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

Aaron Gjerde gjerdea at pconline.com
Mon Feb 4 16:49:33 EST 2002


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?

Bernie wrote:
>IMO, it is this angular acceleration force 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?

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?

What did the engineers that designed our cars not know about?

- aaron

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