Primary Drive

One God-given rule in the vintage/historic racing scene today is that it is allowed to use a toothed rubber belt instead of a roller chain to transmit the rotary power from the engine to the clutch/gearbox. No idea how this has come about but it does make life easier. In the days when I was racing my Norton Commando powered side car outfit, I had replaced the standard Commando triplex roller chain with a simplex. This was in the interest of saving weight and reduce friction losses. The chain was running in the open air, with only a rudimentary chain guard and a drip feed oil line that siphoned engine oil off the return oil line between the engine and the oil tank. It worked but whatever oil did not end up on the inside of the left leg of my leather overall ended up in Mother Nature. A “Schweinerei” as the Germans would say ( svineri på svenska and filthiness in English) and now we can use a rubber toothed belt instead, which runs without any lubrication whatsoever and therefore helps keeping both nature and the leathers clean.

the Norvil company sells primary drive kits, including the engine pulley with two washers that keeps the belt on the pulley, the belt and the clutch basket that fits onto the gearbox. Somebody told me that Dave Nourish used 40 teeth on the drive pulley and 68 on the clutch on his bikes, so that is what I ordered without giving it a second thought.

Well, I knew there had to be catch somewhere. In the case of the belt drive the catch is to attain the right belt tension after having installed it. As you can see from the picture of the gearbox, it has two points to fix it to the engine plates. The bottom point bolt goes through a round hole while the top mounting bolt goes through a radial ( from the bottom hole ) slot which allows the top bolt to be moved back or forth in the engine plate, so stretching or slacking the belt tension before tightening the bolts/nuts. Now, in comparison to a roller chain, the tension of the rubber belt is critical. Too little will make it jump over the pulley teeth while too much tension will ruin teeth and maybe also bearings. The belt manufacturer’s instructions for how to calculate the proper tension is too elaborate to bore you with. In fact it is too elaborate to bore myself with too. I will try to get the number from somebody and buy myself a spring tensiometer and then we are in business. Or maybe I’ll just try to stretch the belt according to the picture below and see how it works.