Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 How hot does the M3W drive belt get?

I have noticed on a very hot day in the south of France (it is over 40ºC today in the small town in the valley below my house which is up in the hills at 2500 feet altitude), I cannot quite use full throttle at the torque peak in second gear or I will get the very occasional belt skip, so I just avoid full throttle or change up early. The whole car has just been serviced by the excellent M3W Services, which included taking the whole back apart to fit a replacement fuel tank, so the belt must have been re-tensioned. I only notice this down at low altitudes on very hot days, so I am guessing that like almost all materials, the composite belt must expand with heat and maybe just enough to allow a skip at max torque. Now the belt is also in an area with not a lot of air flow and must be generating its own heat from hysteresis due to flexing. I wondered therefore if anyone had measured the belt temperature on a hot day. I would not be surprised if it was running at 20-30ºC above ambient. The pulleys will also be hot from friction and lubricant stirring in the bevel box plus braking heat transfer to the rear pulley from the rear brake drum. I assume the belt will be a superior material one made from SRS or similar type synthetic rubber on a kevlar/aramid core. I have not brought my IR remote thermometer with me to France or I would have measured the temperature today. If the belt is at say 70ºC, that is close to the operating limit for standard industrial toothed belt drives.

Wilson

I would say it was very unlikely that your belt is slipping a tooth unless it is VERY loose. As you have a new chassis do you now have the NVH kit for the Bevel Box mounting? If so I would suspect it is that causing the banging noise as you accelerate in 2nd gear. Not sure what temperature the drive belt runs at but not had or heard of any heat related problems with it.

The NVH made mine bang around and when I switched to the Bleazey BB Mount the banging went away. BTW, i have heard that the belts shrink a bit when they get hot!

Repeating hearsay is not helpful in a technical discussion. I very seriously doubt the belts shrink when hot. Much more likely the drive and driven pulleys expand due to heat (especially the original alloy driven pulley) causing the belt tension to rise with temperature. Which could be misinterpreted as a 'shrinking' belt.

I am fairly sure it is not the bevel box banging (I have the flexy mounted one on the later chassis), as I am familiar with that noise when I creep over the very uneven and rocky surface of my car parking area. It is a "twang" sound combined with a lurch, which I understood was the belt skipping a tooth. It only happens at max torque in second gear. In sympathy to the drive train, I never get near to full throttle/max torque in first. I will check the belt tightness tomorrow, where I believe, I should just be able to twist it through 90º at mid point.

I know Vee belts stretch when they get hot, so I would expect toothed belts to behave in similar fashion. When I used to work as the engineer at the family textile mill, where there were hundreds of vee belt drives of anything up to 10 belt wide drives for the larger machines. We had to use a Venner supplied table to get the correct drive specs and belts, dependant on transmitted power, distance between centres and ambient temperature. Some the drives were inside steam drying machines where the temperature from the 200 psi high pressure steam pipes would be 195ºC. We had to use special material belts there.

Wilson

probably true but the end result is the same, a tight belt.

You should only be able to twist the belt 45 - 90 degrees.

As for synchronous drive belt slippage, if there is ANY significant tension in the belt at all, it doesn't and can't happen unless teeth are shearing off. What you are likely experiencing is belt 'slap' and/or 'snatch' from rapid load changes changes caused by throttle inputs and/or shifting. These types of events can be somewhat violent and extremely noisy, especially if you have a machine with the ill conceived factory NVH 'upgrade'.

Tightening the belt is not the answer. There really should be some sort of tensioner/anti slap device on the slack side of the belt. I believe someone on this forum has posted their installation.

If the banging is under full load but is neither the nodding bevel box nor a drastically loose belt then it will be insufficient tightening of the swing arm pivot bearing adjusters. They need to be tighter than you would expect taper rollers to be.

I still doubt it is the drive belt jumping teeth, it just won't happen. I could go with Calum's swing arm bearing adjustment, belt slap or (more likely) the nodding BB with the NVH. Have a check around the BB and see if there are any signs of fouling?