Back-End Weave: BMW R90

| 6/25/2014 9:39:00 AM

Motorcycle Classics tech expert Keith Fellenstein

Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Back-end weave

Q: I have a 1976 BMW R90/6 that weaves to the left on acceleration and to the right on deceleration. This is definitely not a front end problem as the steering head has been checked and greased and is adjusted properly. I have installed new fork springs and a hydraulic dampener. The front end, in my opinion, cannot be the cause. When I ride I can see the frame snake underneath me as I’ve described. The frame is not damaged. I think it has to be the shocks or the swingarm bushings. From what I can gather, it is highly unlikely to be the swingarm bushings. How can I check the health of the shocks? They have been easy enough to dismantle, and they feel equally resistive when I test them by hand, but they appear to have resistance in only one direction. — Ralph Parsons/via email

A: You may be too quick to dismiss the swingarm bearings. They seldom give trouble on old BMWs, but if the locknut on either side is loose, the spindle that tensions the bearing can come loose, too. I would put the bike up on the centerstand so that the rear wheel is free to spin. Use a board under the stand if you must to get clearance. Then try pushing the swingarm left and right while watching the pivot point where the swingarm joins the frame. If you see any play there, the pivot bearings are misadjusted or worn. MC

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