BSA Gearbox ratios - BSA M33/B33 etc

[Barrieh]

Hi,

I have a 1955 BSA M33 Plunger. I am trying to workout how to get the most extra MPH out of the old girl. Its got a standard 4 speed gearbox that I rebuilt but didnt think about the internal ratios. Things can improve the speed include.

1. Larger engine sprocket
2. Smaller Gearbox Sprocket
3. Gearbox ratios.

Attached to this message is the 1955 BSA Sales Brochure with the gearbox ratios for both BSA B and BSA M, with and without side cars. Can someone explain how the gearbox ratios relate to the internal gears, number of teeth, and layshaft number of teeth etc. People advertise the 4 speed gearboxes as Goldstar's but may not have the right ratio's

Ideally someone out there will have a standard 4 speed gearbox with higher ratios that the one I have at the moment and might sell me them?

Thanks

[JulianS]

A larger engine sprocket raises the overall gear ratio. The standard size solo sprockets are usually well chosen for best overall performance. Increase it too much and it will reduce acceleration and flexibility and the bike might be faster in 3rd than in top.

A smaller gearbox sprocket will  lower overall gear ratio and may improve acceleration.

Dont confuse the plunger and swinging arm gearboxes. They do not interchange and neither do the individual parts.

[idie]

You want to go faster? Tune the engine. Higher compression piston change the cams for a set of Gold Star ones this will give you some increase in speed. Otherwise you will just end up with a top gear to high for anything other then down hill.

[Rog1]

What speed are you getting out of your M33? I have a M33 plunger that's about a year older then your one. It cruises quite happily at 55mph and I'll blatt it up to 65 now and then. I think it'll go faster, but that's as far as I'll take it. The M33's top gear is already slightly higher than the B33's, I think by one tooth on the engine sprocket. Raising it any more might make the bike over geared.

[MGI]

The gearbox ratio that would affect maximum speed is the top gear ratio and that is always direct and cannot be changed. The easiest change would be the rear sprocket, or perhaps the engine sprocket, the gearbox sprocket involves the removal of the gearbox. Perhaps you might purchase one of the readily (Cheap approx £12) available electronic rev counters which operate off a wire wrapped around the plug lead, you might then gas it in 3rd and observe the revs & then gas it in top & do likewise... if the revs in top are lower than in 3rd it would be pointless changing to a smaller rear sprocket as the engine cannot pull enough revs as it is without over gearing it!

[Greenfield]

Our BSAs of the 50's were designed for the road conditions at the time. There were no motorways and few dual carriageways and they were pretty well sorted regarding engine power and gearbox ratios. Most vehicles cruised at 45 to 50mph.
I can remember fitting overdrives to my 1300cc Triumph spitfire and 2 1/4l Landrover, and they were transformed. Not they they probably went any quicker, I've never been interested in testing the limit, but with what amounted to a 5th gear, they were much quieter when cruising with more mpg, and more suited to modern roads.
Recently I fitted an engine sprocket +1T to my A50. This gives approximately 5% less revs at the same road speed. Its a small difference but it must be a little quieter and a touch more mpg than before, and I detect no difference in acceleration. My B33 has an engine sprocket +2T, giving approximately 10% less revs at the same road speed. It now has the same overall ratio as a B34 GS but with significantly less power. Going down hill with a following wind, yes it will probably go faster.
The faster we go the more wind resistance there is and thus the more power is needed to overcome it. So, if you need to go faster there's no alternative but to tune the engine and then you can start lifting the gearing.

[Barrieh]

Thanks for all the feedback.

I spent the weekend testing out the bike to see whats what. I would add that the 1955 Advert for the BSA M33 does states a top speed of 70mph.

I got the bike up to 58mph and it seemed pretty stable but I think it was a bit lumpy... i note that when slowing down it started to pop meaning that too much fuel is ending up in the exhaust. This could be a timing issue but I did also notice a small amount of soot in the exhaust pipe.

I know this will annoy some of you but I am running a premier carb 928 but I have to say that it does do a good job. I suspect that if I drop a main jet size - 3/4 throttle open, it might run less lumpy. I have a idle jet running a treat but I will need to play with the needle and main jet a bit more... perhaps try and run it a little leaner.

I have taken on board the comments on the gearbox and will focus on the engine tuning.

Hamish

[Jim S]

If you feel the engine is reving too high, compare your sprocket tooth count vs the manual. On my 1949 B33 I had a 48 tooth rear sprocket which I changed for a 44 tooth. This resulted in a 9% reduction in rpm. Now the bike cruises comfortably at 50mph and as an added bonus, the speedometer gives the correct reading.