How to calculate forces on a wheel

[Maertens]

I have seen a lot of different formulas about the forces acting on a wheel. So now I don't know what to use.

So, I have a vehicule with 4 wheels ( Ø 200 x 50mm) with a load of 900KG. I want to know 3 things:
1. What power I need to start the vehicle to move.
2. What power I need for constant speed 6km/h? The rolling resistance.
3. What force ( pressure) the wheel gives on the floor? When the wheel stand still and when the wheel is rolling? In N/mm².


The wheelcore is made in cast iron and the tread is polyurethane 92 shore A. Tread can be Rubber 68 shore A. The wheel can also be completely made in polyamide 72 shore D.
The floor can be concrete, wood, linoleum.

What are the influences when we double the wheels?
What are the influences when we increase the diameter by 2?

I'm almost sure it's impossible to calculate this. But, I'm satisfied when I have a good 'direction'.

 

[CWatters]

I've never done this type of problem but I think you would need to know what losses were involved when you compressed and released the tyre material. In theory (ideal spring) there are no losses. In the real world tyres are heated by the process an that represent a loss that has to be overcome.

 

[mikeph]

There are some promising empirical answers on the wikipedia page
http://en.wikipedia.org/wiki/Rolling_resistance#Depends_on_diameter

"For pneumatic tires on hard pavement, it is reported that the effect of diameter on rolling resistance is negligible (within a practical range of diameters).[31][32]"

For doubling the number of wheels, my best guess would be that since the rolling resistance seems to be modeled on a linear coefficient, doubling the wheels (and therefore halving the load per wheel) would have no first order effect.