Author Topic: Trials specifications for C15  (Read 281 times)

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jonboy

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Trials specifications for C15
« on: 18 February, 2018, 18:48:50 »
I have good sound C15 G that I intend to put into a 70s trials rolling chassis... sorry purists, it probably will be a Beta , but I also have a a couple of 'Taco and Cota frames to play with...not intending to compete, just a green laner really but I was wondering whether to try and locate C15 Trials spec gear cluster, or should I just experiment with the final drive sprocket ratios..?. Also, did the C15 T have other  different specifications  to the roadster  G ...? comp ratio perhaps, whatever....

Any info, or point me in the direction  of relevant stuff most welcome... thanks J

AWJDThumper

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Re: Trials specifications for C15
« Reply #1 on: 19 February, 2018, 13:44:32 »
The attached table gives the overall gear ratios for a standard 1961 C15 (Col 1) and C15T (Col 2) with the gearbox (GS) and rear wheel sprocket (WS) sizes indicated. Column 3 shows the C15T gearbox ratio comparison with standard C15 if the 4th gear ratio is made the same and gives some idea of the differences in ratios provided by the C15T lower gears - obviously, the C15T gearbox offers much higher 1-3 gear ratios. In all cases, the engine sprocket and clutch sprocket are assumed to be same; namely, 23/52T.

For a trials bike, which is intended for both road and off road, it's difficult to get the balance right with the gear ratios. You need to work out what 4th gear ratio you can get away with on the road and then see whether the resulting 1st and 2nd gears are then low enough for off road. As a comparison, the B40WD has similar gear ratios to those shown in Column 3 and can do 50 mph in top but the lower ratios (especially 1st and 2nd gears) are next to useless on the road but not too bad off road. Changing gearbox sprocket would obviously change that balance between the gears.

In terms of the difference between the standard (1961) C15 and C15T, the latter used a different camshaft. It also used an oversized inlet valve. There was also a higher compression piston (10:1) available compared to the standard 8:1 piston.

jonboy

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Re: Trials specifications for C15
« Reply #2 on: 19 February, 2018, 21:43:11 »
Thank you AWJD  its set me off on some new routes....I had come across some of the info but wasn't sure how relevant the early 60s stuff was to a late 67 C15G as it had some later mods to  the bottom ends and gearing of the earlier bikes.    internal gearbox gears with parts numbers are listed but I  wasn't sure how compatible they were to the later models.. Thanks again and I'll keep you all posted on the progress. Maybe . JB

AWJDThumper

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Re: Trials specifications for C15
« Reply #3 on: 20 February, 2018, 06:57:01 »
As far as I can see, the gear ratios for the standard C15 didn't change much between 1961 and 1967 - it was identical up to 1966 and then changed marginally in 1967, at least according to Rupert ratio.

JulianS

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Re: Trials specifications for C15
« Reply #4 on: 20 February, 2018, 09:24:07 »
Some information on C15 trials spec from this 1965 season USA catalogue - smaller carb - same compression as standard C15 and different gears.

By 1966 the Trials model was no longer in the catalogues.

No parts book issued for C15G model, as far as I can see, just a parts bulletin supplement to earlier catalogue.