Wheel offset for an R12?

schrader7032 · Post by **schrader7032** » Fri Jun 23, 2006 10:08 pm

Darryl -

I don't know if this applies to the R12, but here's the website for a recent article on /2 wheel building. Down towards the bottom is a discussion about offset.

http://agwalker.com/wheelbuilding.html

Kurt in S.A.

Darryl.Richman · Post by **Darryl.Richman** » Fri Jun 23, 2006 10:24 pm

Thanks, Kurt! I'm familiar with Lonnie's website -- in fact, I have a copy of it printed out to use when I go to try lacing up these wheels. Right now, my best idea is to take the average of the two old wheels and lace up the new wheels to that offset. I'm hoping that someone can give me a better idea.

schrader7032 · Post by **schrader7032** » Sat Jun 24, 2006 9:46 am

Darryl -

For my own education, what's the purpose of the offset? Is it to put the hub in a position that "works" for the rim and also "works" for the frame? Obviously, the hub should be somewhere between the two outside edges of the rim. But I guess it could be moved closer to one or the other in order to mate with the final drive all the while keeping the two edges of the rim more or less centered between the frame, or at least in the position to track with the front wheel. Is that about it? Is there any way to compute what the offset should be based upon the known dimensions of the frame?

Kurt in S.A.

Darryl.Richman · Post by **Darryl.Richman** » Sat Jun 24, 2006 10:49 am

Kurt, I believe you have the idea correct. The wheel offset should get the tires onto the centerline of the bike, from where the hub sits on the axle. And there is a drawing "out there" of the special tool to use for /3 and /2 wheel building, to get the offset correct -- each frame, motor and wheel type combo has its own value. So, there ought to be a correct value for the R12. I just don't know what it is.

There's a two page document that shows all of the set up information for the R12, but it doesn't include this value. Likewise, neither do the various "Betriebsanleitung" manuals I've perused. There are some drawings indicating the centerline of the bike (for sidecar setup), but curiously, the war bike drawing contradicts the civilian drawing about where the centerline really is.

Finally, the reason I'm so interested in this is that I'm having the frame straightened by The Frame Man in Sacramento, and they wanted me to build a wheel so they could use it to align the frame!

A supplier I use and rely on in Germany has told me that it's not critical, and a few mm either way won't be noticable. He suggests that I lace up the wheel counting the number of turns on the spoke threads to get to a "natural" position. That might be true, too, but I'd like to hear about it from more than one source, before I make a mistake with spokes and nipples I paid to have painted...

schrader7032 · Post by **schrader7032** » Sat Jun 24, 2006 11:33 am

Still not any help specific to the R12, but Vech posted this on the VBMWMO site...he shows the offset for a number of different rims and bikes...all about the same. Duane Ausherman also has a write up but it focuses on the /2/5 rims.

http://www.vintagebmw.org/current/news- ... tech13.htm

Kurt in S.A.

schrader7032 · Post by **schrader7032** » Sun Jun 25, 2006 1:26 pm

Darryl -

I ran across this on the /2 forum...maybe he could do a measurement for comparison??

http://www.bungeebob.com/R12Photos.html

Kurt in S.A.

Darryl.Richman · Post by **Darryl.Richman** » Sun Jun 25, 2006 2:38 pm

Thanks for your keen eye, Kurt! I have sent a follow up message to Bungee Bob, so perhaps something will come from that.

Considering the conditions of roads when these bikes were current, the low top speeds they attained, and the apparent lack of detail in the manuals regarding rebuilding them, I have to suppose that the numbers aren't that critical to the bike's performance. Still, after the time and money I've sunk into this, I'd like to make the best of it I can.

Bruce Frey · Post by **Bruce Frey** » Mon Jun 26, 2006 3:12 am

My next R12 task is to remove the wheels and check the brakes, but it will be two or three weeks before I can get to it.

I would ask Vech, Huggett and the Archives.

There has been discussion in the Kradrider group whether the rims should be centered in the frame or not, but I can't imagine German engineers designing them to be off center.

Best regards,

Bruce

Darryl.Richman · Post by **Darryl.Richman** » Mon Jun 26, 2006 6:15 pm

Erik Bahl, who has restored an R12, was able to definitively answer. He said:

27 January 2004 - I contacted the BMW Historical Archives in Germany to see how much higher the hub should be relative to the rim. They found an original dimensioned wheel drawing and the distance is 7mm. Tonight I disassembled the rear wheel. Some of the spokes came loose. Others were rusted solid and had to be cut off.

VBMWMO · Post by **VBMWMO** » Mon Feb 15, 2010 2:51 pm

I am embarking on a wheel building project. The wheels on my R12 were not original (or correct, or straight, for that matter) for the bike, so I have procurred a new pair.

Before I disassembled the old ones, I measured the offset of the hub from the wheel. Needless to say, the wheels had different offsets. The really crooked wheel varied by nearly 4 mm depending on where I measured, but even on average, it was 6 mm different from the straight(er) wheel.

So, I wonder if anyone out there knows what the correct offset really is?

Vintage BMW Motorcycle Owners