For no special reason at all, I decided to expend the money to put a car tire on the back of the 85 K100 rig, since all 3 wheels need replacing anyhow. I had spoken to Claude Stanley a number of years ago, and gave him a shout about adapters. Short story, Claude found an old adapter at his supplier and I ended up with it after a week or so. Since the 4 bolt K100 adapters haven't been used in awhile, Claude couldn't really remember what wheel was used, except it came from Summit Racing, and I would need shorter bolts to attach the adapter to the bike. He did give me a tire size... 165/80R15.

I found some pictures in another forum of the adapter and bolts, plus pics of the wheel used, as mounted. Figuring out the Summit wheel was easy, SUM-531P-5434N19 (150 bucks w/shipping) Sourced all the adapter and wheel bolts locally (just a few bucks). Got a Nexen tire (90 bucks) and had it mounted/balanced at Discount Tire (22 bucks). The tire is dang near the same diameter as the oem size. No re-rigging needed. I don' want to say what I offered Claude for the adapter, but my whole total end of day was a bunch under 800 US dollars.
The whole thing mounted perfectly, with the rear brake caliper nestled inside the wheel with about 1/8 inch clearance, plenty of clearance on the right side to the swingarm, but with a slight interference possibility to the sidestand bracket on the exhaust, which I was not going to remove. I figured a way to space the footpeg bracket outwards with 1/8 inch worth of washers, as the footpeg bracket also positions the exhaust. Problem solved. The problem with the sidestand bracket I had remembered from years past, so looking for it was forefront in my mind, considering I want the bike to be able to be returned to stock.

It is worth noting the exhaust needs to be displaced quite a bit to mount or remove the wheel, or to remove the rear brake caliper.

The tire as positioned is a bit left of stock, which is apparent from a rear look. What I like about the overall solution is that is was thought out to solve all the problems involved... the tire, the wheel, the caliper, everything... even though the solution was old and Claude could not remember everything about it. I could not have started from scratch and done this.
Pics later.

800 $? Wow...
At least, the money you're going to save now on tyre wear will pay for that wheel.
And the rim is beautiful!
On my 2 K100 with sidecars, I have the same solution, an adapter to fit a car wheel.

 The K1 runs a 6 inches wide, 14" rim with a 175 65 14 tyre. A bit wide (but no interference with the exhaust), but the small diameter needs machining the caliper to lower it.

The K100 runs a 8 inches wide, 15" rim with a 215 55 15 tyre. VERY wide, needs to fit the exhaust the other side. I suspect it to create excessive stress on the bevel drive bearings and it's much too wide to be safe on wet or snow. 
But it looks terrific!

The cost has to be taken in context... I sold an expensive guitar, and got a less expensive guitar and had the money left over for this rear wheel. 
Also, when I got the bike back in 88, I wanted to do the EML route, but could not afford it. In 2003, I was able to get a used Unit leading link, a really low cost Dnepr tub, a new subframe from Dauntless (now DMC), and a sidecar wheel that looked like the K100 rims, since the welded Dnepr wheels are not even round. Put a car battery in the back of the Dnepr.

The rig has been sitting for awhile in the garage. I need to clean it up, but it is low miles and runs really good.
Now I need to get flat treads for the tub and front.
(EDIT) - a pic of the kit.

DMC is just a few neighbourhoods away from where I am living now. I've been selling them taillight assemblies, ball joints for Telelevers and other Beemer bits for years. They do a good job. The guy who runs the place is a bit of a handful, such is his primary focus. A former colleague, all-round fantastic mechanic, lasted all of a half a week there.

Two Wheels Better wrote:DMC is just a few neighbourhoods away from where I am living now. I've been selling them taillight assemblies, ball joints for Telelevers and other Beemer bits for years. They do a good job. The guy who runs the place is a bit of a handful, such is his primary focus. A former colleague, all-round fantastic mechanic, lasted all of a half a week there.

I like the company a lot, and when I got the K100 subframe and the sidecar wheel, I was dealing with Jay Giese who was very helpful. We spoke many times before I ever bought anything. I considered Jay to be in similar category with, Neff, Bingham, and the folks at Unit. All would talk to me and give helpful advice.
Lot's of things have changed since 2003.

jbt wrote:800 $? Wow...
At least, the money you're going to save now on tyre wear will pay for that wheel.
And the rim is beautiful!
On my 2 K100 with sidecars, I have the same solution, an adapter to fit a car wheel.

 The K1 runs a 6 inches wide, 14" rim with a 175 65 14 tyre. A bit wide (but no interference with the exhaust), but the small diameter needs machining the caliper to lower it.

The K100 runs a 8 inches wide, 15" rim with a 215 55 15 tyre. VERY wide, needs to fit the exhaust the other side. I suspect it to create excessive stress on the bevel drive bearings and it's much too wide to be safe on wet or snow. 
But it looks terrific!

My '87 K100RS sidecar has a 15 x 4 custom wheel fitted, no adapter.  The tyre is a 165/80-15, the offset is just right it has around 3-4mm clearance on the swingarm and i can wriggle the tyre through between the caliper and exhaust without removing anything.  I think the original cost of the wheel was around $1000AUD but i didn't pay anything like that. 

I paid $4000 for a sad uncared for, unregistered K100 with sidecar, including EML subframe, leading link and billet front and rear 15" wheels.  I took the subframe, LL and wheels and fitted them to my bike, gave the sad bike some TLC and got it running properly, fitted the standard front suspension my old subframe and sidecar chassis then sold it on as a sidecar project for $2000.  So $2000 and a bit of work netted me all the good bits and made my outfit much better than it was

Two Wheels Better wrote:DMC is just a few neighbourhoods away from where I am living now. I've been selling them taillight assemblies, ball joints for Telelevers and other Beemer bits for years. They do a good job. The guy who runs the place is a bit of a handful, such is his primary focus. A former colleague, all-round fantastic mechanic, lasted all of a half a week there.

DMC make and sell left hand fitted sidecars and associated components(for some commonly used sidecar bikes).  Though it's expensive it's still tempting as you can buy virtually nothing off the shelf here in OZ.  I've seen a few here now, they look like reasonable units but I get really mixed reviews on the level of support when trying to bolt it together here on the other side of the planet.  Doesn't make you confident.

Al