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vibration/shudder at 70 - possible alternatives (other than wheel balance?)

Subject: vibration/shudder at 70 - possible alternatives (other than wheel balance?)
From: marc_scirocco@sympatico.ca (Marc Scirocco Québec)
Date: Tue, 22 Apr 2003 00:05:16 -0400
In-reply-to: <003301c30880$be420660$5fe0a818@main>

Simple laws of physics. Your left wheels will not exactly turn at the same
speed as the right wheels, Unless you are really going absolutely straight.

As an exemple, you have your left wheels turning at 60.00 mph, and your
right wheels turning at 60.10 mph. Two frequencies will do the following:
they will add up, and they will substract.
So you have:
F1 + F2 = Fadd
F1 - F2 = Fsubstract

So in this case what you are hearing is the difference between two very
close frequencies, hence the ±1 Hz thing.

Same deal if you are driving (or stopped at a red lioght) side by side with
a loud car, try this:
Match your rpms with the other car, and when you get close, you will hear
the modulation until you are at the same rpm (frequency). Same principle
when you tune a guitar, or a piano.

Clear enough? well...

Marc
'83 Scirocco
'88 Scirocco Slegato

-----Message d'origine-----
De : scirocco-l-admin@scirocco.org
[mailto:scirocco-l-admin@scirocco.org]De la part de Allyn
Envoyé : 21 avril, 2003 23:39
À : scirocco81@vwmail.net; Patrick Bureau; Rhett Farnum;
scirocco-l@scirocco.org
Objet : Re: vibration/shudder at 70 - possible alternatives (other than
wheel balance?)


hmm, ok, so we got the height issue taken care of, there goes my cv theory.
patrick, i know you mentioned your rear toe was perfect with the ehims
removed, how about the toe?

new theory:
shudder gets worse under acceleration due to weight transfer onto rear
misaligned wheels.
now to figure out why it modulates at ~1 hz
Al

----- Original Message -----
From: "Eric S" <scirocco81@vwmail.net>
To: "Patrick Bureau" <txrocco@sbcglobal.net>; "Rhett Farnum"
<rocco_crossing@hotmail.com>; <amalventano@sc.rr.com>;
<scirocco-l@scirocco.org>
Sent: Monday, April 21, 2003 11:24 PM
Subject: Re: vibration/shudder at 70 - possible alternatives (other than
wheel balance?)


