flywheel balance question

Bernie Benz b.benz at
Fri Feb 27 23:28:16 EST 2004

> From: "Rick Houck" <rhouck at>
> Bernie,
> Your logic makes sense on balancing individual parts, but I saw the flywheel
> being checked for balance, and accepted their premise and the imbalance.
> Either they were correct, or the imbalance I witnessed in their test makes
> no apparent difference in operation. I can not detect any abnormal vibration
> at all, even though I run it up to redline regularly on the track.
> As to lightening the flywheel, the original 7A design is a heavy compromise
> I was not willing to live with on a dedicated track car. Of the three engine
> configurations I have (7A, MC, & 3B), the 7A flywheel is by far the
> heaviest. Cutting this one will serve to bring it down close to the original
> factory weight for a 3B.
Of course and as you know, the flywheel weight is not the criteria of
interest.  The critical factor is the Moment of Inertia of the flywheel,
which is proportional to the weight element times the square of its distance
from the rotational axis.  Hence, weight removed from the outer diameter of
the FW is most effective, good or bad, depending upon your perspective.

> This may be academic, since I may be able to use the 3B flywheel afterall.
> The two flywheels have different hub and pilot bearing designs, to work with
> their stock transmission shafts. The 3B has a needle bearing installed in
> the crank, while the 7A has a ball bearing mounted in the flywheel hub. Now,
> I may have found a way to use the 3B flywheel with the smaller coupe
> transmission shaft.
> Rick
> ----- Original Message -----
> From: "Bernie Benz" <b.benz at>
> To: "Rick Houck" <rhouck at>
> Cc: "audi-20v" <audi20v at>; "200q20V mailing list"
> <200q20v at>
> Sent: Thursday, February 26, 2004 11:12 PM
> Subject: Re: flywheel balance question
>> Rick,
>> IMO, components must be balanced at the component level, otherwise
>> subassemblies must be balanced together as if a single component and tied
>> together in the documentation.  Such is not the case, so your vendor took
>> the easy way out and lucked out, meaning no unhappy customer.
>> An as side point, if there were no need for flywheel inertia, why would a
>> mfgr. add the additional material to a flywheel beyond that required for a
>> suitable clutch plate surface?  IMO, your gain in acceleration is trivial,
>> considering the vehicle momentum as transposed back to the engine.
>> Bernie
>>> From: "Rick Houck" <rhouck at>
>>> Date: Thu, 26 Feb 2004 20:10:41 -0500
>>> To: <audi20v at>
>>> Cc: 200q20v at
>>> Subject: flywheel balance question
>>> I need advice from someone who has lighteded an I5 flywheel. Should the
>>> flywheel actually be balanced perfectly when removed from the crank?
>>> Background: A few years ago, I took my coupe flywheel to a local speed
> shop
>>> for cutting. They did the job, but when I picked it up, they told me it
> was
>>> out of balance before cutting, so they left it that way after cutting.
> Not
>>> knowing Audi 5 cylinder engines, they were not sure if this was done on
>>> purpose to counter some effects from the rotating mass of the crank and
>>> reciprocating parts. After 5-6 years of track duty behind my 7A and MC
>>> engines, I can say that it is very smooth all the way to redline. I
> think
>>> they were probably right.
>>> Fast forward: I am now getting ready to do a 3B conversion on another
> car,
>>> and again will be using a lightened coupe flywheel. My experience tells
> me
>>> it is OK to just remove metal evenly, but I have never heard this
> discussed.
>>> Anyone else go through this?
>>> Thanks in advance,
>>> Rick
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