There is approximately 10%-20% slippage between the braked wheel's Apparently the "maximum braking force is obtained when The transition from static toĭynamic friction coefficient is not a discontinuous So that the relative speed of their contact with the road does notīut why should it work on a dry road ? According to Wikipedia, there As soon as the wheel starts slipping, the friction reaction force of the wheel that preserved some motion in the brakes may become too low for the brakes to allow for motion, and the brakes block, no longer providing any dissipation, and increasing further the skidding speed of the wheel.ĪBS prevents blocking the brakes by removing briefly the friction, and allows the wheels to turn some, The ideal situation is dissipating energy both in the brakes and in the rubber.īut that is not easy to attain, because the static friction coefficient is usually greater than the dynamic coefficient. Hence, it take a longer time to dissipate, meaning In the brakes, and the wheels may be skidding too fast to dissipateĮnergy efficiently. So, with the brakes blocked, there is no energy dissipated by friction This happens much faster if you block the brakes. On a thin layer of water, and the friction goes down, thus dissipating Beyond a certain speed, the tire can even sort of surf Speed of the wheel part in road contact is not too important relative On some surfaces, like a wet road (butĪpparently not all surfaces) the friction is more important if the If you brake, this may no longer be true, because the wheel When the car is rolling normally, there is no (or marginal) kineticįriction because the wheel is at rest relative to the road in theĬontact part. The word kinetic is in parentheses, because it may require some further precision (see below). In the case of cars there are two possible frictions :īretween the brake and the wheel (not the rubber itself hopefully) and betweenīut there is energy dissipation only if there is motion with (kinetic)įriction creating a resisting force (in the case of friction braking). However, the most common way to dissipate kinetic energy isįriction. Which may sometime be reused, or is dissipated into heat as eddy (or Some trucks or bussesĪctually brake by transforming part of their KE into electricity, Of the wheel as you said, but the kinetic energy of the car, even though you mayĭo that through transmission to the wheel. The whole point of braking is to dissipate kinetic energy. 10 km/h, and it shortened the stopping distance, but with this speed we should not think about thermal destruction of tyres. This explanation is not confirmed by experiment which I performed myself on ice with the speed ca. So the ABS changes "used" part (hot) of tyre to a "fresh" part (cold). If it would be hot too much, it becomes more liquid and the friction force is smaller (this is why these tracks come from). The only thing I can imagine is that this small angle of rotation prevents tyre from getting hot. So the way to stop the car should be longer.Īgain, I understand how the ABS prevents wheels from blocking and allows maintaining control, but I don't how it shortens this distance. If the ABS allows the wheel to rotate for some angle, in this moment there is no friction between wheel and ground (well, actually there is, because the wheel rotates, but in this case it is useless friction). I heard that such act can shorten the time to stop a car, but I don't know why. This is intended behavior, and again the ABS stops the wheel. If now ABS starts to work, it moves the metal parts away, so the wheel can rotate again for some angle, even if I am still pushing the brake. Sometimes stopping car leaves a black trace on the ground. The kinetic energy of moving car is (please confirm) dispersed as thermal energy. This force depends on (among others) car's mass. When it happens, there is a friction force between a tyre and ground. It does not matter how strongly I press the pedal, if only it makes wheel stop rotating. Because of friction force between these metal parts and wheel, the latter stops rotating and its kinetic energy is dispersed as thermal energy and both parts are hot. When I push a brake pedal, there are metal parts that touch a wheel. The system ( a Wikipedia link) is intended to prevent locking of wheels and thus allows keeping control of the car (this I understand). The web has lots of explanations about the system, how it works, but I don't understand how it shortens the way of stopping. ABS, from German Antiblockiersystem, is a device put in almost every new automobile.
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