Dear Tom and Ray:

While watching a baseball game, an ad on the backstop said to change your shocks at 50,000 miles. I realize this national shock-replacement company is trying to sell shocks. What I want to know: Is 50,000 miles a hard and fast number? If not, how does one know when to change the shocks?

--Ken

RAY: No, there's no reason to automatically change your shocks at 50,000 miles. We see lots of cars these days whose shocks last more than 100,000 miles. So these guys are just hoping to double their income.

TOM: If it works, we're going to try it, too!

RAY: Even to estimate how long your shocks will last, you have to evaluate them on a car-by-car basis. First, you have to factor in the quality of the shocks that came with the car, which varies. Then you have to consider the type of driving the car is asked to do.

TOM: If you do a lot of smooth highway driving, the shocks mostly are just sitting there, doing very little. They'll last a long time when they're doing nothing.

RAY: My brother's hoping for the same result for himself.

TOM: Whereas if you drive on a lot of unimproved roads with potholes and bumps and lots of dead armadillos, you easily could go through a set of shocks in 50,000 miles.

RAY: And how does a mechanic know if you need new shocks? He looks for leaks. If a shock is leaking, it's either worn out or it's well on its way to being worn out. That's the definitive sign.

TOM:There's also a practical test you can do at home. You get someone big (try a mother-in-law, for instance) to push hard on a corner of the car and get it bouncing down and up, down and up. Then, when it's at the bottom of its cycle, you let go and see what happens.

RAY: If the corner of the car comes up and stops, without going down and up again, that shock is good (assuming it hasn't started leaking). If the car keeps cycling up and down, even a little bit, after you let go of it, the shock is worn out.

TOM: Shocks don't really degrade over time. They're usually either good or bad, so there's no need to replace them unless they've actually stopped working. Good luck, Ken.