Assuming you're not driving over pea gravel or experiencing an earthquake, unwanted vibrations coming through the steering wheel or floorboards can not only be a nuisance, but also detrimental to your vehicle and all its thousands of parts. Trying to pin down what causes unwanted shimmy and shaking is a broad subject, so what's it going to take to eliminate the shakes? Is that vibration coming from the rearend? Maybe it's only a tire. We're offering some generalities to help whittle down what it can be and how to amend it.
Torsional vibrations, or harmonics, come in three different varieties: engine, driveline, and the dreaded none-of-the-above category.
Engine Vibrations
The easiest way to know if you're experience engine harmonics is to park the car on a level surface, place the transmission in neutral, and let the engine idle. If you feel vibrations without the car in motion, then it's in the motor. What this usually means is that the engine is off-balanced, most likely in the rotating assembly.
Imbalanced rotating assemblies are typical when a motor has been built away from stock specs. Changing the pistons, crank, and connecting rods alters the original blueprinting, and, ultimately, the weight distribution of the engine. Even many OEM parts may not be precisely balanced to match the identical factory specs, as they are balanced on the average weight of the OE part.
With high-performance engines, specifically blocks that have seen some hard racing, boring over-size or even sleeves with new pistons will help cure such problems. Yet, changing the dimensions of the block, accompanied with new lighter pistons, definitely alters internal balancing.
Nearly every professional engine builder agrees that regardless if you're building a small-block cruiser or a ten-second street mauler, you should always conduct a full engine balancing. Balancing reduces internal stresses and vibrations that strain parts, cause excess friction, and may cause engine failure. make sure accessory items, such as the flywheel, are of the same balance as your engine. If you have an engine that is externally balanced and use a neutral balance flywheel (or vice versa), you will definitely feel some vibrations.
If you refuse to believe that your engine is out-of-whack, it might be a few things you missed, possibly low-operating temperature (usually in colder climates) or low oil pressure; other symptoms will alert you of these problems, such as internal knocking and poor throttle response.
Driveline Vibrations
Harmonics found in the driveline can be found anywhere between the flexplate or torque converter, and the rear axles. Usually driveline vibrations are felt only when the car or rear wheels are in motion, typically at a specific rpm range. The list of possibilities is a mile long, so we'll give you the more prevalent ones.
When having your engine's rotating assembly balanced, a good machine shop will ask for the flywheel to balance with the rest of the rotating assembly.
The possibility of a damaged or imbalanced flywheel is rare, but it does happen. Usually if the flywheel isn't mounted dead center or if there happens to be a heavy spot on it, the misalignment will cause severe harmonics. More often than not, driveline vibrations are due more to mechanical failure than human error, but don't pat yourself on the back just yet. We've seen some really stupid mistakes (usually ours).
Very rarely do torque converters give out without some other indicator, yet a converter whose internals have separated will definitely cause vibration. aside from a damaged output shaft or its bushing, there will be major vibrations caused by the internals of a transmission without other serious signs such as slipping or poor shifting. Be careful to check for any vertical play in the output shaft.