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

Bernie Benz b.m.benz at prodigy.net
Mon Feb 4 23:20:22 EST 2002

Thanks, Alen.

> From: "Cordeiro, Alan" <Alan.Cordeiro at mts.com>
> Date: Mon, 4 Feb 2002 21:09:26 -0600
> To: "'200q20v at audifans.com'" <200q20v at audifans.com>
> Subject: [200q20v] RE: timing belt/dist rattle/cam chains
> In reply to the timing belt resonance discussion....
>> From: "Aaron Gjerde" <gjerdea at pconline.com>
>> To: "Bernie Benz" <b.m.benz at prodigy.net>
>> 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?
> A more correct engineering term would be "TORSIONAL RESONANCE"
I stand corrected, but then I've been out of school longer than you, and Old
Fart's Desease takes its toll over time.
>> 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."
> The spring rate should not vary much with tension. Yes the frequency
> of resonance of the belt itself if "plucked" like a guitar string
> would change, but the torsional tension on the camshaft against
> rotational forces would remain similar, the belt has a reasonably linear
> spring rate over tensions in the operating range. Hence the torsional
> resonant frequency would only increase slightly (10-20%) with increased
> tension.
I'm contending that some (or possiblly all) belts causing the problem are
opperating well below the intended operating range.  Otherwise, agreed.
>> 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?
> The torsional resonant frequency is related to rotational inertia
> and torsional stiffness. Think of a weight at the end of a spring
> for a linear analogy to this rotational resonance. The resonant
> frequency of the weight/spring would depend on the SPRING CONSTANT
> of the spring and the MASS at the end of the spring. Hence
> the shape of the cams might have a slight effect, but it would
> be a few percent...
Hardly that much.  Those coming onto the ramp would be offset by those
coming off in a symeterical system.

>> What did the engineers that designed our cars not know about?
> This is a VERY WELL KNOWN problem, even mechanical engineering
> students know about this.. even the most rudimentary simulation
> program would immediately show this effect. Audi engineers were
> completely aware of it.
>> - aaron
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