Correct rim width for 1972 BMW R75/5 SWB

[jrussakis]

Hello all,

I'll keep it quick. I'm in the process of a pretty extensive rebuild. I'm preparing to rebuild the my wheels (SS spokes and nipples and new Akront/Morad rims). I just disassembled the old rims 1.85 x 19" on the front no surprise, but I found a 1.85 x 18" on the rear wheel. Zoinks! I had always believed the airheads ran 2.15 x 18" in the back. I did a little 1972 R75/5 spec sheet research and saw that while the rear wheel for this model was always 18" it could be either 1.85 or 2.15. If my research was diligent enough, it seems that the R75/6 was the first to consistently have the wider rear rim.

So my question: now that the wheels are apart and I'm replacing all but the hubs, which width of rear rim should I get? What is gained or lost by going wider staying narrower? Is it a question of intended use? While I know the SWB /5 were the sportier, "quicker handling" of the wheel bases it was always my intention to take this bike on some longer trips. Also if it helps for someone trying to lend advice, I plan on using the stock tire sizes.

I'm sure my questions on rim width difference only scratch the surface of aspects to be mindful of, so if someone on here wants to break it down into some heavy rim theory, I would not begrudge them .

Thanks in advance for the help! Apologies if this topic has already been covered somewhere on the forum, if so I'd appreciate a link to the thread.

Best,
Josh

P.S. if anyone can comment on the quality the SS spoke and nipple sets sold by Speichen Spokes in Germany that would also be greatly appreciated.

 

[20774]

From what little checking I did, I think you're right on the rim choices for the /5 and that the /6 went to the 2.15 rim. Snowbum has some discussion about these rims here:

http://bmwmotorcycletech.info/section2A.htm

 

[jrussakis]

Thanks Kurt!

From Snowbum's article it sounds like I could go either way given that the offset was done right. I plan on sending the wheel assembly off to woody's to be expertly laced and trued. I assume they'll know exactly what is needed to do if I provide them with the right vehicle information. I'm typically a big DIY amateur mechanic, but the talk on the forums has made me hesitant to undertake the /5 wheel.

So with that said, is there a case for going wider in the back? From listening and reading the forum and tech articles around the net it always seems like BMW changed things from model to model if it would improve function and performance. Would it be safe to think that this is the case for the wider rear rim in the /6 series?

Best,
J

 

[20774]

If I understand the use of wider rims, I think it lets you install a wider tire, but one really only should install a 4.00x18 tire in the back. Maybe 4.00x18 tires come with beads that are designed for specific rim widths...not really sure. It appears that fits both so I might be inclined to go with the wider rim.

 

[jrussakis]

Hi Kurt,

Thanks again for being my guru on this question. I did more research and consulted the specs on the tires I plan on running (Avon AM26's 3.25x19 Front and 4.00x18 rear) the minimum rim width spec for the rear tire is 2.15 so it looks like I should play it safe and adhere to the manufacturer's standards.

I'll have to keep the original WM 1.85 x 18 around (I keep reading here and there that they are rare?) for another time. I'll see if I can figure out what the idea was behind it and perhaps build another rear wheel to put onto the bike later. A very curious discrepancy in the /5 factory specs...

Thanks again!
Josh

 

[69zeff65]

I am told

I have been told that a lot of the old time cafe bike builders (the real ones) look for those skinny rims from the early /5 bikes. The marrow foot print of the rear tire apparently makes a difference in the handling in tight and twisty riding.

 

[jrussakis]

I have been told that a lot of the old time cafe bike builders (the real ones) look for those skinny rims from the early /5 bikes. The marrow foot print of the rear tire apparently makes a difference in the handling in tight and twisty riding.

hmmmm.... I am intrigued. Did you by chance hear any talk of what these builders use for rubber on the skinny rims? I'd be all down for keeping the rear rim in use since it's in great condition. It just seems like the challenge is finding a tire meant for the narrow width.

If I keep it, I guess it would mean finding a useable WM front rim (long story short a DIY rim un-dinging attempt went horribly awry...).

Thanks,
J