MadSci Network: Physics

Re: Can some brakes stop a car faster than others ?

Date: Tue Apr 18 22:34:52 2000
Posted By: Arnold Anderson, Staff, Tribology/Friction systems, retired (Ford Scientific Laboratory)
Area of science: Physics
ID: 955744820.Ph

Tom, you have asked a number of good questions about vehicle braking.  
However, rather than addressing each of your questions separately, I will 
describe the fundamentals of vehicle braking for a car or light truck.  
These have four brakes, one at each wheel.  To stop in the shortest 
distance, each brake needs to produce the proper torque.  By that, I mean 
the torque that maximizes tire friction.  Brakes slow down the wheels.  
The tires slow down the vehicle.  Braking is fastest when the tire-road 
friction is greatest.  This is the tough part.

A tire that is locked (in a full skid), usually does not provide as much 
braking effort as when it is rolling, but with some 'slippage.'  The 
optimum slippage varies with many things, such as tire loading, tires 
design factors, tire inflation pressure, road surface conditions, and 
vehicle speed.  However, for normal vehicle braking, it is about 20 
percent.  This slippage provides about a 4-centimeter deflection of the 
tire tread as it contacts the road.  When the tire tread surface separates 
from the road surface, at the rear of the tire contact, it has to slip.  
This slippage is what makes the tire braking noise--and a similar noise 
when cornering.  It is important to remember that most of the tire contact 
patch has static friction, when braking effort is maximum.  Braking noise 
is generated at the rear of the tire-road contact.   

Ideal emergency braking requires all four brakes to make their tires 
produce the maximum retardation.  This is very difficult to achieve, since 
the loading of the tires varies with passenger and cargo loads, 
deceleration rate, and several other factors.  Modern vehicles use brake 
proportioning valves to compensate for the vehicle weight shift when 
braking.  Some vehicles, especially trucks, use load sensors to compensate 
for different axle loadings.

Antilock braking systems, as their name indicates, prevents wheel lockup 
during braking.  This is very important for vehicle control, as locked 
wheels provide no steering effort.  Antilock brakes do not optimize 
braking, but do improve it for most vehicle usage conditions.

When brake linings are replaced, the brakes may have a different 
effectiveness.  That is, they may generate a different braking torque for 
the same brake pedal input.  Some brake designs are very sensitive to the 
choice of brake lining friction materials.  Disc brakes are less sensitive 
to friction material differences than most drum brakes.  For this reason, 
a vehicle with four-wheel disc brakes tends to have better 
braking 'balance' than one with drum brakes.  Many passenger cars need 75 
to 85 percent of the hard braking effort on the front brakes.  Some of 
these have disc brakes in the front, and drum brakes in the rear, with 
very good braking balance.

Disc brakes are very popular, largely because of their friction stability 
and their faster cooling.  Brakes are temporary energy storage devices.  
Eventually, all of their energy must be released to the ambient air in the 
form of heat.  Surprisingly, the brakes do absorb as much heat with rapid 
braking as with light-to-moderate braking.  This is because of the tire-
road 'slippage.'  If a person were to rapidly lock all four wheels, the 
brakes would absorb only about 5 percent of the total vehicle energy.  
This is the energy of rotation of the wheels and tires.  The remaining 95 
percent of the vehicle's energy would be generated as heat at the tire-
road interface.  This is what makes tires smoke during skids.

I hope this helped answer your questions, Tom.  You may wish to check the 
following web sites for additional information.

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