[200q20v] Re: [audi20v] RE: timing belt/dist rattle/cam chains

Bernie Benz b.m.benz at prodigy.net
Mon Feb 4 22:05:17 EST 2002

You wanted answers.  Here is more than you ever wanted to know.

> From: "Aaron Gjerde" <gjerdea at pconline.com>
> Reply-To: Aaron Gjerde <gjerdea at pconline.com>
> Date: Mon, 4 Feb 2002 16:49:33 -0600
> To: "Audi 20V" <audi20v at rennlist.org>
> Subject: [audi20v] RE: timing belt/dist rattle/cam chains
> Bernie,
> Thanks for your thorough reply.  I am not and never have been an engineer,
> so would you mind explaining a couple things for me?
> Bernie wrote:
>> 2200 crank RPM, 1100 cam RPM being the resonant angular
>> frequency of the valve train rotary system.  If there ever is to
>> be a floppy T belt condition, this is it!
> Being a fan of Nikola Tesla's work, I understand resonance and basic wave
> theory just fine, but I am not sure what is meant by "angular" resonance?
There is only one basic electrical resonant circuit system, degree of
freedom, if you like.  That is any circuit containing reactive components, L
and C combined.

There are two mechanical degrees of freedom, linear and rotary.
You may be more familar with the linear mechanical system, involving Force,
Mass, Spring Rate, and Linear Friction, in which the dynamics are described
in terms of Displacement, Velocity and Acceleration.  The mechanical
equivalants in the rotary degree of freedom are Torque, Inertia, Angular
Spring Rate, and Rotary Friction, the dynamics of which are measured in
terms of Angle, Angular Velocity, and Angular Acceleration, respectively.
> Bernie wrote:
>> IMO, it is this angular acceleration force
I have carelessly mixed my system terms for clarity(?) here.  I should have
said "this angular acceleration Torque required to - -"
>> required to drive
>> the distributor shaft inertia at this angular vibration frequency that
>> stresses the plastic gear to eventual failure, not just the steady state
>> torque required to turn the distributor shaft at half crank speed.
> This certainly seems plausible and if that is the case, we want to isolate
> the variable that creates the resonant frequency, right?  Are suggesting (in
> the paragraph below) that this variable is the belt?  Is there any practical
> way to test this?
My point is that the TB spring rate is a principle part of the cam drive
dynamic system, and it can't be eliminated, only altered.  But Audi did
change the TB on the 20V re. the 10V, so maybe it is a stiffer belt under
optimum belt tension conditions.  But with a higher, 2x valve train inertia,
the 20V needs a highest spring rate belt available, IMO.  Therefore the belt
tension must be sufficient to provide this maximum spring rate capability.
> Bernie wrote:
> "Timing belt spring rate is very low and nonlinear at low belt tensions.  At
> some higher belt tension, when each tension cord is equally stressed and the
> cover materials are fully compressed, the belt spring rate settles into a
> much higher and linear value, the design opperating area.  Because of the
> high valve train inertia of the 20V system, it apparently is critical that
> the timing belt be run at some rather high tension to avoid this resonant
> valve train syndrome."
> This makes sense as well.  Are there any other plausible scenarios?  Could
> different cams or even a different cam grind affect the cam inertia enough
> to affect the resonant frequency?  Is anyone out there with different cams
> experiencing the rattle?  Has anyone mega-tightened their tbelt and still
> received the rattle?
 Forget this tangent!  The only significant difference in cams on valve
train res. f. would be one vs. two cams.  As I said before, the only
difference in valve train res. f. among different (like new condition) 20V
engines is the timing belt spring rate (belt tension).  Points right to the
problem, IMO.

Further on resonant frequency of a spring/ inertia system.  Increasing the
r.f., by either increasing the system spring rate or lowering the system
inertia will also decrease the amplitude (I should say angle) of the
oscillations.  So either or both the increased natural frequency or reduced
amplitude may move the valve train critical factors outside of other engine
or distributor critical points.
> What did the engineers that designed our cars not know about?
Assume that the design engineers knew everything (being the brightest), but
that the maintaince/shop manual designers/writers just carried the TB info
forward from the 10V.  I've not found anything in the Bentleys that
specificly specifies a TB change interval or tension spec differing from
that of the 10V.  IMO, it should have been addressed.

Tighten your belt, or rattle on!

> - aaron

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