2011-11-18

braking - slowing down - stopping distances

Braking / stopping distances

If you go out to learn just one thing about your car, try to get to the point where you instinctively know what the stopping distance will be for different speeds at maximum braking effort. Quoting figures are mostly useless in real life situations, this is something that needs to be a split second decision, not a calculation.
The graph below show approximate braking distances in a standard road car in the dry.
Braking distances
Source: UK Highway Code
Speed
Reaction distance
Braking distance
Total
70
21
75
96
60
18
55
73
50
15
38
53
40
12
24
36
30
9
14
23
20
6
6
12

A real example:

Ariel Atom 2 300BHP Supercharged 100-0 = 3.8 seconds = 84.3 meters
Source: Autocar Magazine 0-100-0 tests
This figure is marked in the graph above in red. As you can see it comes well under trend line for standard cars but this would be expected for a performance car that weights as much as a small fish. This inconsistency nicely illustrates that fact that you need to get used to the braking performance of your car before you can start to push the limits.
Source: “How Long Does It Take to Stop?” Methodological Analysis of Driver Perception-Brake Times (2000). http://www.visualexpert.com/Resources/green_transportation_hf.pdf


 

 

What is the fastest method of slowing down in ideal conditions?

The following list shows the quickest methods for an experienced driver to slow down in high grip conditions such as a dry racing track. Threshold braking in the wet is much more difficult, and in this case ABS may well be the quickest method.
1. Threshold braking
2. ABS braking
3. Locked wheels
4. Cadence braking
5. Parachute (technique not discussed)
Diagram 1: The fastest methods of slowing down in high grip conditions
Braking methods
Just because a braking technique is not the quickest to bring you to a halt doesn't mean that it shouldn't be used, cornering ability while braking also needs to be considered. Locked wheels do not permit steering so this is only an option when you have a clear straight line ahead of you, however ABS equipped cars and the cadence braking technique both allow you to steer at the same time.

Which is the best technique for slowing down?

This depends on what you're trying to achieve. On the track drivers should aim for threshold braking, however even the best drivers in the world occasionally lock up wheels, and it is at this point the decision needs to be made whether to use cadence braking (if ABS isn't fitted), or reattempt threshold braking using slightly less pressure. In general, locked wheel braking should be avoided where possible*, as it does not allow steering control to be maintained.
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