Why would the engine shake more when idle speed is lower than 800rpm?

Experiment Leads to Conclusion

Physics Will Help Explain

If you place a force onto an object, there is always a feedback that is pushing back at you with an equal amount of energy.

If you take a rope and attach a weight to the end and swing it around at the man powered equivalent of 2000RPMs(high energy). It tends to give more of a smoother movement all the way around the full rotation. This happens because the force being placed on the rope by the weight's speed is enough to keep the rope tense in which you can use the full potential of the gained (tension/feedback)energy. Anytime a rope is stretched it becomes stressed and is trying to return to it's relaxed state, the rope starts to pull back as a simple law in physics says it should do. This makes using already gained energy(tension/feedback) easier to keep the weight from loosing speed and reducing the stress placed on the rope. Rope is very flimsy while relaxed but if you stretch it(thats what the weight is doing) it gains feedback then it becomes tighter, more stable, and stronger increasing with the amount of force placed on it. But when you stop swinging fast and slow down to a speed that would be the man powered equivalent to 800RPM or lower. The rope looses tension so now it lacks the feedback energy to successfully push and pull the weight through a good full rotation. The rope will still spin around due to the relaxed state, but soon as the weight reaches 12 o'clock in the rotation and there is just enough momentum left to make it to 11 o'clock. Then the weight just lazily plops down which creates a sudden jolt of tension in the rope which makes it jerk back up. The weight applies force to the rope which converts potential energy into feedback which can be used to stabilize the rope so it can be used most efficiently. The harder you push, the harder it pushes back. This concept directly applies to your engine jittering all about in the chassis.

Im not saying every little detail applies to your engine in exactly every shape and form explained in the express Newtonian Physics course....I had to retrace my steps just to finish it, got confusing....

The motor-mounts are large springs that are meant to hold the engine in place while running and reduce the amount of shaking. Im assuming the car isnt brand spanking new so obviously those springs have a bit of wear and tear. They have been compressed and relaxed many different times to the point that they are beginning to bend inward and loose potential energy as a whole. Plus they have lost most of their ability to hold the engine still at Low RPMs(low energy, low force applied and even less feedback, the shaking is a direct result). When the car gets going at High RPMs(High energy, large amount of force and equal or greater feedback) it places enough stress on the motor-mounts so they will recoil(feedback) and force the engine to stay in place. The wear and tear on the mounts has greatly increased the threshold marker for the amount of force needed to be placed to get the desired effect.

Dont mind the glorified physics explaination..... Got bored with trying to explain in detail something as dull as large springs called motor-mounts. Needed some flare!

First answer by Walama. Last edit by Walama. Contributor trust: 128 [recommend contributor]. Question popularity: 4 [recommend question]