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

Derek Pulvino dbpulvino at hotmail.com
Mon Feb 4 20:20:57 EST 2002

The only thing I can think of to add as fuel to this discussion is the 16v
VW head.  I may or may not be correct on this, although I believe the first
is true...  The VW 1.8 16v is the rough equivalent of a NA 20v 5-cylinder
minus one cylinder, but as far as the dist. drive is concerned, it is off of
a straight slot at the end of the camshaft.  The dist. to cam interface
relates basically like a flathead screwdriver and a flat screw, only more

If I'd owned one of those I could more likely remember whether the dist.
rattle was common in those vehicles, but I do however remember that these
dist. are blessed with a "you can bet on it" oil seal leak.  I could see
oscilations on the chain/cam/distributor agravating this situation, but I
think the completely horizontal position of the distributor is a large

Might be an interesting case study to add to the hypothesis.

If I read on of the post correctly, I think I saw you also mention the
plastic gear as possibly being a designed in "weakest link?"  How then does
the updated metal geared dist. fit with this?  Maybe the t-belt was isolated
as root cause, and they hoped to keep the situation in check by way of
accurrate shop work?  That seems like the biggest variable.

Aaron,I apologize if I misread your post, but one variable easily checked in
the situation is the timing belt tension.  If that can stop the noise, no
reason to look elsewhere.

For what it's worth, I thought I have also heard that on the reverse side of
the situation, the timing belt tensioner pulley can start whining if

I guess we all need a calibrated 90 degree thumb and forefinger combo.

Derek P

Message: 5
Date: Mon, 04 Feb 2002 14:42:19 -0800
From: Bernie Benz <b.m.benz at prodigy.net>
To: Aaron Gjerde <gjerdea at pconline.com>
Cc: audi-20v <audi20v at rennlist.org>,
	200q20V mailing list <200q20v at audifans.com>
Subject: [200q20v] Re: timing belt/dist rattle/cam chains


To answer your questions, let me expand on my "hypothesis", since apparently
there is some interest among the 20V lists.

An underdamped resonant system, electrical or mechanical, is a docile system
except when driven at or near its resonant frequency.  Driven at this
resonant frequency, the vibration amplitude increases sharply, dependant
upon, and limited only by the Q (quality factor) of the system.  (Q is an
inverse function of system friction.)

The infamous 20V distributor rattle apparently ocours, and is at its worst,
around 2200 RPM(?) and at a slightly negative engine torque condition, i.e.
no applied power and with some small engine braking.  (I haven't experienced
the problem, just guessing that this is the consenses. listening to the
descriptions of you rattlers.)  Under these conditions, timing belt static
running tension is at its minimum, inasmuch as there is a slightly negative
valve train decceleration force being transmitted which is just equal to the
positive force transmitted to overcome valve train friction.  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

Remember, this resonant angular vibration is a cyclic speeding up and
slowing down of the cam system inertia superimposed upon its steady state
value, the 1100 cam RPM.  This resonant angular acceleration of the cam
system and the resultant rotary forces generated are transmitted by, or
coupled through the timing chain, the distributor drive gear system, and the
timing belt.  The distributor shaft inertia is forced to follow this
resonant angular vibration because it is tightly coupled to the cam by its
gear system.  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.

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.  I don't know what the correct tension is or how to
define over tension, but I'd elect to error on the high side.

John L., will you please further elaborate, from your 30 year "feel" base,
on 20V belt tension?  I would say that it should be considerably higher than
that required for a 10V because of much higher valve train inertia.  Will a
20V belt complain audibly if over tight, as apparently the 10V does?


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