TOM: They don’t just fail in collisions. They’re under stress all the time, and eventually, motor mounts just wear out.
RAY: When your engine is just idling, and not trying to move the car, it probably does sit quietly. But when it’s delivering its “twisting power” to the drive train, that creates one of those Newtonian situations where every force has an equal and opposite reaction. And if the crankshaft is twisting one way, the engine tries to twist the other way.
TOM: The job of the motor mounts is twofold: One is to hold the motor in place. The other is to provide some isolation between the engine and the passenger compartment, to cut down on vibration. So if the mounts are broken, you’re likely to feel more vibration from the engine, you might hear the engine bump around when you go over bumps, and when you step on the gas, the engine literally will try to lift up out of the engine compartment.
RAY: That’s not necessarily bad for the belts, but it can cause other problems. In an extreme case, the engine can move so far that the axles can fall out and cause the transmission to leak.
TOM: But just looking at the engine while it’s idling is not an adequate test for the mounts. The test we do is that we find someone brave or dumb enough (e.g., my brother) to stand off to the side of the engine with the hood up.
RAY: And then a mechanic will plant his foot on the brake, put the transmission in gear and give the engine some gas without letting the car move. That “loads up the engine” and simulates the conditions that take place when the engine is trying to move the car. And if the motor mounts are bad, you’ll see the engine jump up several inches.
TOM: At which point my brother tries to catch it. No, not really. If the mounts are good, the engine shouldn’t move more than an inch or so.
RAY: So go back and ask the dealership if they’ll do that test for you. My guess is that they’ve already done it. But if you see it for yourself, you’ll feel a little less bad about spending hundreds of bucks for new mounts. Good luck, Marsha.
Pebbles-on-ice theory has a couple of flaws
Dear Tom and Ray: The other day, I installed winter tires on my car (I have those tires on separate rims, so I can do this myself). Prior to attaching the wheels to the car, I checked the tires for any damages I might have overlooked last spring. Apart from one or two pea-size pebbles, I noticed quite a few very small pieces of gravel that had lodged themselves in the small crevices of the tread. I cautiously removed all those gravel bits — but while doing this, I wondered, don’t those embedded pieces of stone somehow resemble the studs or spikes we used to have on winter tires (which have since been outlawed where I live)? Maybe I should intentionally allow small pieces of debris to collect there. My question is: Am I doing the right thing when I remove those bits of stone, or should I instead take the car for a spin in the gravel pit to create “instant studded tires (just add stones)”? — Wolfgang
TOM: It’s “Eureka!” moments like these that lead to great inventions, Wolfgang.
RAY: Notice we said moments LIKE these. Not this one in particular, Wolfgang.
TOM: Unfortunately, the pebble theory has a couple of flaws. If you managed to embed enough pebbles in your tire to make a difference on ice, you’d run into the same problem that studded tires have: That is, they’re awful on wet and dry roads — or any non-icy surface.
RAY: Think about it. You’re essentially driving on little pieces of metal (or stone, in your case) instead of on rubber. And to make it worse, the pebbles are much more random in size, and therefore are less controllable and predictable than even the lousy studs.
TOM: But you don’t have to worry about that, Wolfgang, because the pebbles won’t stay in there anyway. As soon as you reach any kind of decent speed, centrifugal force will throw most of those stones out, banging them against the underside of your car and the windshields of the poor schmoes driving behind you.
RAY: So file it under “interesting theories, still in development,” and take out the pebbles, Wolfgang.
Send your comments or questions for Tom and Ray to: Car Talk Plaza, P.O. Box 3500 Harvard Square, Cambridge, MA 02238. Listen to them Saturdays at 10 a.m. on 91.3 FM or 88.5 FM. Visit them on the Internet at www.cartalk.com.
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