Another speedo drive question

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MikeG

MikeG

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How do you know if you have the correct ratio speedo drive? The one from my Commando is marked BG5330/164 Ratio 1.251. I think this is a repro but not sure, and my speedo has always read fast. Looking at Old Britts site I see several different ratios. My bike is a 71 Roadster with the stock 19 inch rear rim and speedo head.
 
The ratio is 15/12 which is usually marked on them. That would be 1.25 : 1
 
Basically, the speedo drives are the same for British bikes except the ratio and hole diameter. I regularly swap parts between them to make a good one - best not to count on the number on the ring. If your hole is a close fit on your axle and the ratio is 15/12 (1.25/1), you're good. The goofy part is that Triumphs with 18" rear wheels also use a 15/12 ratio but then the speedos are different.
 
If the speedo is reading inaccurate by a reasonably small amount, it can be adjusted using a small brass calibration screw sitting near the cable connection beneath the housing. I used a bicycle speedo with magnet and inductive pick up on the rear wheel to determine true speed and then adjusted smiths speedo to read right. Also verified with gps displayed on phone mounted to handle bar while riding along.
 
marshg246 said:
The goofy part is that Triumphs with 18" rear wheels also use a 15/12 ratio but then the speedos are different.
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"Different" in that Triumph Smiths speedos generally have a trip odometer and obviously don't have the NV blob/globe logo but those used with the 15/12 (1.25:1) drive ratio gearbox are still '1000' (cable turns per mile or '600' turns per km) instruments.
 
L.A.B. said:
"Different" in that Triumph Smiths speedos generally have a trip odometer and obviously don't have the NV blob/globe logo but those used with the 15/12 (1.25:1) drive ratio gearbox are still '1000' (cable turns per mile or '600' turns per km) instruments.
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Yep, which is why I said it is goofy. Each turn of an 18" wheel w/tire is covers about 3.14 x 22" whereas a 19" wheel w/tire covers about 3.14 x 23" (roughly, don't kill me). At a given rear wheel RPM, a 19" wheel goes about 4.5% further. Clearly either the Triumph or Norton (or both) reads wrong given compatible speedo innards.
 
marshg246 said:
Yep, which is why I said it is goofy. Each turn of an 18" wheel w/tire is covers about 3.14 x 22" whereas a 19" wheel w/tire covers about 3.14 x 23" (roughly, don't kill me). At a given rear wheel RPM, a 19" wheel goes about 4.5% further. Clearly either the Triumph or Norton (or both) reads wrong given compatible speedo innards.
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Speedos can read up to 10% fast but should not under-read so they normally over-read to a certain degree and I don't think odometers have to be particularly accurate.
It's possible speedos could have been calibrated to suit the wheel size but somehow I doubt that.

If making one speedo mechanism with either a 1000 (MPH) or 600 (Km) odometer and 1.25:1 drive gearbox gives readings within acceptable limits (0 to +10%) for both 18" and 19" wheels thus reducing production costs seems reasonably sensible.

Edit:
Triumph twins with 18" rear wheels often had wider, (therefore taller) 4.00 tyres.
http://www.classicbike.biz/Triumph/Brochures/Brochures.htm

The 18" Dunlop 4.00 K70 as fitted to many Triumph twins being a similar diameter to the 4.10 x 19" TT100 and the triples had 19" rear wheels.


https://www.longstonetyres.co.uk/tyres/400-18/400x18-dunlop-k70.html
"OVERALL DIAMETER INFLATED 671 mm / 26.4"

https://www.dunlopmotorcycletires.com/tire-line/k81/
4.10H19 TT Overall Diameter 26.46"
 
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