How to size spindles for solar car

I'm working with a group of high school students to design and build a solar vehicle for a regional competition. The entire vehicle will weigh about 600 lbs, with the weight distributed to 4 high-pressure bicycle tires on custom hubs and rims. For various reasons, the team has decided to use spindle axles, attached at one end, at each corner of a frame, and we're trying to work out the required diameters. The track is a straight 'drag strip', quite flat, so shock loading is not a worry. I'd appreciate any advice on sizing the spindle diameters. I've noticed that trailer spindles are about 1 inch diameter for a 1000 lb rating, but that's my only reference point. Paul Mathews