Information About Crankshafts - CHP How It Works

Crankshaft Experts Talk Metallurgy, Race Balancing, Counterweight Profiling, Heat Treating, Production Techniques, And Much More.

Stephen Kim May 28, 2010 0 Comment(s)
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Alan Davis: "When balancing a rotating assembly for a street motor, the goal is to equalize the rotating mass and the reciprocating mass. However, in race motors it's not uncommon to overbalance the crank. A balancer generally spins a crank 500-750 rpm, and for obvious safety reason, you can't replicate the actual rpm the crank will experience in a running engine. However, if you spin a motor at very high rpm, say 7,000-8,000, parts can stretch and move around. Aluminum rods might stretch as much as 0.030-inch. This stretch increases load on the crank and bends it, making the pistons and rods behave as if they're heavier than they really are due to dynamic inertial effects. To the crank, the pistons feel heavier. So if you have a rotating assembly that calls for a bobweight of 1,800 grams, a motor may run more smoothly if you overbalance the crank by two percent. In other ways, you'd compensate for the inertial loads the crank endures at high rpm by balancing the rotating assembly to a bobweight of 1,836 grams instead of 1,800 grams. The balancer will indicate that the crank isn't balanced, but the bearings will actually look better when you tear the motor down. On a 6,500 rpm street motor there's no need to overbalance. It's more for race engines than run 7,000 to 8,000 rpm all day."

Dwayne Boes: "Crank overlap is something you should always pay attention to. If you were to stand a crank up vertically, overlap can easily be seen as the portions of the main and rod journals that overlap each other. The more overlap you have, the stronger the crank. To keep production costs down back in the '60s, the OEs used cheap casting materials in their cranks, but compensated for it by making the main and rod journals very large, which increased overlap. With stroker cranks, however, moving the rod throws farther away from the mains inherently weakens the crank by reducing overlap. Likewise, high-end race motors often reduce the rod journal diameters of a crank to reduce bearing speed and friction. This also reduces crank overlap as well. Fortunately, with today's quality aftermarket forgings overlap isn't as big of an issue as it used to be."


Coast High Performance
Torrance, CA 90505
Callies Performance Products
Fostoria, OH 44830
Eagle Specialty Products, Inc.
Southaven, MS 38671


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