Why is my car shaking?

There is no secret that shaking isn’t good for your car. It loosens bolts, wears out your engine or suspension, and causes squeaks and rattles in the cabin. It also makes even a short trip a chore…

Shaking is a symptom that something is wrong. At the same time, shaking can be doing damage. It’s a symptom of a problem and a cause of many others.

Like a doctor, we need to see past the symptom and isolate the cause before it makes things worse.

Given the number of parts in your car, figuring out what is causing the shaking can be time consuming, even for the experts.

But fear not, narrowing down the cause of that worrisome shaking is easy to do.

First, a word of warning. Cars aren’t meant to shake. If something is shaking on your car, make sure you get it looked at before something drastic lets go.

The most likely scenario is that your wheels need rebalancing at your nearest tyre shop, but it could be something much more dangerous.

A tech talk about shaking wouldn’t be complete without a story. Perhaps my most dramatic shaking story was while competing in the Targa High Country rally in an FPV F6 that had just had a new engine installed.

About halfway through the first day we noticed a wobbling noise near the end of a stage. My first thought was that one of the wheels was coming loose. Not a nice feeling while driving a car flat out down a tree-lined road…

I tried playing with the steering to see if the wobbling stopped while we were cornering. No difference. It seemed to change a little when I was on or off throttle. Odd. But it wasn’t affected by the brakes.

We finished the stage and I jumped out of the car and checked the wheel nuts. They were all tight.

We had one more stage before we reached our service crew and could take a proper look. This stage opened out and we were reaching speeds of 200km/h. Thankfully, whether it was the vehicle speed or being higher up in the rev range, the wobbling stopped when we were really motoring.

Actually, it seemed more related to the engine speed – it was actually at its worst while idling.

We made it to the service in one piece and furiously tried to diagnose the problem in the 20 minutes allocated. We looked at engine speed related parts: harmonic balancer, accessory driven parts like the alternator. Nothing.

We looked at vehicle speed related parts: wheels, tailshaft, diff, wheel speed sensors. Zip.

We knew something was loose but were no closer to figuring it out. Time was almost up. I jumped in and went to start the car. The starter motor was free spinning… The flywheel :o !!

The rotating inertia of a flywheel in a high-revving engine is a scary thing to behold. It will literally cut its way through a bellhousing, transmission tunnel and anything else that stands in its path in its bid for freedom.

We made sure the replacement flywheel had thread lock glue applied and was triple torque checked.

Before we get into how to troubleshoot shaking we need to classify which type of shaking it is.

Broadly, there are two types: rotating imbalance and impact aftershake.

The type we saw above was a shake or wobble that was related to the speed of the flywheel’s rotation.

The second type, impact aftershake, is effectively the vibration in your hands when you are holding a metal bar and your friend dongs the other end with a hammer.

There can also be a third type: aero resonance.

If you are in a high-speed chase in a convertible supercar with the roof down, have a stern word with yourself for not putting the roof up before antagonising the authorities. For the rest of us, the most common aero resonance can be solved by opening a front window when the back seat passenger has his head stuck out the window barking at passing cars.

Like all troubleshooting, figuring out what is causing the shake is often a case of ‘seat of the pants’ best guesses and trial and error. First we need to figure out what kind of shake it is.

Is it a continuous wobbling? That is something that is rotating. It’s either loose or out of balance – the accompanying noise will normally tell you which.

Does the shake change speed as something else changes speed? If it matches the tachometer, then it’s something attached to the engine.

Does it seem to match the speedo more, wobbling faster as you speed up? Try sitting at a constant speed and change gears. Did it change with the revs or stay constant with the speedo?

Safely try loading up the car in some cornering. Does it come from the steering wheel or from the seat?

All of these tell-tales are clues that will point us to the solution.

Does it get worse as you increase speed? It could be wheel balance. Does it get worse when you slow down? You might have a loose wheel. Is it only while you are braking? It could be a warped brake disc.

Sometimes loose things hold themselves in position when they are spinning quickly with the gyroscopic effect – like a spinning top, or the centre bearing of a Ford Falcon tail shaft.

Once you have a few clues it’s time to either pass by a tyre shop for a wheel balance or do an inspection for anything wobbly, loose or out of shape.

On the flipside, maybe it isn’t a continuous shake, but rather a wobble that starts after hitting a pothole and slowly decays.

This is your classic impact aftershake – like a guitar string in slow motion. It might just be the natural behaviour of your rickety old jalopy or it could be a sign that something has worked its way loose.

Vibrations like these are much harder to track down.

It might be that you need to tension up your spare wheel holder on your cab-chassis ute to damp out the resonance. (Yes, that is a real thing!) Or it could be that your wishbone or leaf spring bushes are shot. Instead of spanner checking, some careful tapping under the car with a nylon hammer will give you more of an idea of what’s going on.

In many cases you may need to seek professional assistance to diagnose and remedy vibrations and wobbles.

In reference to the explanations above, your mechanic will likely ask you to answer the following questions. Being prepared may save them time, and you, money…

• Is the vibration or wobble continuous or intermittent?
• Is the vibration related to engine speed or road speed?
• If it’s road speed, does it increase when you speed up or when you brake?
• If it starts after a pothole, do you feel the vibration through the steering wheel or the seat?

After going this far down the rabbit hole of shaking, it’s worth mentioning two fundamentals: balance and natural frequencies.

Balance is what causes the shaking of something that is rotating. In effect, the sum of all of the centripetal forces don’t match the centre of rotation.

Like the spokes on your bike connecting to a point in space rather than the hub. They don’t meet in the middle because the weight of the rim isn’t evenly distributed. Time for some wheel weights.

Natural frequencies relate to impact forces. The frequency you hear when you dong something with a hammer is its natural frequency. It is related to the mass and stiffness of whatever it is you’re donging and whatever that is connected to.

Take guitar strings; your heavy top E string is less stiff than your light bottom E string. That combination of weight and stiffness means the strings naturally resonate at different frequencies, it transfers those frequencies into a nice wooden box and out comes a note.

Same thing when you hit a pothole, it plucks the string of your chassis, you feel the vibration in the driver’s seat and you hear noise amplified while sitting inside the metal box of the car body.

Like gold panning, shaking gets things moving, but with cars, there is no pay dirt. The movement caused by shaking in a car wears things out, or worst-case, completely undoes things.

Car manufacturers have specific shaker rig testing where they make sure the bolts will stay tight and the interior won’t start squeaking after you drive the corrugated Gunbarrel Highway.

Either way, it’s always better not to tempt fate and get that shake attended to.

So there you have it. Shake 101 without a single mention of Elvis. Well, almost.

Once you have that shake fixed you will be back to many hours of safe, enjoyable motoring. Safe travels.

via GIPHY