Typically its done in V6's and V4's and some straight fours depending on
their design, to offset the forces caused by the pistons flying around at
high speed that cannot be counter balanced by the more traditional methods
of counterbalance weights or harmonic balancing on the crankshaft. The
balance shaft counterbalances these forces usually rotating at half engine
speed with counter weights causing a reduction in these noticeable
vibrations. In four cylinder engines, usually over two liters, the forces
of the rather large pistons at high speeds cause similar vibrations that
cannot reasonably be dampened out by conventional means, and a balance
shaft provides a reasonable way to counter these forces. Straight sixes,
and v8's are inherently more balanced by their design (straight sixes being
the more balanced of the two) and can be dealt with using the more common
counter weights and crankshaft dampeners -
Barry Schwartz (San Diego) bschwart@pacbell.net
72 PI, V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
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