Crossmember Bars POLL

[nlowe79 at gmail.com (Nate Lowe)]

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