Quote:
Originally Posted by ib4200
how is that possible if theres oil gallys running from the crank main journals to the rod journals? also does the weight of the piston and rod get calculated into the counterbalance on the crank?
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The short answer is Yes, to answer your original question, Yes, if you changes the weight of your rods, you are basically taking the crank out of balance.
The way I understand the process ( and I'll agree with Josh that JohnJZ would know tons more then I and could probably explain it far better ).....
You aren't actually balancing the crank, but more so balancing the crank TO THE reciprocal weight.
Cranks are basically built with an assumed weight of the rotating mass, but it still is never exact. On a standard V8, the crank is balances so that the counter weight matches the weight at 4 - 90 degree points so that the mass is balanced as it spins these points are the counter weights and material is removed or added at these points to balance the rotating assembly.
The overall balance is done by calculating mass in two sections: the piston, rings, wristpin, top of rod and 1/2 of bearing (remember the rod journal is a pivot because there is mass on the other side). So rod cap 1/2 bearing, bolt heads ect are weighted separately.
From the weight the machinist will determine the weights (bobweights) to attached to the crank ( with what I'll call ) given inner and outer or upper/lower weights. That is the weight always above the throw of the crank and weight always below it. Remember two rods/pistons to each throw/journal, so it's a combined weight of the two, attached to a journal. (why it's also very important to put rods+pistons back in the holes they can out of). Typically you can machine some of the weight out of the rods so that the 8 rods/pistons are near the same static weight, but most often they won't be exact, but will be paired to better balance then once the engine is assembled.
This is one of those areas where more expensive cranks/rods are closer/better balanced during manufacturing/machining. Balancing a rotating assembly can be expensive and the more weight needed to be remove or more importantly added (in the form of Mallory) can drive up the cost.
The machinist gathers the info of the weight of the mass, if you are externally balanced then you must install the flywheel/flexpalte and the harmonic balancer. Attach the bobweights on the journals and spins the crank to determine where weight needs to be added or removed from the counter balances. The bob weights only represent the weight being exerted on the crank, they don't need to free spin like a rod would, the entire unit, crank, bobweights, etc are spun as a whole. The machine wil indicate at what location of the counter weights, that the weight would need to be removed or added. I would assume much as a tire balancer would. you could add weight at one point, or remove it from the throw that is across ( 180 degress ) from that point.
Little side note, with my engine and the concern of weight of the street twin clutch setup the machinist balanced the flywheel to a tighter tolerance then McLeod, and then and balanced the crank with my flywheel attached to ensure it was correct.
if you look at the pic, the counter weight on far right of the crank has a hole drilled where weight was removed.
