The whole point of braking is to dissipate kinetic energy. Not the kinetic energy
of the wheel as you said, but the kinetic energy of the car, even though you may
do that through transmission to the wheel. Some trucks or busses
actually brake by transforming part of their KE into electricity,
which may sometime be reused, or is dissipated into heat as eddy (or
However, the most common way to dissipate kinetic energy is
friction. In the case of cars there are two possible frictions :
bretween the brake and the wheel (not the rubber itself hopefully) and between
the rubber and the road.
But there is energy dissipation only if there is motion with (kinetic)
friction creating a resisting force (in the case of friction braking).
The word kinetic is in parentheses, because it may require some further precision (see below).
When the car is rolling normally, there is no (or marginal) kinetic
friction because the wheel is at rest relative to the road in the
contact part. If you brake, this may no longer be true, because the wheel
may not turn fast enough. On some surfaces, like a wet road (but
apparently not all surfaces) the friction is more important if the
speed of the wheel part in road contact is not too important relative
to the road. Beyond a certain speed, the tire can even sort of surf
on a thin layer of water, and the friction goes down, thus dissipating
less energy. This happens much faster if you block the brakes.
So, with the brakes blocked, there is no energy dissipated by friction
in the brakes, and the wheels may be skidding too fast to dissipate
energy efficiently. Hence, it take a longer time to dissipate, meaning
a longer time to stop.
The ideal situation is dissipating energy both in the brakes and in the rubber.
But that is not easy to attain, because the static friction coefficient is usually greater than the dynamic coefficient. 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.
ABS prevents blocking the brakes by removing briefly the friction, and allows the wheels to turn some,
so that the relative speed of their contact with the road does not
get too high.
But why should it work on a dry road ? According to Wikipedia, there
is another phenomenon to be considered. The transition from static to
dynamic friction coefficient is not a discontinuous
phenomenon. Apparently the "maximum braking force is obtained when
there is approximately 10%-20% slippage between the braked wheel's
rotational speed and the road surface", beyond which "rolling grip
diminishes rapidly" to kinetic friction. So that is where the heat
dissipation is at its maximum, since maximum dissipation requires maximum motion with
the greatest motion compatible friction (actually, it is the product that is to be maximized). The role of ABS will be to
let go when the slippage becomes too important so that the slippage
remains in the optimal range (in addition to above issues).
But apparently some surfaces behave differently, and ABS may actually
brake more slowly. I would guess that this is due to the specific properties of the function that relates the friction force and the slippage speed for
that kind of surface in contact with rubber wheels. But on such surface, the advantage of keeping
better control of the car, by slipping less, is also an issue.
Another role of ABS systems is to distribute the braking effort
between front and rear wheels. Front and rear wheels have different
internal pressure, thus different contact surface with the road. They
are also subjected to different forces as the car is braking (more
force in the front), so that the friction coefficient acts more
effectively where the force is greater. Hence slippage control has to
differ in the front and in the back. It may also balance left and
right if for some reason the two sides behave differenlty.
A last issue was actually raised by @tohecz. Where should the energy
be dissipated, or according to what ratio between brakes and
rubber-road? His opinion is that it should be in the braking system,
not in the wheel-road contact. I did not find any information stating
that, if there is a choice, it should one more than the other, but it
may indeed be preferable to spare the tires (I do not really know).
It is however worth considering the issue and the degree of freedom of
We can analyze somewhat this ratio by considering extreme cases. If
you block the wheels (assuming no ABS), no energy is dissipated in the
brakes. Thus it is all dissipated in the rubber-road contact. On the
other hand, if you brake slowly, the wheels surface remains in static
contact with the road (no ABS needed) and all the energy is dissipated
in the brakes. This runs contrary to some belief that violent braking
could heat the braking system: frequent and slow braking will,
while violent braking without ABS will heat and wear the rubber.
So the question of the ratio, with an ABS system, occurs really only
when you brake strongly enough so that wheel slippage will occur and
the ABS can be used to control it. Here a proper analysis would really
require working on actual figures, as there are many possible
It should be the case that optimal braking, with fastest energy
dissipation, will impose a precise pressure on the brakes resulting in
a precise dissipation ratio between brakes and rubber. However, given
the hiccup behavior of ABS system, this corresponds probably to an
unstable setting requiring a dynamic control of the pressure so as not
to leave the optimal dissipation zone. I did not find any information about this ratio.
If the pressure on the brake pedal does not indicate urgency for fast
braking, the ABS system can probably choose, according to its programming,
what amount of pressure to apply, and when, so as to determine where
most of the energy will be dissipated, between brakes and rubber. But
there does not seem to be much public information on that.
A last remark is that the choice of optimal pressure for whatever
result is desired should also depend on the current speed of the car.
It is probably hard to get any slippage from a very slow car. Hence
the process has to be dynamically controlled for that reason too.
Note: In this analysis of ABS braking, the careful reader will have noticed that I
talk of forces, when actually it should be torques in many cases. My
reasons for doing this are the following:
the main issue is friction and friction forces, which become torques
because of the structure of the devices considered;
talking of torque would necessarily require the description to
introduce size considerations (wheel and brakes radius), which
would complicate the analysis without bringing in any essential
insight regarding ABS;
this is just a qualitative analysis, without using any actual
figures. Developing complete formulae would of course require to
bring in size issues, and to consider torques. But I deemed it simpler
not to do that here.
To dissipate any misunderstanding and any heat that could result from
it, I should make it clear that this looked to me like an interesting
problem to work on, but that I have no particular expertise, and I did what I could with the information I could find. Comments and criticisms are welcome.