Author Topic: BSA Ratios  (Read 7935 times)

Offline Beesaboy

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BSA Ratios
« on: December 30, 2006, 05:12:06 PM »
Standard ratios for a B31 (as stated in BSA Singles Restoration) are E17 C43 G19 & R42, for an overall ratio of 5.591. I am replacing the gearbox of my B31 based road race bike with a C11G box (the original is worn out and the clutch is very basic). The rear sprocket fitted to my bike has 43 teeth (not listed in Bacons book at all) but I guess it lowered the ratio slightly, as the bike was scrambled in the past. As a road racer,a higher speed (ratio) is needed. I need to change the gearbox sprocket anyway (C11G uses a different pitch chain) so can I go to a 20 tooth sprocket on the gearbox to counter the effect of the one extra at the rear? and then go to a 19 tooth at the engine,or would 19 increase the overall ratio too much. The math's involved is beyond me, my brain hurts  ???

Offline L.A.B.

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Re: BSA Ratios
« Reply #1 on: December 31, 2006, 12:30:55 AM »
Which clutch are you intending to use (as you said the original is worn)?
Do you intend to use the C11 clutch?
If so, how many teeth does the chainwheel have, or are you going to rebuild the B31 clutch? (I have the C11 engine sprocket size and gearbox overall ratio gearing data, but no clutch sprocket/gearbox sprocket/rearwheel data.

I take it the C11G gearbox in question is the four speed one, as many C11s were three speed?
« Last Edit: December 31, 2006, 12:31:28 AM by L.A.B. »
L.A.B.

Offline geoffus

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Re: BSA Ratios
« Reply #2 on: December 31, 2006, 01:48:47 PM »
 ;)   I'm not sure a C 11 G gearbox would be strong enough if you are considering a quick B31 as it was just a commuter 250 with swinging arm and the gearbox goes back to the late forties I'm sure there must be 350 500 and gearbox is around as for clutch you could look at triumph as a very often fitted BSA unless your gearbox is shattered I would be looking at spares i.e. new Bearings shafts etc.
happy New Year to all.
regards GEOFF

Offline Beesaboy

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Re: BSA Ratios
« Reply #3 on: January 01, 2007, 03:53:40 PM »
Hi L.A.B & Geoff, Thanks for your your response. Yes, I will be using the C11G clutch, the b31 clutch is a taper fit, where as the C11G is a spline. The C11G is a much better clutch all round, it has rubber cush shock absorbers, the b31 has none, the c11g has a good strong roller bearing, the b31 has a puny double row ball bearing that looks like it came out of a bicycle! The clutch (and gearbox) in my road going C11G is light and positive, just the opposite of my race bike. The chainwheel has 43 teeth, the same as the b31, only the gearbox sprkt is different, but I have to change that anyway. That would give sprkts of...E19? C43 G20 R43...for an overall ratio of ???
 The b31 gearbox may well have worn dogs etc, it works perfectly at normal road speeds, but throw's in the towel when faced with a few more horses and the need to change gear at pace. I think that this is a combination of a very basic clutch, a gearbox that is maybe getting worn out and a less direct system of gear selection, pawls etc. Most of the B31's in the clubman class that I race with have had similar problems, poor gear selection or missed gears ( in my case leading to bent valves through a huge over rev'!) and  the fix is either welding and recutting selector dog's and teeth and fitting a Triumph clutch (with an adapter for the different tapers) or as at least 4 other guy's that I know, have done, use the c11g. A better clutch, a more positive gear selection and they seem to stand up to the extra horses and punishment no problem. They also beat me across the finish line every time, dang it!!
Cheers and all the best for the new year

Offline L.A.B.

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Re: BSA Ratios
« Reply #4 on: January 01, 2007, 09:12:39 PM »
Quote
The chainwheel has 43 teeth, the same as the b31, only the gearbox sprkt is different, but I have to change that anyway. That would give sprkts of...E19? C43 G20 R43...for an overall ratio of

As you say the C11G chainwheel has the same number of teeth (43) then basically that will not change the primary gearing, but the gearing combination you suggest (19/43/20/43) I think will be far too high.

43 divided by 19 = 2.263

43 divided by 20 = 2.150

2.263 x 2.150 = 4.865

So that would result in a *top gear* ratio of 4.865 which is far higher than what you are using now (43/17[2.529] x *43*/19[2.263] = 5.723.

17/43/20/43 would give an overall top gear ratio of 5.437 so that could be a good starting point?  

      
L.A.B.

Offline VintageBike

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Re: BSA Ratios
« Reply #5 on: January 01, 2007, 09:48:51 PM »
wow!  I think I need some classes!  ??? ??? ???

Offline Beesaboy

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Re: BSA Ratios
« Reply #6 on: January 02, 2007, 06:43:01 AM »
Thanks L.A.B, I think you're right, better to start with caution and work up if necessary, cheers for that.
and I agree with the administrator, classic bikes and maths!!!