Standard Gearing

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Re: Standard Gearing

Postby greasemonkey62 » Fri Sep 26, 2014 11:50 am

What I had thought was if a bike can do 90mph ish, then at two thirds of that maximum speed 60mph it should not feel as if it was revving so much, its definitely not the clutch slipping, all I am concerned about is causing damage to the old girl, if she had a rev counter clock I would know what the revs were and just go up to 4,000 of the maximum 6,500 stated.
I tend to cruise about 50/55mph but on rare occasions I have done 60/70mph the engine vibrations do become smoother, but I am concerned that the revs sound so very high, I suppose that riding a modern twin and an old twin has this effect as the modern bikes engine does not vibrate so much with higher revs as does the Ajay, so I guess the combination of higher revs plus the extra vibration is making me wary of breaking something, cranks and crankcases are not easily replaced, should the worst happen.

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Re: Standard Gearing

Postby MikeM. » Fri Sep 26, 2014 2:53 pm

I fitted a NON standard 17 tooth gearbox sprocket to my '59 Model 31 not long after I first got it as I was going on long distance rides down through France, it gave a more relaxed ride on these trips but when riding around the local lanes on VMCC and club runs I tend to use third gear more when on the hilly areas.

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Re: Standard Gearing

Postby Groily » Sat Dec 13, 2014 4:09 pm

Ray's original Q was 'had anyone tried a 19 toother on a G12?'.
Well . . . I just have, on what is a very high-geared beast now, with a 24 on the crank as well. Madness, some might well say, and I'd accept the verdict - apart from the fact that it's what I personally want.
Is she OK off the mark? Yes, but there's no doubt it will reduce clutch plate life, especially hill starts. Is she comfortable on the road? Yes, she is. A very comfortable indicated 70mph, superb through 2nd and 3rd, and absolutely excellent at a steady 65 on the clock in top, with enough torque to tackle normal hills. Now, I don't live in a very hilly part of France, so this would be a bad plan for an Alpine dweller. Nor do I do many miles in town, where it would also be unwise probably.
In this config, she is about the same in rpm:roadspeed as my P11, also fitted with a 19 toother. Obviously the 750 has a few more horsepower and probably a better torque curve at low/mid rpm, but with 8.5:1 pistons and a near-new motor, the Model 31 is going to be OK, I'm quite confident now. Rear sets and flat bars help a bit, I suspect, as the profile of the thing plus me is a bit less than the proverbial lead brick.
I tried an extra tooth on my completely standard Model 20 a while back - and it was no good at all on hills and into the wind. Not helped by my being tall and heavy, but there just was not enough torque, and with a Burman 'box the big rev rise dropping into 3rd made progress painful in certain circs. On the bigger bikes, with AMC boxes, it's no hardship to drop into 3rd anyway, but there's enough torque not to have to too much.
I reckon most classic 650s can take an extra tooth on the gearbox to be honest, to help in modern road conditions. My standard 7.25:1 iron head A10 also runs one extra on the 'box, is really relaxed on the road and handles headwind and hills OK; and a big Enfield twin has the biggest sprocket I could get on there, and has an almost absurd amount of torque to handle it.
It's a question of what compromises you want to make I suppose. For me, relaxed open road cruising is important, owing to the uncluttered nature of French roads of all sorts. Acceleration off the mark is worse, obviously - but things pick up fast enough (for me anyway) once rolling. Another downside with all of this messing about is the impossibility, on dynamo bikes particularly, of balancing electrical loads at modest road speeds in high gears. Again, OK in rural parts, not OK in towns or for doing runs out at pootling speeds.
Just my tuppence worth on this long-running debate.
Cheers, Bill

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Re: Standard Gearing

Postby greasemonkey62 » Tue Dec 16, 2014 9:34 pm

I have had just the 19 tooth gearbox sprocket on my 31 now for a while and I find it is exactly what I wanted, 60mph on A roads is fine now and the old girl doesn't sound as if she is revving too much, the bottom end, crank etc as far as I know are original and so are 52 years old now and I think lower revs will keep it going longer, the clutch is new this year so it pulls away just fine even on Sheffield hills and I have no problem in town in fact trickling along was somewhere between first and second gear but now in heavy traffic first gear is fine at very slow speeds, its perfect for me now and suits me down to the ground, I may even put the 17 tooth sprocket on he buys.

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Re: Standard Gearing

Postby mumbray » Wed Aug 10, 2016 5:00 pm

hi,could somebody tel me what size the gearbox sprocket nut size is please?

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Re: Standard Gearing

Postby bjorn » Thu Aug 11, 2016 8:19 pm

From memory:

Spanner width 1 1/2" or close enough to 38mm.

regards
Bjørn

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Re: Standard Gearing

Postby mumbray » Fri Aug 12, 2016 4:42 pm

thanks,i am new to this site and i'm not good with computers,i want to fit a 17t gearbox sprocket to my g12,hope it can pull it,the bike seems very high reving.

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Re: Standard Gearing

Postby TrevorCSR » Fri Aug 12, 2016 6:41 pm

You need to fit a bigger gearbox sprocket to gear it up. Fit a 19 tooth, you won't regret it.

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Re: Standard Gearing

Postby mumbray » Thu Aug 25, 2016 3:31 pm

will this be ok on a g12 or would that be the csr model?

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Re: Standard Gearing

Postby TrevorCSR » Thu Aug 25, 2016 6:09 pm

Re-read Groily's comments, six posts before yours.
Fitting a 17T on the gearbox LOWERS the gearing. You need to fit a BIGGER gearbox sprocket. You're complaining that your bike revs high and you're not going very fast. Gear it up as high as you can, you'll be surprised how your motor will be able to cope.

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