and axle in place, torque the axle and pinch bolts and then move in on the brake.
Give the caliper pin a few turns just to get a feel for how it moves the caliper. Your goal here is to align the fixed pad, which is closest to the wheel, with the brake rotor. Eyeball it to start with, and then zero it in after that. One creative method suggested to me for aligning the caliper is to use a felt tip pen to draw a line on the rotor (on the inside) and apply a little pressure while rotating the wheel, or more specifically the line, throught the brake. Get it to rub off the line as evenly as possible and you’re set!
Button everything back up, fender, bars, etc., and you’re ready for the test drive. Make sure the front brake is pumped up all the way, wouldn’t want any surprises when the neighbors dog starts chasing you out of your driveway. You might accidentally run over it while accelerating. Wait, we’re talking about braking aren’t we, never mind…
Time for a nice long ride to enjoy your like new forks!Joe Senner