Final Drive Unit

[EMSimon]

Why is it so difficult to find a /6 or /7 final drive for a drum brake with a 32:11 ratio?
Was this a rare combination? It is on my 1977 R100S.
I would like to find a good condition replacement, ready to install, while I take mine out and rebuild.

[lmo1131]

If you intend to have your's rebuild pick up any final drive, a 32/10 even. Use it, and resell it when you get your OEM back; it shouldn't take more than a week or two to turn around your FD. There are three of four 32/10s on eBay today.

The FD must be the easiest R&R on our old bikes.

[pyropete@sympatico.ca]

There are at least 2 for sale on ibmwr.com marketplace. 1 says for drum , other does not state.

[lkchris]

32/11 is 2.91 and this only ever was fitted in conjunction with the "big valve" heads R100 motors.

BMW is very particular which motors get which rear drive ratio.

You'll only ever find a 2.91 on a '77-'84 R100S, R100RS, or R100RT. All of those are rear disc brake models, save for your '77 S. There never was 2.91 on a /6.

It's possible (can't find my literature at the moment) 2.91 was also fitted to the R100T from '79 on, as the motors were all "rationalized" from then through 1984, meaning they all had the same cylinder heads and compression ratio regardless of model in a given year. But, these bikes were VERY slow sellers in these years, so you won't find many.

Of course, the '77-'78 R100/7 had small valve heads and lower compression ratio and therefore less power and needed a bit more pull from the rear drive, so they didn't get 2.91.

You are indeed looking for something that's really rare.

[20774]

FD 32/11 mentioned on this site for the R100T.

http://www.bmbikes.co.uk/specpages/R100T.htm

[EMSimon]

Thanks guys, interesting information! So it is a rare item.
The ideal scenario, of course, would be to find a unit that I could continue to use and not re-install the original one and then keep my own rebuilt FD as a spare. I don't mind keeping parts for my Vintage bikes.
You never know when they become really, really rare.

[20774]

Check with Airhead Salvage.

[jforgo]

I would suggest you widen your search to 33/11, which is very close, and was used on more drum bikes. Only 97 more RPM at 55mph versus the 2.91 unit.

[20774]

This showed up in the Flea Market:

http://www.bmwmoa.org/forum/showthre...e-70-up-R-Bike

No vested interest.

[lmo1131]

Just my $0.02USD Mike.

Pull your final drive and have it rebuilt. You'll know what you have; the parts will be new, and set-up to specs, and you'll also have a service warranty. Buy used, and you have.. .. used.

Where to have it rebuilt... . ? Ask anyone here.

[20774]

Should the wheel splines be rebuilt along with the FD? Seems like you want nice splines to match up rather than one good and one not so good.

[lmo1131]

Should the wheel splines be rebuilt along with the FD?

I guess logic would say yes. But it prolly depends on the degree of wear overall. That said, old splines would start to adversely wear new splines.

If they look like this. Yes.



IF they look like this... maybe not. If you want to be totally confident that climbing on the loud handle won't do any harm, then I'd say replace the set. Spelled $$$