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beardy2000

BSA B40 GB trials gearing info

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Hi All,

I'm building a B40 trials bike and wonder if you could help with primary and secondary sprocket selections?

i don't have a crank primary sprocket or gearbox output sprocket yet or a rear wheel sprocket.

any help would be much appreciated and save me some money and a lot of time getting it right!

 

cheers

Michael

 

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22 minutes ago, beardy2000 said:

Thank you for that, if I use 18t and 52t for primary what would you recommend for the chain sprockets.

cheers

Michael

Will check and post tomorrow,if nobody else chimes in. (read notes about modifying seal and housing when using 18t  crankshaft sprocket)

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@beardy2000 15 : 62 on 428 chain is 4.13. A decent 428 chain is fine for weight / power of bike, and is narrower than 520. Check chain / tyre clearance before going 520.

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I think you also need to take into account of what internal gearbox ratio's you are using.My C15 needed a different rear sprocket after fitting the B40 GB cluster in it.I'm away on holiday till next week,so can't tell you what I'm using.The only thing I would say is that although its a wide ratio GB cluster,first is fine,just where I need it,so is second. Bit of a gap to third,but top is bloody useless on the road.Much over 24mph is horrible, so I wouldn't want to use it for a road trial. For that I'm going to build another engine based on an early distributor type engine with the original trials ratios, giving the biggest jump from third to top you can get - Info from the main man Rupert Ratio.

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Many thanks for the info, I'm slowly building the engine and it has a GB gear cluster in it so is wide ratio, I would mainly use the bike for pre 65 trials, it would be nice to do the odd classic road trial but will be mainly trials sections. I'm waiting for the frame at the minute so I will have to wait for chain clearance measurements. I have cub hubs front and rear and have just got them back from the powder-coating after making the rear hub 35mm wider. I will get the 18T primary sprocket and seal, it would be nice to fit the drive sprocket and build up the clutch basket etc.

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Ok, home again and just getting my C15 ready to ride tomorrow. Its running a 520 chain with 13 teeth on the gearbox and 48 on the rear wheel. WD B40 cluster and standard C15 engine sprocket and clutch basket. That combo works fine for closed events, but would be cruel for any distance on the road.

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Hi all,  just jumping onto this topic as it's kind of related to the problem I have with a 1962 BSA C15 that I recently acquired.

A previous owner fitted PVL ignition (Rex Caunt) and changed the engine sprocket from the standard 23T to 18T.

She starts very easily and pulls well in low gears, but my problem happens when in 4th as she revs very highly but never achieves more than around 30mph and this is the main subject of this post.  Sounds similar to problem that @jon v8

refers to above?

I was thinking that to address the high revs/low speed issue in top gear, but retain the low speed performance in low gears, I should leave the engine sprocket at 18T, and fit the C15 Trials 4th gear, wide ratio pinions on the lay-shaft and main-shaft.  Although there would be a big jump between 3rd and 4th, at least I’d be able to cruise on the open road at a more respectable speed without it revving so highly.

Is my thinking sound?  Has anyone else had the same issue and resolved it in a different way?

On a related point, when I stripped down the gearbox, I discovered that the 1st to 3rd gear sprockets are from a standard road gearbox, but the 4th gear uses the cogs from the Scrambler gearbox (main-shaft: 26T / lay-shaft: 17T) .  Not sure if this could be a contributing factor to my top speed issue?

If the approach I outline above would work, is it just a matter of swapping the 4th gear pinions on the layshaft & mainshaft over?  Or will I need to change anything else - eg the gear selector?

For reference, I have listed all the sprocket sizes on my bike in the table (not sure how to embed it into this text - so added as an attachment). 

For others that might be interested in the future, I've also listed the standard sprocket sizes for C15 variants - road, scramber, trials.

Thanks all - hope I've explained this simply and it all makes sense - and thanks in advance for anyone that help me out.

Cheers!

BSA C15 Sprocket Sizes.JPG

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Bit confused about what you are saying about 4th gear.  Surely it is 1:1, basically straight in, through the dogs, and straight out.

Only 1st 2nd and 3rd go via the lay-shaft.     ( Also the total of the number of meshed teeth always = 50t.)

Edited by scifi

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Hi scifi, thanks for the feedback - however, I'm confused too - I took this information from page 4 of the BSA bulletin that can be found in this link:

BSA C15 Gear Ratio Bulletin 1963

I've added a snapshot of the section of the document below - hopefully that makes more sense?

Thanks

 

667007412_ExctractfromBSAC15GearboxServiceSheet.PNG.57e2de996b0ae5f76203a9abfe96acd6.PNG

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I think the confusion is that the top gear ratio as said locks the clutch to the front sprocket - 1-1.  In the charts the top gear ratio has to be used to calculate 1st,2nd,and 3rd overall ratios as they all drive through the layshaft back up to the sleeve gear. So what we need is the smallest teeth count on the layshaft going to the largest tooth count on the sleeve gear.That way the gap separating the intermediate gears from top gear is biggest. The early distributor type engines  have a better choice,as the trials sleeve has more teeth than the WD B40 type. I always ask Rupert Ratio for the info,he is a walking BSA info system !

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Ok, thanks for the teeth numbers, it looks as if the scrambles A to B uses a different tooth profile as they don't add up to 50.

So if you want to know your overall ratios, you need to multiply / divide four cog teeth.

1st   H/G x A/B        3.17  (trials.)

2nd  D/C x A/B        2.44

3rd   F/E x A/B         1.63

4th is 1:1 as C Dog Locks to A

Looking at the BSA publication, it suggests different selector forks and spring for the different ratios.  When I last swopped cogs (1970s) I don't think I changed any of that.

Edited by scifi

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Thank you all for the info posted.

The engine is coming together slowly, abit too slowly really but things get in the way!

 

cheers

 

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Thanks Scifi and Jonv8 for your comments back on 23/24 May and please excuse this tardy reply.  Finally I know how to work out the ratios - so thanks for the clear explanation.  

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