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trialsrfun

Triumph Tiger Cub Gearing/sprockets

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with a smaller rear sprocket you also get the added advantage of the chain not getting damaged on large rocks so you gain all round will

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with a smaller rear sprocket you also get the added advantage of the chain not getting damaged on large rocks so you gain all round will

Good point aawil, another reason for a smaller sprocket.

Edited by trialsrfun

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On a comparative test way back when, which involved an ex-works Triumph rider, we proved that using combinations of crankshaft, gearbox and rear wheel sprockets that gave very similar final overall gear ratios, it was much easier to break traction on slippery going with a larger rear wheel sprocket - in every gear.

 

It took a lot of doing, a lot of rapid spanner work to keep the comparison valid, in case the surface conditions were changing - but we had received a challenging question from an engineering student who felt, as a result of purely mathematical calculations, that the radius at which the driving effort to the wheel was applied had a significant effect.  And our test supported his maths, the bigger the rear wheel sprocket, the more difficult to find grip.

 

So the answer must be fit the smallest crank and gearbox sprockets, that will give you the smaller rear wheel sprocket and you should find grip better............unless you can prove different.

 

I'm highlighting the crucial part of the text in red.

 

I think that what is missing from the discussion that is developing around the original post is consideration of the direction of and what the "driving force" x "radius of rear sprocket" (the moment or torque) is acting on in relation to the gearbox sprocket and the suspension geometry.

 

On a bike with a rigid rear end, does it matter what the rear sprocket size is, apart from larger ones are more vulnerable?

 

On a bike with a swinging arm consider the forces acting on the swinging arm through the swinging arm pivot as a function of the relationships in space between the centre of the rear wheel axle, the centre of the swinging arm pivot and the top of the gearbox sprocket from where the chain is being driven. For the time being just sketch these out for a particular bike and I think the issue is the degree to which "squat" is induced as power is applied through the driveline for a given rear sprocket radius; and the situation changes with suspension movement....and then consider suspension geometry, forces and damping....

 

If anybody has a reference to an engineering paper on this please post. I've tried to do my own sums to determine what the ideal is but I've done this purely out of interest; I've long come to the conclusion that I am the weak link in any rider/bike combination. I agonised for about 30 seconds about the pros and cons of 9/44 or 10/48 on my TLR200; should I have agonised some more?

 

Related to this do an internet search for Silk, JJ Cobas in relation to jackshaft, front sprocket concentric with swinging arm pivot.

 

If I used a 1964 Honda C90 motor do you think I'd get a Pre 65 Scottish entry for this? Obviously I'd get rid of the pedals. The saddle should count in my favour.

 

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Before I go ahead and change my Triumph Tiger Cub from the standard 19t to a 16t crank sprocket will I also need to make any changes to the clutch.

Looking at websites selling Tiger Cub trials parts I see there are thinner clutch plates available to enable 4 to be fitted instead of the normal 3, are these needed with the smaller sprocket to handle the increased torque.

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You can get the 3 plate clutch to work but you need heavy springs and careful setting up. Without some sort of clutch lightener, you will end up with a heavy clutch lever.

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My recently acquired Cub has 14/58 secondary ratio, with a 15 tooth primary. It was built up in 1997 and hardly used since (apparently) Yes, the clutch struggles if you are heavy on the kickstart, but a gentle stroke gets her started. I have recently fitted the four plate clutch with new springs and pressure plate, which made a considerable difference, and I have since found that the friction material on the back of the clutch basket has all but disintegrated. This is away to be re-lined, and I'm confident this will be another step forward.

As for the effect of the gearing, I'm very happy with the low speed this gives. The bike has a low 2nd gear as well, which is quite usable in sections. Unfortunately, the trade off is that top speed is about 25mph!

Mark

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Thank you Alan so I need to get the thinner plates, do they then work ok with the existing springs?

The standard clutch effectively has 4 plates.. The back of the drum acts as plate, adding another set of plates increases theoretical torque capability by 20 % for the same spring load. This should just about compensate for the increased torque due to the reduced engine sprocket. I havent used a four plate clutch so cant say for sure.

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