Crossmember Bars POLL

[scirocco16vr32 at gmail.com (Chris Bennett)]

I got under the 85 with the 16v conversion today. The starter apparently has 
died. (good charge on the battery, click click with the ignition) I thought 
about trying to push start it but I have had two knee surgeries in the last 
year and I don't push like I used to... I removed the starter and performed 
the INFAMOUS "mexican jumping starter dance" (that would be a set of jumper 
cables on the starter and a jumper...er I mean wire to test to see if the 
bendix works or not. I forgot to put my foot on it and scared the shit out 
of myself.) The bendix extends and it spins very weakly sometimes not at 
all. Jumper wire gets REALLY damn hot really damn quick. So I decided to end 
amatuer hour and take it to a FLAPS. I walk in and dude asked me if that was 
the turbo from my scirocco. I realized at that point that I was in deep 
shit. After discovering what it was he goes to the test bench and punches in 
the the code and tells me that he can't test it. So I was like COME ON 
MAN!!! There's only four wires... So he hooks up the +12v and the two 
smaller wires, (ignition and something else--don't remember off the top of 
my head.) and tells the thing to go... As politely as possible I remind 
supergenius that it needs a ground. You know how dem dere "turbo's" act when 
thays ain't properly grounded.
 So once grounded he hits the button and the bendix extends once again and 
then the machine tells us that it is a bad starter. So I still don't have a 
warm fuzzy since this guy told me he couldn't test it and he didn't know 
what the hell it was, so I asked him to go and get a new one off the shelf 
and prove that the machine is doing what it's supposed to. Well he wasn't 
"allowed" to do that until I pay for it. So I was like fine, what's the 
damage?
 $117 plus a $60 core. I think I stared at him in disbelief for a good 12 
seconds. I am thinking: WHY THE FUCK WOULD YOU CHARGE ME A CORE WHEN I HAVE 
THE DAMNED THING SITTING IN FRONT OF YOU?!? and then WHY THE FUCK WOULD I 
PAY THAT MUCH FOR A STARTER?!? I resisted the urge to shove the starter up 
his ass, thanked him with all the nice I could muster, and left. I don't 
have that kind of money anyway.
 Anyhow, here's the bottom line of this story. While I was under the car I 
took a real hard look at the cross member and I do not see any cracks. 
There's a lot of sludge and crap and I wiped it off the best I could but I 
still didn't see anything. I was looking from the bottom where the two bolt 
for the fwd mount plate are, and didn't see anything. So for the time being 
I am safe or is there somewhere else I should look?
 Chris

