How Vehicle Type Affects Stopping Ability

Todd O. Hutchison, ACTAR.

 

The size and configuration of a vehicle will affect the ability of that vehicle to stop. Whether the vehicle is a car or pickup, a motorcycle, heavy vehicle or a passenger vehicle pulling a trailer, the type of vehicle needs to be considered before calculating the stopping distance of that vehicle. Each vehicle has its own set of special circumstances which need to be considered.

"The way the brakes were applied on a motorcycle is important, the way the brakes were adjusted on a truck, whether the vehicle is equipped with an ABS braking system all need to be considered."

Passenger Car with ABS Braking

A passenger car such as a Ford Fusion or Chevrolet Impala will vary on its ability to stop in a panic stop. Expert AutoStats and manufacture testing shows that on dry pavement at 60 mph using a hard application of the brakes that engages the ABS braking system, the 2015 Ford Fusion has the ability to stop in 136 feet while at 60 mph the 2015 Chevrolet Impala can be stopped in 119 feet. This is 87.5% of the Ford Fusion’s distance.

Motorcycle with Front and/or Rear Braking

And while this is a noticeable difference it’s nothing compared to a motorcycle where the driver is trying to stop at 60 mph by using rear braking only. In that circumstance on dry pavement, depending on the friction factor it could take 500 to 600 feet to stop. If the motorcyclist was an expert experienced driver with a modern motorcycle and used hard front and rear brakes the stopping distance at 60 mph can decrease dramatically to around that of an ABS equipped automobile.

Heavy Vehicle Braking

Heavy vehicles such as semi-tractor trailers, dump trucks and buses all utilize air as opposed to hydraulic braking systems that automobiles and motorcycles use. The stopping ability of vehicles with an air brake system depends largely on having it properly maintained and adjusted. Since the airbrake system has mechanical and electrical components that work together to supply good quality air to the individual chambers that in turn are used to apply the brake linings against the drums or rotors which activate the brakes each of these systems need to be working properly. If they are adjusted and functioning properly the trucks stopping ability at the same speed will at best less effective and take a longer distance to stop than that of a passenger car.

For a well-adjusted fully loaded semi-tractor trailer equipped with an ABS braking system, the stopping ability is generally no better than 70 to 80% of that of a passenger car. Federal Motor Carrier Safety Regulation 393.53 requires that property carrying vehicles with a GVWR of more than 10,000 pounds have a minimum deceleration ability of 14 feet per second per second (f=.434 g’s). The braking force is to be 43.4 % of the gross vehicle or combination weight. So on level dry asphalt with a coefficient of friction of .7 g’s in order to meet the minimum braking requirements the brakes have to be able to be adjusted such that they have at least 62% of a car (62% times 70% = 43.4%).

Once the stopping capability of the vehicle is established by the vehicle inspection then the scene data can be applied to a particular crash for analysis. In addition to the brake system inspections the electronic data from the truck’s Engine Control Module (ECM) or the Event Data Recorder (EDR) of a passenger vehicle can be imaged to determine the speeds in the seconds leading up to the collision and the crash pulse (Delta V) experienced during the collision. The scene inspection and a preserving of the physical evidence through photographs and total station measurements or other measurements will be used to verify and validate the electronic data.

 


 

 

 

 
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Using Event Data Recorder "Black Box” Information

What is an Event Data Recorder (EDR)? In short it is the "black box" of the vehicle. It has the capability to save pertinent crash data before, during, and after a crash.

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