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Repair a cracked set of chain stays on a Specialized StumpJumper M2? TIG/Braze/Scrap?

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Repair a cracked set of chain stays on a Specialized StumpJumper M2? TIG/Braze/Scrap?

Old 07-04-14, 12:28 AM
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89Trek970
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Repair a cracked set of chain stays on a Specialized StumpJumper M2? TIG/Braze/Scrap?

Hi,
I have just gotten back into riding with my family after a 10 year hiatus. Currently we are riding Rail-Trails in the Poconos / Lehigh Valley. I ride my trusty 89 Trek 970, towing a toddler in a trailer. My elementary child rides a Trek 6spd 20", and my wife got rid f a larger framed 26" MTB to a lower stand over 24" Murray Venom. When we get home from riding, she seems wiped from the heavy Murray.

I saved a small framed Specialized Stump Jumper M2 (98/99?) from my local recycling yard, complete minus the rear wheel, intending to pull the parts to lighten her Murray, but realized how much lighter this could be for her as a rail-trail bike. realized the previous owner had split both chain stays, approx. 1-1.5", one in the center and one near the dropout. Everything worked fine (including the suspension forks) once I added a rear tire. I'm not sure if this was a seamed tube or drawn tube, and thus unsure if it was worth repairing on my very limited budget.

I spent VERY little from the yard, but I am currently on a limited budget and she would never go for me spending $3/400 on a brand-new entry level bike for her. She will never use this for more than a recreational rail-trail bike, and my limited knowledge of these frames has me looking for help. IS this something I could have Brazed, and be safe/strong enough for a recreational, low speed bike for the next year or so until I can budget her a NEW MTB? I thought this might be a repair for a TIG welder and surveyed my local welding shop. Sight unseen they quoted approx. $75 to TIG. I was hoping to stay around $50, figuring it was a quick/easy repair. Or should I scrap it and look for another option...CAN I get a compatible alloy bare frame (HardRock) from Specialized or another MFG in my budget ($100) to transfer the components over to, or look for an aluminum major brand MTB in need of TLC to fix using the Stumpy parts?

I've been out of this for too long, and althought I know automotive repair & welding, this is not in my capability (beyond the Brazing as I have Propane, MAP & Oxy/A ).

Any thoughts and suggestions will be GREATLY appreciated and accepted.
I am hoping to make riding a more enjoyable experience for her, and hopefully more frequent for the family.
Thanks,
Mike

BTW- Happy 4th of July!
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Old 07-04-14, 06:06 AM
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To my eye these cracks are not typical to Alu bikes. Esp. the LH stay one. The mid tube location and the crack running lengthwise suggests that the stays may have been made from strip (as in seamed tubes) and you're now seeing the weld stress cracking the stay open. I would suspect the remaining of the stays. Andy.
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Old 07-04-14, 06:07 AM
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That looks like the stays got filled with water and froze, causing those splits. Repair is not practical, but for rail trail riding it might last a while. When it fails, it won't be catastrophic. For an inexpensive frame, look at Nashbar. They have a $99.99 MTB frame that is pretty light. I have one that i bought to build up a spare bike from my parts bin and I think it weighed around 3.7 lbs.
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Old 07-04-14, 06:11 AM
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Originally Posted by dsaul View Post
That looks like the stays got filled with water and froze, causing those splits. Repair is not practical, but for rail trail riding it might last a while. When it fails, it won't be catastrophic. For an inexpensive frame, look at Nashbar. They have a $99.99 MTB frame that is pretty light. I have one that i bought to build up a spare bike from my parts bin and I think it weighed around 3.7 lbs.
I hadn't considered this possibility. Still the frame is trashed. Andy.
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Old 07-04-14, 06:37 AM
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I agree it looks like damage from freezing water. Given your ability to braze that is what I would try. Hammer the splits shut and braze them. It will be a delicate operation but what have you got to lose? I will always try a no cost repair before I trash something, if only for the learning experience.

The chain stays are in tension and with your intended use of the bike I think it will hold up just fine. And after all it's a bicycle, not an airplane.
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Old 07-04-14, 07:54 AM
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Originally Posted by JBC353 View Post
I agree it looks like damage from freezing water. Given your ability to braze that is what I would try. Hammer the splits shut and braze them. It will be a delicate operation but what have you got to lose? I will always try a no cost repair before I trash something, if only for the learning experience.

The chain stays are in tension and with your intended use of the bike I think it will hold up just fine. And after all it's a bicycle, not an airplane.
Brazing is for steel, not AL.

You would need to find a TIG guy experienced with thin walled aluminum. Not any hack used to welding thick steel.

