Thank you for all the feedback. I'm responding without quoting because I find the resulting quote windows annoying.
I think that a 90-degree V-Twin with a single crank pin and long rods should be balanced with 100% rotating and 100% reciprocating weight in the piston. I am guessing that secondary vibrations for short rod engines make the balance factor (the % of reciprocating weight) somewhat less than 100%. In any case, if we model the V8 crankshaft as four V-Twins with flexible plug in the mains, we'll get very big counterweights. I am not sure, however, what happens to the rocking motion if each V8 crank throw is fully counterweighted with a 100% balance factor and then put into the regular V8 balancing maching with 50% balance factor bobweights. Would it show perfect balance, or would more weight still have to be added to the end counterweights (or externally to flywheel and damper)?
In terms of my crankshafts, the front end counterweight is not exactly behind the 1/5 crank throw. It has maybe 1/3 of the weight on the trailing side and 2/3 on the leading side, relative to the plane that goes thru both the 1/5 crank journal centerline and the main journal centerline.
If you look at the crankshaft from the front such that your line of sight is parallel to the line going thru the mains, you can see the 1/5 and 2/6 crankpins forming a 90-degree angle (duh, obvious). If you bisect that angle with a line and continue that line to the counterweight, the counterweight number #3 is about centered relative to the line. In other words, the counterweight #3 seems to be placed half way between balancing 1/5 and 2/6 crank throws. If you think this thru with the mental model that assumes flexible mains, I am thinking I would like to have the cw #3 aligned more behind 2/6 crank throw and less behind 1/5 crank throw. Now, the mains are pretty rigid and other parts of the shaft flex too, so maybe that four-V-Twins doesn't capture everything.
Given all this, I'm considering adding heavy metal to counterweights #3 and #6 (4 and 5 are missing since this is a six counterweight crankshaft) on the side that lines them up behind 2/6 and 3/7 crank throws better. Then, offsetting this added weight by cutting the end counterweights from the side that is currently less aligned with the adjacent crank pin. Maybe about half the added mass, eyeballing the lengths? If the math is done right, this should result in a balanced rotating crankshaft with the stock bobweights. Then, considering the lighter pistons and rods and resulting lower bobweights, I'd further cut the fan angle of the end counterweights from both sides to get the crankshaft to balance.
Does this make sense or have I misunderstood something?
Circles are locations yo which one might be able to physically install heavy metal:
E47B3058-FA5F-4538-9637-85667996D30D.jpeg
64502141-3FD8-463D-9954-FA31741F4629.jpeg
You do not have the required permissions to view the files attached to this post.