MadSci Network: Engineering

Re: Will bigger brakes stop an ABS-equipped car in shorter distance?

Date: Wed Jun 7 23:56:01 2000
Posted By: Arnold Anderson, Staff, Tribology/Friction systems, retired (Ford Scientific Laboratory)
Area of science: Engineering
ID: 959965680.Eg

Brakes do not stop cars--not directly.  That is why they are called wheel 
brakes.  The wheel brakes stop the wheel. Tires stop the car.  Antilock 
brake systems (ABS) stop cars faster (most of the time) because they 
prevent wheel locking. A locked wheel typically has a lower tire-road 
friction than when rolling with some slippage.  

Tire-road friction normally reaches a maximum when the slip rate is around 
20%.  The reason for this slippage is that the tire is elastic, and must 
deform to transmit the tire-road tread force to the wheel.  When the tire-
road friction is greatest, this deformation (rolling 'slip') is about 4 
centimeters (1.6 inches) for a typical passenger car tire on a good road 

Present antilock brake systems do not optimize the tire-road friction.  
However, they do prevent wheel locking.  This does two things; it permits 
vehicle steering control (very important) and usually increases the tire-
road friction, compared with locked-wheel braking.

If you want to stop a car in the shortest possible distance, you first 
should apply the brakes very rapidly.  The rotating components of a car, 
mostly the wheel-tire assemblies, provide about 5 percent of the vehicle's 
kinetic energy.  Rapidly braking the wheels helps reduce this energy and 
also starts the tire tread deformation needed to maximize tire-road 
forces.  Since the wheels are directly bolted to the brake drum or disc, 
almost any torque can be used.  More is better in this operation.

When the tires reach maximum tire-road friction, you then should reduce 
the brake pedal force to maintain maximum friction.  Experienced 
performance test drivers can do this surprisingly well.  Some use the tire 
sound to help indicate the degree of slippage. 

Now you have enough knowledge to answer your own question.  Brakes that 
are large enough to just lock the wheels (but no more) will complete a 
stop as fast as larger brakes.  However, larger brakes may reduce the time 
to initiate braking of the tires.  Since this is at the beginning of 
braking (when the vehicle is moving fast), a small savings of time can 
shorten stopping distance by a significance amount.

If your goal is to stop even faster, you might wish to consider increasing 
the load on the tires.  Racecars use airfoils (small wings) to increase 
the tire loading.  If this added down force is equal to the initial car 
weight, the stopping distance can be cut in half.  With this scenario, you 
definitely will need bigger brakes to generate the additional torque.

Some racecars have used air 'dams' that are employed during braking to 
increase aerodynamic drag.  Such devices have little value, however, at 
legal highway speeds.

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