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Old 531 frame - Paint scratch? Crack?

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Old 531 frame - Paint scratch? Crack?

Old 05-12-14, 07:23 PM
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pgoat
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Old 531 frame - Paint scratch? Crack?

Hi all,

about a year ago I noticed a small scratch on my 1986 Trek - 531 butted main tubes. Scratch is about 1 cm long, curved, and is on the side of the down tube, right behind the lower area of the head tube lug. EDIT: Not sure why my pics keep uploading upside down - sorry about that! The files are right side up on my computer...)





I have seen online pics of cracks in lugged steel frames before, but they usually look like a seam separation right ON the lug-tube join...this line is a good couple of mm away from the lug, but it does seem to follow the curve of the lug pretty exactly, hence my worry.

I bought this bike used on Craigs List in 2005 - used it for occasional commutes and charity rides for a few years, then weekly club rides and midweek training rides for the past 4-5 years, putting about 2000-3000 miles a year on it.

When I bought it the bike was almost 20 years old but looked very very clean and all original. But, it did have a blip in both front rim walls - my impression was it may have hit a curb or pot hole. But everything else looked fine, so I added new wheels and all has seemed well since.

Last year I brought the bike to a respected local shop and asked their opinion about the scratch. They said they thought it was fine.

This year I recently went from a flat bar arrangement I'd been using for club rides for about 5 years back to a drop bar, and I noticed a lot of flexing in the front end when braking hard (going downhill, etc). I wasn't sure if this was due to my weight being more forward braking on the hoods (with a flat bar I tended to stand on the pedals and get my butt back off the seat to get my weight back over the rear wheel on such braking). I have not overhauled the headset for 3-4 years but it does not seem loose at all..no obvious play in the head set. No indexing or other signs of bearing wear...


I also recently noticed some front end creaking noises. I tightened down the bar clamp and stem, but the noises continued - on today's commute I held down a badly routed gear cable (I botched the smooth cable run when swapping bars) and when I did the creaking went away - this was consistent, so I am confident the creaks are just the cable housing tip rubbing on the downtube's cable stop, but I am still concerned about the scratch line and the front end flex.

I should add, this bike was just demoted from club rides to just commuting/utility use, as I got a new road bike for fast weekend rides, and sold my older commuter. I'm hence concerned about the pounding rough commute routes will give the bike if the frame is compromised...

The line does not look like it has grown any longer in the past 10 months or so since I first saw it, but it might look a bit more distinct/sharp if that makes sense.

Any thoughts?

Is the one way to know for sure to scrape off paint and look for a line in the bare metal?
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Old 05-12-14, 09:57 PM
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Originally Posted by pgoat View Post
Is the one way to know for sure to scrape off paint and look for a line in the bare metal?
That's what I'd do. Once the paint is removed and the area cleaned you can use Dye Penetrant Inspection to see if it's really a crack. The dye penetrant is available in aerosol spray cans for about $20.
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Old 05-12-14, 10:34 PM
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Another technique to monitor a crack's growth (and if a crack never grows will any one hear it?) is to make a scratch mark across the visible ends of the "crack". If the 'crack" is stable or non existent then the cross marks and the 'crack's" ends will always coinside. If there really is a crack and it's propagating then the crack's ends will travel past the marks. The neat aspect of main triangles and steel is that failure is rarely without warning. Andy.
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Old 05-13-14, 04:40 AM
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It's hard to tell in the pictures but it looks like a scratch in the paint to me. It looks like it starts abruptly at the end closest to the lug tip and the paint appears pushed up at that end. Look at it closely with a magnifier and you can probably see more than I can. I think a crack would generally taper at each end, follow a more crooked line and simplt crack the paint, not push it up.
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Old 05-13-14, 06:00 AM
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that is not a direction that a crack would generally take. the lug/tube shoreline is the big stress riser in that area, that's where your cracks would be. There is nothing to make a tube crack where that scratch is. Don't ruin that paint
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Old 05-13-14, 07:23 AM
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Thanks, all. the last two posts are very reassuring, I appreciate that.

I have a theory; I don't clean my bikes as often as I should, and when I do I have to really scrub to get the grit and grime off. This bike got ridden a lot in the rain and of course a lot of road grit collects behind the fork crown and head tube. I wonder if I may have been rubbing a rag along the lines of the lug and simply rubbed some grit into the paint, making the scratch....

Short of sanding it (I agree on not wanting to ruin the paint, the rest of the frame has a few minor rust spots from chipped paint but otherwise looks fantastic for a 1986 bike!) am I right in just riding it and keeping an eye on the crack? I figured between taking photos to get a baseline and measuring it, I'll know if it grows.

Also, I did try looking at it with a 8x magnifying loupe - kinda hard to get in close enough for a good solid look but even so, it was hard for me to tell - it basically looks benign to me (fwtw), like a surface paint scratch.
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Old 05-13-14, 09:31 AM
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i can't see anything, anywhere. looks like new. sounds like someone wants a reason to buy new bike to me.
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Old 05-13-14, 09:40 AM
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Originally Posted by hueyhoolihan View Post
i can't see anything, anywhere. looks like new. sounds like someone wants a reason to buy new bike to me.
LOL. No, just got a new one, as mentioned, which is why this one got reassigned from weekend club rides to beater duty...I just want to keep all my teeth and my brains, so didn't wanna take a foolish risk in doing a face-plant on the next pothole going to work.
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