I was told that a belt drive will increase the load on the twin gearbox my calculations concerned just the gear ratio I had to make an adapter for a smaller than standard range of rear sprockets. I would be interested to know how much more torque per revs is transmitted by the gearbox
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I had a good discussion with Bob Newby on this subject. His original belt drive for the Vincent gave a lower geared primary vs stock Vincent. It was something like 1.8 to one vs the stock Vincent at 1.6 to one. This would increase torque on the gearbox.
When he made my belt drive for the 1360, he tried to make it the same as the stock Vincent, as he was concerned about the gearbox load.
As I recall, he had to abandon his first attempt which was actually higher geared than stock. He got it nearly finished then realized something wouldnt fit, probably the bigger engine sprocket.
He ended up with a belt drive close to the Vincent 1.6 ratio, which, as far as transmission loading, was an improvement over his older belt drive.
So it makes sense that if your primary ratio with belt drive was lower geared, (higher reduction) then you would need either a smaller rear sprocket or a larger gearbox sprocket, maybe a bit of both to get back to the desired overall ratio.
Glen