PD: Primary Drive Primary Drive Ratios and Torque

bmetcalf

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VOC Member
I agree with you, Norman, the lower gear box gears are to increase the torque to the rear wheel to get you going vs. trying to start off in top gear.
 

passenger0_0

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I'm not an arbitrator however I think this misunderstanding is all due to terminology defining 'high' and 'low' ratio.
I think we can all agree that when we have a speed reduction there will be an increased torque delivery.
 

Chris Launders

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I will try and explain my thinking.
If the engine produces 50 lb/ft of torque, and there is a reduction of 4:1 there will be 200 lb/ft at the clutch. Speed is divided by 4 and torque multiplied by 4
If there is a 2:1 reduction there will be 100 lb/ft at the clutch.
Speed is divided by 2 and torque multiplied by 2.
So the Suzuki clutch would be handling less torque on the Vincent (if the engines produced the same)
 

timetraveller

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I am always reticent to say too much on mechanical engineering matters. A back ground in astronomy does not qualify me to pontificate on mechanical matters. In addition I know from when Chris was testing one of the first JE steering heads I had made that he is not only an astute rider, he is also a brave rider, having done some of the early testing in the snow. I was hoping that this discussion would bring one of the VOC's 'proper engineers' out of the wood work but no luck so far. I am not sure, but I think that the difference between me and Chris is as follows. It concerns the relationship between torque and power. It does not concern any gearing in the gearbox itself or ratios between the gearbox output and the rear wheel.
First an equation but don't worry. It will not cause brain damage.
TORQUE = (HP x 5252) / RPM
The 5252 figure is just a constant going back to the days of horses pulling barges and so on and the definition of horse power.
Putting some values into the equation then we can say that
TOQUE = (50 x 5252) / 5,250 (arbitrary rpm) which gives 50 foot pounds of torque
Now double the revs to 10.500
This now results in the Torque value dropping to 25 foot pounds, i.e. double the revs needs half the torque for a fixed value of horse power. So the way that I was envisaging this was that if we go from a standard Vin primary reduction of about 3:2 ( I don't have the exact figure to hand) to one of about 4:1 for the original Suzuki ratio then the torque would be reduced by a ratio equal to the difference between the Original Vin ratio and the Suzuki ratio.

Is there an M.I.Mech E. in the house?
 
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vibrac

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VOC Member
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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Chris Launders

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VOC Member
I cannot see why a belt drive would increase the load on the gearbox, the engine is producing the same power and the belt shouldn't be tight, in fact there would probably be less load due to the weight reduction of the components, Norton boxes manage them quite well when paired with a Vincent engine and I'm sure the Vincent box is stronger than a Norton one.
 

timetraveller

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VOC Member
If my understanding right, and don't ask me to bet money on it. then it will depend upon the reduction in the primary drive ratio between a standard Vin and one with a belt primary drive. If you can give me the original ratio and the one with the belt then I can give you the percentage increase/decrease in torque as applied to the input gear to the gearbox main shaft. e.g. suppose there is a reduction in engine to gear box ratio of 10% then there will be a reduction in the torque applied to the periphery of the clutch drum. Where is that M.I.Mech E.?
 

Monkeypants

Well Known and Active Forum User
Non-VOC Member
I am always reticent to say too much on mechanical engineering matters. A back ground in astronomy does not qualify me to pontificate on mechanical matters. In addition I know from when Chris was testing one of the first JE steering heads I had made that he is not only an astute rider, he is also a brave rider, having done some of the early testing in the snow. I was hoping that this discussion would bring one of the VOC's 'proper engineers' out of the wood work but no luck so far. I am not sure, but I think that the difference between me and Chris is as follows. It concerns the relationship between torque and power. It does not concern any gearing in the gearbox itself or ratios between the gearbox output and the rear wheel.
First an equation but don't worry. It will not cause brain damage.
TORQUE = (HP x 5252) / RPM
The 5252 figure is just a constant going back to the days of horses pulling barges and so on and the definition of horse power.
Putting some values into the equation then we can say that
TOQUE = (50 x 5252) / 5,250 (arbitrary rpm) which gives 50 foot pounds of torque
Now double the revs to 10.500
This now results in the Torque value dropping to 25 foot pounds, i.e. double the revs needs half the torque for a fixed value of horse power. So the way that I was envisaging this was that if we go from a standard Vin primary reduction of about 3:2 ( I don't have the exact figure to hand) to one of about 4:1 for the original Suzuki ratio then the torque would be reduced by a ratio equal to the difference between the Original Vin ratio and the Suzuki ration.

Is there an M.I.Mech E. in the house?

Other way round on the last paragraph, if I read it correctly.
But the numbers seem off.

The standard primary ratio for the postwar twin is 56 /35 or 1.6 to 1.
I don't know what reduction the Suzuki arrangement has, but if it gives a higher reduction number than this, ( lower geared primary), then torque on the gearbox would increase.

Some of the belt drive makers offer multiple gear ratios. For high output engines where the gearbox is highly stressed, it's quite common to use a high geared primary ( low reduction) to reduce torque load on the gearbox. You spin the gearbox faster so there is less torque going thru it, same horsepower coming out ( other than tiny increased friction losses)
The final drive ratios can be altered to give the same overall ratio.
This kind of change takes some load off the gearbox and puts it on the rear chain, which is generally a good thing.

Glen
 
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