Take wheel of vehicle to be towed. Its diameter and width will be in line with the weight it is carrying. Therefore the trailer needs to be able to cope with that weight safely. Its wheel/wheels needs to have an equal or greater contact area with the road which is the width and diameter of the wheels on one side to do that. It is the largest trailed wheel, value x versus the entire side of trailer wheels, value y. This should offer generous excess in most cases.
So, yes a motorcycle has large diameter wheels but no axle. Therefore calculate half the bike wheel. The tyre section is narrow compared to the trailer wheels on a sensible trailer, which will have a larger area on the road. Only bike trailers with piddly wheels need worry. Why would you put a Giraffe on roller-skates for safety anyway?
Tractors are not cars. Tractors with large front wheels, not sure if there are models that were offered with in only that form or with smaller front wheels, see above. But a few and half tons needs to be spread across a few tyres and area of the road. I can think of a few bridges round here where a tractor on a single axle trailer would be illegal due to axle weight restrictions nor would I want to tow it. Also Tractor tyres are not a road section so there must be a diffing sectional value on off road tyres, a bit like gear and engine oil grades, or a multiplier used, as clearly the section is to increase traction in loose ground at the expense of drag. Its not totally a weight of machine issue.
Modified wheels on racing cars, hot cars etc refer back to standard manufacturers spec unless clearly modified beyond that spec. Clearly a single seat race car weighs next to nothing - and technically is not a car anyway. Q registered vehicles do have a problem as in the main there is no data.
Trikes, before you ask. The size of registration or number plate designates if it is a bike or a car. That is why they are different and your not allowed to mix them. I know the law now has trikes as a separate class but as we all know the laws governing trikes are a mess.
