Can I use R100 rear drive on my R90S?

[p2squared]

As my '74 R90S rear drive is seeping hypoid pretty badly, (fairly certain it needs a rebuild and seals, already tried cleaning out the small breather hole, to no avail) figured I'd see if the rear drive from my '83 R100 parts bike would work.

Fits onto the swing arm o.k. it seems. Axel seems same size as well. Haven't tried a wheel, so that may be the rub, so to speak, looks like more flange around the wheel splines. And the S's final drive ratio I guess is 34/11 and the R100's appears to be 32/11. So it would be a bit higher/lower, (can't recall which) final drive ratio, right?

Bottom line, will it work as a straight swap?

Thanks for any thoughts,

p2

[SteveD]

I've used a 78 wheel on a 81 bike. It does need some modification. An 81 wheel will fit a 78 without any mods.

I expect if the fd fits onto the shaft then a 74 drum rear wheel will fit the 83 ok.

[jimmyg]

The stock R90s rear end ratio is 33/11 and matches to the stock speedo. There will be a number at the bottom of the speedo face. (1.112 is stock for the R90s with the 33/11 rear end).

If you change the rear end ratio, without changing the speedo, it will be off. Some says it's always off anyway.

Years ago, I purchased a 32/10 rear end for my R90s, thinking it would be more fun, use for the track, etc. The speedo was at least 15mph high at highway speeds and it taught me that the stock R90s gearing is well thought out as I was always looking for another gear when the 32/10 was installed. FWIW.

jimmyg

[p2squared]

Just fine. True the brake shoes are slightly…different---in that they're narrower in width on the pivot side and adjuster side.

Was easy enough to machine the new shoes to ones I had to install on the old 90S to match the ones that came off R100, so have full contact on the wheel drum.

Only other thing---R100 rear is 32/11 final drive ratio. Seems slightly taller ratio in the little bit of around town riding I've done. Which is fine as I've upped the power over stock, and lightened weight considerably, (I know, blasphemous, but I've
cafe-ed my R90S a bit).

Long & short---was easily do-able, (for me).

Thanks for the input guys.

p2

[mattcfish]

I have a 32/11 on my R90. No mods necessary. Better MPG and the original speedo, which had been 10mph off with the stock gear was much closure (at freeway speed) with the taller gears. Only drawback is that in the twisties there are times when 3rd is too high and 2nd seems too low.

[disston]

I used a 2.91 final on my R90/6 for a year or more. The plain Jane /6 came with a 3.09. I really liked the higher gearing of the 2.91 on the highway and didn't miss the off the line acceleration any after I got used to it.

I have a speedo to match either final but with any combination of final and speedo I still don't know how fast I'm going. Maybe a small GPS speedo would help. Mounted out of the way it could be referred to only sometimes.