Originally Posted by wa8yxm
With ALL WHEELS LOCKED. But if you have wheels still turning free.. the distance goes up.. Way up, as much as 30% per US-Gear.
I think you have your physics a bit jumbled, and you might be misreading the US Gear charts.
There is one coefficient of friction when the tires are rolling, called static friction when the tire contact patch is not sliding relative to the road surface. There is another coefficient of friction when the wheels are locked and the tires are sliding across the road surface, called sliding friction. Sliding friction is always less than static friction. The most efficient braking (shortest stopping distance) is when the wheels are just about to lock up, but are still turning. Apply less braking effort than necessary to lock up the wheels and your stopping distance increases because the brakes are not working to their maximum potential. Apply more braking effort than necessary to lock up the wheels and sliding friction takes over, and the stopping distance increases again.
For the shortest stopping distance, you want all wheels to be on the verge of locking up, but still rolling. This is can be hard to accomplish, which is one of the reasons that automatic antilock brakes have been mandated in most situations. (The other has to do with maintaining steering control, once your steer tires lock up you can't control your direction anymore.)
The coefficient of friction says how much friction you have between two surfaces (tire and road) as a function of the force pressing between those surfaces (weight on the wheels.) As the weight on the wheels goes up, the friction between the tire and road goes up, and the braking distance stays the same. However, you add a towed vehicle, and you are adding more mass that needs to be stopped, but because the toad has it's own wheels, you are not increasing the weight on the wheels that have the brakes. So, you've increased the weight that needs to be stopped, but not increased the friction on the wheels with brakes. That means you have the same braking ability, yet more weight, therefore you have a longer stopping distance.
The other issue is that brakes work by turning the kinetic energy of motion into heat. (The prime law of thermodynamics days energy cannot be created or destroyed, only converted from one type to another.) The more work the brakes have to do to stop the vehicle, the hotter they get. The hotter they get, the less they work (because they can't absorb as much heat when hot) The brakes on a vehicle are designed to safely stop a certain amount of weight. This is called brake fade. If you go over that limit (perhaps by adding a toad with no brakes) you increase your chances of overheating your brakes and running into brake fade, which again increases your stopping distance.