I am planning my next drivetrain. I want to get an aftermarket, flywheel. If 2 (or 3 or 4) lightweight flywheels have the same weight or close to the same weight, but one has more mass, will the one with more mass absorb/positively affect harmonics better then the one that weighs the same but has less mass?

I am comparing lightweight steel, to aluminum, 153 tooth, 2 piece main seal flywheels. Ultimately connected to a internally balanced 400 or 377 sbc.

 

Two objects can't have the same weight but different mass in the same gravity.

 

Should I change mass to physical size?

 

If physical size is what you mean instead of mass, then yes.

As long as all of the components of your drivetrain are balanced, then vibration (harmonics?) is not an issue. If you have a component that is out of balance, then a more massive/heavier flywheel will dampen it to some degree, and one of greater physical diameter (physical size?) but same weight and mass will dampen it a small, probably negligible amount.

If vibration is really a concern of yours, then I would focus more on balanced driveline components than the size or mass of your flywheel.

 

I think Pyroviking has explained the important issues well, I would just like to point out that I think what you're looking to compare is moment of inertia vs mass/weight. Two parts can have the same weight but different moment of inertia values. This is because moment of inertia = mass x radius^2. However, (and please correct me if I'm wrong here) radius can't change because the flywheel still has to interface with the starter which has a fixed location. If that is the case, mass/weight would be the only important property to consider (other than material type).

 

@86 Firebird - you are correct about Inertia.

You are partly correct in that the starter ring gear will be in the same place. BUT, the flywheel's inertia is also dependent on WHERE the manufacturer puts the rest of the mass. Drill several holes near the outer edge (remove mass from the larger R) and a heavier flywheel, despite more overall mass, could have a lower inertia than a lighter flywheel, due to more of the mass being closer to the axis of rotation.

Look at the cutouts on the Centerforce DYAD flywheel. This is how Centerforce reduces inertia to counteract the extra mass of their dual disk system.

 

I thought I remembered location of the mass being important but looking at the simple equation on Wikipedia (yeah I know I should've used a more credible source) it didn't seem to indicate that location mattered. And since flywheels are symmetrical/balanced I thought "well maybe it doesn't affect it".

So that being said, I would think that for most of us we would want the flywheel with the smallest moment of inertia, even if it has a higher mass than another flywheel.

 

Just did another quick search to refresh my memory. I think you also have to look at the Second Area Moment for location in addition to the Moment of Inertia.

 

Balance has more to do with how much weight is at 12 o'clock vs 3,6,9 o'clock. Meanwhile inertia is how much mass is as the center vs out at the edge. Here's a demonstration: Hold a sledge hammer by the head. This keeps the mass close to the axis of rotation. Then swing the handle... really easy to start, stop and change direction. Now hold that same sledge by the handle. This puts the head (mass) at a longer Radius from the axis of rotation... really hard to start, stop and change direction.

High inertia = slow rev'ing engine... the engine doesn't change speed (rpm) quickly.
Low inertia is good for a road race car, where you are always running up and down through the gears.
BUT, for a street car, you can have too little inertia, or a flywheel that is "too light". It will stall frequently moving the car from a stop, and/or you have to give it a lot of gas (rpm) to keep from stalling it.

As a rough rule of thumb: super cheap flywheel will probably be heaviest and highest inertia. A very expensive, "race", "road race" or "aluminum" flywheel will be lighter/lightest and probably lowest inertia, and may be too light for a daily driver. Look for something in the middle... not too cheap, and not super expensive.

 

so as a conclusion, with a given radius AND weight (=given mass at a given gravity), the MoI can still be different depending on the radial distribution of that mass.. right?

 

@ownor - yup. Spoken like an engineer!

 

that's good because i happen to be one LOL thanks for the confirmation anyways. lots of stuff I know now I didn't learn in collage but more in forums like this great one here, and actually applying knowledge

 

For now I'll go with physical size. I disagree however that vibration/harmonics/resonisne is not an issue in a properly balanced engine. All engines naturally create these "vibations" or whatever they are. Maybe someone can shed some more light on that?

That's not exactly what I meant, but I wonder if a higher or lower moment of inertia (or specific flywheel design/compostion), is suited to better reducing the resonance of a specific rotating assembly? Meaning the specifications of say, the crankshaft stroke, bob weight, rod length, piston specs, plus the intended operating rpm band and maybe it's physical shape?

 

Defining your terms and understanding their meaning might help here. I think that swapping the term mass for inertia would make more sense.
Inertia is a function of (mass) x (acceleration). The more mass/weight, the more force it takes to accelerate/decelerate.

