M-21 top speed incresed with larger drive sprocket?

[wzuccarello]

Hello all, I have a nice running 1954 M-2. It tops out at 48 mph. Has a 19 tooth drive sprocket on the tranny. Will a larger sprocket increase top speed or is the motor too underpowered to pull a taller ratio?

If a taller ratio helps, how many teeth over 19 would be advised.

Thanks, Wayne

[BoschZEV]

Hello all, I have a nice running 1954 M-2. It tops out at 48 mph. Has a 19 tooth drive sprocket on the tranny. Will a larger sprocket increase top speed or is the motor too underpowered to pull a taller ratio?

Although you wouldn't want to run it for extended periods any faster than 50 mph, an M21 in good condition will be able to reach at least 60 mph. If yours really won't go any faster the first place I would look would be the carburetor. If the choke isn't being fully withdrawn that alone could explain the issue you are having with it.

[wzuccarello]

Thanks BoschZEV for the reply. I may have a carb issue, but it might be too lean instead of too much choke.
At full choke and cold start, it starts easily and idles good, but it spits and sputters like too lean for the first mile or so until it warms up and then runs fine.


Timing checks good, just a little high at full advance. So I studied how much to back off to be close.

From my minimal research, I read a couple places that the M21 with it's longer stroke, couldn't attain the same top rpm as an M20, and the M-21 normal top speed was considerably lower for that reason.

So anyways, If I had a perfectly running M-21 no matter what it's top speed, would it go faster with a larger drive sprocket? If it will make a faster top speed then it will also make a faster cruise speed, which is what I am after.

[BoschZEV]

So anyways, If I had a perfectly running M-21 no matter what it's top speed, would it go faster with a larger drive sprocket? ...

OK, taking everything as stock and at face value, your M21 produces its max. 15 h.p. at 4000 rpm, has 20T engine sprocket, 43T clutch, 19T gearbox, 42T rear, and 3.50x19" rear tire. Note, though, that if it has sidecar gearing it will have a 13T engine sprocket, in which case that's the issue to address. Anyway, top speed will be when all 15 of those raging horses is balanced by wind resistance, i.e. when the engine is at 4000 rpm. So:

4000 rev./min. x 20 / 43 x 19 / 42 x 84" (the circumf. of the tire) x 1 ft./12" x 1 mile/5280 ft. x 60 min. /hr. = 66.9 mph.

The fact that a perfect running M21 might just reach 67 mph says that the current gearing is optimum. If you changed to a 20T gearbox sprocket the top speed seemingly would increase to 20/19 x 66.9 = 70.5 mph, but the wind resistance increases as the cube of the velocity so it would take 17% more h.p. (i.e. 17.6 hp rather than the claimed 15) to make use of that larger sprocket. So, the answer to your question is, no, it would not go faster with a larger gearbox sprocket. In fact, it would go slower. Because it would not have developed the full 15 h.p. at 66.9 mph because it would be at less than 4000 rpm at that speed due to the larger sprocket, and it would need the full 15 h.p. to get to that speed.

[wzuccarello]

Wow, Thanks for the excellent post! Love all the the tecnical info. This bike is an old restoration. I'll hafta check and see what engine sprocket it has. Saved me a lot of wasted time and trouble experimenting with changing sprockets.

[BoschZEV]

Glad to be able to help. I own an M21 myself, but its top speed is, and very likely always will be, 0 mph. I rebuilt the engine shortly after getting it as a rolling basket case nearly 40 years ago, but it just kept getting shoved lower and lower on my to-do list as other bikes came into my life.

[wzuccarello]

OK, my bike must have the small engine sprocket. I know the tranny sprocket is 19 teeth.

I just got back from a test run using a GPS and an induction powered electronic tach (tinytach) and the numbers are about 47 mph and 3900 rpm. Haven't done the math but I'm guessing a 16t engine sprocket. So, I may try a 19t so I can get a good cruise speed around 50-55 mph.

So, BZ, it would be a great favor to me if you would run those numbers for me to determine what sprocket is on the engine.
Thanks, Wayne

[BoschZEV]

run those numbers for me to determine what sprocket is on the engine.

Keep in mind the calculation depends on the circumference of the rear wheel, so if you have anything but a 3.50x19" on it the numbers will be off by a large amount. Even if you do have a 3.50x19" there are variations between manufacturers as well as due to wear so the numbers won't be precise. That said, assuming your GPS and tach are accurate, you have a 14.4T engine sprocket (hmm, with that extra 0.4T on your sprocket, no wonder it isn't working right...).

