calculating bob weight

[RDY4WAR]

I've wondered about the role inertial plays on balance. A 600 gram piston (including pin, ring, and retainer weight) in a 383ci SBC (3.75" stroke, 6.00" rod) at 6000 rpm will have 3,328 lbs of upward inertia and 1,743 lbs of downward inertia. What factor does this play?

What about combustion pressure? That same 383ci SBC street engine can be expected to see around 1,200 psi cylinder combustion pressure near peak volumetric efficiency. The square area of a 4.03" bore is 12.756 in^2 which gives a total force of 1,200 x 12.756 = 15,307 lbs. That's in 1 cylinder at a time, while another may have a negative force (-1,743 lbs) near BDC of the intake stroke and another at a negative upward force (-3,328 lbs) near TDC during overlap. Just thinking about it makes me wonder how a V8 crankshaft doesn't get ripped apart by torsional vibration.

[Tom Walker]

The link to Jeffery Diamond’s write up about balancing is very much worth reading. It explains how complex and intertwined the process actually is.
Also talks about the physics of how we determine a rods weight and how that might affect why some engines may want a percentage or so of over or under balancing.

[panic]

Triumph "B" range 650cc twin pre-unit (separate engine and transmission) and unit (engine and transmission in the same case set) engines use different balance factors despite the same internal components, rod ratio, and weights: 71% for pre-unit and 85% for unit. The frames, CG, wheelbase are all very similar

???

[racear2865]

I play with balance machines every day all the way up to 100 ton and more if they pay for it. To my knowledge, there is no fixed fact of what and how a crankshaft should be balanced. The 50% rule came into being across the years and has been pretty well established that it "worked" and thusely became the norm. No engineering facts or calculations to confirm that. It is just accepted that it works. I ve played with a remote vibrometer on the dyno but is hard to justify data because of the firing pulses. You can write data that will eliminate those pulses to get a better picture of what your dealing with. Over and underbalanced has seemingly found its place in racing in 2 different places. One in and engine that is accelerating thru a range and an engine that stays in a high range most of the the time. Either can be used but the monitor is , what do the bearings look like when you make that change. A dyno can lie if you dont have good software to eliminate what you dont want to read. Also in a few situation, Ive seen gain in HP but it hurt the engine. Pick your medicine. I do not deviate from 50% unless I am able to track that engine or a class of engines. Many many theories exist but I want to see the science. Facts dont lie but people do.
reed