mtuandrew wrote:
More or less, a train has more ability to slow down if its wheels are still rolling at the very limit of their adhesion (before they break loose and skid), than it does for them to skid. The rolling friction coefficient limit of steel on steel (or any material on any other material) is higher than the sliding friction coefficient limit, if I remember Physics I correctly after 10 years.
You mean static friction -- not rolling friction.
Static friction is what keeps objects from sliding. However, once an object has already started sliding, kinetic friction acts against the motion and prevents it from sliding forever. Kinetic friction is lower than static friction (for standard materials). Think about sliding a cardbox box on a floor -- it is harder to get it started moving, than to keep it moving.
Wheels are confusing because static friction applies
even when the wheel is rolling. But it's still doing the same thing -- acting to prevent the wheel from sliding. But if you exceed the resistance offered by static friction, then the wheel will start slipping, and kinetic friction will take over.
Rolling friction acts against a rolling wheel (but not a sliding one). Just like kinetic friction acts against a sliding wheel (but not a rolling one). The rolling friction for solid objects is
much, much, much lower than kinetic friction -- which is why the invention of the wheel was such a great thing.
The coefficients of friction for steel-on-steel are:
- Static friction: 0.74
- Kinetic friction: 0.57
- Rolling friction: 0.001 to 0.002
In other words, you can apply 30% greater deceleration force if you can prevent the wheels from locking up. The effect on stopping distance is magnified, because stopping distance scales as the inverse-
square of deceleration. It takes 70% more distance to stop if the wheels lock up.
Antilock braking also helps you to maintain steering ability when driving a car. This doesn't matter quite as much for trains.
