Got a question on a problem that I've been going round and round for a bit.. My '74 R90/6 used to be my daily driver. I though I'd had that bike tuned to perfection. Recently bought a Transalp, then a V-Strom, and haven't been riding the Beemer regularly for about 2 years. Every time I take it out now, I'm a bit annoyed with the front end. Feels stiff as hell. I don't know if it's always been like that, or now that I've been riding something with a bit more modern suspension, I'm starting to notice the 33yr old short comings.. When I sit on it, and compress the forks, I can feel tons of stiction. I've tried many things to alleviate this. I've done the plate glass trick on the tubes to make sure they are aligned. I've changed the fork oil, and I've compressed the forks many times before tightening the axle pinch bolt. Here's what I've found. Changed the fork oil, and found lots of metalic particles in the oil. I fully tore down each fork, and thoroughly cleaned out all oil and what not. Fork tubes seem to have some scoring(polishing) on the bottoms, but not anything I can even get a fingernail in. Rolled fork tubes on kitchen counter(while the SO was out...) and saw no noticeable wobbling. The damper rod rings, and bore seem good. I've rebuilt the forks, and they work fine when off the bike, sliding back and forth very smoothly. When I put everything back together I get stiction. I've tried just snugging up the lower triple clamps, and leaving the top triple clamp loose. Tighen the axle nut, but leave the axle clamps loose. So everything would have room to move if the tubes were bent. Still stiction. I'm thinking that the bushings in the sliders are worn, and the tubes are getting ever so cocked with the weight of the bike, causing them to bind up. Maybe I've got too little sag? With me on the bike, the front sags about 2 inches. I've been thinking about taking 1/2"-1" out of the preload spacers to let the forks ride lower in the sliders. Anyone have anything similar to this with an airhead? And is it possible to replace the bushings in the sliders with something new? Thanks..
I have a '75 R90/6 which I haven't ridden in a while. I'll be interested in comments from others. Any chance it could be a fork brace issue? I suppose you have the SS one the fender attaches to. Although those seem to me well made.
Sounds like you need to read this article on Duane Ausherman's site:http://w6rec.com/duane/bmw/fork/title.html
With that flat plate top tripple clamp assembling the forks is a two man job , and you must have a big bit of angle bolted to the clamp and held by your assistant to stop the forks getting twisted out of line as you tighten them up. The alignment where the centre nut clamps down on top of the tripple clamp/ top bearing is critical, as is the torque of the nut- around 80 ft/lbs from memory. The later forks had a proper alloy triple clamp which clamps on to the stanchions and make keeping everything in line a bit easier - they fit the earlier bikes and help the handling a bit too.They also let you adjust the projection of the stanchion so that it is exactly the same each side - you can check this by seeing how freely your wheel spindle turns in its clamps - when every thing is lined up it should be quite loose.
I don't think so.. I've tried assembling the front end with everything loose, and the fork brace removed. Basically loose axle pinch bolts, loose top clamp bolts, loose lock nut on the top clamp, and loose lower clamp pinch bolts. Still get lots of stiction, even with everything loose enough that it should all be able to self align as I compress the forks.. Next I'm going to try it with loose axle nut, and I'm also going to try the rotate wheel spindle thing..
Cool site, huh? Guess they didn't see my post and check out that site. It's got answers for them on this exact topic. I've owned bikes my whole life and while they were mine, I was the mechanic who fixed them. Until this summer I'd never even thought about the tech presented in that article. I never would have thought that the forks could be farther apart or closer together at the ends than at the triple trees, and how to fix it. It sure helped me with a stiction problem on a new 77S I got.
That is a great article.. I remember reading it years ago, but I didn't check all the alignments of my forks when I had them apart this summer. I think I'll be looking into this in the next few days..
I would suggest that you may have a triple clamp alignment issue if the forks slide easily and smoothly without springs in them and before they are bolted into the bike. The inner bushings can be changed and some tuners even go so far as to insert shim stock under them to reduce the play inside the tube, but that isn't your issue. I would start by reassembling the front end without the front wheel or fork springs in place: ensure you have stiction free motion with each leg before inserting the axle, then add axle, fork brace, tighten tripple clamps, etc., until you can isolate what is providing a bending load and causing the forks to stick. It looks like you have done most of this allready, but perhaps without removing the springs...... all that will do is make it easier to isolate the component which is out of alignment.
This is good advice.. I wasn't so systematic the this summer, because I though changing the oil would solve the issue. This time, I'll go back and do it all in steps until it starts binding up.
That would be the weird replaceable split ring gizmo on the fork tubes which forms a bearing surface between the slider and tube. On some makes of fork there is also a similar thing at the top of the slider. As these wear the working gap between the slider and tube grows, making the fit looser and leading to more slop in the forks.
