MAGLIOZZI

Dear Car Talk: I have puzzled over a feature that all of the vehicles that I have driven over the last 65 years possess. And I have never understood it.

When I turn a corner, in either direction, and loosen my grip of the steering wheel, the vehicle somehow returns to a “straight” direction to follow the road. How does that work? — Steven

I don’t suppose you’d believe that these vehicles are sentient beings, Steven? Didn’t think so. And I’m assuming you’re asking for a mechanical explanation — not a treatise on modern-day self-driving Waymos that use sensors and motors to follow the road without drivers.

The answer is something called caster. Caster is one of three alignment variables on cars (along with camber and toe) and defines the angle between the upper and lower steering pivot points.

Here’s the easiest way to understand it: Take a shopping cart in your local supermarket. Start turning it and then give it a push. Notice how it also straightens out?

Now, go and help the clerk restack the 400 cans of Campbell’s soup you just knocked over, and I’ll explain why it does that. If you look at one of the front wheels, you’ll notice that there’s a rod that goes up and connects the wheel to the cart.

And if you look at it from the side, that rod is toward the front of the wheel and forward of where the wheel touches the ground. In other words, most of the wheel is behind that connection point. That’s known as positive caster. And because the cart effectively “drags” the wheel behind it, the wheel follows the cart’s direction and straightens out.

Why do we use positive caster on our cars? First, it creates stability. It provides a self-centering force, so you’re not having to constantly correct your steering to keep it straight as you drive. This is particularly useful at higher speeds.

It’s also responsible for the steering return you ask about. So, you have to actively work to turn the car, rather than actively work to keep it straight.

And, finally, it improves handling. Because of the physics of positive camber, it allows more of the outside, cornering tire’s footprint to be on the road.

What’s the downside of positive camber? It requires more effort to steer the car outside of a straight line. That was a problem 100 years ago. But that was solved with power steering. So now, positive camber steering is effortless. And the benefits are considerable.

Got a question about cars? Write to Ray in care of King Features, 300 W. 57th St., 41st Floor, New York, NY 10019, or email by visiting the Car Talk website at www.cartalk.com.