The physical size has to do with density which has little effect on anything. Density is a factor of volume and mass.
The radius position (distance from the center) of mass is a factor of inertia on a round object. In that case then yes a larger diameter flywheel/harmonic balancer will make a difference because it will take more force to accelerate/decelerate. As in the example of the hammer mentioned earlier.

Weight is a factor of mass, with gravity being a constant. In most cases people use mass and weight interchangeably even if it's incorrect.
Weight = (mass) x (force of gravity)

The force of gravity is an acceleration. If you think of Earth as expanding really really fast then you can get some kind of understanding on how gravity works. So from that, weight is a force, or the amount of force that an object pushes against the surface of the planet. Inertia is also a force created by accelerating/decelerating mass, it's called something different because it's not being produced by gravity, and gravity is also a constant.

Most of the vibrations of an engine are absorbed by the harmonic balancer (or damper). It has its mass/weight on what you could think of as an outer ring, with a rubber piece in-between, to absorb the vibrations.
Say a vibration is introduced to the crankshaft. This vibration “signal” goes out to the harmonic balancer, accelerating the outer ring a very slight amount. (Again, the acceleration is a factor of inertia.) However, the rubber in-between delays the transfer of energy from the crankshaft to the outer ring, and then also from the outer ring back to the crankshaft. This time delay of the transfer of energy is key to understanding how the harmonic balancer works. You could also think of it as being similar to a shock absorber.

Harmonics (resonance) has to do with a vibration being generated on a timed interval (or a waveform, like a sine wave), but with harmonics/resonance, the vibration is enhanced by the design making the vibration larger and larger, or in other words compounding the energy over time.

The flywheel has some effect on dampening but that is more of a side effect and not the only purpose of its design. It is used more to keep rotational speed constant, and also sometimes used to counterbalance to reduce harmonics/resonance. You could also weigh each piston, rod etc., to help reduce the effects. The crankshaft actually deflects too and the amount is not something that can be controlled which is why the harmonic balancer is needed.

 

Thank you for the explanation and insight. It's funny because I couldn't find the word "volume" in my head when I was trying to ask these questions. Explaining "resonance" and how the harmonic damper work have shed some light for me.

"but with harmonics/resonance, the vibration is enhanced by the design making the vibration larger and larger, or in other words compounding the energy over time. " - Do you mean the design of the engine, or the actual design of the waveform - or something else?

When weighing the pistons, rods, ext. with a goal of the least resonance, is it purely matching each components weight? or can you go further by assembling a reciprocating assembly with serten features that are known to reduce harmonics? Like maybe it's known that in general longer rods/shorter pistons has some positive effect? Or heavier pistons/shorter rods is better? Or crankshaft stroke? Is it possible to go further than just balancing an engine?

As a side note - MN represent!

 

You're welcome. I've had a little bit of engineering and some of the terms and ideas get interchanged which also makes it confusing when trying to do anything theoretical. It's very common.

Yes, I mean the design of the engine, the waveform is more of a byproduct. I was explaining it in general terms though, not anything specific.

Yes, weighing the components so that they are close in weight. I'm not an engine designer or anything close so I'm limited on what I can say. The general idea in a lot of this is to get things within what they call acceptable limits. Other things like cost and practicability factor into it as well. Matching components is something that you do have control over -but there are also design features like the angle of the cylinder banks, crankshaft design and other things that probably affect harmonics even more. The reality too is that some of the internal parts like the crankshaft actually move/warp when in operation so even if you do design everything perfectly, the actual result can be thrown out of whack. GM designed the engine within specifications with things like cost in mind. Some of the later hotrod tricks used includes increasing the tolerances to enhance performance, and at higher RPM's the harmonics can play more into it, but not a whole lot. The increase of compression and resulting explosions are always going to send huge amounts of forces through he crankshaft at specifically timed intervals. So in that sense if you were designing strictly for harmonics you would want a very weak explosion with a huge amount of damping which ends up being impractical imo.

On the practical side, here is an article explaining internal/external balancing and weighing the components. It includes some talk of the cylinder bank angles and crank distortion/warping (Ie. “dynamic effects”) that I mentioned earlier. They also mention internally balanced as being better for longevity and fatigue life, which is also what I was getting at by trying to design so that the damper doesn't make up for the difference.
http://www.eaglerod.com/index.php?op...d=27&Itemid=25

Yeah I saw that! I have a Berlinetta too.