Originally Posted by CarEXPERT
Crank angles determine balance of engine?? I thought the couterweights and the flywheel balances out the power strokes.
They are used to balance out the forces created by the pistons.
Think about it this way ... (btw everyone, i know this is a dodgy example)
If you've got a ball on a piece of rope. Tie the end of the rope around your wrist and throw the ball. When the ball hits the extent of the rope and starts coming back you will feel it tugging on your wrist. This force is why an engine needs to be balanced.
Imagine that all the pistons went up and down together. Everytime they hit the top of the stroke (all at the same time), they would all try and pull the crankshaft upwards. Then at the bottom of the stroke, they would all try to push it downwards. This makes for a whole lot of shaking in the engine.
The counterweights effectively apply a similar force to the pistons, so between the force of the pistons and the force of the counterweights, there should (in theory), be no overall force trying to shake the engine.
In theory (if I remember correctly), a straight 6 cylinder engine can be balanced without counterweights at all.
Balancing comes in 2 parts, static and dynamic ... but that might be a bit complex for this thread.