Today: The Bouncing Fender Test

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Dear Car Talk

Feb 3, 2015 (Archives)

8 Comments

Dear Car Talk:

I went to one of those "drive in for an oil change" places. Almost immediately after arriving, the young man started mashing down repeatedly on the fender above the left front wheel of my van. Then he told me that because the car kept bouncing after he stopped mashing on the fender, I need new struts. He said he also noticed an oil leak coming from my strut, which is more evidence that the struts are shot. Do struts really have oil in them? Wouldn't I have noticed extra bouncing when driving around? Is this a con?

-- Leisa

That's actually a legitimate test, Leisa. My brother used to do a similar test when he was buying a used car: He would mash down on the fender, and if the fender fell off, he'd know that was the car for him, and he'd make an offer.

When a strut (also called a shock absorber) is working correctly and somebody pushes down hard on one corner of the car, that corner should go down and compress the spring, and then come up once and stop. If it keeps going up and down, then the strut is worn out.

And yes, struts are filled with oil. If this guy noticed some oil on the side of the strut, that's another sign that the strut has failed. You wouldn't notice the leak yourself; it's a small amount of oil that seeps out, not enough to create a puddle in your driveway, or to lease to BP. 

And you might notice the extra bouncing when you drive, but you might not. You know the story about the frog they put in water, and then slowly bring the water to a boil? The idea is that if you had dropped the frog into already-boiling water, of course he'd notice. But because the water heats up gradually, the frog doesn't notice he's being cooked until some foodie is putting extra-virgin olive oil on his legs and tucking in his napkin. 

By the way, please don't sic PETA on me, folks. No actual frogs were harmed in any of my analogies.

Anyway, the same can be true of worn-out shocks, faded brakes, scratched-up windshields and live-in mothers-in-law: You don't notice how bad these things have gotten because they get just a little worse every day, over a long period of time.

In any case, I'd suggest that you get a second opinion on the shocks. You clearly don't trust this guy who changed your oil, and you don't have a relationship with the shop. In fact, you're right to be cautious, because there are shops that put mechanics on commission, so they have an incentive to urge you to do additional repairs.

I'd recommend that you either go to your regular mechanic, if you have one, or find one on www.mechanicsfiles.com. That's a database of mechanics personally recommended by your fellow Car Talk readers and listeners. 

If a mechanic you trust does the same test and tells you that you need shocks, you can then be pretty sure that the oil-change guy was on the up and up with you.

And if you do need shocks, we recommend that they get replaced in pairs. 

• 8 comments
• Car Talk

sooieeehog • a day ago

So you seriously think Ray Magliozzi doesn't know the difference between a shock and a strut? I think he said "strut (also called a shock absorber)" to simplify things, and didn't use it as an opportunity to show off all the knowledge had on the subject. Like, you know, some people might.

George_San_Jose • a day ago

"Shock" and "strut" seem to be terms used interchangeably among the experts here at the Car Talk Forums in informal suspension system discussions. I'm sure all of those folks know the difference, and how that difference applies to specific make/model/year vehicles. But no harm done to point out that there is in fact a difference.

SalesGeek • a day ago

I'm surprised nobody has mentioned this but the old "oil leaking from a shock/strut" is the MO for shady mechanics that prey on travelers and folks that know nothing about their cars. They bounce the cars then get underneath with a hidden oil can, squirt a little oil on the bottom of the shock/strut, swipe a finger in it then hold it up for the rube to see.

It's an old, old scame and a huge red flag.

Get a second or even third opinion.

• 2

clack jr • a day ago

whoa now fella's.... A shock cannot be used to replace a strut and a strut cannot be used to replace a shock. A vehicle will have either a shock or a strut at each wheel, never both. Every vehicle was designed with either shocks or struts and your vehicle's suspension cannot be changed to use the other.

The major difference between shocks and struts is that a strut is a structural part of the vehicles suspension system where a shock is not. A strut is also crucial part of the vehicles steering system and greatly affects alignment angles. Camber and caster angles are usually adjusted right on the strut itself. A strut is also a pivot point for the vehicles steering system and contains a coil spring. Because of this an alignment is always needed when replacing a strut. This is also the reason that struts are typically more expensive. Long and short.. a shock is not a strut and vicey versey,,,

• 2

I Would Think There Are Gas Or Air Shocks Along With The Other...

Art B • 6 hours ago

Surprised nobody is talking safety. When traveling down the road, or even worse a curve a car with a bad strut or shock will have the tire double dribbling on and off the road way. The idea for these devices is to keep the tire flat on the road. And another side effect as the tire is intimating Michael Jordan is it is causing the tire to wear unevenly. So not only do you need new struts/shocks, you will need new tires.

johnbarry • 15 hours ago

I think it is a good idea to get a second opinion. I went to one of those "drive in for an oil change" places once (and never again) when they tried to sell me something I had replaced at the Mazda dealer only the month before.

• •

I have never known a car of any age to pass the test as described here. Always, they at least come up above the rest point once, then below it once again, before stopping. Including brand new vehicles and those that have recently had the relevant parts (whatever you want to call them) replaced. But maybe I bounce heavier than some people