I'm a games programmer, trying to write a simple car physics simulation. I'm aware that a car travelling in a straight line will exert a traction force that drives it forwards (by turning the wheels and pushing back against the ground), and that the main forces that act against this acceleration are aerodynamic drag and rolling resistance. But I can't seem to find a sensible way of calculating the rolling resistance for a given vehicle.
This page here claims that
$F_{rr} = C_{rr} * v$
That is, the force is proportional to a rolling resistance coefficient multiplied by the velocity. In my tests, this seems wrong, and the page itself (and the sources it cites) admit that they're on shaky ground in terms of explaining or proving that formula.
This page can't make up its mind. For most of the page it says that
$F_{rr} = C_{rr} * W$
That is, the force is equal to a coefficient multiplied by the weight ($mg$) of the vehicle – i.e. the force is the same regardless of velocity. It even provides a table of coefficients for different circumstances. But if the force is constant, won't a car in neutral with the engine off be accelerated backwards by this force? What is rolling resistance at velocity 0?
Then, for a bit of that page it claims that velocity is a factor in calculating the coefficient:
The rolling coefficients for pneumatic tyres on dry roads can be
calculated as$c = 0.005 + 1/p (0.01 + 0.0095(v/100)^2)$
where
$c$ = rolling coefficient$p$ = tyre pressure (bar)
$v$ = velocity (km/h)
This makes no attempt to explain what all those "magic numbers" mean, and still produces a coefficient of ~0.0088, which in a 1500 kg car would yield a force of 129 N whilst the car was standing still. That can't be right…
So, which is right? Given basic information about a vehicle (mass, velocity, information about the wheels and the surface they're rolling over), and ignoring aerodynamic drag, what's a sensible way to come up with a broadly plausible rolling resistance?
Best Answer
If you are still interested in this question (and hopefully still have uses for the answer), I will try to answer it in an efficient manner. First of all, The rolling resistance force is an interaction between the ground and the wheel, which is independent of speed ONLY when the ground surface is completely flat and rigid. If the terrain is bumpy/hilly, the rolling resistance does depend on the speed.
On bumpy ground: It would usually be best to classify the resistance into a few different stages... At low speeds the wheels stay in steady contact with the ground and do not suffer from impact resistances with the tiny hills (like stones) on the ground. At medium speeds the wheels bounce off the top of each bump and land in the valley between the bumps and impact at the base of the next bump. At high speeds the wheels ski over the bumps, only hitting the very tops of them. As a general rule of thumb, on rough ground, the rolling resistance is least at a low speed, is most at a medium speed, and becomes less once the speed is high enough to reach the last stage (which I call rockoplaning... like hydroplaning on rocks).
On smooth/flat and rigid ground: the rolling resistance force is greatest when the wheel is not rolling... but this does not cause the vehicle to roll backward because the rolling resistance is acting in two directions as once... i.e. it is pressing forward and backward with equal force. The rolling resistance force decreases once the wheel begins rolling, but it changes so that it is only acting against the direction of motion. Once the wheel is in steady motion, there is a rolling resistance force acting against the wheel which is not dependent on speed whatsoever. The aerodynamics are greatly dependent on speed, but the rolling resistance is not... which is why the rolling resistance matters a lot at low speeds, but barely at all while driving at high speeds (the aerodynamics are so much more powerful than the rolling resistance when at high speeds). I can vouch for the fact that the rolling resistance is best expressed as Crr*Fn, where Fn is the force pressing the wheel against the ground. The force Fn may not be equal to the weight, if something makes the wheels press on the ground more or less force (like aerodynamic lift/downforce). I have researched, experimented and studied rolling resistance sufficiently to verify that as a general rule, all you need to know is the load force and the coefficient of rolling resistance. The coefficient should be easy to find online, and for a typical car on asphalt, it should be between 0.015 and 0.02.
Some online sources will tell you that rolling resistance is dependent upon speed, but in those cases, they are usually using rolling resistance as a broad term that applies to more than just the wheels rolling... i.e. they mean either the total resistance or the rolling resistance + the resistance of the axles. Because the axles on a train cause more resistance than the wheels, engineers consider the axle resistance to be part of the rolling resistance. Because axles tend to use fluid lubrication, axle resistance does depend upon speed. While there are exceptions, generally fluids cause friction that depends on speed, while solids do not.
It is incorrect to relate rolling resistance to aerodynamic resistance by some arbitrary factor (like thinking 50% of the resistance is aero, 50% is rolling), because the fact that aerodynamics get stronger and rolling resistance doesn't... So at a very low speed rolling resistance may be >95% of the total resistance, but at a very high speed rolling resistance may be <5% of the total resistance.
Suggestions for your program: The key is that the rolling resistance should be a force trying to make the vehicle stop... not trying to make it go backwards. And the force should still exist when it is not driving, but the force should just try to keep it from moving, like it is held still by a magnet. Crr should be around 0.02?