Help! My Car Shakes and Nobody Knows Why!

Dealing with a Mystery Vibration

Car in auto mechanic shop
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Almost any tire tech could probably tell you a story of a mystery vibration. The customer who comes in with a shake that won't go away no matter what you try. The guy who puts on new tires and comes back the next day, and the next, and the next... and walks away convinced it's either crappy tires or something you did. Those are the ones that keep you awake at night wondering “What did I miss?” Sometimes you track it down.

Sometimes it's a hub-center issue. Sometimes the load rating on the tires is too low. Once after a months-long and sometimes tense process of trying to diagnose a mystery vibration, the customer called me to sheepishly report that one of his engine mounting bolts was broken, and the unsecured engine had been shaking the car.

Part of the problem here is that there are simply too many variables. There are quite a few possible reasons for a vibration; tires, wheels, alignment and suspension being the four most probable. Let's take a walk through my diagnostic process.

I start by taking a history

A: Is the vibration more in the steering wheel or more in the seat?

B: Do you feel a vibration in the pedal under braking?

C: Do you hear an odd tire song?

D: Does the car pull to one side or t'other?

Answers

A: Steering wheel = Might be anything. Seat = Probably a back wheel.

B: Probably a warped brake rotor.

C: New tires = Possibly alignment. Old tires = Probably something out of round.

D: Alignment. Possibly other things too, but definitely alignment.

Next, I check the wheels and tires: The idea is to hand-spin the assemblies on a balancer. You're looking for a wiggle in the edges of the wheel or across the surface of the tire that indicates one or the other is out of round.

Look at the tires straight on – if the treads are moving back and forth, that would usually indicate an alignment issue, or less often, a wheel that is “center-bent” by a sideways impact.

See Diagnosing Wheel Vibration for more information.

Then I rebalance and rotate. Use a road force balancer that can read out the surface of the tire to see if any are vibrating excessively at speed. Once the tires are rotated, don't just check to see if the vibration goes away, check to see if it changes. Does it get less or go from the steering wheel to the seat? The problem was in front and is now in back. Stays the same? Probably alignment.

See The What, Why and How of Wheel Balancing for more information.

Still vibrating? An alignment that is even slightly off can easily cause vibration, especially when new tires are first put on the car. New tires have better lateral grip than older tires, and can pick up an alignment vibration much more forcefully. In addition, by the time your tires are worn enough to be replaced, you need a new alignment. You've gone what, 20-30,000 miles on those tires? You've hit potholes, bumps, bridge joints, you've driven around hard curves - believe me, your alignment is out.

I know it's an expense, but it should be reset for the new tires.

Were your tire treads moving back and forth on the balancer? An incorrect alignment means that the tires are either not quite flat or not all quite parallel. This creates constant lateral pressure on the tire treads, leading to vibration and irregular wear. Sometimes - especially when the car is vibrating due to alignment, bushings or control arms - the tires will “wear into the shake” in a way that damps out the original vibration. New tires can pick up this type of irregular wear amazingly quickly, within days or weeks of being installed. Over the long term it will substantially shorten the life of the tire, but, after a while, the actual vibration may go away as the tire wear compensates.

So when you put on the new tires, that nice firm even tread lets the vibration come through nice and clear.

But if you put the old tires back on, even if you're not putting them back on in exactly the same places, you might still see the vibration disappear. The worn tread has less grip to shake the car, and the funny wear injects random noise into the vibration harmonic. It doesn't mean the cause has gone away, it's just lost in the general noise.

If you decide not to get an alignment with new tires, and you then have a vibration that won't go away with balancing, get an alignment as soon as possible. If the tires have already started to wear, you may still get some residual vibration for a few days.

See Irregular Tire Wear: Causes, Indicators and Remedies for more information.

Worn or broken suspension components can also cause certain vibrations, so if alignment doesn't fix the issue we start looking at control arms and bushings in particular as a last-ditch measure.

But sometimes you just get a vibration that can only be blamed on gremlins. Something where you've checked everything and the vibration laughs at you.

I'm not saying tire techs are always innocent; certainly we make mistakes. But I spent 10 years specializing in finding and fixing vibrations, and there were still times that I had to tell a customer, “These are the wrong tires for your car, for reasons that I just don't fully understand.”