Speedometer accuracy and final drive

[smellybeard]

I used a GPS speedometer/odometer on a recent run and got very different readings from my 1969 R60/2's own instrument.
The bike is recording a significantly greater speed and distance covered than the GPS. It's out by enough for me to question which final drive is fitted.

Speedometers on most vehicles over read but is the odometer on the R60/2 normally accurate.

I'm running a 3,50 by 18" ME22 on the back.

[Kurt in S.A.]

What final drive do you have fitted...what are the numbers stamped into the top of the drive? Also, what is the small number on the speedometer face? It must be a proper match to the final drive ratio.

Kurt

[smellybeard]

I never knew there was a number on the outside of the final drive.

"27/8", it says. The speedometer number is "0,75".

[Kurt in S.A.]

Does the speedometer measure KPH? If so, then the 0.75 number is correct. However, the compatible rear drive should be 25/8. I think the stock drive is indeed 27/8 but is usually with an MPH speedo with the small number like 1.2. Is it possible that either the drive or the speedometer has been changed?

Kurt

[smellybeard]

I've been looking up the numbers since. Much becomes clear and some becomes muddy.

I bought the bike 25 years ago when I lived in the Netherlands. According to it's papers, it was dutch from new. It's a very late bike by its numbers - amongst the last few made. Being dutch and the Netherlands being flatter than a flat earth, I can't see why anyone would have fitted 27/8 by choice. It's a great bike to ride on hilly, twisty lanes and will do 15km/h to 60km/h with ease in third. When I'm going places, I'm frequently struggling trying to get fifth gear. I like it as it is even if it is slow on a motorway.

Maybe it's what they had to hand when they were clearing up bits on that particular Tuesday in July 1969. Maybe they were watching the men on the moon. Anyway, it's what I got. Frankenbeemer.

https://daviddreesen.blogspot.com/p/bmw ... -r69s.html lists a W=0,77 as being right for a 27/8. I don't know. I'm not sure I trust a number printed on a dial face. Bits get swapped. The speedometer error is about +10km/h at 50km/h. I'll have to do another run to test the odometer but I'm pretty sure it's out by far more than 0,77:0,75.

[smellybeard]

Sorry - I forgot to mention the actual numbers.
27/8 is 3.375:1.
25/8 (solo R60) is 3.13:1.
27/7 (s/c R60) is 3.86:1.
25/7 (R60US) is 3.57.

27/8 (what I have) seems to be standard a late type R69S.

[Kurt in S.A.]

I'm not sure that it works that way just dividing the ratios...never really thought about that. Another thing is that you're running 3.5x18 tires. My R69S also with a 27/8 final drive is using a 4.00x18 tire in the back and has a 1.2 ratio...speeds seem to be fine. There was a service bulletin suggesting that the 4.00x18 tire with the 27/8 drive. In a different bulletin, BMW suggested using this combination for better coordination with engine power and road resistances. At the end of this new bulletin, it suggests that the 0.75 should be replaced with the 0.77.

https://beemergarage.com/documents/243.pdf
https://beemergarage.com/documents/267.pdf

But you're running a 3.5x18 and a 0.75, so maybe you should be OK?? Apparently not...kind of confusing.

Of course, there's a transmission in there as well, though that might not affect things in higher gears. Hopefully you have a solo transmission and not one for a sidecar.

Kurt

[smellybeard]

0,77:0,75 is 1.02666 and that's fleas in a gnat's knickers. Exactly the kind of change you want for a four inch tire.
+10km/h at 50 is 20%.

[smellybeard]

Thanks Kurt - 267.pdf provides the answer. It might also be the answer as to why I spin the back wheel in the wet and eat three and a half inch rear tires at the rate I do.