In the United States alone, more than 5,000 people are killed each year in truck-related crashes. A study sponsored by the US Department of Transportation suggests that truck collisions are 50% more likely to be fatal as the weight of the truck increases from 65,000-lbs to the current legal limit of 80,000 lbs. While it may be intuitive that the stopping distance for an 18-wheeler would be longer than for a passenger car, understanding the effects of vehicle weight on stopping distance requires more explanation.
In tests conducted by the Insurance Institute for Highway Safety (IIHS), a passenger car traveling 55 mph required 133-ft to stop, while an 18-wheeler with a fully loaded trailer stopped in 196-ft. That is a 47% increase in stopping distance. While it is tempting to attribute the increase in stopping distances to the comparative weights of the two vehicles, there is more to the picture than meets the eye.
On trucks with properly adjusted brakes and correctly sized components, there will be enough friction produced between the brake shoes and the drums to cause wheel lockup. When this occurs, the tires skid along the pavement, and the resulting stopping force is proportional to the weight of the vehicle. However, the kinetic energy of the vehicle is also proportional to its weight, and since the stopping distance is equal to the kinetic energy divided by the stopping force, the weight variable in the stopping-distance equation cancels itself out. Therefore, theoretically, increasing the weight on an 18-wheeler should have no significant effect on its stopping distance.
There are other factors, however, to consider when evaluating the effect of weight on stopping distance. The most obvious is the mechanical condition of the brake system itself. While the frictional force between the tires and the road is proportional to a vehicle's weight, the frictional forces between the brake shoes and the drums are not.
If the truck's brakes are out of adjustment, the force available to push the shoes against the drums, and therefore, the available frictional force, is reduced. In this condition, the weight of the vehicle could have an effect on stopping distance.
In the next article in this series, we will explore some specific ways that out-of-adjustment brakes cause reduced braking efficiency.