Fork rebuild advice

[daleweisman]

I'm a Forum newbie so pardon me if this topic has been thoroughly addressed.
I'm seeking advice on a potential airhead fork rebuild. I have a 1979 R100RS with only 40K miles. Whenever I ride over a rough road surface (pot holes, train tracks, etc.), I feel and hear a distinct rattling "thunk," which seems to come from the forks (not the steering head bearings, which seem solid). A year ago a local independent BMW mechanic replaced the fork oil and the plastic bumpers (or bushings) at the bottom of each fork. Progressive springs were fine, not replaced. He did NOT rebuild the forks per se. Nor does he want to because he thinks the late-70s forks are antiquated and doesn't believe parts are available (especially for the various springs, washers, etc.) at the top of the forks. He thinks the "thunk" rattle is caused by the forks topping out, not bottoming out. Also, I never asked him about adding spacers (i.e., short pieces of PVC pipe) which are apparently advised for Progressive springs.
I am no mechanic and don't have the skills or patience to rebuild the forks myself.
How difficult is it to remove the forks so that I can ship them to a fork/suspension specialist for a rebuild? Is a rebuild even necessary? Final question: will I need to remove most of the RS fairing panels to remove the forks?
I appreciate any advice. I love this old airhead, and it's solid in every way -- except for the irritating fork noise.
thx
dale in Austin, TX

 

[pglaves]

One thunk that is very common is caused by the fork springs slapping the inside side of the fork tube at full extension. When even slightly compressed the fork spring will be slightly curved and one side of the spring will be aginst the inside wall of the fork tube. If at full extension the spring straightens it may slap the opposite side of the inside of the fork tube. The cure for this is to add a slight amount of additional preload to the fork springs.

 

[stevenrankin]

Preload

I would try Paul's suggestion before sending the forks off for rebuild. There is not a whole lot involved in rebuilding them and your mechanic looks to have done a fairly good job of it.

To add preload you can use pieces of PVC pipe of the correct diameter. The longer the pipe, the more preload. I think in my RS I have about 3/4 inches of pipe in each fork.

To add the pipes, you have to have the bike up and all weight on the front forks removed. Ie, the wheel completely off the ground. You will have to move the handlebars out of the way to get to the top of the forks. I believe the caps are 32mm in size. As long as there is no tension on the forks, you should be able to remove the caps, insert the PVC and be done. It's not an easy job but not overly complex. You can fine tune the handling by trimming or lengthening the PVC.

Give this a try and if the problems persist, the, you will of course have to look closer at your system to find the problem. Good luck, St.

 

[20774]

Welcome to the forum! Adding the PVC spacer is probably 2-3 wrenches out of 5! I went to check the cap nuts on my /7 and they are 36mm across the flats. The last time I did it, I had to use an air gun to break the nuts free. It would be best to get a 36mm 6-point socket and mill off the first part of the socket to reduce/eliminate the chamfer on the inside of the socket. The cap nuts are very thin and doing this gives you more bit on the nut. I recall just hitting the nuts with an initial burst from the air gun...that impact helped to loosen them.

I'd do one at a time. It's somewhat of a "blue air" situation getting the nuts back on. You'll be fight a lot of spring tension especially with the spacer. You'll need to push down hard and enough to begin to engage the first thread or so...once you have that, then you can easily turn the nut back on. I forget the torque...probably around 80 ft-lbs...I see that's what's on Snowbum's site:

https://bmwmotorcycletech.info/torquevalues.htm

 

[stevenrankin]

I apologize

I apologize, Kurt has the better idea as to just how big a job this will be to do. I have not done it in a long time and my memory left out the worst bits.

I just removed the forks from one of my bikes as I am doing an overhaul. Yep, I needed the impact wrench to get the nuts loose. I suppose I could have done it with a long breaker bar but, I have an impact wrench.

I have not reached a point where I am ready to put it back together again, thanks Kurt for reminding me it is not as easy as I thought it was. LOL.

Regardless, you will be able to do it yourself without having to send it to someone, it won't take as long as pulling the forks off to ship them, and if it solves your problem it is cheap. Good luck. St.

 

[jchussey]

Disintegrated Poly Top-Out Bushing

Sounds like a top-out clunk to me. My guess is that #7 in the Max BMW fiche titled "31_0314 - RISER PIPE/ABSORBER" has deteriorated, and that you also have too much preload on the notoriously too-stiff progressive springs. Too much preload (not enough sag) will cause the forks to return to full extension over most every bump, and if # 7 is missing, which I imagine it is, there will be nothing to cushion the metal to metal contact. Clunkity clunkity clunk

By the way your mechanic is wrong about parts for the '79 forks. Last I checked, every single part was available at the dealer. These forks are not bad at all when rebuilt correctly by someone who knows what he is doing. He probably didn't have the confidence to pull the damper assembly out of the fork. The lowest (bottom-out) bushing is much easier to replace.

Put a zip tie around the fork tube when the bike is on the center stand and slide it all the way down against the boot. Then take the bike off the center stand and get on it. Put it back up on the center stand and measure the distance the zip tie traveled. It should be 2" -3" ; 2" being on the firmish end of preload and 3" being on the soft end.

Jim

 

[ccolwell]

The hardest part here is pressing down on the cap and turning it at the same time to reinstall. Before starting in do you like the way the fork works as it is? If not, you could take the opportunity to change the springs. Once the caps are off, it's just a matter of pulling them out and replacing (well, perhaps pulling them half way out and letting the oil drip off them for a bit). As Jim notes, lots of folk hate progressive springs because they are too stiff. I liked them on my R100 but generally don't like them because of the way the fork stiffens under braking. The last time I redid forks on an airhead (my R90S) I went with straight would springs from Race-Tech and was very happy.

 

[kbasa]

If the springs don’t have the little white plastic caps on both ends, they can sit a little crooked in the tube and knock around, especially at the top. Use a tubing cutter on the spacers so they’re nice and square and don’t have burrs.

If you’re going through the forks and have some budget, consider adding RaceTech cartridge emulators and get modern fork adjustability and compliance in old school appearing forks.

Set compression damping and preload for your specific weight and usage and enjoy a dramatically better front suspension.