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Slingshot effect? What happens when a car hits a pothole

Hitting a pothole can turn a swell day into a lousy one, sometimes requiring a costly and unexpected trip to the repair shop.

We asked one expert Jennifer Bastiaan to weigh in on what these craters can do to an automobile. Bastiaan is an assistant professor in mechanical engineering at Kettering University in Flint, Mich., with a Ph.D. in mechanical and mechatronics engineering. Her teaching and research focus is on ground vehicle systems.

The following interview was edited for brevity and readability and is not verbatim.

Question: What happens to your vehicle when you drive over a pothole?

Answer: It's kind of a slingshot effect, where there's a spring that's the suspension that's compressed and basically the wheel and tire are propelled into the pothole. There's an impact and that impact is damaging to the vehicle systems.

Q: What can happen?

A: All things that are expensive in the end. Tires and wheels are the primary sufferers when it comes to these impacts, but ultimately you can have damage to the vehicle body, and there can be suspension components that are damaged if they come in contact with broken pavement.

Q: How much is speed a factor?

A: The speed is a factor, so the faster the speed that you're traveling, the shorter the time duration of the impact. At highway speeds, you basically travel through the pothole very, very quickly whereas if you're just tooling around town you'll travel through it slowly. At highway speeds or fast travel speeds, the impact is very short in duration, and this is more painful where the vehicle system is concerned.

Q: What about damage?

A: So what happens is we put big loads into components like tires and wheels and the suspension, and what happens on a technical level, we say that these systems and components experience stress. When we load up a component like a tire, it develops stress. Lower stress is better. Higher stress is worse. So big loads from big potholes result in big stress and the materials cannot withstand so much stress, especially due to repetitive stress events.

The materials that make up the components, like steel and rubber and nylon and all the textile materials in the tire, can only be subjected to so many cycles of stress before they fatigue and fail. So there's not only the issue of how much force goes into the component in one pothole, but then there's also what we call the fatigue or durability aspect, which is how many potholes a car has to be subjected to. So it's bad to run through one, but it's worse to go through many.

Q: Is it much better if you're going very slow?

A: You're still going to be putting the forces in, but the thing is that to go slow enough to basically avoid damage any real damage of significance you would have to be going so slow that you would never get to your destination in a reasonable period of time. You'd be better off walking.

Q: Anything else to consider?

A: If you drive around southeast Michigan like I do, you know that some roads have such broken pavement that there's a tendency to pay attention to just immediately what's in front of the car, where driving certain streets and highways is a kind of steeplechase. There's a distraction from looking down the highway from where our eyes are supposed to be. Some of our roads are bad enough that it is honestly at the level of serious distraction.

One other thing ... Bastiaan says that low-profile tires, which are increasingly being paired with large wheels even on economy cars, are probably a bad idea for most vehicles trying to navigate treacherous road terrain. It's too easy for the metal to contact broken pavement, which can, you guessed it, cause more damage.

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