> Let me chime in with a "me too" on this.  The mk2 has done this since I
got it, and it bugs the crap outta me.
>
> And, patrick, I *just* put new a-arms with poly bushings in....no
difference.
>
> Its done it with 13s with 175/70s and now with 15s with 205/50s, it
already has new shocks, I was going to change the rear wheel bearings next
and get a *true* 4-wheel alignment (shim the rear backing plates).  I have
new half-shafts that I thought about trying...but now from hearing from you
guys that it made no difference, for some reason, I think its coming from
the rear....don't ask me why, just a gut feeling.  I will let you all know
how it goes with the 4 wheel alignment.
>
> --
> Eric
> Northglenn, CO
>
> 79 VW Iltis
> 81 Scirocco S (*TDI* swap and complete restoration in holding pattern)
> 81 Rabbit Truck LX Diesel (Waiting for BAE Turbo Install)
> 83 Scirocco Wolfsburg
> 91 Cabriolet (2.0 Crossflow 8v swap and mechanical rebuild in progress)
>
>
> --- Patrick Bureau <txrocco@sbcglobal.net> wrote:
> >Nope it did this with spider wv 13" , with Konig;s
> >15x7 with new rubber, and still doing it with
> >speedline 14" with new rubber as well...
> >
> >it is not a tire/rim related issue I am pretty sure of
> >it, Oh yeah add change bearings, cv joint, shocks and
> >springs, ball joint and tied rods...
> >
> >I am thinking that it could be the sway bar bushings
> >or the a-arms bushings that need to be changed (small
> >bumps would cause because of deterioration of the
> >bushing a "loose" vibration....
> >
> >I have the same problem on the prowler and I will know
> >soon anought as I change the bushings for poly's on
> >a-arms, and new sway bar bushings (oem)...
> >
> >
> >
> >--- Rhett Farnum <rocco_crossing@hotmail.com> wrote:
> >> HOw about flat spots? I have at least one tire with
> >> a flat spot on my Cut'
> >> and she likes to shake between 76-80mph. Just a
> >> thought.
> >>
> >>
> >>
> >> Peace Out,
> >> Rhett
> >> Rocco_crossing
> >> '83 Rocco (autocross ready, tune up, harness bar,
> >> rear wheel bearings, and
> >> finally I may get to makeing it look better!!)
> >>
> >>
> >>
> >>
> >>
> >> >From: "Allyn" <amalventano@sc.rr.com>
> >> >To: "scirocco list" <scirocco-l@scirocco.org>
> >> >Subject: vibration/shudder at 70 - possible
> >> alternatives (other than wheel
> >> >balance?)
> >> >Date: Mon, 21 Apr 2003 20:28:24 -0400
> >> >
> >> >ok, the rieger has had the same shudder at 70ish
> >> for the longest time.
> >> >
> >> >things changed:
> >> >a-arms, bushings (all), bearings (all), half shafts
> >> (replaced with a
> >> >questionable set), engine/tranny, engine mounts,
> >> and a totally different
> >> >set
> >> >of wheels. not to mention rotors and hubs (i think
> >> this covers just about
> >> >every moving part).
> >> >
> >> >...and yet i still get the shudder...
> >> >
> >> >exact symptoms:
> >> >
> >> >- 0 vibration felt in wheel, even with the shudder
> >> at its worst.
> >> >- alignment is close enough such that it couldnt
> >> cause a problem like this
> >> >- shudder is not constant, the vibration is
> >> consistent with wheel rotation
> >> >at 70 mph, but it comes/goes at a 1 second cycle
> >> (kinda like its amplitude
> >> >modulated).
> >> >- shudder seems proportional to throttle
> >> application. the other day i did
> >> >several runs on the highway, coasting from 90-60 in
> >> neutral, and the
> >> >shudder
> >> >seemed to go away. shudder returns when holding
> >> speed at 70, and gets worse
> >> >under acceleration.
> >> >- feels like an out of balance wheel, but i've also
> >> felt the same feeling
> >> >when i had a bad inner cv (however the bad inner cv
> >> had a constant
> >> >vibration).
> >> >- also seems like bad inner cv in that its almost a
> >> side-to-side vibration,
> >> >felt in the floor under the center console.
> >> >
> >> >possibilities:
> >> >1. other set of half shafts (rebuilds from
> >> potterman) had the exact same
> >> >problem with the inner cv.
> >> >2. frame/engine is misaligned such that one of the
> >> half shafts is at its
> >> >compressed limit when installed / car on ground.
> >> possibly forcing tranny
> >> >over as it rotates, causing the vibration.
> >> >3. both sets of wheels/tires have had the exact
> >> same imbalance, after being
> >> >balanced several times between both sets
> >> (doubtfull).
> >> >
> >> >suggestions???
> >> >
> >> >(thx in advance for info)
> >> >Al
> >> >
> >> >
> >> >_______________________________________________
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> >> >Scirocco-l@scirocco.org
> >> >http://neubayern.net/mailman/listinfo/scirocco-l
> >>
> >>
> >>
> >_________________________________________________________________
> >> Protect your PC - get McAfee.com VirusScan Online
> >>
> >http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963
> >>
> >>
> >> _______________________________________________
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> >> Scirocco-l@scirocco.org
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> >
> >
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>
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References:
- vibration/shudder at 70 - possible alternatives (other than wheel balance?)
  - From: amalventano@sc.rr.com (Allyn)

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