Truck Braking

portrait photo of James

James I. Middleton, Jr., M.E., P.E.

If I’ve heard it said once, I’ve heard it said 1,000 times. It takes longer for a truck to stop than an automobile because the truck weighs more. But, fact is, a truck’s weight has nothing to do with why a truck takes longer to stop. There are many factors that affect the stopping distance of a tractor-trailer, but weight is not among them. This results from one of the quirks of physics. The more weight an object has, the larger the friction force (the force actually responsible for stopping) produced. As the weight of an object is also present in the determination of the energy present, the weight from energy cancels the weight from friction. What this means is whether the object (or vehicle) weighs 4,000 pounds or 80,000 pounds, it will slow down at the same rate.

So, why do trucks slow down more slowly than other vehicles?

 

Well, the simple answer is tires… The differences in tire compounding (makeup of the rubber) between a truck tire and automobile tire increases the service life of the truck tire but decreases the coefficient of friction (the measure of the resistance) between the tire and the road. This lower coefficient of friction is around 80% of the value of an automobile tire. In addition to tires, there are multiple other factors that increase stopping distance.

Fundamental differences in the makeup of the braking systems between trucks and automobiles also create differences in stopping distance. First among these is the working fluid. For automobiles, braking systems operate using hydraulic fluid (brake fluid). This fluid provides instantaneous application of brake force. In addition, the fluid provides feedback to the brake pedal that gives the driver a “feel” to how much brake force is being applied. Unfortunately, the system is not resistant to leaks and the lack of fluid can lead to brake system issues. To work around this issue in vehicles that continuously change their brake system, such as hooking to a trailer or trailers or operation without any trailers (known as bobtail), a truck uses compressed air as the operating fluid. This allows small leaks to be overcome and continuous changes to the vehicle configuration to be undertaken without compromise to the system. The downside to this approach is that air is a compressible gas instead of a liquid. As a result, application of the braking system has a measurable time lag between application at the brake pedal and the individual brake components achieving operational pressure; this brake lag can be up to ½ a second. In addition, the use of air removes the feedback feel from the brake pedal. Therefore, to simulate this feel, air brake component manufacturers make use of a spring within the pedal assembly.

Lastly, there are fundamental differences in the brake system components. Modern automobiles tend to use disc based foundation brake components. There are many reasons for this usage which will be covered at a future date. However, in this context, the use of disc brakes allows fewer moving parts and a more consistent brake force application. Trucks still commonly use drum based foundation brake components (although the use of air disc brakes is expanding). Drum brakes also have a lag associated with the mechanical play (give) in the linkage. As a result, there is an additional time lag associated with the removal of the mechanical play.

So, as you can see, trucks do take longer to stop than automobiles. But, I hope you can also see that that increased stopping distance isn’t a result of the truck weighing more.


About the Author

James I. Middleton, Jr., M.E., P.E. is a Consulting Engineer in our Columbia, South Carolina Office

Mr. Middleton's areas of expertise at the firm involve engineering analyses including metallurgical analysis, loading analysis involving manual calculations as well as Finite Element Analysis (FEA) techniques, as well as failure and fracture analysis of varying types of mechanical components and systems. In addition, Mr. Middleton also is the senior vehicle engineer within the firm. In this role he analyzes issues involving passenger and commercial vehicles, heavy trucking, as well as off-road and construction equipment. This work includes accident reconstruction in addition to mechanical evaluations. You may contact him for your forensic engineering needs at jmiddleton@edtengineers.com or (803) 791-8800.

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