Hi - I'm trying figure out if crankshaft counterweights can be effectively used on other engines besides 90 degree v8s with crossplane cranks. In reading about crossplane v8s, it is explained that the crankshaft causes pistons to go in opposite directions at each end of the engine which would normally cause the engine to rock. This is supposed to be counteracted by using crankshaft weight to balance each pair of pistons. In the diagrams that I have seen, this arrangement of a counterweight works nicely with two pistons that are 90 degrees from each other. That is, the side-to-side motion of the weight falls in the plane of the other piston, so the weight is always cancelling one of the pistons and is not causing extra vibration by itself. The diagram typically just shows two pistons. My question is then, why can't the counterweight system be used for any number of cylinders as long as they are in a 90 degree vee? I've often seen that the v6 is notoriously hard to balance. Why not make a crank with 3 throws that are 120 degrees apart and have 3 pairs of pistons (each pair connected to the same throw) and have counterweights that will balance each pair? Since you've balanced each pair, you should not have to worry about the engine rocking, just as the rocking was corrected in the crossplane v8. Of course, you wouldn't want to use counterweights on an engine that really doesn't require it (eg V12). Now I know I'm missing something, otherwise this would have been done already. What am I missing?

Uh it IS. Counterweights are used on just about every type of engine. Either on the crankshaft itself or attached to the crankshaft externally such as a weighted flywheel. Uh that has been done since about 1964. They are already there. I have yet to see any crankshaft (other than a simple flat 4 or 6) that didn't have counterweights. Many even use balance shafts to cancel out the rocking coupling that occurs in the engines. Has a bolt on external weight.

Why not use crankshaft weights on any vee engine?

Nov 25, 2008 42 Replies

Rate this thread:

jim 17 years ago

Actually, no that is probably incorrect. Those bosses are probably what foundries call "locators" There are actually 3 that I see in that picture and their purpose is to provide a way to reference and coordinate the orientation of the crankshaft for first machining operation, quality control fixturing and tooling design at the foundry.

Yes probably true, but since all this can be modeled in computers nowadays the weight distribution can be fairly easily incorporated into the whole design and as a result what is added or subtracted for weighting may not be so obvious. The locators play an important role because they enable each part to be fixtured and machined in a repeatable way. Which means the weight distribution will be pretty close to the same.

-jim

Vote

Steve 17 years ago

Opposed-piston engines do have vestigial balance weights on the outboard ends. It compensates for the fact that the pistons aren't actually DIRECTLY opposite each other, but are offset by one throw distance. Compared to the weights on other crankshafts, they're practically nonexistent. Without them the engine would have a rotational vibration making it want to twist back and forth around an axis perpendicular to the plane containing the cylinders.

Ben91932 obviously hadn't followed the whole thread through or he would have seen that we'd been through that already.

Vote

Steve 17 years ago

IIRC the GT40 crank is just an ordinary 90-degree crank. What it has different is a set of exhaust plumbing that criss-crosses a pair of cylinders so that each of the dual exhaust tubes is even firing, unlike any configuration where you just take the 4 left cylinders for one pipe, and the 4 right cylinders for the other pipe from a 90-degree crank v8.

"Flat-crank" or "180-degree crank" v8s (think late 90s IROC or CART, I forget which) fire that way naturally, but have worse high-order vibration than an inline 4, because they are TWO inline 4s set at an angle. Thats why flat-crank v8s are never used for street cars.

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required