Mono Rear Wheel Question...

[justabill]

Hey folks,

I am in a rebuild of my 85 R80RT and looking at the wear on my rear wheel's brake cylinder. I am curious what the factory diameter on these wheels is. Mine is currently measuring at just shy of 201mm which converted gives me .007" of remaining wear given the 7.92 max diameter. As a rider, I am never heavy on the rear brake, but don't have enough experience with these to know what to expect from so little remaining. Other than wear, the wheel is in great shape for 163K miles.

Thanks!

Bill

 

[20774]

Bill -

My Haynes says the maximum diameter is 7.933 inches! That's a little better!! Still doesn't give much room to work with. Most braking, approaching 70%, is done with the front brakes, and I think most of us just "trail" brake with the rear. There is a Wheels and Brakes section in the link in my signature line. You might poke around and contact say Woody's or Buchanan's and see if they offer a service, or know of, someone who can sleeve the rear drum area and then mill it back to proper size.

 

[justabill]

Thanks Kurt. Not sure how I missed your resources list. That's excellent. I'll poke around. As luck would have it, there's a shop not far from me that specializes in resleeving, but they only do auto hubs. I'll call around.

 

[88bmwjeff]

My Clymer manual is similar to what Kurt indicated for Haynes. Clymer indicates:

Standard 200.00 mm (7.874 in)
Maximum 201.50 mm (7.933 in)

I'm not so sure what the standard represents.

 

[manitou_commando]

Hey folks,

I am in a rebuild of my 85 R80RT and looking at the wear on my rear wheel's brake cylinder. I am curious what the factory diameter on these wheels is. Mine is currently measuring at just shy of 201mm which converted gives me .007" of remaining wear given the 7.92 max diameter. As a rider, I am never heavy on the rear brake, but don't have enough experience with these to know what to expect from so little remaining. Other than wear, the wheel is in great shape for 163K miles.

Thanks!

Bill

Bill, interesting that you have a 1985 RT. What's interesting is that a BMW did not manufacture an R100RT after 1984 until manufacture resumed in 1988.
I bought a 1988 RS with cast wheels and the drum diameter was beyond spec. It never really mattered as the rear brake is not much use.
I ultimately replaced the rear wheel with a new one about 10 years ago for the princely sum of ~$700 after putting an irreparable dent in it.
Online, new cast and/or spoked wheels are now ~$800.

In my experience, you will not notice any difference if you upgrade the drum or buy a new wheel. Just ignore it until you put a dent in it.

 

[justabill]

My Clymer manual is similar to what Kurt indicated for Haynes. Clymer indicates:

Standard 200.00 mm (7.874 in)
Maximum 201.50 mm (7.933 in)

I'm not so sure what the standard represents.

Thanks! This gives me some hope. If the total life of the wheel is only 1.5mm of use, then it's only 2/3 gone. Assuming Standard = New. The most curious thing in all this is that Haynes and Clymer are not agreeing with the stamp on the wheel. Given that there's light use on the rear, maybe I'll run with it for now and keep an eye out for a good used one down the road.

 

[justabill]

Bill, interesting that you have a 1985 RT. What's interesting is that a BMW did not manufacture an R100RT after 1984 until manufacture resumed in 1988.
I bought a 1988 RS with cast wheels and the drum diameter was beyond spec. It never really mattered as the rear brake is not much use.
I ultimately replaced the rear wheel with a new one about 10 years ago for the princely sum of ~$700 after putting an irreparable dent in it.
Online, new cast and/or spoked wheels are now ~$800.

In my experience, you will not notice any difference if you upgrade the drum or buy a new wheel. Just ignore it until you put a dent in it.

Yep. I was able to find a couple places selling them in the high $600 range, but mostly nearly $800 like you say. For what it's worth, I am adding the Siebenrock power kit, so it will be a *100 in the end.

Thanks for the advice!

 

[pglaves]

The most curious thing in all this is that Haynes and Clymer are not agreeing with the stamp on the wheel.

I can't be positive in this instance but here is how that usually used to happen. BMW engineers decide on a specification and publish that specification. They might even stamp it on a wheel. Later in the lifecycle they discover that that specification needs to be changed and they do so. Example - the volume of lubricant to be in a "big hole" final drive - reduced from 235cc to 180cc.

Any publication before the change is going to have the formerly correct but now bogus specification.

 

[justabill]

That makes a lot of sense.

Any way we look at it, this wheel still has some life at this point, so I won't have it on my critical path to replace it.

Thanks everyone.