Crank Dynamic and Static Balencing

[Rob H]

For a given balance factor can anyone explain the difference between static and dynamic balancing of a crank and if a crank is static balanced first and then dynamically balanced what diferences may one find.
Look forward to hearing and learning from the more technically minded members, regs

 

[BigEd]

Dear Rob,
You may soon get bombarded with answers from the more technically minded posters. In the mean time here is my very simplistic and probably incorrect understanding of it. A crankshaft may be "in balance" when placed on knife edges so that it will stops however it is placed. If it is "out of balance" the heaviest point will always move to the bottom. This crankshaft is said to be in balance statically.
Dynamic balance is achieved when the crankshaft is in balance when it is actually rotating. A crankshaft that is in balance statically may be out of balance when rotating in the engine. This might be because although the flywheels may be the same weight their individual heavy point may not be at the same point on the circumference. e.g. the heavy point one one flywheel could be at 3 o' clock and 9 o' clock on the other so statically they would be in balance. When rotating fast they would be out of balance.
Cars with very wide tyres benefit by being dynamically balanced because if one side of the tyre is heavier, e.g. the outside, this would tend to make the wheel turn in.
I'm going to the Coventry VOC meeting now while everyone else gives the correct answer.

 

[roy the mechanic]

My thoughts -dynamic balancing sounds good on paper. In real life it is only relavent to seriously modified hot-rods. The crank has to be assembled without the rods +bearings. Every time the assembly is stripped and reassembled the "fit" of the crankpin suffers. So the lifespan of said crank has to suffer. So. unless you have a bunch of mismatched rubbish, leave the poor old goat alone, ducks to avoid "incoming" !

 

[Vincent Brake]

a single, twin, triple, 4 cranck / piston assy, is never in balance , by phisical law it cant, as there are rotating and reciprocating masses at the small end, whom turn at the big end, only a straight six comes a good way, if it would be a very very short one.
anyway thats not the question:
static balancing will set a cranck/piston assy. at a given % of balance.
dynamic will do ???? it a different way ha ha (yust had a nice red wine)

 

[Monkeypants]

If you have a Norton twin, you likely know who Jim Comstock is. Aside from all of his other training, he actually went to am "Engine Balancing School" at one time. Even so, for years he believed that static balance was all the was needed and that dynamic balancing was a bit of smoke and mirrors. Not anymore, apparently:

Glen

 

[davidd]

Neville goes into a lot of math, but he seems to acknowledge the need for dynamic balancing:

http://www.voc.uk.com/net/docs/3.2d/3.2-590-18.pdf

I thought this Service Bulletin by S&S was interesting.

David

 

[roy the mechanic]

Thanks for the s+s pdf, it reinforces my previous post about the down side of re-assembly and its negative consequenses. Roy.

 

[Monkeypants]

There is a company in the US with a setup for dynamically balancing pressed together Harley cranks without disassembly. There is a "reciprocater" built in to the balancer such that the pistons and rods are left in place for balancing. The pistons run up and down in the reciprocater during dynamic balancing just as they do in the engine.
Great idea. I'm trying to convince them to set up for Vincent cranks.
Glen

 

[highbury731]

If you have a Norton twin, you likely know who Jim Comstock is.
Glen

I have, but I didn't... I followed the links to another Jim Comstock video, and found this one on long-chain polymer oil additives. He did a test using oil with 25% STP, and quickly got valve seizure:

I am not sure, but I think that the valve used had a special coating which may be some kind of ceramic.

 

[roy the mechanic]

The coating mentioned was Diamond Like Coating. -D L C. Used by all sorts of modern racers for cams +followers. Also used by me on camfollowers in my new "vin" . Roy.