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Rear sprocket size for trials/offroad

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rear - Rear sprocket size for trials/offroad Empty Rear sprocket size for trials/offroad

Post  rob spence on Wed Nov 15, 2017 3:24 pm

Hello,
Does anyone have a recommendation for rear sprocket size for an offroad 20T engined bike with 18 tooth gearbox sprocket.

rob spence

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rear - Rear sprocket size for trials/offroad Empty Trials gearing

Post  piston 197 on Wed Nov 15, 2017 4:43 pm

You are better off dropping your gearbox sprocket to about a 15 tooth, this will enable you to keep the rear sprocket to around road size. The bigger your rear wheel sprocket is the more chance of chain derailment and chain stretch.
All you need to do to down size the gearbox sprocket is put it in an open fire ( or woodburner) over night, take it out when cool in the morning and turn off the teeth and reduce diameter, Do the same with a 15 Tooth blank from the nearest bearing stockist, but this time turn out the middle leaving a chamfer, press the old centre into the new 15 tooth sprocket and weld together.
Heat up the new sprocket to red hpot and cool in a tin of oil to re-harden ( it may go on fire briefly so do it outside !)

JH
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Post  john87 on Wed Nov 15, 2017 6:11 pm

Hi There,

I do believe that villiers services have a range of gearbox sprockets in stock including 14 teeth and lower.. This might help...

john..

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Post  piston 197 on Wed Nov 15, 2017 6:16 pm

They do indeed, but they are Villiers ones , this is an AMC engine and different sprockets, i do have them for the 15T engine which are 16 tooth, and the same spline as the 20T but they are for a narrower chain, so no use either
JH
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Post  john87 on Wed Nov 15, 2017 6:47 pm

Ahh, that will teach me to read threads properly!!!

Ok, as JH said, New teeth on the old one!!

john..

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rear - Rear sprocket size for trials/offroad Empty Re: Rear sprocket size for trials/offroad

Post  rob spence on Wed Nov 15, 2017 6:50 pm

I'll get down to the dentist with the sprocket.

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Post  john thomason on Wed Nov 15, 2017 8:47 pm

i believe it to be 58 teeth for trials, but it is a matter of choice and what you intend doing.,

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Post  rob spence on Thu Nov 16, 2017 11:12 am

Thanks John, I'm sure that gear ratios are an art in the off road area...I just need some starting point. Many thanks.

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rear - Rear sprocket size for trials/offroad Empty Gearing

Post  piston 197 on Thu Nov 16, 2017 11:31 am

You have
1 engine sprocket
2 gearbox sprocket
3 Rear wheel sprocket

all of which can be changed, i would advise keeping the rear wheel sprocket as small as you can and reducing the primary and gearbox ratios as 2 teeth here ( smaller) will make a much bigger percentage than 8 teeth on the rear wheel will do ,
going up from 50 Teeth to 58 is 16% reduction whereas 2 teeth on the gearbox 18 down to 16 will give approx the same reduction and shorten rear chain length .
If you look at a modern trials bike they all have tiny rear wheel sprockets 30/32 teeth and the gearing is done prior to the rear wheel
JH
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Post  rob spence on Thu Nov 16, 2017 12:00 pm

Thanks JH.

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