Topic: Secondary Vibration?

[Jay B]

I'm no engineer, but I think I understand the forces that make a motor vibrate fairly well. My question to the experts here is what causes the secondary vibrations typical in an inline 4? Everything seems like it should balance out, no rocking couple, etc. Just curious.

[ofreen]

The secondary vibration occurs because piston speed and acceleration of the two outer and the two inner pistons is not symetrical thoughout the crankshaft's rotation.

[hcritz]

Hey Jay...
The vibration I think you are talking about is caused by what is called a "Rocking Couple"...It would take a while to explain here, but you could do a google search...you might get a few "Strange" sights too!<G>
Most larger in line four cylinder engines now have counter rotating balance shafts to eliminate it. They run at twice crank speed, so In a high rev engine like these...you are looking at some substantial rpm!