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PD: Primary Drive Primary Drive Ratios and Torque

bmetcalf

Well Known and Active Website User
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
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 ratio.

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

Well Known and Active Website User
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
1585482113566.png
 

Chris Launders

Well Known and Active Website User
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

Well Known and Active Website User
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.?
 

Glenliman

Well Known and Active Website User
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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vibrac

Well Known and Active Website User
VOC Member
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
 

Glenliman

Well Known and Active Website User
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
View attachment 33112
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
 

Glenliman

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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.
 

Glenliman

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
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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