PD: Primary Drive Primary Drive Ratios and Torque

vibrac

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The standard ratio is 1.6 i used the 36T so its 1.89
I just managed to get a standard cover on with 10mm spacers to crankcase, and nail biting milling to clutch area
 

Monkeypants

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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
 

Monkeypants

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The standard ratio is 1.6 i used the 36T so its 1.89
I just managed to get a standard cover on with 10mm spacers to crankcase, and nail biting milling to clutch area

So you have 36 teeth on the engine vs 35 stock?
How many teeth on the clutch sprocket?
If standard at 56 then you have reduced the ratio ( geared it higher)
to 1.555 to 1
 

vibrac

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I know the ratio is 1.6 I dont know the clutch teeth number (I think all Bobs are the same I suppose its 36*1.6? I only know the front one because its the largest that will allow a chain case that looks like standard.
If anyone is thinking of doing this I am using the 30mm belt with a bob supplied stronger belt (the egli Racer used 40mm) here are the details
The clutch will fit inside the chain case but only if the clutch recess inside is milled out until about 1/8"-1/16" wall is left and the clutch cover screw pillars are left about 1/4" deep at the top of the recess. Some of the inside face wall where the belt runs is reduced the inside, the oil level boss and part of the clutch "wall" is also removed. the main-shaft where the thread for the clutch nut runs is shortened by about 1/2"-5/8" because the clutch is thinner and the end of the shaft would hit the clutch spring plate also this means I could reduce the height of the clutch cover to a minimum to increase cornering clearance all in all that leaves 10mm (sorry about metric) gap between cover & crankcase I was going to get some 'gaskets' made out of water cut alloy but in the end I used longer screws and thin tubular spacers Ben made a thin alloy strip that exactly fitted round the gap painted black you would be pushed to see the join
I am very happy with the result but making that first cut without knowing for certain it would work on a component costing on the way to £500+ to replace was a bit of a worrying moment.
 

Monkeypants

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The 40 mm Newby belt drive on my bike has 71 teeth on the clutch pulley and 43 teeth on the engine pulley. This gives a ratio of 1.65 to one vs stock at 1.6. He was hoping to get to 1.6 ratio or even less, didn't have room to do it.

Glen
 

davidd

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On the single, which uses the small Newby clutch, the ratio with a 36T front and a 68T clutch is 1.889 as Glen mentions. With the TTI transmission, I have 18T,19,20,21 and 22T CS sprockets.

David
 

vibrac

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The 40 mm Newby belt drive on my bike has 71 teeth on the clutch pulley and 43 teeth on the engine pulley. This gives a ratio of 1.65 to one vs stock at 1.6. He was hoping to get to 1.6 ratio or even less, didn't have room to do it.

Glen
That's what I had on the Egli racer when I did not have to worry about the cover also the clutch went inbord a long way and to obtain enough clearance on the pressure plate and retain enough thread on the clutch nut I screwed a face plate of 8mm alloy over the standard pressure plate
 

vibrac

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VOC Member
I am very happy with the result but making that first cut without knowing for certain it would work on a component costing on the way to £500+ to replace was a bit of a worrying moment.
And then what do I find today on evilbay?
It may suit a belt conversion or some multiplates but not a standard set up its definitely 'original' but not in the way normally implied
And not a mention that the item has been cut open !
you need to go in to EB and purchase with a lot of care....
 
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