Counterwighted cranckshaft

The non-counterweighted crankshaft is balance but only across its latitude. When it spins the higher weight at each rod journal tries to put the crank into a "S shape. (and does). A counterweighted crank has an offsetting weight on the opposite side of the rod journals. Thus, the crank is balanced on its latitude and longitude (at least much closer)

Jim SR Racing

Someone smarter than I am can show you how a couple static ounces of inbalance becomes hundreds of pounds at 6,000 RPM, thus tearing your engine to pieces.

The stock crank is statically balanced. That's true. A stock crank CAN be dynamically balanced, but I don't think that they originally were.

The real advantage of the counterweighted crank is that each of it's parts is balanced. In a standard crank you can have a heavy section (the #1 rod throw) counterbalanced by the #3 rod throw. Since these are not actually in the same axial position along the crank, at high rpms the crank will actually bend some as a result of the forces from these 2 weights.

With a counterweighted crank, the bending forces are minimized. In the stock crank the #2 main bearing tends to get pushed out of shape due to the crank bending there as it's rotating.

You're right. A CW crank has a higher moment of inertia, but it's not much higher since all the extra mass is at a rather small radius, compared to the flywheel, for instance. So, while there IS a penalty here, it's minor compared to the benefit of not damaging the case at the #2 main bearing web.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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