National Highway Traffic Safety Administration (NHTSA) accident data reveals that 30 percent of all heavy truck accidents are caused by brakes being out of adjustment. The most common brake system used by heavy trucks is the air brake system. It is also the most complex.

The air brake system is designed to store compressed air in a reservoir. The driver slows the truck by pressing the brake pedal, which releases compressed air. The brake chambers and slack adjusters convert the energy of the compressed air into a mechanical force. This activates the brake camshaft, forcing the brake's shoes against the brake drum. This action slows the truck.

Properly adjusted brakes help to create a stable vehicle with the ability to stop properly. A truck with brakes out of adjustment can produce an unstable vehicle that is difficult to stop.

Braking systems performance is significantly affected by maintenance practices. The braking torque of air braking systems on heavy trucks is very sensitive to brake adjustments. Most heavy truck brakes must be adjusted, even though some trucks are equipped with automatic slack adjusters.

Poorly adjusted brakes affect the thermal capacity and time lag performance of the vehicle. As a result, the braking performance of a loaded truck can be seriously affected.

Speed from Skid Marks

Speed estimates of heavy trucks can be determined utilizing the knowledge of the distance the vehicle moved while sliding, the ending velocity, and its rate of deceleration. The ending velocity and the length of skid marks can often be determined. The real difficulty is determining the rate of deceleration of the vehicle, which is affected by the drag factor.

The coefficient of friction is the same as the drag factor if all wheels are locked and sliding on a level surface. Then the acceleration rate becomes the product of the drag factor and the acceleration of gravity. Using this information in a velocity formula, the speed can be calculated.

However, the conditions noted above rarely apply to heavy trucks. Generally, heavy trucks do not lock all their brakes. One or more of the wheels may have no brakes, or the brakes may not be able to lock the wheels. If all wheels do not slide, the drag factor on these wheels will be different from the wheels that slide.

Drag Factor

The total drag factor is complicated when different wheels have different drag factors. During braking, a truck's weight shifts from the rear wheels to the front wheels. Braking friction tends to force sliding wheels toward the rear at the road surface, and the center of mass tends to keep moving forward. This causes a rotation of the vehicle as it pushes down at the front and up at the rear.

When you apply the brakes on your car, the nose dips down. This is due to the weight shift toward the front of the car. The wheels try to stop, and the center of mass near the mid-point of the car tries to keep moving. When this weight shift is transferred to wheels with brakes, it makes no difference to the skidding distance. If the weight shift is to a front axle having wheels with no brakes, then the stopping ability of the rear wheels is lessened because weight is transferred to wheels that are free-rolling and not braking.

A truck loaded with cargo to the ceiling of the trailer will generate more weight shift that a flatbed truck hauling steel products only a few feet high. The center of gravity is higher off the ground. The higher the center of gravity, the greater the weight shift. A resultant drag factor must be calculated, taking into account the weight shift of the vehicle and its cargo.

Summary

By evaluating the braking system, determining accurate drag factors, having accurate skid mark data, and applying good judgment in using the analytical procedures, good results can be obtained from the investigation of heavy truck crashes.

Copyright 1999 Professional Investigative Engineers. All rights reserved.