Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 Drive belt tooth jumping? maybe a solution...

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.

Nice on Callum, will follow with interest......

Perhaps you can fit a GoPro or similar camera in the rear suspension area to observe what's happening in real time?

Thanks for the marathon post. I guess there was no football worth watching on the telly?

I was interested to hear that you are going to make up a low profile dust cap. As far as I know, MMC never fit a dust cap on that side (adjacent to the front pulley) so that the swing arm can be positioned a gnat's away from the pulley. Certainly on mine there is probably only around 0.5mm clearance between the pulley guide screws and the swing arm. I know that Planenut covers the bearing orifice with aluminium tape of some sort which would seem to be a good solution.

How about replacing the front pulley guide button head bolts with countersunk machine screws? That should buy you another couple of mm extra clearance to play with?

It's aircraft "Speed Tape" (the answer to many aircraft problems 😁) I use to keep the dirt out of the bearing housing but a recessed and sealed bung will work just as well. I found the swing arm needs to run as close as it can to the drive sprocket to get the belt to track without putting pressure on the front sprocket guide plate. If it is running hard up against the guide plate it will eventually fail, usually the screws break. A long straight edge between the front and rear sprockets, with the LH shock absorber removed, will help get the best alignment. You may have to sacrifice a bit of rear wheel alignment with the chassis to get the best compromise. I think Phil Bleazey recommends about a 1/4 turn loading on the swing arm bearings, it seems to work for me, no bangs or clunks. I have Phil's BB mount as well. There is always the chain drive conversion?

Definitely nothing worth watching on the box! but I might be able to spare a couple of hours on Saturday evening to work on the car again!

At the moment I don't think I need to go to countersinking the flange and using screws to suit but it is on the radar if I want to move it that far over. The plastic dust cap I have in the swing arm is on zero clearance at the moment having been set to slightly negative clearance it soon moved out of the way of the end of the output shaft once it was spinning. The foil tape sounds like a easy option, I might give it a go with any more permanent solution neatly shelved for the moment.

That's pretty much what I'm aiming at; minimal clearance between the swing arm and the sprocket. I've probably got slightly more than a 1/4 turn of load on the bearings, but it is quite difficult to judge just where that 1/4 turn should start! I've found that I can get the straight edge on from underneath without removing the shock absorber but it does involve getting the car jacked and supported up high then much rolling around underneath it with lights and the straight edge plus a healthy amount of head scratching to get things moving in the right direction.
I have some aluminium tape that is similar to speed tape that I may well seal up the swing arm with, it's a lot easier than making a custom recessed seal for it.

For the first time in a very long time I had a good fun run out in the countryside without any banging on acceleration nor was there any belt squeal and believe me, I was really trying to provoke it with some properly hooligan low gear, full throttle, antics on grippy but bumpy roads.
I am now fairly damn convinced that the acceleration banging is due to inadequate load on the taper roller swing arm bearings causing a nasty resonance that manifests as something like a heavy machine gun firing: the solution? Tighten the adjusters up to a ridiculous degree. I have the female and male threads on the adjusters cleaned up, in good condition and greased so that they turn smoothly but still had to tighten them to a very high torque to finally get rid of the last occasional individual bang. A very strange way to treat taper rollers but I'm going to plead "what works, works".
The belt squeal that has varied from nuisance to unbearable for a long time has now gone. The last bit of adjustment that I've done was to move the swing arm as close to the bevel box output shaft end as possible. This has involved making a flush fitting dust seal for the swing arm bearing housing and a new front sprocket side flange attached to the sprocket with countersunk screws. Even with the countersunk screws the clearance I now have between the swing arm and the sprocket is best described in proper Scottish technical engineering terms as little more than a "baw hair".

The flush fitting seal, machined from some Delrin and a couple of cone bolts of just the right length to secure it;


Pressing the seal in;


Seal secured with the cone bolts;


Flush seal on left and ordinary dust cap on right, the swing arm is of course upside down for added confusion;


Countersunk stainless flange on sprocket without swing arm;


...and with swing arm though before last couple of adjustments taking up the last of the clearance;


Just for interest, here's a close up of the original carbon steel front sprocket flange showing just a hint of wear!

You have been busy! It must take the better part of a day to just get that out and back in, not to mention the remachining time. Kudos!

Good research Calum. Maybe Mr Bleazey's ears have pricked up?

I seem to recall that you have had several different chassis? Do you think this problem has existed with all of them?