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

Cordeiro, Alan Alan.Cordeiro at mts.com
Mon Feb 4 21:09:26 EST 2002

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"

>Bernie wrote:
>"Timing belt spring rate is very low and nonlinear at low belt tensions.
>some higher belt tension, when each tension cord is equally stressed and
>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

>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...

>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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