Does anyone know the efficiency differences in respect of bhp figures between belt drives and chain driven primary drives for Commandos?
Two factors come to mind.
Weight differential being one, the other, friction losses between the triplex chain and sprocket teeth being the other.
Another benefit would be less stresses incurred due to reduced mass.
A bit of Googling shows that…
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What I see in classic racing
Quite a lot of belt drives on Norton singles and twins, G50 and some others. Main advantage is less problems with chain lubrication. As the primary chain rotates much faster than the rear, much of the lubricant is thrown away. You can't use the original method of feeding oil to the chain as race officials hate oil leaving the bike. I use chain drive on my Manx as I try to keep it mostly as it was. Though the chain guard is dented since the primary chain broke on the 6th lap at the TT in -63.
One drawback with belts is that they must align better than a chain. So it is common to fit adjusting screws on both sides of the gearbox to keep a straight line.
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A bit of Googling shows that a well maintained chain will have losses of 1 to 4%
a belt drive could be around 9%
Is the reduced weight worth the hassle of changing from a perfect good drive train?
belt drives can last a very long time if the shafts are perfectly aligned but whereas a chain tends to give you some indication it is getting tired (adjustment required) belts tend to go from perfect to snapped.