christurner Posted June 30, 2011 Report Share Posted June 30, 2011 Hello readers. I am building a pre-65 James trials bike, converting a 1955 Captain to Commando spec. I need some information about the optimum size for the rear sprocket as I don't want to buy more than one if I can help it. At the moment I have the standard clutch and engine sprocket with a 17 tooth 420 size gearbox sprocket. The rear wheel sprocket is 46 tooth 420 which gives road ratio. Clearly this is too high for trials use so does anyone have recommendations? I would prefer to change just the rear sprocket if possible but all offers of advice will be appreciated. Post here or feel free to use my email address christopher.turner at tiscali.co.uk replacing the at with @. Many thanks. Quote Link to comment Share on other sites More sharing options...
stickinthemud Posted July 3, 2011 Report Share Posted July 3, 2011 Hi Chris - Which engine are you using? Is it 6/8E or 9E/32A? Quote Link to comment Share on other sites More sharing options...
christurner Posted July 4, 2011 Author Report Share Posted July 4, 2011 Hello Sticky. Thanks for the response and apologies for delay - away for the weekend. It is an 8E engine and the hubs are full width ones by the British Hub Company. I was interested to see that you had a Tandon at one time. I too had one back in 1968 and searched in vain recently for another, purely for nostalgic reasons. That is how I discovered the James which was beyond realistic restoration and hence will be converted to trials use. Chris Quote Link to comment Share on other sites More sharing options...
stickinthemud Posted July 6, 2011 Report Share Posted July 6, 2011 Hi Chris - still got the Tandon and it's still undergoing constant improvement and is getting better all the time. Much better now than it was in the Pre65 Scottish this year. Very interested to hear from other owners or the whereabouts of any more Tandon 'Kangaroos'! The only other Mk2 (conventional sprung rear) that I know of is in California! For your James, you should be aiming at between 35 and 38 to 1 overall ratios for bottom gear. So the next question is which gearbox are you using, 3 or 4 speed and are the ratios wide or standard or a mixture. If you are using the 23/51 teeth primary, I would recommend you get hold of an 18t engine sprocket from a 12D 125 engine to keep the rear wheel sprocket size down and the primary chain tensioned perfectly. You will have to put a step in the woodruff key as the 125 engine sprocket has a narrower key. A 20t from the 9E will also fit but the gearbox will need packing out to sort out the tension. 13t x 428 gearbox sprockets are available which will mean a rear wheel sprocket of somewhere between 54 and 60 t depending on the gearbox. So many questions. The various answers lie by putting the ratios on a spreadsheet if you can. I have one that covers all the above options if you fill in the missing blanks! Quote Link to comment Share on other sites More sharing options...
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