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More than Engines and Tires

Changes are coming in automotive safety—and not just for the occupants.

Picture it: you step into the street from between parked cars, distracted by your thoughts. Suddenly, a horn … a screech of brakes … you turn and see a car just a few feet away.

Unfortunately, this is all too common—over 100,000 pedestrians and bicyclists are injured in car crashes every year in the U.S. While safety for people inside the cars has steadily improved since the advent of seat belts, similar advances have been slow in coming to those outside the vehicle. But, times are changing.

Safety researchers have been studying specifically what causes leg injuries during a pedestrian crash. OK, that may seem obvious … but when you are an engineer working to make a vehicle more 'friendly' to a pedestrian, you need to get down into the details. You need to understand the injury biomechanics. Researchers study pedestrian injuries by collecting data at accident scenes and trying to simulate the crash in a laboratory. They use a variety of techniques, including specially designed crash test dummies and detailed computer models of the human body. The goal is to define when and how injuries occur, to enable other engineers to prevent them. It turns out leg bone fractures are caused primarily by the stiffness of the bumper, while knee injuries result from insufficient support below the bumper.

Computer model of lower limb, emphasizing knee joint.

Simulation of knee joint injury.

Funded by the American Iron & Steel Institute and the Office of Naval Research, Peter Schuster's team at Cal Poly State University has been working to develop bumper designs that take advantage of this information. They are changing the bumper stiffness, rearranging the support structures on the front end, and coming up with tools to test the resulting designs. The challenge is to do these things and yet still allow the bumper to work as … well … a bumper: The last thing you want is a bumper system that will protect a pedestrian, but will fall apart if you bump into a shopping cart in the parking lot!

Examples of benefits of computer modeling-predict injury sequence related to applied loads.

One design proposal includes a steel beam hidden behind multiple layers of energy-absorbing polypropylene foam. This foam may look like Styrofoam packaging materials, but is far more durable and stiff. Both foams are designed to do the same thing—absorb energy during an impact—whether you're protecting a computer monitor or a person's leg.

In addition, support below the bumper is needed to protect the knee joint. So, the team has developed different methods to push the lower part of the leg forward with the bumper. These lower stiffeners have been dubbed 'cowcatchers.' The nickname is appropriate — they have roughly the same purpose as the cowcatcher attachments on the front of old steam engines.

Yes, changes are coming to improve pedestrian safety. But, the best option will always be to just stay out of the way of cars!

Contributed by Peter Schuster, Associate Professor, Mechanical Engineering
California Polytechnic State University.

www.calpoly.edu