How Things Move and Why: Springs

Cars come with more springs than you may realize because your tires are springs, too. Fundamentally speaking, all springs have a rate. U.S. folks usually have it in pounds per inch, and it’s simple enough to imagine: If a spring has a rate of 100 pounds per inch, that means if you put a 100-pound load on the spring, it will compress 1 inch; 200 pounds would squeeze it down 2 inches; and on from there. In Europe, by the way, it’s kilograms per millimeter. Different units of measurement, otherwise the same.

Tires have spring rates the exact same way, as the sidewall compresses more or less based on the amount of weight the car places on it. And, better still, it’s an adjustable spring because the amount it compresses changes with how much air you fill the tire with. And that means that every car in the world with inflatable tires—you know, like, every car—comes with adjustable springs. How cool is that?

In fact, the compound of materials used in a tire, the way it’s constructed and its tire pressure have a huge impact on your car’s ride, handling, noise levels, fuel economy—just about every aspect of the vehicle. That’s why most cars sold today come with a set of tires specifically designed to work on that car. Vehicle dynamics engineers work with tire engineers for years to design and “tune” the tire to meet desired performance metrics.

It goes deeper than that, though. Different wheel designs are tested and chosen based on how they work with the tire. While wheels look totally stiff, they’re not. A wheel flexes and bends against forces applied to it—like taking a corner or hitting the brakes—just like the rest of the car, and the tire relies on the wheel to allow it to work well. Engineers will choose the wheel based on how well it supports tire performance. It also means tire performance affects what springs and shocks they choose. Tires play a critical role.

It’s why I cringe when I see people buying the cheapest tires possible from Costco or where-ever; “You’re ruining your car!” I always scream in my head. When it’s time to replace a set, if you don’t buy tires from the dealership, you most likely are not getting the actual specifically tuned tire for your car. That’s not the end of the world; in fact, it can be an improvement. But no matter, be sure to buy a high quality set of tires. This is the last place to go bargain hunting.

But if you’re a proper enthusiast, you probably already knew that, right? In that case, let’s get back to the “adjustable spring” bit. If you want to take your car to a track day, know that the tires play an important role in its performance. Speaking with Woody Rogers, director of tire information at Tire Rack, he offers a few tips:

Tires work best when they stay on the tread blocks, not rolling onto the shoulder or sidewall. This requires higher pressures, or pounds per square inch (psi), than usually recommended. Typically, a rear-wheel-drive car will want pressures in the high 30s, front- or all-wheel-drive in the low 40s. This is a hot measurement, which generally is 6 psi higher than cold, as you’ll see about a 1 psi gain for every 10 degrees of heat.

The goal is to find the pressures that keep your car on the tread blocks. Pay close attention to them and check pressures immediately after every session while tires are still hot. Once you’re in the ballpark, start playing around in 2 psi increments to improve balance. Too much understeer, add pressure to the front tires; you’re probably rolling them over the shoulder. Oversteer? Do the same to the rear.

Want even more good track advice? Check out this Engineering Explains video on how good tires improve track performance much more than engine power.

Tires are springs. And so much more. They are the single most important part on your car!

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