Destroying the dogma: lightweight crankpulleys

[MK_94_3000GT]

I have the only dampened underdrive pulley in existence.................. Let the bidding war begin

False, I have one too

[i3igpete]

To an extent, but not entirely. As the article summarized... it has very little to do with power, but the design and relationship between the pistons, rods, crank, and material coefficients. If you change the mass or frictional coefficients of any these items in an engine you will change the natural frequency of an engine.

If you modify your engine from stock, an oem dampener serves no purpose, the natural frequency of your engine will be different then stock so what purpose does a stock dampener even serve? Since the stock balancer is designed with a specific vibration frequency in mind, with a modifeid engine you are just as well off without one. (until fluidampr makes us one )

you have a valid point - changing the mass/inertia of the system does change the natural frequency, and yes, this needs to be tuned for. i never said it didn't and the presentation mentions it as well. I can post the tuning methodology from my own notes well. you are incorrect that a slightly mis-tuned damper does "nothing". it still works, just not as perfectly as it should - see the page on undertuned or overtuned dampers.

however, the power output of the crank is directly related to the stress imposed on the system; it is the forcing function. UTracerX was saying that the power did not matter. It does because as you increase power things you could "get away with" before, no longer work. again, you are not changing the system, but you are changing the loads and therefore the corresponding stress. recall from your vibrations class that when designing a damper, you are changing the free vibrations of the system (the eigenmodes, which have unitless vibration amplitudes). the forced response (the real, measurable response) depends on the free system and the loads applied to it.

edit:
while looking thru my notes, i see that i have points on how to tune a fluid damper. i'll post this as well.

[J. Fast]

i see that i have points on how to tune a fluid damper. i'll post this as well.

Love to see the notes, any chance you can post any info on possible variations of destructive htz ranges with differing rod and piston weights on our crankshafts? I can send you some info on my wrist pins, rod bolts, rod, and piston weights / composition, etc.

Jeremy

Edit: hell everyone should throw some info in there and run some different scenario... how cool would that be.

[UTRacerX9]

UTracerX was saying that the power did not matter.

No, what I said was...

The power of the car has little to do with it.
My car still spins to a 7k redline, and if harmonic balancing is what we are debating here, it makes little difference how much peak power an engine can put out, what matters is whether the engine is balanced or not.

I'll take the reduced stress on the engine from less rotational weight than worry about a dampener that was originally designed for a 9b, heavy flywheel car.

[i3igpete]

here's the thing though, on an engine, rotational weight has no effect on the crankshaft unless you mean centrifugal (unbalanced) loads. a heavy flywheel and a light flywheel are the same thing unless it is unbalanced (and makes centrifugal force).

again, torque and unbalanced force are two different things. one is a twist, the other a shove. Both act on the crankshaft in different ways.

Jeremy,
the notes on the fluidampr are page 5.

Another fun point is that people don't realize that it's possible to tune a crank with a solid pulley (page 2). Though it's nowhere near as good as with an elastomer, there is a more subtle kind of takeaway point: a crankshaft with a solid welded steel pulley would actually protect your engine better than a aluminum pulley.

[UTRacerX9]

I'll admit I'm no engineer here, but I damn sure can tell that having a light flywheel and my old stock heavy one did indeed affect the operation of the crankshaft! The stock flywheel allowed the engine to slowly return to idle after being revved. With the UR one, my rpms drop like a rock. It takes more energy, (and thus more work) for my crankshaft to turn the heavier flywheel, and the same principle would hold true for the crank pulley, it just is not felt nearly as much.

Here's my non-engineer example. Say I have a toothpick. I stick a piece of round cheese on the end of it. I turn the toothpick with my fingers and the cheese rolls back and forth. Take the same toothpick (crankshaft) and stick it to a heavy rock. I turn it, and it takes a lot more force to move it, in fact, I may break it.

As I stated originally, it comes down to personal preference. I KNOW my engine spins to redline faster than a comparable turbo car with a stock flywheel and pulley. My engine has to work less to turn. Now, am I possibly doing some harm to the engine by not having a harmonic dampener? Sure, that's why they are there. But my point is that I've never seen ANYTHING proving how much damage it may be doing vs the benefits of having a lighter drivetrain. And in my case at least, we're talking 50k miles with upgraded turbos, the underdrive pulley and the UR flywheel. If it's made it that far (now having 91k on it) with no issues, that to me says that it is not doing enough damage (if any) to warrant removing it.

[i3igpete]

Actually, your toothpick in cheese is a good example. let me expand on it and explain how a torsional damper works.

Imagine that the cheese wheel is your flywheel, transmission, and vehicle body. for all intents and purposes it is - the time scale between a torsional vibration (~10 vibrations per engine cycle), and the number of cycles per second (6000rpm = 100 engine cycles per second), means that reducing the weight of the flywheel is relatively insignificant. The crankshaft vibrates so many times per second that reducing the drivetrain inertia or vehicle body has no effect on the vibration of the crankshaft. the time scale is completely different - a crank vibration is a snapshot compared to a run down the drag strip.

now, let take a look at cheese wheel example in a different way. say you apply the torque to that tooth pick until it snaps. what if there was a cheese wheel on the other side of the toothpick? if you grab the toothpick in the middle now, it will probably take more torque to break, right? That is how a solid (heavy) disk tunes a crankshaft. It adds inertia to the front end of the crankshaft, so that both ends are stuck into a cheese wheel. torques get absorbed in two directions, meaning that the total twist in the toothpick is less. however, the total output torque going out the clutch is the same, because the cheese wheel pulley is a dead end. there's nowhere for it to go.

Now, I'm not saying that a damper is always needed in every situation. I can think of plenty of normal applications where the crankshaft is relatively stiff/strong compared to it's output torque. Small engines like harley engines or small NA engine cars like civics can get along just fine without one. We have a relatively large crank. Lightweight pulleys are fine for SOHC and NA/DOHC applications on our cars, because they don't have as much torque pulsing through the crank. as brake horsepower goes up, so does the output torque thru the clutch, and so does the vibrational torques in the crank.

[UTRacerX9]

I see what you're saying. The vibration of the crankshaft though, to me is somewhat inconsequential. You just said yourself that reducing the weight of the flywheel is relatively insignificant, so why then do you make the argument that having a lighter pulley is significant and bad for the engine? You can't have it both ways...

For your second paragraph, I also see what you you are getting at, and agree. However, by lightening both the front wheel of cheese, AND the rear, total twist should remain somewhat in proportion. (In fact, the flywheel I have is supposed to be about 40% of the weight of the stocker from what I remember, and the 3SX pulley was about 33% the weight of stock I think.) So... still not seeing why a lighter pulley is a bad thing.

[i3igpete]

here's the thing though, you "can't" reduce the rear wheel of cheese because that is the transmission, driveshaft, and vehicle moving thru the air. no matter how light you can get the car, drag force is always there, making that rear cheese wheel grow bigger and bigger as you speed up. it will never go away.

[UTRacerX9]

Good point... Hmm. We need a thumbs up smiley...