Don't Go Nuts With The Lighter-Weight Lug Nuts

Dear Car Talk:

Can you explain the physics and engineering for the concept of "un-sprung weight?" I see people who drive cars like mine (a 2006 BMW Z4) talking about how to "reduce un-sprung weight."

What are they looking to do?

Thanks. -- Russ

"Un-sprung weight" is an automotive engineering term. So don't feel bad that you didn't learn it in 6th grade vocabulary class, Russ, along with "egregious."

In the simplest terms, "un-sprung" is everything below the car's springs. So, it's things like wheels, tires, axles, and brakes.

If you want to get picky, there is some weight that's considered partially sprung weight. There are some suspension parts that attach both above and below the springs. But for the purposes of explaining the concept, think of it as mostly wheels and tires.

Why do people want to lose un-sprung weight? Well, aside from making your car look better in a bathing suit, people with performance cars want less un-sprung weight because it improves the car's handling.

When you drive over bumps, the tires and wheels get bounced up in the air. The lighter they are, the quicker the springs can force them back down onto the pavement and keep them there. The more time the tires spend in full contact with the road, the better the car's handling.

Heavier wheels and tires react more slowly, and when they hit the ground, they're more likely to bounce up again. It's more weight for the springs and shocks to manage. This is not a perfect analogy, but imagine you were friends with my Uncle Nunzio, and he gave you a pair of cement shoes. How maneuverable would you be? Not very, right? Your feet would be slow to pick up and move.

So, to reduce un-sprung weight, manufacturers (or car owners) replace steel wheels with lighter, aluminum alloy wheels. That's probably the most cost-effective change you can make. And for people who drive on the streets (as opposed to the track), doing more than that is probably not worth the money.

Still, some zealots will look for narrower wheels that can be fitted with narrower, lighter tires. And they might even replace steel brake calipers with alloy calipers and use carbon ceramic rotors. You can go nuts with this stuff and even try to save a few ounces with lighter-weight lug nuts.

But by then, your family will have left you, Russ.

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