Why does my plastic air deflector scrape the road in the cold?

Dear Car Talk

Dear Car Talk | Jan 01, 1990

Dear Tom and Ray:

I purchased a new 1987 Chevy Celebrity Eurosport. During the winter months, the front plastic air deflector (below the front bumper) scrapes the road whenever I go over a bump. The scraping only occurs when the temperature drops below 35 degrees. What do you recommend?

TOM: That air deflector is low to begin with, so it's probably almost scraping even when the weather is warm. What's causing it to make contact with the road in the cold weather is probably low air pressure in your tires.

RAY: The air pressure in your tires drops when the outside temperature drops. You lose about one pound of pressure for every ten degree drop in temperature. So if you last filled your tires to 30 psi in the summer when it was 85 degrees out, when it's 25 degrees out, you're tires are down to 24 psi! That's why it's important to check your tire pressure throughout the year.

TOM: The other factor that could exacerbate the scraping is the funny thing that happens on roads in the winter. Roads develop two tracks where the tires go, and in between those two tracks, a heap of stuff called ice piles up. If the road (that is, the ice and debris) is getting closer to the bottom of your car, that would also make bottoming out more likely.

RAY: I wouldn't worry about it, Ken. Any day now, this air deflector will break right off and wind up with the rest of these things by the side of the road. You won't have anymore scraping this winter, and when the snow melts in the spring, you can find it and put it back on.

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