Spitfire Front Sprocket RPM

[Big Phil]

Hi All,
          I am new to this forum as a proud owner of a recently acquired fully restored 1967 BSA Spitfire MklV Special. I'm running-in the Spitfire in at present and all is going fine although I've only covered 70 miles so far. I'm staying local for now doing short 10 miles circuits and checking and resetting torque settings etc. as I go along. However one thing I have noted is she seems to be geared a little low and could easily pull along in top gear at lower revs. Currently I find I'm at 3,750 rpm at approximately 50mph in 4th gear. I know you can fit 21t front drive sprocket as an alternative to the standard 20t as originally fitted but don't know if this has already been carried out. So does 3,750rpm at 50mph equate to a 20t or 21t front sprocket or even perhaps a sprocket of less than 20t?

Thank you in advance for any and all replies.

Regards
Phil

[Gup holland]

Hello phil, got a lightning an use a 21t sprocket for many years now, did it to lower the revs. Engine copes with it, on the back 47 t.
When u use it, you must make the hole slightly bigger because it doesn't fit through the case, see photo.

G gup

[Big Phil]

Thanks Gup that's good to know your bike runs well on the 21T setup and that I will have to enlarge the hole slightly. From my best efforts determining the revs I should expect from a 21T front sprocket with the standard 47T rear I think the engine should run at 15mph per 1k rpm in top gear. Therefore at at 50 mph the revs should be 50/15 x 1000 = 3,333 rpm but with my set up it's running at 3,750rpm at 50 mph. Therefore it would appear my bike has a 20T or probably even 18T front fitted sprocket fitted. I will dismantle primary drive, clutch and rotor etc. and check it out over the winter.

Thanks again.
Kind regards
Phil

[mikegt4]

According to a period Cycle World road test my 1966 Spitfire Mk11 with the stock US spec gearing (21T/47T, 4.64 overall) runs 16.5 mph per 1000 rpm.

In the Cycle World road test of the 1967 Spitfire Mk111 (US spec) with 4.87 overall gearing (20T) runs 15.7 mph per 1000 rpm. Then again in their Lightning test they listed 4.87 overall gearing yielded 15.9/1000.

The 21T sprocket probably looses a bit of acceleration but the reduced rpm at cruising speed is always welcome.

IIRC, CW attached a attached a calibrated bicycle size wheel to the motorcycle to do it's speed/gearing tests so their info would be more accurate than the bike's speedometer.

[Big Phil]

Thanks Mike that makes me very confident the front sprocket on my bike must be 18T. As stated I will investigate over the winter.