Frame Crack?

Nubo said:

I noticed a crack in the paint on the steertube beginning near the weld
for the downtube, and extending downward and slightly towards the front.
Given the location I am concerned to know whether or not this was due to
a crack in the tubing itself. Under about 30X magnification I could see
that it was not a simple scratch due to small-scale irregularity. I
scraped away the paint from the area with a blade and checked again. At
this point I was thinking maybe a crack since there was a slight groove
visible that followed the contour exposed by the paint crack for part of
its length. There was a slight amount of reddish brown rust. The
groove itself was maybe 0.3 mm wide and did not appear very deep at all,
maybe 1 or 2 thousands of an inch. At this point I still couldn't see
anything that I could definitively call a crack.

Using a Dremel tool and fine sanding disk, I removed more paint and
attempted to clean and polish the suspect area. Checking again I could
not see any defect in the metal using the 30X magnification and a very
strong light source (tactical flashlight) at various angles.

At that point I wiped down the area and applied a couple of coats of
bare metal primer and rustoleum enamel to protect the metal. But, I am
having lingering concerns.

Is it possible that the paint defect could have formed independently of
any metal damage, then allowed water to contact the metal, which then
resulted in the grooving I initially saw? Or is this more likely an
actual metal crack?

Seeing as how I could not see any sign of a crack at magnification, Is
it judicious to rely upon the new paint as an indicator, checking
diligently for any new paint cracking at the same spot? Or should I
make the effort to get it checked with definitive methods (dye)? Is
this a location that would be prone stress cracking? If suggesting a
dye examination, would this be something a good LBS would typically
handle, or is it best done elsewhere? Could it be a DIY?

The bike is a Trek520, about 10 years old, so is a fairly beefy Cro-Moly
steel tubeset. Probably about 15 thousand miles on it, some wet riding
and I am a heavy rider, around 265 lbs. Late this summer and through
the fall my commute route changed to include a length of road which has
been abandoned for nearly 25 years (Carquinez Scenic Highway) and has
fairly rough sections with some high speed descents and spots that
demand heavy braking. I replaced the headset bearings about 18 months
ago and took a close look at the inside of the steertube at that time.
There was no significant corrosion that I could detect.

Here's a picture with a red line overlayed where the original paint
crack was noticed. I don't have any photo from before the fresh paint
was applied.

http://home.pacbell.net/lososos/STEERTUBE.JPG

if there's a crack in the metal, it'll break through the paint almost
immediately next time you ride around the block. if that happens, stop
riding immediately & either repair or replace the frame. if it doesn't,
continue close observation with increasing use. if no further evidence
appears, it was just a paint blemish.

on steel, paint blemishes can occur a significant distance from a defect
in the surface due to undercutting. it looks almost as if a worm has
burrowed under the paint and woven a crooked trail away from the
starting point. kind of fits your description.

in answer to your dye penetrant question, you'd not typically do it on a
painted surface; in this instance, your clean new paint will show any
problems real quick.

On Tue, 14 Dec 2004 06:00:56 GMT, Nubo <[email hidden]>

Quoted message said:

I noticed a crack in the paint on the steertube beginning near the weld
for the downtube, and extending downward and slightly towards the front.
Given the location I am concerned to know whether or not this was due
to a crack in the tubing itself. Under about 30X magnification I could
see that it was not a simple scratch due to small-scale irregularity. I
scraped away the paint from the area with a blade and checked again. At
this point I was thinking maybe a crack since there was a slight groove
visible that followed the contour exposed by the paint crack for part of
its length. There was a slight amount of reddish brown rust. The
groove itself was maybe 0.3 mm wide and did not appear very deep at all,
maybe 1 or 2 thousands of an inch. At this point I still couldn't see
anything that I could definitively call a crack.

Using a Dremel tool and fine sanding disk, I removed more paint and
attempted to clean and polish the suspect area. Checking again I could
not see any defect in the metal using the 30X magnification and a very
strong light source (tactical flashlight) at various angles.

At that point I wiped down the area and applied a couple of coats of
bare metal primer and rustoleum enamel to protect the metal. But, I am
having lingering concerns.

What you have described is possibly consistent with a creeping
corrosion runner that burst the paint from below. It can form as a
result of pre-painting contamination among other things. If there was
no visible evidence of a crack after the area was cleaned, and if the
paint remains sound after a reasonably energetic ride, than it's very
likely that there's nothing to worry about. If there is an actual
crack in that location, the fresh paint will split along it when the
tube is loaded enough to put the crack interface into motion.
--
Typoes are a feature, not a bug.
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If you are really concerned, get in touch with Trek. The frame might be covered by a lifetime warranty. They are pretty good about helping you out.
Good Luck