B Series rear sprocket.

[MGI]

I am contemplating (amongst many other things) changing the ratio of the final drive on my B33. This intention is somewhat hindered by the fact that the bike is a committed bitsa and I am experiencing difficulty deciding which sprocket is suitable. Unlike my 56 B31 which appears competely original, my B33 is the opposite. The existing sprocket has the brake drum operated by a rod that is on the left of the bike (Unlike the 350) and appears to have a series of nuts around the periphery of the drum in close proximity to the teeth of the sprocket. The diagrams in the 'Spares' books I have (B group 58-60 & 1954/7) only appear to have holes more or less in the middle of the drum and there are either 4 holes (58/60) or 6 (54/57) My frame is a 58 but the rear wheel is not full width as per the relevant info. The sprockets I have looked at appear to have the teeth integral with the drum, not bolted, they are described as being suitable for the swinging arm models but I have 2 swinging arm models and they are completely different to each other. The front wheel ass is actually Gold Star and has the 8" brake, could the rear wheels be similar? Helpappreciated...

[idie]

It sounds as if you have an A65 rear drum. These have a bolt on sprocket.

[MGI]

I will attempt to post a photograph!

[MGI]

Here is the sprocket detail that I promised that I would send.

[idie]

I would say that it is a A65 drum fitted with a A10 steel brake plate. Funny that as my Shooting Star has the proper drum fitted with a A65 alloy brake plate.

[MGI]

My problem would be that the sprocket retaining bolts are so close to the teeth that it appears impossible to fit a sprocket with less teeth as the existing bolts would obstruct them. Is there a practical way of reducing the total teeth count by two (say) Failing that, is it feasable to increase the teeth on the gearbox output sprocket?

[idie]

You can get a 20 tooth gearbox sprocket. I have one this size on my Shooting Star. This will help to make your speedo more accurate. When you lower the gearing via the rear sprocket or the gearbox one you cause the layshaft in the gearbox to turn faster. The layshaft drives the speedo, helps to stop you getting a speeding ticket though.

[MGI]

Are you referring to the gearbox sprocket driven by the engine sprocket, which would be good - or are you referring to the gearbox sprocket which drives the rear chain which would be bad!

[idie]

It would be easier to up the size of the engine sprocket. How many teeth on the rear wheel sprocket that you have. A normal sprocket is 42 teeth. The front should have 19 teeth there are plenty of sizes available to fit. I would try a 20 or 21. A larger engine sprocket would make kicking the bike over easier.

[MGI]

Unfortunately Idie, the size of the engine sprocket is a given by virtue that Phil Pearson only supplies one size of engine sprocket when he installs his starter conversion. My best bet is probably to remove the existing rear brake ass and replace it with the early QD unit. It may be a winter operation, also I would like to convert the front brake to a TLS version, another winter task, although in this case I have the axle and the lower fork shrouds required.

[idie]

He does three sizes of front sprocket. 21t, 22t, and 23t. On a B33 it does raise the gearing as it should have a 19t front sprocket. I have the same set up as you with a 21t front sprocket that is why when I fitted the kit I put on a 20 tooth gear box sprocket to help lower the gearing. But it seems that your gearing has been lowered by fitting a A65 rear sprocket. You are saying that you feel your B33 is still under geared. the easy answer is to change the rear drum to a standard one with 42 teeth, then you will have much higher gearing. This is ok if your B33 is tuned or has a Goldie engine fitted. If you want twin leading shoe brakes the easiest way is to change the fork legs to fit the later full width hub.

[MGI]

I appreciate your comments Idie, Phil gave me the impression that was no wiggle room on the size of the engine sprocket. Prior to the conversion the bike had a 1st gear that was a trifle tall but the remaining gears were sound. Now it takes off like a rocket (figuratively) but is revving hard at 60 to the extent that I feel uneasy going any faster although I have seen an indicated 70 on the clock a couple of times.I had some problems with the starter with the fact that I had a 4 cam cush drive originally which transpired in me having to buy a sleeve device at great expense, would this reduce the ratio options at the engine end? I like the sound of changing the drum and I think I will explore that avenue first - cheers!

[craigc11g]

you would need to come down 3 teeth on the rear to make a difference , gearbox is the way to go , if you go up on the crank your spining the main shaft faster which can lead to gear change issues , any size gearbox sprocket can be made .

[idie]

If as I suspect he has a A65 rear sprocket 45t, changing to a A10 or B33 drum would bring it down to 42t. This would save ripping down the clutch and the front sprocket to remove the rear chaincase to access the gearbox sprocket. Just depends on how much he loves fettling.