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If a well balanced system starts moving, it should not vibrate ! Or if it does, it should vibrate even more at high speeds !

But a car clearly vibrates hard during the startup, to then stabilize.

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Not all cars do this, I6 and V12 engines have opposing cylinders that fire simultaneously and they are considered "balanced", at any RPM. But even still, we're talking about explosions shooting metal slugs at insane speeds: there's going to be some vibrations. –  Nick Aug 14 '13 at 17:18
    
@Nick: Thank you for this valuable comment. I did not now that specificity about those motors. –  Skippy Fastol Aug 19 '13 at 20:13
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2 Answers

The core reason is that the core of an engine is a series of explosions moving asymmetric components by pressure and then venting gases. At speed, these happen so fast that there is no obvious bang before the next one, but think about what happens at the initial stages of starting your engine:

  • 1 explosion forces a piston down, which pushes the connecting rod to rotate the camshaft

You can see from the diagram that there is a counterweight, but to get it started is a bit asymmetric. Usually the firing of the cylinders is not smooth at first as well, so until the idle speed is reached, there may be explosions happening at irregular times - leading to vibration.

enter image description here

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It is a well balanced system, but remember that it is not purely circular...the main shaft is circular, but the pistons are at an angle to the circular shaft, thus during low engine RPMs (startup, stall, etc) the vibration is increased as the system is cast out of balance.

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So the reason is the non-circular distribution of the cylinders geometry... I see. Makes sense ! –  Skippy Fastol Sep 21 '12 at 16:29
    
Pretty much as I can best explain it –  jsanc623 Sep 22 '12 at 6:41
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