I’ve have had something of a long battle with the dreaded toothed belt jumping, machine gun type banging from the back of the car on hard acceleration. The usual stock answer is to adjust your drive belt; get it tight, get it straight and the problem will go away. Sometimes it does go away, I’ve had it happen a long time ago now when I didn’t have the rear spindle tight enough which allowed the spindle to shift giving me a slack belt on skewed sprockets which of course gave a horrible series of bangs on acceleration but was easily sorted out. Sometimes it doesn’t go away which is the problem that I’ve had for a while which has led to an inordinate amount of time and effort involving straight edges, careful measuring, lots of methodical adjustments from rear wheel skewed to the left through the central straight position to skewed to the right at a variety of belt tensions from definitely a bit slack to unhappily tight, plus adjusting of the swing arm position from as far left to as far right as it can go, shimming the bevel box to alter its line in the chassis, spacing the rear sprocket further left away from the tyre, new belt (the old one was noticeably worn) new rear sprocket (a Fabian lightweight) and even a nearly new front sprocket (the old one was surprisingly worn; they are available new…but only with a new bevel box…and new bevel boxes don’t seem to be available!) and I even made up three new, slight larger flanges for the sprockets with nicely radiused edges to give the belt an easier time when running a bit off centre. I’ll add that I have a chassis with the rigid bevel box mounting so it wasn’t the box moving about on its rubber mounts and hitting the chassis tubes and neither was it loose mounting bolts or cracked brackets or tubes.
All of these things had no real effect on the loud banging on acceleration, some adjustments made it worse, change it back and it would stay the same, sometimes it would get slightly better but was always there when pressing hard in second gear and generally in third gear as well which was both a bit alarming and confusing; what was causing it?
Just before the weekend I was having another go at moving the swing arm sideways on its adjusters when I decided to have a play with the tension on the bearing adjusters. Normally the wisdom is that when setting up taper roller bearings as found in the swing arm pivots you tighten them till you feel tension, back them off a bit to be able to lock the adjusters and that’s about it. This doesn’t seem to work with the M3W swing arm. I tightened the bearings up a good bit tighter than I would normally consider correct for taper rollers and took it out for a run, result; dramatically less banging on acceleration. Another two adjustments tighter and I seem to be back to full throttle being useable with only the occasional bang from the back end when hitting a bump on full throttle in second.
My guess is that what is happening is that when there is any play at all in the bearings then a combination of the load from the belt acting on the back sprocket causing twist around a vertical axis through the pivot bearings plus the inherent lack of rigidity of the swing arm and the chassis mounting points then there is a resonance set up that manifests itself as loud banging like the teeth of the drive belt are jumping the sprocket. My next move is to make and fit a lower profile end cap/dust seal for the sprocket side swing arm bearing housing so that I can move the swing arm slightly further over towards the front sprocket and to slightly reduce the tension in the drive belt; I think the residual occasional bang on acceleration over bumps may be due to the belt tension going way too tight at full bump travel of the awing arm; at least on my car the swing arm pivot and the front sprocket are not coaxial so the tension varies with wheel travel. Might be a little while before I can do any more adjustments as real life is getting in the way so I won’t have much chance to get down into the garage or out and about for about the next three weeks.
This is getting to be a marathon post so here’s the summary; if you are getting unexplained banging from the back end of your M3W then try tightening the awing arm bearings, it might just sort it out.
