Triumph Tiger Cub Gearing/sprockets

[trialsrfun]

What size of sprockets are you fellow Triumph Tiger Cub riders running on the crank, gearbox and rear wheel also rear chain size.

My bike is 199cc currently fitted with a 420 chain.

Edited April 13, 2016 by trialsrfun

[trialsrfun]

Does anyone know if the smaller crank sprocket is essential to obtain trials gearing or just a way of reducing the diameter of the one fitted to the rear wheel?

I think the standard crank sprocket is 19t but I have seen both 16t and 17t crank sprockets, would these require a half link or cause clutch slip problems.

[trialsrfun]

Just found an American Triumph Cub website which says the 16t engine sprocket is to enable a 14t gearbox sprocket to be fitted for greater swing arm clerance, with this info the options/confusion list just gets longer, what do you use on your Cub.

Edited April 13, 2016 by trialsrfun

[stanmet]

I run a 16 engine.sprocket 13 gearbox and 52 rear wheel on 420 chain you need a chain tensioner on front chain

[davetom]

I remember this being discussed before ( quote below), was this the same experiment or a different one ? I notice the one involved 2 bikes and the other lots of spannering..

Hi,

 

There was a lot of discussion at one stage that the larger the diameter of the rear sprocket, the easier it was to lose traction under slippery section conditions, the argument resting on the point of the diameter of the rear wheel at which the tractive effort of the chain pull was exerted.

 

To settle the argument two machines were chosen one with a larger gearbox sprocket and huge rear sprocket, the other with a minute gearbox sprocket and a smaller rear sprocket - but both with exactly the same overall gearing.

 

Then a multitude of riders compared the two bikes in an afternoon of various sections.

 

Realising it wasn't a genuine comparison, one of the rear tyres might have had better grip - one of the footrest positions could be affecting the result, etc., etc., but the overall view at the end of the day was universal, every single rider was convinced the bike with the smaller sprocket gave far better grip.  It was universally agreed that the minute gearbox sprockets would wear quickly - but ever one of the riders involved gradually modified their own bikes over the following season to get the smallest diameter rear sprocket that they could.

 

Proof - several riders went down a chain size because that gave them an even smaller sprocket...............

[davetom]

Ok thanks. If larger sprockets are acting as torque multipliers then, it sounds like a good mod for low powered, low torque bikes, like the smaller two strokes ?

[trialsrfun]

Hi Deryk an interesting comparison, do you recall the sprockt sizes please.

Are we talking sizes that are used for trials with maybe a six or eight tooth size difference which would be simiar to possibly two at the gearbox or a much greater difference.

[davetom]

I thought Sammy went to smaller chain sizes for weight reasons rather than sprocket diameter? I bet that maths student was grateful that you carried out all this work to validate his calculations ? It's a shame that no numbers were kept on the test examples

[trialsrfun]

Some interesting observations here, many small capacity modern trials bikes (those ridden by youngsters) have sprockets the size of a frying pan but the 199a and 199b Bultaco bikes had smaller rear sprockets than previous models.

The post was started because I think I may have my bike a little overgeared and wondered what ratios others were using but it has become very thought provoking which is positive, it just goes to show that motorcycle trials is actually a branch of physic's.

[aawil]

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

[trialsrfun]

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 April 17, 2016 by trialsrfun

[esteve]

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.

 

[stan wellback]

you will struggle to get trials tyres in that size.

[trialsrfun]

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.

[trialsrfun]

Should the Cub clutch handle the loading from the 16t sprocket or are the thinner plares needed as well.