I don't know offhand if BSA made a 14T (or 15T) engine sprocket for one of their bikes from the 1950s, nearly all of which interchange with the one on your M21, but if yours actually has a 13T sidecar sprocket on it, for the numbers to work out, and if you have a fairly new 3.50x19" rear tire on it, either you actually were going 42.4 mph when you measured the rpm, or you were going 47 mph and your tach should have read 4300 rpm. Personally, I would suspect errors in the rear tire circumference and tach before I would suspect a problem with the speed from a GPS unit.

[wzuccarello]

OK, I did some tinkering and some more test runs.
I took off the air cleaner and found that the slide was only opening half way! The throttle spring was too stiff and too long, obviously not a stock one.

Fixed that with another spring I borrowed from an old .32 pistol LOL and moved needle clip from the first to the 2nd groove from the top did a test run. Boy, it runs WAY better now!
Tucked in and got it up to 58 mph in the limited strait run space I had. Didn't seem like it would go too much faster though.
Don't trust the tach though, at 58 mph it was still reading around 3900. Guess I'll have to pull the primary if I want to know the tooth count.

This bike was imported here from Egypt about 10 years ago, might have had a sidecar at one time. It had a flat steel bracket secured to one fork leg with the fender bolts, maybe for a splash guard?
Also has a clevis head bolt thru the frame below and forward of the crankcase.

P.S. It has a 19t on the tranny

[BoschZEV]

P.S. It has a 19t on the tranny

Yes, you wrote that in your first post, but how many teeth are there on the rear sprocket? And does it have a 3.50x19" rear wheel?

A bike like yours builds speed v e r y s l o w l y so the fact it got to 58 on a "limited straight run" almost certainly means there are more mph on tap even though it might have felt like it didn't have much more left to give.

[wzuccarello]

Didn't check the rear sprocket. I assumed they were all the same from the literature and forum posts I have studied. It does have 3.50 x 19 tires though.

[BoschZEV]

Didn't check the rear sprocket. I assumed they were all the same from the literature and forum posts I have studied...

When trying to sort out problems you just can't make such assumptions. Further, it spent time in the Egyptian army, and I doubt you ran across much information on how those machines were equipped. Is the apparent lack of top speed because of carburetion issues, too short of a straight to reach actual top speed, or a rear sprocket a few teeth larger than standard? Three potential causes, with three very different solutions. That's why I've been pressing you to find all the relevant information needed to rule out as many causes as possible.

[wzuccarello]

OK, just checked, 41 teeth om rear wheel sprocket. I'm pretty sure the limited carb slide travel and needle in the top notch was causing the performance issues. Difference in performance now is very dramatic.
I'm pretty sure it'll make 60 or better on a longer stretch of road. Was very windy and gusty here today, that could have influence on performance today also.
Thanks for perseverance in keeping me studying all the possible influences to it's performance.

[BoschZEV]

OK, just checked, 41 teeth om rear wheel sprocket...

That's close enough to the 42T from the manual that the difference is probably due to a miscount, which is easy to do. Glad to hear your M21 is now producing every bit of h.p. it was designed to...

[AFJ]

OK, my bike must have the small engine sprocket. I know the tranny sprocket is 19 teeth.

I just got back from a test run using a GPS and an induction powered electronic tach (tinytach) and the numbers are about 47 mph and 3900 rpm. Haven't done the math but I'm guessing a 16t engine sprocket. So, I may try a 19t so I can get a good cruise speed around 50-55 mph.

So, BZ, it would be a great favor to me if you would run those numbers for me to determine what sprocket is on the engine.
Thanks, Wayne

From a road test of a new 1939 BSA M21 (MotorCycling June 7, 1939) in solo form it did 69 mph in 4th gear, 53mph in 3rd and 34 mph in 2nd. Gearing was 4.8 in 4th, 6.3 in 3rd and 9.9 in 2nd. Flying 1/4 mile speed was 67.14 mph, speed at the end of a 1/4 mile from a standing start was 62 mph. Time from a standing start to 69 mph was 39.2 seconds. From the graph given it looks like a zero to 60 mph time is about 18.5 seconds. Braking from 30 mph to 0mph on a tarred road took 31.5 ft.
Gearing of 20 tooth engine sprocket, 43 clutch sprocket, 19 gearbox output sprocket and 42 tooth rear wheel sprocket gives the 4.8 overall top gearing so that is what you should have for solo use. For sidecar use the engine sprocket would probably be around a 17 tooth, giving a 5.6 overall top gear. This would be about 15% shorter and reduce the top speed from 69 mph to 69 x 0.85 = 59 mph. I believe that is about what you got when you sorted out the carb and timing. So I would suspect the engine sprocket to be a sidecar one rather than a solo one.

AFJ