Lionel,
I know that this is covered by Hanno's post but in WW2 a truck was specified by its carrying capacity not its unladen or gross weight. A 30cwt truck would carry 1.5 tons but probably have an unladen weight of about 2.5 tons or even more if 4x4. If empty there would be rather more than half the weight on the front axle so say 0.75 ton per wheel.
If the front were supported by two dollies, each with four castors, each castor is supporting nearly 0.2 ton. However, if you are pushing the truck and a castor meets an obstruction, say a bit of rubbish or a ridge in the floor, the load on it could easily double. The bigger diameter the castor wheels are the better in terms of climbing obstructions and of course it will be much easier to push with bigger castor wheels but cost will be a big driver here. If you have dead smooth concrete I would tend to not worry too much and use the 225kg castors but if the concrete has ridges in it left from tamping it, bigger castors might be worth it. I agree about polymer tires getting flats.
I would probably make the dolly frame out of 50 x 50 x 4 mm or 40 x 40 x 5 mm square tube. Both are very easy to get here but possibly not where you are. I prefer tube as it is much more rigid than angle for a given weight. I would just make a rectangular frame, say 500mm x 400mm and bolt the castors onto the corners. A truck wheel would sit directly inside the frame or you could put timber on top and sit an axle on that, possibly with a ratchet strap to stop it falling off.
I think that you would be amazed at how much use the dollies get for all sorts of jobs.
David
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