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#701232 - 07/09/17 6:03 pm BSA b31 engine sprocket  
Joined: May 2016
Posts: 16
stevenick Offline
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stevenick  Offline
BritBike Forum member

Joined: May 2016
Posts: 16
east yorkshire uk
Hi, I have a b31 motor fitted into an m20 frame gearbox. , The other year I bored the engine out to 400 cc and fitted touring goldstar cams with an 19 engine sprocket.
I have just got the primary cover off and while I am in the I was wondering if a 21 engine sprocket would be a good idea . I have one .
The bike runs fine but it's high in the rev,s when on the open road . 55/60 mph

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#701350 - 07/10/17 4:16 pm Re: BSA b31 engine sprocket [Re: stevenick]  
Joined: Aug 2001
Posts: 29
Josef2 Offline
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Josef2  Offline
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Joined: Aug 2001
Posts: 29
Germany
hi,

first you have to calculate the overall ratio.
Internal gear ratio for the top gear is always 1, as its a direct gear.

Engine/Clutch and Gear/Rear differs widely.

M20 had 19/43 and 18/42
or 18/43 and 19/42, depending on the year.
The overall ratio is the same = 43/19 * 42/18= 5,281

B31 had some 5,591, B33 had 5,003 and there are some hundred other ratios for B32/B34, sidecars, Gold Stars ..
So you have to calculate your current setting.


The you need the tyre diameter , an Avon 3,50-19 has 682 mm or 26,85 inch.
And the speed, for example 60 mph

The formula to calculate with mph is now
rpm= speed * ratio * 334,4 / diameter.

With ratio 5,281 its 60 * 5,281 * 334,4 / 26,85 = 3946 rpm - So your B31 runs with 3946 rpm at 60 mph in the top gear.

With ratio 5,0 its 60 * 5,0 * 334,4 / 26,85 = 3736 rpm

Josef

#701764 - 07/14/17 9:34 pm Re: BSA b31 engine sprocket [Re: Josef2]  
Joined: May 2016
Posts: 16
stevenick Offline
BritBike Forum member
stevenick  Offline
BritBike Forum member

Joined: May 2016
Posts: 16
east yorkshire uk
Thanks Josef, going to give it a try and see how it goes

#701838 - 07/15/17 3:22 pm Re: BSA b31 engine sprocket [Re: stevenick]  
Joined: Aug 2015
Posts: 932
Andy Higham Online content
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Andy Higham  Online Content
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Joined: Aug 2015
Posts: 932
Bolton Lancs UK
It's easy to get trapped into thinking higher gearing=more relaxed and better MPG.
Very often what it really means is at touring speeds you are constantly shifting between top and third, worse MPG because a wider throttle opening is required for a given speed and a slower top speed because the engine will not pull the gearing


1955 BSA B31 400cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350cc
1967 Greeves 360cc Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
Modified Nu-Trak GM500cc sprint/LSR bike "Deofol"
2003 Aprilia RSV Mille "Lo Stregone"

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