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Half VW Balancing

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Hot Wings

Grumpy Cynic
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Would it help this problem since there will be no blade corresponding to any combination at tdc?
No. The opposed 2 cylinder shakes along the Z (vertical) axis. A 2 blade that is phased with the crank is going to have less stress on the crank and less shake than any number of blades, other than a single bladed prop.

The counter weights on an opposed 2 cylinder, like the 1/2 VW and the O-100 have their counterweights arranged to reduce this shake - thus the external counter weight on the prop flange of some.
 

Pops

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I built a 1/2 engine many years ago. I bought the counterweights that Scott sales and welded on the crankshaft. Not an easy job, had to buy a different TIG head to get in the close space. Then the crankshaft had to be balanceed and adding the weight at the prop flange. Lot of work and I would never do it again. I would pay Scott to balance it for me. At the prices Scott charges for a 1/2 VW and all the work on building one, I would just buy a complete engine from Scott. Its a deal.
 

slociviccoupe

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Hotwings please bear with me. Im still not understanding. Z axis as in up and down? So if crank throws are flat (horrizontal) the weight has to be 90* to it.

Also my not understanding the prop is its not a disk of equal weight all around. Its more like 2 levers of weight 180* apart.
If the prop were phased where the weights should be wouldnt you essentially be using prop as counterweights?

And the reasoning on thinking of 3 blades is they are 120* apart and dont correspont anywhere near the crank 180* throws.

I just cut my crank this week and left rear counterweight behind middle bearing. Will see how it goes. Just trying to understand the logic. So prety much doing a dynamic ballance internally and then doing an external ballance after. No way to throw off the internal ballance intentionally to make up for the external ballance. And also from another post it seems in the location nedding the external weight it wont fit on crank or in the case? Id like to see onr of scotts cranks. Might just have to eat it and send him mine.
 

Hot Wings

Grumpy Cynic
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Visual aid given to me by another HBA member in a time before HBA:

Grab a yard stick and hold it, on edge, out in front of you palm down. Now wiggle it side to side by moving your wrist right/left (about the vertical axis). That is how an opposed 2 cyl shakes.
Now twist your wrist 90 degrees (clockwise for right handed persons, counterclockwise for south paws) and rotate the wrist about the same vertical axis as in the first test.
Gyroscopic precision enters the test once things start spinning but trying to include that in a typed form makes my brain hurt. 😖

The crank can be fully rotation-ally balanced with bob weights to simulate the rotating portion of the piston/rod mass but the pistons and rods are still off set and the reciprocating mass introduces the shake. There just isn't enough room in the VW crankcase to add enough counter balance weight to match the piston/rod reciprocating weight on the pulley (now PTO) cylinder. This is why the external weight gets added - and needs to be there when the crank gets balanced.

The above is also probably why Pete with the O-100 is using lighter rods and pistons then the stock O-200. Lighter rods and pistons lets him use smaller counterweights.

On the full VW the forces are still there trying to bend the crank. They just aren't experienced as external shake. The stiffness of the center main bearing is good enough to keep things in check for stock RPMs with out counter-balance.
 
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