I had originally hoped to use a set of spare wheels from the abandonded V-Ger robot that my friend Derek and I had started several years ago. After looking at the wheels and their related axle solution, I decided to go with a heavier duty wheel and axle solution instead.
After doing a bit of research, it seemed that most similar skateboards had used racing GoKart slicks. The most common tire size seemed to be 11 X 7.1 X 6. After doing a bit of reading, I learned that this meant the tire was 11" in diameter, was 7.1" wide, and needed a 6" wheel to mount to. I decided to go with a 6" wheel that was 8" wide. Even though the wheel was wider than the tire, this is considered normal in the Karting world. For rear wheels on a Kart, people typically use a wheel that is 1" wider than the tire.
To attach the wheel to an axle, I needed a wheel hub. They make wheel hubs with different bolt patterns that fit on axles of varying diameters. I decided to go with the "American" bolt pattern, which has 3 bolts on a 2.5" circle. I decided on a 1" axle, which is the smallest size that you typically see. I would have preferred to go with a "dead" axle set up, but that doesn't seem to be a common design in Kart racing, so I went with a "live" spinning axle. For axle material I ordered a 1" diameter keyed aluminum shaft from McMaster. It was more expensive than its steel counterpart, but I thought it would be worth it for the weight savings and ease of working.
To spin the shaft, I first looked at timing pulleys. I couldn't find any that were large enough in diameter to give me a decent reduction between the motor and wheel. Instead I went with a Kart sprocket and and sprocket hub, using #35 chain. I was able to get a 6.4:1 reduction pretty easily. I already had #35 chain, the necessary spur gear, and a few other key pieces, which made the choice easier.
I ordered all the Kart related items off of Americian Power Sports which seems to have a good selection of kart related parts. You could save some money by buying off of EBay, but I didn't have the patience to wait for the right wheel to pop up there.
Once I received the parts, the next trick was to mount the tire on the wheel. This is not a trivial thing to do. I read the forums over at Bar Fly Racers to get a good idea on how to accomplish this. I used Windex to lubricate the tire and the wheel, then pushed the tire over the wheel at a 45 degree angle. Once the tire was on the wheel, I placed tie-down straps across the tread of the tire so that it could not expand during the bead seating process. Next more Windex was used on the tire, and a high presseure air compressor inflated the tire the rest of the way until the beads popped up into the proper portion of the wheel. I strongly suggest reading up on this process, as it can be quite difficult for the unwary, and quite dangerous as well. Luckily, by following the advice found on the Barfly Racers forums, I was able to get the tire mounted without too much effort.