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Offline Innerzoomer

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Fitting a 15T sprocket and speedometer readings.
« on: June 05, 2021, 08:51:02 AM »
I've read through various threads on this topic. Due to the speedometer being driven from the gearbox output shaft, any increase in the mph/000 rpm will not affect the speedometer readings. The ratio of the speedo reading to the tachometer reading will be unaltered. It is understood that the speedometer overreads with the 14/46 final gearing and that changing the gearing to 15/46 will increase the road speed for a given rpm figure, and that the true road speed will be closer to the indicated road speed. I have done some calculations intended to show the true effects of fitting a 15T front sprocket with regard to the accuracy of the speedometer readings and the reduction in engine RPM at cruising speeds. I observed that my bike showed 58mph@7000rpm in 6th and 62mph @7500rpm. I do not know, of course, how accurate the tachometer actually is, but I calculated what the true road speed should be at 7000rpm in 6th. I measured the circumference of the new Pirelli 14/70-17 rear tyre, which was 77.25" (not 77.65" as suggested online for a tyre of this size). The number of rotations of the rear wheel in one minute is 7000 X 0.3088235 (primary reduction) X 1.2376237 (overdriven 6th) X 0.3043478 (final drive 14/46). Multiplied by 77.25 that gives the distance in inches travelled in one minute at 7000rpm. Multiply that by 60 to get the distance travelled in one hour and then divide by 36 and 1760 to convert to miles per hour (36" to the yard and 1760 yards to the mile). At 7000rpm the actual calculated road speed is 54.976032mph on my Pirelli. So, assuming the tachometer is accurate, the speedo is 3mph fast at 55mph true, showing 58mph. Changing to a 15T sprocket would result in a calculated true road speed of 58.6mph @7000rpm, so that the speedo would underread slightly, again assuming that the tachometer on my bike is accurate. At an indicated 60mph, true speed would be 60.61. At indic. 70mph, 70.72. At indic. 80mph, 80.82. The reduction in engine speed resulting from the 15T sprocket is from 7678rpm to 7167rpm at 60mph, 8958 to 8361 at 70mph, and 10237 to 9555 at 80mph. It is worth noting engine speed reduces by 899rpm when changing up from 4th to 5th at 7000rpm, by 778rpm when changing up from 5th to 6th at 7000rpm, and by 467rpm by changing from 14T to 15T at 7000rpm, so it is almost equivalent to what a 7th gear would give with the 14/46 final drive, as the higher the gear, the smaller the ratio gap from the previous gear. These calculations regarding engine speed reductions due to the 15T sprocket do not depend on the accuracy of my tachometer. In conclusion, the replacement of the 14T with the 15T does give a worthwhile reduction in engine speed when cruising and, assuming the accuracy of my tachometer, results in the speedometer underreading fractionally. The mileage recorder will presumably underread to the order 6.7%, if it was previously accurate. I understand that GPS observations suggest that the speedometer may, in practice, still overread fractionally. I have yet to put that to the test on my bike. I have yet to receive my 15T sprocket, for that matter!

Offline Robell

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Re: Fitting a 15T sprocket and speedometer readings.
« Reply #1 on: June 09, 2021, 06:45:41 PM »
Wow, lost me  :008:

But I can confirm the final sentence
Compared to GPS the speedo, with a 15T over reads very slightly, literally 1 or 2 mph
Certainly more accurate than with the 14T
" it's better to burn out, than to fade away "

Offline tookadum

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Re: Fitting a 15T sprocket and speedometer readings.
« Reply #2 on: June 09, 2021, 07:16:32 PM »

All makes perfect sense to me.   I think  :745:

Just know it was a smoother ride with a larger front sprocket.   :821:
I was born with nothing and I still have most of it left !