BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: AWJDThumper on 30 January, 2017, 08:51:47

Title: B40 WD gear ratio
Post by: AWJDThumper on 30 January, 2017, 08:51:47
I've assumed I have to re-start the previous topic from the old forum to continue with the discussion?

I had a quick look at one of my B40WD's and, surprisingly, it is fitted with a 19T gearbox sprocket rather than the standard 17T. I am still restoring this bike and have not yet had a chance to ride it but assume the previous owner was happy with the much higher gear ratio. My other B40WD has standard gearing which is much too low for my liking and I will almost certainly change to an 18T gearbox sprocket to improve things slightly.
Title: Re: B40 WD gear ratio
Post by: STAR TWIN on 30 January, 2017, 10:12:44
My WD B40 has a Victor engine sprocket rather than the standard one. Makes the bike much more usable. Sorry, can't remember the number of teeth.
Title: Re: B40 WD gear ratio
Post by: JulianS on 30 January, 2017, 10:47:25
The WD engine has a wide ratio gear cluster to deal with off road use as standard and what ever change is made to the sprockets first will still be very low and gaps between the gears will still be wide. According to my calculations a 19 tooth gearbox sprocket will give a 6.18;1 top and first will be 18.2:1 which is still very low for road use but perhaps not for off road.

Title: Re: B40 WD gear ratio
Post by: MASTERS on 30 January, 2017, 11:08:25
Thanks all for taking the time to count sprockets and post the info found on this. Today I will count teeth on my own 78 MARK 1. Even though I live in an extreme mountainous region I don't need to do off road hill climbs in 1st. I would still appreciate any more info on this. kind regards Michael. :)
Title: Re: B40 WD gear ratio
Post by: AWJDThumper on 30 January, 2017, 11:31:18
The WD engine has a wide ratio gear cluster to deal with off road use as standard and what ever change is made to the sprockets first will still be very low and gaps between the gears will still be wide. According to my calculations a 19 tooth gearbox sprocket will give a 6.18;1 top and first will be 18.2:1 which is still very low for road use but perhaps not for off road.
I believe top gear will be 5.83 (6.53 x 17 / 19) with a 19T gearbox sprocket. Using a 20T sprocket would make the B40WD top gear similar to that of a standard B40. However, you'd need a 23T sprocket to give the B40WD the same 1st gear as a B40.

Basically, the B40WD is not that good on the road - unfortunately, it's also not very good off road either!!
Title: Re: B40 WD gear ratio
Post by: JulianS on 30 January, 2017, 11:58:45
Calculations go like this for overall ratio -


Clutch sprocket/engine sprocket x rear wheel sprocket/gearbox sprocket.

With standard sprockets of 52 each for clutch and rear wheel and standard sprockets of 23 for engine and 18 for gearbox then the result is 6.53 with 18 (not 17) tooth ;

52/23 x 52/18=6.53

For 19 tooth gearbox;

52/23 x 52/19 = 6.18
Title: Re: B40 WD gear ratio
Post by: AWJDThumper on 30 January, 2017, 12:58:44
Julian.

You're quite right about the values. Rupert Ratio gives 17T/49T for the B40WD which is clearly wrong. I have just checked the B40WD parts book and, as you indicate, this gives a 18T/52T combination. That's certainly the last time I rely on Rupert Ratio!!
Title: Re: B40 WD gear ratio
Post by: JulianS on 30 January, 2017, 20:36:47
The standard Victor sprocket is 28 teeth which would raise gearing to 5.36:1 in top with 18 tooth gearbox sprocket.
Title: Re: B40 WD gear ratio
Post by: STAR TWIN on 31 January, 2017, 06:47:58
Thanks Julian. Of course you need to replace the primary chain when you fit the 28T engine sprocket. It's the best modification you can make to a B40. And you can still ride long distance trials!
Title: Re: B40 WD gear ratio
Post by: MASTERS on 16 May, 2017, 01:21:02
Arthur, this is really helpful. I have decided to install a 19" rear wheel which will give some 9% gain. I will also look into the 18T sprocket if I can find one for my WD40. My engine as is seams to enjoy lugging under load. Mike
Title: Re: B40 WD gear ratio
Post by: STAR TWIN on 16 May, 2017, 06:27:24
With a 19" rear wheel, it might become a bit wobbly on its centre stand. Worth a check.
Title: Re: B40 WD gear ratio
Post by: Pauldemob on 16 May, 2017, 22:24:52
hi
if you want to improve  gear box on a WD b40 fit one from a road bike.
I fitted a 19 tooth gearbox sprocket and a box out of a 1968 250 cc starfire, direct swap. bike pulls a treat, still flat out at about 60.
Keep the back sprocket as standard
Sell your old box to pre 65 guys and enjoy the ride
Paul
Title: Re: B40 WD gear ratio
Post by: STAR TWIN on 17 May, 2017, 06:25:18
Cheapest option is still to fit a Victor engine sprocket and primary chain. Works wonders.