 On 6/18/05, Nate Lowe <nlowe79@gmail.com> wrote: 
> 
> On 6/18/05, Allyn <amalventano1@tds.net> wrote:
> > i knew that one was coming. the way this bar works is similar to that 
> bar
> > you see the big-ass weight lifters using on tv. the guy lifts the bar 
> near
> > the center, with massive weight at either end. these bars get used for 
> years
> > and years. it may weaken and bend easier over time (yield strength /
> > ultimate strength lowers as it is fatigued), but you never see one just
> > snap. the failure mode in this case is to become more bendable, not to
> > brittle fracture.
> 
> Interesting analogy, but I don't think it's completely applicable. I
> don't know anything about those bars used in weightlifting, but I do
> konw what you're talking about. The thing is, those bars can't last
> forever, they will break if they are used too much. The same goes for
> the crossmember bar (well, maybe not, if you crunch the numbers it
> very well come out that the bar is never pushed passed the fatigue
> limit). I will also add another quote from my textbook: "Fatigure
> failure is brittlelike in nature even in normally ductile metals, in
> that there is very little, if any, gross plastic deformation
> associated with failure. The process occurs by the initiation and
> propegation of cracks, and ordinarily the fracture surface is
> perpendicular to the direction of an applied tensile stress." That
> sounds like the cracks people are seeing in their crossmembers. I have
> to admit that I have never seen one in person, but I have seen
> pictures and those pictures seem to fit what I'm thinking..
> 
> > > Do you think the crossmember is breaking due to being
> > > pushed past it's yeild strength?
> >
> > yes, this is the exact reason. the metal in the cross member is somewhat
> > hardened steel. my guess is that it is hardened enough such that the RTT 
> is
> > near ambient temperatures. if you've ever seen the cracks we speak of, 
> they
> > are jagged and sharp at the edges. basically brittle fracture. brittle
> > fracture is 'sudden, catastrophic failure of a metal under tensile 
> stress
> > with little to no plastic deformation (bending)'. it requires a 
> pre-existing
> > flaw (small crack, nick in the metal, etc). when a crack propogates in 
> this
> > mode, the only way to stop it from propogating is to stop drill it. its
> > basically the same as a crack in glass. the stresses are amplified 
> greatly
> > at the leading edge of the crack, so it spreads that much easier.
> 
> You are right in that once a crack is formed it will spread much,
> much, much easier (I can't emphsize enough how easy it is for a crack
> to spread once it's there). Again, the problem with your reasoning
> here is that you aren't considering fatigue, you are only considering
> brittle fracture alone. Lastly, I don't know what you mean by RTT,
> I've never seen that abbreviation before and I'd like to know what it
> means. I don't think it will change my opnion on this though.
> I also left my responce in here (noted by the double arrows (whatever
> they're called) because I think it's necessary to understand the
> conversation if you havn't been reading every post. Allyn's words are
> noted by the single arrow.
> 
> > > I don't, because if that was the case
> > > drilling holes would do no good and the entire crossmember would break
> > > in half before you even noticed the cracks. The crossmember is
> > > cracking due to fatigue.
> >
> > easy stop to that argument is the fact that only the _newer_ (~>85) 
> cross
> > members are the ones that are cracking. wouldnt you think that a similar
> > designed piece in a 30 year old scirocco would have more fatigue than 
> that
> > of a 15 year old one? mileage and horsepower dont seem to have much of 
> an
> > impact on it either. from my words above - i think vw hardened this 
> steel a
> > bit more or used a different mix that had a lower fracture toughness / 
> RTT.
> > i also believe that the cracks don't occur unles some major shock type
> > stresses are applied to the member (i.e. accident, bad side mounts, 
> etc).
> 
> I think that there are a couple different things that contribute to
> this point. First, I'd agree that there was most likely a change in
> the material used for the crossmember sometime during the mid 80's.
> You must also consider that a 30 year old scirocco didn't have the
> horsepower of a 16v out of the box. I thought, from what I've read,
> that most people with this problem had 16v versions (not swapped, but
> real 16v's). I may be wrong on this, that's just the impression I got.
> It would be intersting to see who had cracks, when they started, and
> what had been done to the motor over the car's life. I'm still
> inclined to believe the cracks are from fatigue though.
> 
> > > Good, now I know how thick the plate is. It didn't look that thick in
> > > the pictures. But if you look at the geometry of that plate, the
> >
> > whoa there, dont quote me on it :) its either 1/4 or 3/16. im going on
> > eyeball gauge only.
> 
> Oh, I thought you had the exact measurement. I won't quote you on it
> then ;). I'd lean toward the 3/16 if not 1/8, but that's just eyeball
> from me too, and from pictures only at that. This is nothing to argue
> about since the designer is on the list and can just tell us what it
> is.
> 
> > gotta run, party at my place :)
> > p.s. no flamage here, just opinions, backed by some navy training.
> > Al
> 
> Have fun with your party. No flamage from me either. I openly admit
> what I'm saying is opinion and can be proven wrong. I don't mind at
> all if someone points out where my reasoning is flawed, it certainly
> won't be the first time :)
> But what I would like to say is that my main focus here is that the
> bar hasn't gone through any rigorous testing so nobody can say for
> sure how much it helps (and testing can be as simple as doing a few
> calculations to see what it will withstand. Granted some assumptions
> will need to be made, but that's engineering). That said, I'd suspect
> that any car without any existing cracks would probably be fine with
> this bar, it's the cars who already have cracks that I really
> question. That is because of the major loss of toughness (engineering
> meaning of thoughness: resistance to fracture) for those unfortunate
> cars that already have cracks.
> Nate
> 
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