I was recently helping to crew Mark Thornton's effort at the Silver State
Grand Prix in Nevada. Mark had built a beautiful car with a theoretical top
speed of over 200 miles per hour for the 92 mile time trial from Lund to Hiko.
Mark had no experience driving at these speeds and asked me as a physicist if I
could predict what braking at 200 mph would be like. This month I report on the
back-of-the-envelope calculations on braking I did there in the field.
There are a couple of ways of looking at this problem. Brakes work by
converting the energy of motion, kinetic energy, into the energy of heat in the
brakes. Converting energy from useful forms (motion, electrical, chemical, etc.)
to heat is generally called dissipating the energy, because there is no easy way
to get it back from heat. If we assume that brakes dissipate energy at a
constant rate, then we can immediately conclude that it takes four times as much
time to stop from 200 mph as from 100 mph. The reason is that kinetic energy
goes up as the square of the speed. Going at twice the speed means you have four
times the kinetic energy because 4 = 22. The exact formula
for kinetic energy is ½mv2, where m is
the mass of an object and v is its speed. This was useful to Mark
because braking from 100 mph was within the range of familiar driving
experience.
That's pretty simple, but is it right? Do brakes dissipate energy at a
constant rate? My guess as a physicist is "probably not." The
efficiency of the braking process, dissipation, will depend on details of the
friction interaction between the brake pads and disks. That interaction is
likely to vary with temperature. Most brake pads are formulated to grip harder
when hot, but only up to a point. Brake fade occurs when the pads and rotors are
overheated. If you continue braking, heating the system even more, the brake
fluid will eventually boil and there will be no braking at all. Brake fluid has
the function of transmitting the pressure of your foot on the pedal to the brake
pads by hydrostatics. If the fluid boils, then the pressure of your foot on the
pedal goes into crushing little bubbles of gaseous brake fluid in the brake
lines rather than into crushing the pads against the disks. Hence, no brakes.
We now arrive at the second way of looking at this problem. Let us assume
that we have good brakes, so that the braking process is limited not by the
interaction between the pads and disks but by the interaction between the tyres
and the ground. In other words, let us assume that our brakes are better than
our tyres. To keep things simple and back-of-the-envelope, assume that our tyres
will give us a constant deceleration of
The time t required for braking from speed v can
be calculated from: t = v / a
which simply follows from the definition of constant acceleration. Given the
time for braking, we can calculate the distance x, again from the
definitions of acceleration and velocity:
Remembering to be careful about converting miles per hour to feet per second,
we arrive at the numbers in Table 1.
Starting Speed (mph)
Starting Speed (fps)
Time to brake (sec)
Distance to brake (feet)
Distance to brake (yards)
30
44
1.37
30.16
10.05
60
88
2.74
120.62
40.21
90
132
4.11
271.40
90.47
120
176
5.48
482.49
160.83
150
220
6.85
753.89
251.30
180
264
8.22
1085.61
361.87
210
308
9.60
1477.63
492.54
Table 1: Times and Distances for barking to zero from various
speeds
We can immediately see from this table (and, indeed, from the formulas) that
it is the distance, not the time, that varies as the square of the
starting speed v. The braking time only goes up linearly with
speed, that is, in simple proportion.
The numbers in the table are in the ballpark of the braking figures one reads
in published tests of high performance cars, so I am inclined to believe that
the second way of looking at the problem is the right way. In other words, the
assumption that the brakes are better than the tyres, so long as they are not
overheated, is probably right, and the assumption that brakes dissipate energy
at a constant rate is probably wrong because it leads to the conclusion that
braking takes more time than it actually does.
My final advice to Mark was to leave lots of room. You can see from
the table that stopping from 210 mph takes well over a quarter mile of very
hard, precise, threshold braking at 1g!