Good thinking.. So far I've removed the springs, wheel, and lower brace/fender. Forks slide well when connected by the axle without the clamps tightened. As soon as I tighten the axle clamps, the forks become a little tougher to move. If I push them all the way up, and tighten the clamps, they will drop under their own weight, until they get about 2 inches from all the way down, then they stop. I can push them down from there. When moving the forks one at a time, the left slider won't drop completely down without assistance when I allow it to fall on its own. So I think first I need to dig into the left fork, and see if anything is amiss..
Gramps- while you have the springs out and the wheel and brace off, loosen the tree clamps one leg at a time with the axle in place and spin the tubes without removing them- this will let you see if there is any change in stiction when the tube is oriented differently, a sign that the tube is slightly bent. If you note no change with this, tighten the lower tree and (since the R90 uses the goofy flat plate upper tree) loosen the upper nuts. I'd probably do this with the forks colapsed and the axle tight.... I think there may be enough play at the upper plate when loose to compensate if the upper plate is twisted. The left fork should move freely as you noted- I suspect you don't have a bent tube causing the sticking as this would tend to get worse as the fork is compressed and the engagement increases. I would look for isues with the bushes and damper rod. Good luck! Let us know what you find.
It sounds to me like the left tube is bent. It is hard to tell much by rolling them, unless they are really bad. The best way to ch eck them is by chucking them up in a lathe. If that isn't possible you can lay the tube on v-blocks and turn it while checking it with a dial indicater. You should check in several places along the length. A small deviation in straightness can cause a big difference in your tubes relationship from one end to the other. I had a tube that only showed .010" out of straight, but because the bend was near the top the tubes were farther apart by over.120" at the bottom. Alot of the stuff on Duanes site is very usefull. He shows you a measureing contraption that by his own admission is hard to use. I have had good results by just using a 12" caliper to measure the parallelness (sp?). The other possibitity is the tube to slider wear pattern. If the tube does not go back together facing the same way in the clamp the wear grooves will no longer match up with the ones in the slider. Don't forget that the oil holes in the sides of the tube face either to the inside or outside.
I'm not sure I have a bent tube. I don't have a lathe, but I may be able to fab some v-blocks, but I still need a runout gauge. Further examination is showing that there is still some very fine metallic particles in the fork oil, like the chroming is coming off the tubes or something. I suspect these particles could be helping bind the forks up. I did the plate glass thing again, with the sliders off, but the tubes in the forks, and they are aligned. Using a caliper, I've managed to get the same result over and over again, so I'm pretty certain the tubes are pinched toward each other ever so slighly at the bottom. I'm coming up with 0.012" difference at the bottom triple clamp, vs the very bottom of the fork tubes. I've stopped there for the moment, but I think I will rotate one of the tubes 360 degrees and measure again at 90 degree increments.
Gramps, If you can't check them on the v-blocks, turning them one at a time will tell you about the same thing. Is the .012" you measure with the top caps fastened down? The stock "triple clamp" is so wishy-washy it could throw you off that much. The ones I have set up with used (loose) parts worked fine if they were within .006-.007" of parallel. To me if one slider by itself hangs up on the tube, it points to abent tube or rough spot. I installed some new tubes that didn't have the oil holes chamfered properly, that acted that way,(tight spot).
Nah, I'm doing this with everything loose. There's plenty of clamping from the lower triple tree to hold everything in place. I get a consitent 5.831" between the inner tubes at the trees. I think I have a bent tube on the right side. This evening, after turning the tube 360 degree, and measuring every 90, I've found the right tube varies from 5.801 with the holes pointing in, to 5.849 with the holes pointing out.. This is all plus or minus .020" or so because I can deflect the forks that much by squeezing them together, or pulling them apart before I pop the wedge out of the lower clamp. Originally the holes were facing rearward, and I was getting 5.820 or so.. I'm thinking this makes sense.. about 4 or 5 years ago I hit a dear, but was going pretty slowly. I caught it just in front of the rear legs, so the fender went right under it, and it bounced squarely off the forks. Spun the deer around, wobbled the steering a bit, and then everything was fine. I was only going about 10-15 mph. I'm guessing it was probably enough to bend the forks back. I must have gotten used to the rough front end, but started to notice it after riding other bikes... Or maybe they were both bent the same direction, and it didn't start binding it until I pulled to forks, and re installed them with the bends non-parallel a year or so later.. anyhow.. I guess I know what I need.. Now I'm wondering if the sliders could have any damage from running bent tubes. And what's the deal with the holes on the tubes? Why do they have to face in or out? I don't understand that one.
