No belt sanders involved. You can buy them like that. Without going out in the cold to the shed to confirm, I'm 99% sure it's a Metzler Block-K Sidecar tyre.
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The tyre on the back of my Kawasaki outfit is a car tyre. I forget the exact details, but I had a car rim grafted onto the Kawasaki hub. I think the car rim was off some small European car like a VW beetle or old Citroen. I think it's 15 x 4" rim. It was for the skinniest car tyre I could find that was approximately the right rolling diameter. I couldn't go wider because of the Kawasaki's drive shaft housing in the swing arm being in the way. As it was, I had to offset the rim about 10mm on the hub and grind down the welded seam on part of the drive shaft housing. There's only a millimeter or two of clearance. It would be a major engineering project to shift the shaft drive system sideways to allow a wider tyre. As it turns out, the car tyre is slightly smaller in diameter than the original Kawasaki tyre, which is not a bad thing on a sidecar outfit because it lowers the gearing a tad.
Incidentally, the bloke I bought the outfit from used the original Kawasaki rear wheel. He had tyres made by getting old bike tyres re-treaded with a square profile. He gave me a couple of tyres he'd had done. When the tyre that was on the bike wore out, I tried his square section re-treads. They were absolutely awful. So far out of balance and alignment that they were unusable. That's when I had the car wheel rim fitted by a sidecar specialist in Melbourne (Ian Souter at Premier Sidecars).