Why do cars vibrate more when still in comparison to when they are moving?

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Why do cars vibrate more when still in comparison to when they are moving?

In: Physics
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Your car likely has a part that exists between your engine and your transmission called a torque converter. Your engine and transmission aren’t directly connected by a solid rod, they’re split by the torque converter, which is full of fluid. It allows the two to slip past one another, and provide bursts of extra torque when your car needs it.

If you idle at a stoplight while still in Drive, your engine is still spinning, but your brakes have your transmission locked up. All of the force of your engine is being dissipated by your torque converter, which is slipping, and this causes a great deal of the shuddering.

Put your car in neutral when you idle. The shuddering will decrease substantially and it’s better for your torque converter. There’s a reason why you can pop your gearshift between Drive and Neutral without pressing the gearshift switch.

Lower RPM’s have a tendancy to cause vibrations, simply because the motor isn’t in its optimal power range. Its worse on lower cylinder cars, for example 4-cyl cars at idle can be very jerky, and its about firing order, cylinder count, and the balance of the motor overall. If you have a stock V8 and a stock 4-cyl (typically straight, or ‘inline’) the v8 has a pattern that keeps the motor mostly ‘balanced’ with the firing pattern of the cylinders. A 4-cyl has less cylinders to fire (and bigger cylinders that are on the same plane), and will create more vibration because of the slower firing pattern. Add in the weight of each cylinder moving up and down in its 4-stroke cycle (commonly verbalized as “suck, squeeze, bang, blow” relating to pulling in the air/fuel mix, compressing it, spark ignition, then exhaust) and that cycle at a low RPM, or low cycle rate, causes the motor to move more because of the lack of counter-balancing actions (given that motors are all counter balanced via the camshaft). Where higher RPM has a counter balancing ‘resonanace’ from the motor and moving parts on the motor, that ‘balances out’ the forces of each cylinder compared to the last cycle (S-S-B-B, as described above) and the vibrations act against each other, making the cabin feeling very ‘neutral,’ until you reach the second resonace where both the noise and vibrations will be noticible in the cabin again for a short time through the RPM range.
In other words: physics of moving objects that are larger and slower will have more of a noticeable effect than smaller ones, or faster ones, until a resonance is reached that matches the given harmonics of the moving objects.
Not really the best ELI5, but its all i can manage.