The rotating mass

Plastermaster · Post by **Plastermaster** » Thu Apr 11, 2002 6:06 pm

It is common to lighten the fly wheel, as it lightens the rotating mass, freeing up power for acceleration. Also it compensates for added counterweight on the crank. Sometimes I see what looks like an extra heavy pully. What is the purpose of this? Do people lighten the flywheel and then put on a heavy pully? After you add weight to the crank and lighten the flywheel, where do you end up weightwise?

Sorry to squeeze in so many questions. Maybe some one can clear up the subject for me.

Thanks,
Ron

Sidewalk · Post by **Sidewalk** » Thu Apr 11, 2002 6:30 pm

Inertia

The purpose of heavy components (especially flywheel) is that it absorbs the energy from the power stroke (lose that acceleration) but between stroke the flywheel releases that energy to keep the movement smooth. Helps make parts last longer for one thing.

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'59 bug-unstockStella
'88 Bronco II-unstock Sonja
"Yes, I am a smart ass."
"I'm a DUMBASS too"

But are you gutless?

Marty · Post by **Marty** » Thu Apr 11, 2002 6:38 pm

Berg thinks that a heavy pully helps to offset the weight of the heavy flywheel. This way, there is more of a front to rear balance. He says that it helps main bearing live longer. I run a heavy pully but I have never had bearing problems. I run it for the "inertia" effect. It helps to get your car off the line.

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Marty Staggs
www.staggsracing.com

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Thu Apr 11, 2002 9:29 pm

My new CNC balancer hates those heavy pulleys, they throw the front to rear balance off way more than a stocker just at 500 RPM.

If they are so good, wonnder why Porsche did not use one, I have never seen a bearing failure with one, but have seen some cranks with bent noses that were so bad that a .010 cut barely cleaned it up, usually .002 will clean something up nicely.

The pulley is not that large, so it doesn't affect balance that much. With balance, radius and plane are the factors, the closer tha plane&radius are to the measuring stanchion,the easier it balances.

I have been balancing to .17 grams,rear and .19 front with a tolerance of .5 Oz In.
anything less than that is considered "zero grams" and is unobtainable without chasing weight for days.

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Jake Raby
Raby's Aircooled Technology
www.aircooledtechnology.com

James2 · Post by **James2** » Thu Apr 11, 2002 11:03 pm

I'm with Marty and sidewalk on this. I just switched back to a full size flywheel.

I think the lighten flywheel is just something else for the venders to sell.

BTW, anybody wanting a 12.5 lbs wheel email me. Got one cheap.

Plastermaster · Post by **Plastermaster** » Fri Apr 12, 2002 12:19 am

It sounds like for drag racing, whatever you loose in bringing the flywheel up to speed is made up for in the high RPM launch. For good acceleration with out a big dump on your clutch, does a heavy flywheel make much difference or are we just trying to sqeeze out that last bit of power to the wheels? In other words if you are going 55 mph in 4th gear and floor it, how much difference is a lightened fly wheel going to make on a 150 HP engine?

Thanks
Ron

Marty · Post by **Marty** » Fri Apr 12, 2002 1:03 am

Probably not too much. For the street guy though I think lighter is better.

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Marty Staggs
www.staggsracing.com

Freq2002 · Post by **Freq2002** » Fri Apr 12, 2002 1:09 am

At 55 & you put your foot in it? Prolly wont make all that much difference,... From what I've gathered from the guys around here that race, lightened flywheels are more of an advantage to situations where you're doing a lot of gear changing & need to spool your motor up/down faster. (i.e.: Rally cross) Otherwise, lighter=quicker, so anywhere you can save weight can give an edge on acceleration.
I've seen those monster pulleys, I always thought they were harmonic balancers, like a Fluidamper or Rattler. Does anyone use them on VW's?

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Where are we going, & why am I in a handbasket?...

rozbug · Post by **rozbug** » Fri Apr 12, 2002 3:55 pm

When I rebuilt my girlfriend 1600dp in her 66 square, I used a DMS CW crank and a 12# fly. The motor would rev up, noticably quicker, but it also seems to take longer to rev back down. It's good. I am going to run autocross w/ my new 1915 and I am going to run a 12# fly to see how it feels.

