4x4Wire's TrailTalk Forums: VIBRATION From Engine?

I found that new mounts had a bit more give to them and helped isolate the motor better, not a cure all but it may help.

Some vehicles without balance shafts use silicone filled mounts with internal chambers to further isolate the motor. On these same engines they design them so that the resonant frequency of the parts bolted onto the motor ( intake manifold, alternator etc) is higher than the second order vibration from the uneven piston acceleration all engines have. This prevents the parts from singing along.

Pistons accelerate faster off of TDC than BDC since we don’t have the luxury of infinitely long connecting rods--- an unequal acceleration exists between each pair of pistons as the motor hits TDC / BDC.

Longer stroke motors can’t grow the rod length enough to keep the vibration reasonable, hence the need for balance shafts on motors like the 2.6.

Balancing the pistons, rods and crank does not get rid of the basic problem. It can eliminate first order vibrations and even out the magnitude of the second order vibration so that each second order vibration is that same as the other but it cannot get rid of it, just even out the size of the pulse from piston set to piston set ( two pistons up VS two pistons down, they travel in pairs on a 4 cyl).

The only way to reduce the size of the vibration is to lower the weight of the pistons, reduce the weight of the rods or use longer connecting rods.

As you get a few miles on the motor it will smooth out at idle bit as the friction level drops. High friction slows the motor more between firing pulses adding to the shake at idle.

At higher RPM's the motors without balance shafts will make more noise since the vibrations are not canceled internally. When the frequency of the vibration is the same as the natural frequency of any component on the motor that part will vibrate in unison, just like a tuning fork ( a harmonic of the frequency will do the same thing but at a lower amplitude).

I stand by claim that the main difference is some people just don’t notice the difference. Kind of like how some people can drive a car around with a low tire and one broken shock and not notice.

Kevin

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87 Dodge

Edited by Kevin C (11/03/07 11:52 AM)

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