Determine final drive gear ratio without disassembly

[huntmiller]

As Ron Slabon says in his how to restore your BMW motorcycle book. You really don't know what gear ratio you have until you "unbutton the case and count teeth". Short of taking the final drive apart has anyone had success turning the pinion shaft and counting the revolutions of the ring gear spline that engages the wheel?

[Beemer100]

Huntmiller,

The counting is easier for the R51 and R67 due to the open shaft drive. For the swing arm models it’s a bit more trickier. Check out the rubber collar at the gearbox output shaft. You can feel the coupling flange and more importantly, you can feel the boltheads of the 4 bolts which connect the shaft drive to the flange of the output shaft.
Make a mark on the tyre and turn the tyre (bike is on the main stand) one complete turn. While doing this, keep your finger on the flange checking the turning of the coupling by counting the amount of time you feel a bolt head. Obviously 4 bolt heads are one turn of the flange.
It’s a bit tricky, but it works. If you end up with a number of bolt heads which is to vage to make the maths, do another full turn of the wheel (means, counting 2 wheel turns against the amount of bolt heads you felt).

Works for me
Klaus

[Daves79x]

I assume this is done when you don't trust the ratio stamped on the final drive case?

Dave

[Flx48]

Agree with Klaus; many of us have checked out our rear ratios just as he describes, or very similarly.

I always wanted to know what ratio I had with most /2s passing through my barn, particularly after finding one that felt somehow different from expectations when riding.

My method was to find a single bolt head feeling through the rubber boot, rotate it to sit it at the 12 o'clock position, (top) chalk the tire at 6 o'clock, (at the bottom) and keep my finger on that same bolt head (the tricky part) while rotating the wheel around for a single tire revolution.
As the wheel gets nearer returning to the 6 o'clock position, noting where on the clock dial the bolt head was as the wheel finishes back at 6 o'clock.
So, the "standard" 25/8 gears (3.13) would have the bolthead at about 1:30 on the clock dial.
And 27/7 gears (3.86) would be about 10 o'clock, 26.6 about 4 o'clock, and so on.

I usually ran through the procedure several times to double/triple check that my finger was keeping good track of the same bolt head.

You don't know if someone has changed the gears or not till you check; and who knows, you might find the elusive 25/7 gearset!
I didn't have stamps, so never hammered the new ratio on the housing after swapping gears, a mystery for the next guy...
Best-
George

[huntmiller]

Thanks to all for advice! I have the ratio that is stamped on the final drive. 32/11 which could be found on 55-60ish R69 bikes. Its quite tall. A mid 60s R69s comes with 25/8 which = 3.125, while this drive is 32/11 or 2.91