ANT · Post by **ANT** » Wed Apr 17, 2002 1:40 am

<BLOCKQUOTE>quote:<HR>Originally posted by Freq2002:
At 55 & you put your foot in it? Prolly wont make all that much difference,... From what I've gathered from the guys
around here that race, lightened flywheels are more of an advantage to situations where you're doing a lot of gear changing &
need to spool your motor up/down faster. (i.e.: Rally cross) Otherwise, lighter=quicker, so anywhere you can save weight can
give an edge on acceleration.
I've seen those monster pulleys, I always thought they were harmonic balancers, like a Fluidamper or Rattler. Does anyone use
them on VW's?
<HR></BLOCKQUOTE>

My feeble understanding of the heavy pulleys is as follows---they are not intended nor designed to aid or affect revability--their
function is to reduce HARMONICS.

Anyone familiar with Chevy V8's know that they choice stock pulley is the heaviest one.

The Fluidamper and Rattlers are all about finding the "sweet spot" of second couple vibrations and reducing them. Remember
when you were a kid sloshing around in the bathtub and you'd get all the water going back and forth in a huge tidal wave, only
to stop it by suddenly going against the grain? That's kinda how these pulleys work--they're phased in the opposite direction as
it were, perhaps not the most accurate description, but I think it illustrates the point.

Bottom line, you want a fast revving engine (not so much a HIGH revving one, but one which will buzz UP to whatever speed
in a quick manner) then you need a lightened flywheel.

If you want to have the maximum life out of your recip assembly and bearings then I suggest using a heavier pulley.

Just my 3 cents.
-ANT

wellington · Post by **wellington** » Wed Apr 17, 2002 7:04 am

Scat does not recomend these heavy pulleys. In fact they have been known to break cranks because they disturb the harmonics in the crank. Personally I trust Scat as they are a major inovator and sell very high quality parts.

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Wed Apr 17, 2002 11:02 am

Hmmn, wonder why I had to turn down 3 cranks, and 6 sets of pushrods in the last month......Things aren't like they used to be.

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Jake Raby
Raby's Aircooled Technology
www.aircooledtechnology.com

ANT · Post by **ANT** » Wed Apr 17, 2002 2:03 pm

<BLOCKQUOTE>quote:<HR>Originally posted by wellington:
Scat does not recomend these heavy pulleys. In fact they have been known to break cranks because they disturb the harmonics in the crank. Personally I trust Scat as they are a major inovator and sell very high quality parts.<HR></BLOCKQUOTE>

And I suppose Berg isn't (a major innovator who sells very high quality parts)? At least they used to be.

Was Scat referring to their Volkstroker III cranks? If so, then I can certainly understand those cranks having problems--they're CAST, not FORGED.
-ANT

Bent · Post by **Bent** » Sun Apr 21, 2002 6:39 am

Hi,In my opinion a lightened fly-wheel with correctly counter-wieghted crank and rod assembly will benefit from a heavy pulley.
The man who manufactures our crankshafts does a lot of testing at Monach uni ,spining parts to the point of destruction etc, our race crank has holes bored through pin end and massive counterewieghts, very milled fly-wheel and a wieghted pulley, he assures me his testing is conclusive. Others have copied his early 70's incorrect desighns and in fact still make them today.
Harmonics is the key word here.
Oh yeh after scat take an order for some part, then supply another useless item then not answer any correspondence, I would not trust them again.
There cranks flexssssssssssssss

Tom Simon · Post by **Tom Simon** » Sun Apr 21, 2002 10:12 am

Ant has it about right, it's about damping torsional vibrations. The heavy pulley thing is suposed to add mass to the opposite end of the flywheel to try and equalize the rotating mass, end to end. Will you destroy your engine if you fail to use a heavy pulley... absolutely not. Do those heavy pulleys work? It seems every racer or engine builder has his favorite recipe for building a fast engine that lives. If you subscribe to the "add weight to the pulley end" theroy, then your best bet is to use a vibration damper as well. Extra mass has the effect of converting high frequency vibrations into low frequency ones. A damper (Fliud, Rattler, clutch disc type, intertia ring type) will convert certain frequency vibrations into friction heat.

I have and use a couple of the clutch disc type on my drag motors, and haven't broken a crank yet. That said, I'm not sure my Berg cranks would have broken if I was using one of those "drag button" degree wheels.