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Brakes and Stopping Distance

Braking systems for vehicles are dependent on a series of individual components. Each of these components is important for the functioning of the brakes, allowing the vehicle to stop when necessary. Hydraulic disc brakes are common for many passenger vehicles, and are the gold standard nowadays for performance and reliability among all cars on the road. Commercial vehicle brakes are a little different; mostly because they use compressed air in the brake lines instead of hydraulic brake fluid. Brake drums at a truck’s wheels handle the actual braking duties, components that are not usually found on modern passenger vehicles but were very common in passenger vehicles 30 or more years ago. One thing is for certain: brakes serve one purpose: to slow down and stop a vehicle as quickly as possible under all circumstances.

The time spent braking before an accident is typically done at maximum braking level. Accident reconstructionists can measure maximum braking by application of a few different methods . The main way is by measurement of tire marks. For passenger vehicles, tire marks can be an indicator of locked wheels usually by application of brakes (not considering the usage of anti-lock brake systems). These skid marks can be used to help determine vehicle speeds at the time of brake application. Typically, a vehicle that is in the process of leaving tire marks on the surface of the roadway, when all four tires are locked, will be capable of stopping from 30 mph in about 43 feet. This is useful to know when asked to determine the speed of the vehicle at the start of the skid. How does an accident reconstructionist come up with this speed? By applying a proper “brake factor” to the calculation of the skid. Brake factors have been studied extensively by many different scientific agencies based on typical tires for standard tire skids. The results of skid testing has resulted in an industry wide accepted range of values for passenger vehicle brake factor that can be used in the determination of vehicle speeds from skid marks.

To complicate things, semi tractor trailers and other commercial vehicles do not share the same brake factors that passenger vehicles do. The industry accepted brake factor for these large trucks is on the order of only 80% of the brake factor for passenger vehicles. What does this mean? In a similar braking situation and at the same 30 mph speed as a passenger vehicle, a semi truck will take longer to stop when emergency brakes are applied. Instead of stopping in 43 feet as mentioned above, a semi truck could take almost 54 feet to stop. This means that, in a similar scenario, a semi will still be moving at a speed of about 14 mph when the passenger vehicle has stopped!

The Difference:

Not only are semi tractor trailers much heavier, they are also unable to stop in as short of a distance as passenger vehicles. Why is this? There are many things that contribute to the differences in stopping distance of these two categories of vehicles. However, the main reason is fairly straightforward: TIRES. Tires for a passenger vehicle will last about 30,000 to 60,000 miles on average. This is acceptable for most consumers, as it means that tires only need to be replaced every few years. Tires for commercial vehicle are designed to last a long time. In fact, it is not unheard of for semi truck tires to last 400,000 miles. In order for these tires to last longer, they are manufactured out of harder rubber that resists wear at the cost of increased stopping distance. The amount of miles that commercial vehicles are driven in a year is significantly more, therefore making it necessary to have tires that last a long time to be cost effective for shipping and vehicle maintenance.

It is important to consider the differences in stopping distance between large trucks and passenger vehicles. Veritech’s team of forensic engineers is well versed in the differences between all different types of braking systems and stopping distances . We can help you understand the effects of this for your accident, so reach out to us today.

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