Reducing gearing for trails and trials

[BultacoBen]

This September I'm taking part in the Arbuthnot trial on my G12 CSR 'sled' - easy route Class F, but still not a walk in the park.

I have plenty off-road and trail riding experience, a very little (1 event) proper trials experience, and based on that the standard gearing on my G12 feels like it's going to be much too tall.

It's a '59 motor in '60 duplex frame, with what I believe to be standard '60 running gear. Is that universal across the Heavyweight bikes? From what I can see the rear sprocket integral with hub only comes in 42T? A 'lay-over' sprocket would work, but I can only find them 43T e.g. https://www.aceclassics.co.uk/products/ ... -sprocket/

So I think the best option would be to fit a smaller front (gearbox) sprocket. Going from 23T I have currently to 21T would match the G12 CS gearing, but basing my logic on the overlay sprocket to turn a road-going Triumph twin into an off-roader by going 43->56 = 30% bigger, I would need to put about an 17 or 18T on the front. Seems a massive jump!

Anyone have experience of the right sort of gearing needed to turn a big Matchless twin into a capable trail-bike? Bike (and rider) below, for context.

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[clive]

You could think about changing the primary drive sprocket. This is easier to do as you only have to take the primary cover off and each tooth change will make a bigger difference to the final drive . The front sprockets were different for the 500, 600 and 650 and of course there may have been additional options for Sidecar use with the 500. I would start with seeing what you have and go from there. The sprockets are not that expensive and you can change them relatively easily.
A few years ago I followed the Arbutnot course on G11CS. I was not actually doing the sections simply following the course to watch. I got to the halfway lunch stop and retired! I was double your age at the time, good luck

[BultacoBen]

You could think about changing the primary drive sprocket. This is easier to do as you only have to take the primary cover off and each tooth change will make a bigger difference to the final drive .

That's a great point Clive. I haven't looked closely at the primary-gearbox yet, but am guessing it's the same as my old BSA where you need to remove the clutch in order to access the gearbox sprocket in classic British style.

I'm used to bikes without a chain tensioner - so the primary ratio can't be changed. But with a pre-unit I take it I can just re-tension via gearbox movement once I've put a smaller sprocket on!

I'd forgotten the difficulty of changing gearbox sprocket on Brits as I'm used to easy gearbox sprocket changes on my Bultacos - we change gearing *a lot* when racing flat track, sometimes multiple times in the course of 1 day trying to get the perfect drive out the corner and keeping revving down the straight without needing to change gear. Our 'short tracks' in the UK are so small (usuaully Speedway tracks) that any gear change from 3rd to 4th for example means losing precious acceleration! Normally we change gearing via a stack of rear sprockets, but sometimes also via the gearbox sprocket too.

[BultacoBen]

I've also just realised I'd been mis-reading the parts book and that it is the engine sprocket that varies between the twin models and whether for sidecar use, not the gearbox sprocket. Explains why I couldn't match the diagram in parts book to the look of the gearbox sprocket fitted...

[clive]

Yes you will need to remove the clutch and inner primary chaincase to change the gearbox sprocket. When changing the engine sprocket you may need to change the primary chain too, as there may not be enough movement in the gearbox adjuster.

[BultacoBen]

Yes you will need to remove the clutch and inner primary chaincase to change the gearbox sprocket.

Yikes, had hoped sprocket would fit through the hole in primary side like it would on my old BSA B50 and A65, didn't think the chaincase would have to be off too! Even more reason to go engine rather than gearbox sprocket.

Thanks for your suggestions, the decision now would seem to be whether to drop from 23T on the engine to:

• 21T (standard for G12CS, a good benchmark, or sidecar on G12CSR)

• 19T (standard for sidecar with G9)

• 18T (smallest listed in the parts book, closest in ratio to what an 'overlay' sprocket would give.

I'm not as cautious as I was of going down to 18/19T which initially seemed like a *very* big jump now that I know it would have been standard gearing for a sidecar hack (albeit with the 500 motor). I'll whip the primary off and count teeth to confirm it's currently on 23T and then ride some more and think further. Availability of these alternative sprockets doesn't seem great, so it may end up being a function of what's available.

Would still be interested to hear from anyone who has an off-roading G12 and what gearing they found to be suitable.

[BultacoBen]

Took the bike out trail riding and was impressed! I mean, it's bouncy and both under/over damped, but very tractable with the engine at low rpm, even with the tall road gearing.

My conclusion is I think I will drop two teeth on engine sprocket, to 21T from the 23T, same as the G12CS, but will save doing so until close to the Arbuthnot as the riding I'll be doing before then is either gentle lanes where current gearing seems fine, or A and B roads round Bristol where the longer legs of the road gearing will be appreciated.

Really enjoying the bike apart from the cursed wet sumping (loses about 2/3 a pint overnight, 1.5 pints in a week!) but I'm happy taking my time and draining the sump each ride, although the regular threading in and out of the alloy does slightly worry me...

I see a bash plate in my future too...

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[ajscomboman]

In my box of spares I have a larger sprocket conversion on a brake drum, I also have 21 and maybe a 22 tooth engine sprocket.

[Eamonn]

... draining the sump each ride, although the regular threading in and out of the alloy does slightly worry me...

I recall reading a few years ago a possible option of taking a standard drain plug and tapping a hole in the centre that would take a hex allen bolt. So instead of continually removing the steel bolt against the alloy of the crankcase, you instead are just unscrewing the allen screw in and out of the drain plug itself.

Not sure I've explained that too well, but hopefully you get my drift.... !

[clive]

I think that "fix" requires doing away with the magnet which would normally reside in the sump plug