|MadSci Network: Physics|
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. http://www.gbhap.com/fu lltext/184/T960002F184.htm http://www.hunter.com /pub/undercar/901701/btest.htm http://www.ingallseng.c om/performance/primer.html
Try the links in the MadSci Library for more information on Physics.