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Thread: M-21 top speed incresed with larger drive sprocket?

  1. #11

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    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.

  2. #12
    Join Date
    Jul 2012
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    361

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    Quote Originally Posted by wzuccarello View Post
    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.

  3. #13

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    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.

  4. #14
    Join Date
    Jul 2012
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    361

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    Quote Originally Posted by wzuccarello View Post
    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...

  5. #15
    Join Date
    Jul 2002
    Posts
    444

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    Quote Originally Posted by wzuccarello View Post
    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

  6. #16

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    [QUOTE So I would suspect the engine sprocket to be a sidecar one rather than a solo one.

    Hey thanks for the data AFJ. Also I think you're right about the gearing. There is some hardware on this bike that makes me think that it originaly had a sidecar.
    Thanks,
    Wayne

  7. #17

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    I have complied some interesting M20 mods on my page here: http://victorylibrary.com/brit/enigmas3.htm

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