Model 18 Trials Sprockets / CP Gearbox

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Bob PA USA
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Model 18 Trials Sprockets / CP Gearbox

Post by Bob PA USA »

I have a 1948 Model 18 trials special with a CP gearbox with trials gears. It has a 16 tooth tranny sprocket and a very large 60 tooth rear, running 530 chain. I'd like to run smaller sprockets and a 520 chain, like many of the AJS trials bikes that I see in on-line photos. I'm new to AJS's and would appreciate some guidance on how to go about this. I haven't put many miles on it, but the overall gearing seems to be OK, so I would need a smaller transmission sprocket. Is there a supplier that sells what I am looking for? Thank you for any advice you can provide.

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dave16mct
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Re: Model 18 Trials Sprockets / CP Gearbox

Post by dave16mct »

I don't know anything about trials but a 530 chain is 5/8x3/8 and a 520 chain is (Norton size) 5/8x1/4. Could you not skim the sprocket down in the lathe to 1/4" thick? Is this to save weight? An alternative might be to ask one of the specialist sprocket companies to supply or alter your sprockets and supply the chain.
Dave
Bob PA USA
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Re: Model 18 Trials Sprockets / CP Gearbox

Post by Bob PA USA »

I believe you are correct, and have read about others doing just that. The core of my problem is needing a smaller transmission sprocket so I can reduce my rear sprocket diameter, which is the size of a small pizza!😄
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dave16mct
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Re: Model 18 Trials Sprockets / CP Gearbox

Post by dave16mct »

You don't say what size your engine sprocket is. It may be easier to find a smaller engine sprocket.
Dave.
Bob PA USA
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Re: Model 18 Trials Sprockets / CP Gearbox

Post by Bob PA USA »

The engine sprocket is 15 teeth, and the clutch 40, which I guess is a constant. That would be an easy fix, if smaller engine sprockets are available - beats taking my CP gearbox apart to change the transmission sprocket.

Thanks,
Bob
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Pharisee
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Re: Model 18 Trials Sprockets / CP Gearbox

Post by Pharisee »

A !5 tooth engine sprocket and a 40 tooth clutch drum, coupled with a 16 tooth gearbox sprocket and a 60 tooth rear wheel sprocket, will give you a 10:1 overall top gear ratio.... Which is very low compared with the 5:1: normal top gear ratio of the touring bikes.
Are you intending to compete in trials? If you want to use it sensibly on the road, you will need to up the gearing appreciably and that means getting the rear wheel sprocket down to something approaching the normal touring sprocket size of 42 teeth. Even so, keeping the other sprockets the same, that is still 7:1 and will make road riding a bit "busy" in the engine department.
My M18S is running a 21 tooth engine sprocket with the 40 tooth clutch drum and a 17 tooth gearbox sprocket with the 42 tooth rear wheel sprocket/brake drum. That's an overall top gear ratio of 4.7:1 which with my very slightly modified engine set up, works very well on the road.
John
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Bob PA USA
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Re: Model 18 Trials Sprockets / CP Gearbox

Post by Bob PA USA »

Thanks John. I'm using it strictly for trials, but the road gearing is a good reference. I would say that about half the top speed sounds right.
Bob
Bob PA USA
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Joined: Sun Jan 15, 2017 2:03 am
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Re: Model 18 Trials Sprockets / CP Gearbox

Post by Bob PA USA »

I ended up buying a 14 tooth 520 Talon sprocket on eBay, and a custom 52 tooth from Sprocket Specialists in the US, to fit my Bultaco hub. It looks like there's enough room over the nut for a 14 toother. Fingers crossed.
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