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I stumbled across this and read with great interest ...and also cringed at this misinformation mixed with correct information. I've spent 10y years of my life training people how to use balancing machines, and have been called into many facilities to balance something when no one else could do it. I have also spent another 8 years in the field of vibration analysis.
**) Balance has nothing to do with the total mass of the object, so a lighter flywheel does not change the balance of the the assembly. Unbalance is measured as mass x radius. There is no such thing as "zero balance". Zero balance only means "my balancer isn't accurate enough to measure this".
**) unbalance ALWAYS takes place at the same frequency as the speed of the rotating shaft ...no matter what speed the shaft is spinning.
**) Harmonics always take place at direct integer multiples of the shaft running speed (2x, 3x, 4x etc ...never 1.23x).
**) Unbalance can causes harmonics, but can never cure harmonics.
**) Assuming the shaft is rigid ...and it is safe to say all crankshafts are, (even though I know of 2 that were not), any Unbalance in the crank flywheel assembly can be corrected in two planes, but is best corrected on the crank, then add each component and balance that item in the same balance plane. This is the preferred way ONLY because it allows parts to be replaced without needed disassembly of the entire engine to re-balance it.
Given those statements of fact, a lighter flywheel will NOT change the balance of the system regardless of how much it is overhung away from the rear Bearing. RMS leaks are probably caused by bearing clearance issues and seal design issues, and can not be cured or made worse by any type of flywheel. A lighter flywheel will always make for a quicker reving engine, but some mass is desired to smooth out the pulses of the engine and more importantly to retain enough inertia to make starting from a dead stop easier. The heavier the flywheel, the more accurately it must be placed on the crank. If you mount a light weight and a regular flywheel that both have theoretically zero unbalance, the flywheel and pressureplate that weights 20 lbs and mounted to a flywheel with .0010" of clearance, would have half the residual unbalance as a 40 lb flywheel & pressureplate that have .0010" of clearance!
Now onto harmonic dampers. These are designed to reduce torsional vibrations. A person needs either 2 rotational accelerometers or 2 Polytec torsional vibrometers to measure these vibrations. While it can be done with one unit, it requires certain assumptions to be made. It can NOT be done with a regular accelerometer.
Piston movement turns the crank because the crank's rod journal is off the crank's centerline. The crank's rod journal acts as a lever arm attached to a shaft. The piston and rod push down on the lever arm, which rotates the shaft. That force is resisted by the entire weight and inertia of the car, which the driveline is attempting to push forward. It is impossible to create a perfectly rigid crank, so some of the energy from the piston bends the lever arm and twists the crankshaft.
When the force is reduced enough for the crank to spring back, it doesn't just return to its original shape immediately. Instead, the force of untwisting causes it to shoot past its original shape and twist in the opposite direction. This twisting goes back and forth a number of times until the crank settles down. The number of times this event takes place and its duration depends on the crank's construction (cast, forged, billet), mass, and quality of materials. When the engine moves through its rpm range, the speed at which the power pulses from ignition of the air/fuel mixture will eventually match the natural frequency of the crankshaft. When this happens, the vibrations moving through the crank will build upon each other and gain strength, often with damaging results. Running an engine without protection from these vibrations can cause flexplate or flywheel failure, loosened bolts, broken timing chains, along with erratic timing and valvetrain functions. The crankshaft will fatigue and eventually fail, usually at the snout. The purpose of the harmonic damper is to protect the engine from these damaging vibrations. Most quality harmonic damper designs reduce the vibrations traveling up and down the crankshaft by absorbing the vibrations and converting them to heat energy. Many racers use the words "balancer" and "damper" interchangeably, but since almost all engines are internally balanced, "damper" is the more accurate term.
There is a good reason that the damper is normally attached to the front of the crankshaft. The flywheel and the resistance provided by the rear wheels absorb crankshaft harmonic vibrations at the back of the crank. So if you run an engine without a damper, the crank will show greater twist the farther you get from the flywheel. Since the timing chain is normally at the front of the crank, running without a damper (or even with a poor-quality, inefficient one) reduces crankshaft life, but also causes the ignition and valvetrain to behave erratically. OEM manufacturers have started to place the damper in the flywheel to quiet transmission gear noise. As manufacturers have moved from very strong billet or forged cranks to sintered metal cranks, they are more susceptible to stress cracking due to torsional loads. Sintered metal is less expensive to produce, but the drawback is they break much easier. The harmonic damper (on the nose of the crank or built into the flywheel are both little more than a heavy rotating mass coupled to the crank by a rubber gasket with a specific damping frequency and with a very low Q. As the flywheel ages that rubber deteriorates and the frequency will change as will the Q. Eventually it will be damping the wrong frequencies and will become so compliant that it will break. This is the reason for not reusing a old dual mass flywheel. Unfortunately they are quite expensive, and the stock units are darn heavy and are not well suited to quick reving engines.
If you have made it to this point, you probably now know more that you really ever wanted to know about harmonic dampers, harmonics, and unbalance, (and according to ISO and ANSI it is UNbalance not IMbalance despite the high number of people in the industry that don't know the difference and even dictionaries that get it wrong).
-Gregg
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