Truck Design and Modifications

May 1, 2000
How truck braking and handling can be compromised by equipment installers was the emphasis during an educational session at the NTEA presented by Thomas

How truck braking and handling can be compromised by equipment installers was the emphasis during an educational session at the NTEA presented by Thomas D Gillespie of The University of Michigan Transportation Research Institute (UMTRI).

Truck design has changed little since the early part of the 20th century, Gillespie said. Most trucks have a ladder frame, a solid front axle with leaf-spring suspensions, and a solid rear drive axle on leaf springs with a U-joint in the drive-line at the front of the differential.

However, body and equipment installers can inadvertently alter the vehicle dynamics by changing the weight distribution or center of gravity. For example, the front/rear load balance must be preserved by keeping the rear overhang of the body less than half of the wheelbase length.

Lateral load distribution can be compromised if the wheel loads are not equal between the left and right side. Ideally, the wheel loads between the two sides are equal or within five percent. Braking instability will be introduced by load imbalance, causing a yaw moment during braking so that the vehicle veers to the light side. Load imbalance on a steering axle causes a pull toward the light side because of the wheel caster.

Suspension modifications also can affect handling behavior directly. If a spring needs to be reinforced to handle high corner loads, correct the imbalance rather than modify the spring, Gillespie advised. High corner loads (even with spring modifications) upset the balance of a truck, causing braking efficiency to suffer and handling to be compromised, and the rollover threshold is reduced.

A high center of gravity reduces the rollover threshold also, causing the truck to roll more during normal maneuvering, and contributing to a loose feel in steering.

Steering system modifications should be approached with caution and in consultation with the chassis manufacturer, Gillespie said. Adding spacers to the front axle will introduce steering geometry errors such as bump steer and roll steer. If a spacer is required at the front axle to level the vehicle, a longer Pitman arm probably is required. This will increase steering sensitivity but will minimize bump steer. Roll oversteer is introduced when the rear axles steer the vehicle around a corner.

Adding axles changes the effective wheelbase; tag axles make a longer wheelbase and pusher axles shorten the wheelbase. Multiple rear axles increase the curb-to-curb turning radius. Off-tracking is increased by the square of the increase in wheelbase.

Tag and pusher axles potentially change the front axle load, Gillespie said. Pushers will reduce the load on the front axle, but tags will increase the front axle load. Add-on axles also reduce the tractive capability of the truck.

In conclusion, truck design and modifications involve many considerations affecting the brake performance, handling, ride, and hill-climbing ability, Gillespie said. The government and legal system will become more critical of decisions equipment installers make in the future. o