You could bang it back closed & cover with bondo and see what happens. I bet it will last a while since that is not a high stress area.
If it starts to crack again, then time for a new frame.
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Old 07-04-14, 09:10 AM
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I was hoping that someone would point that out.

MTB frames are easy to find cheap. You can find whole bikes for $50 or less. Personally, I'd look for steel.
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Old 07-04-14, 09:37 AM
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Originally Posted by Homebrew01 View Post
Brazing is for steel, not AL.

You would need to find a TIG guy experienced with thin walled aluminum. Not any hack used to welding thick steel.

You could bang it back closed & cover with bondo and see what happens. I bet it will last a while since that is not a high stress area.
If it starts to crack again, then time for a new frame.
M2 = metal matrix composite. Aluminum with aluminum oxide particles mixed in. Supposed to give it some desirable properties. That pretty well exhausts my knowledge of metallurgy.
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Old 07-04-14, 10:08 AM
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Good rigid mountain bikes can often be found in the $100-150 range. Sometimes even less. Look for a complete bike so that you won't have to worry about parts compatibility when transferring stuff to a bare frame. You could end up spending more replacing parts that didn't fit a new frame than you might spend on a whole bike.
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Old 07-04-14, 03:12 PM
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Originally Posted by Homebrew01 View Post
Brazing is for steel, not AL.
Wrong, aluminum can be brazed. Google is your friend.
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Old 07-04-14, 04:11 PM
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Originally Posted by JBC353 View Post
Wrong, aluminum can be brazed. Google is your friend.
Well, dang, some good options out there.
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Old 07-04-14, 06:14 PM
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Originally Posted by Homebrew01 View Post
Well, dang, some good options out there.
Just because an option exists does not mean it is a good or prudent one. We all can drive through red lights after all... Andy.
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Old 07-04-14, 06:27 PM
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Originally Posted by Andrew R Stewart View Post
Just because an option exists does not mean it is a good or prudent one. We all can drive through red lights after all... Andy.
Well, some interesting options out there.
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Old 07-04-14, 06:53 PM
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simple

sounds like you got a good deal on some components
trash the damaged frame
keep lookout for another good frame to put the parts on

don't bother with the murray, not worth upgrading
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Old 07-04-14, 08:23 PM
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Originally Posted by SkyDog75 View Post
Good rigid mountain bikes can often be found in the $100-150 range. Sometimes even less. Look for a complete bike so that you won't have to worry about parts compatibility when transferring stuff to a bare frame. You could end up spending more replacing parts that didn't fit a new frame than you might spend on a whole bike.
That's kinda what I'm finding- that "standard" interchangeability is much less standard than back in my 'youth'...why I was thinking maybe just a bare Specialized HardRock frame might work....kinda scared of other avenues.

I am still looking for a good quality entry bike used locally right now, as well as a used frame as backup options.....
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Old 07-04-14, 08:50 PM
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Originally Posted by dsaul View Post
That looks like the stays got filled with water and froze, causing those splits. . . .
+1
I'd stop-drill them, leave exposed, ride, and monitor.
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Old 07-05-14, 06:40 AM
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I knew the Murray was a poor option, but for $20.00 it got her riding again.....I did assemble as much of the alloy components temporarily on the Murray, & it took quite a few pounds off, she's happier but it's still heavy. Like I said I am still going to keep looking for a good quality entry bike used locally right now, as well as a used frame as backup options..... but may try to get the TIG job cheaper or Aluminum 'Braze" it in the meantime.

Thank you for the input, BTW I never would have thought about freeze damage.....


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Old 07-05-14, 07:05 AM
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Since it's metal matrix, welding attempts are likely to end in tears, I'd wager.

Originally Posted by AnkleWork View Post
I'd stop-drill them, leave exposed, ride, and monitor.
This.
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Old 07-05-14, 12:28 PM
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Scrap, too many decent MTBs out there at reasonable prices. Keep or sell the parts.
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Old 07-05-14, 12:57 PM
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Originally Posted by Kimmo View Post
Since it's metal matrix, welding attempts are likely to end in tears, I'd wager. . .
+1 I wouldn't get a torch or arc near MMC, outside of the factory where they made it.

But really, there's no need. Riding did not cause those splits, and the force paths from further riding won't interact with them. At a maximum I'd consider formed aluminum patch plates with structural epoxy.
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Old 07-05-14, 06:14 PM
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Originally Posted by AnkleWork View Post
+1 I wouldn't get a torch or arc near MMC, outside of the factory where they made it.

But really, there's no need. Riding did not cause those splits, and the force paths from further riding won't interact with them. At a maximum I'd consider formed aluminum patch plates with structural epoxy.
Gently re-bend back together & JB Weld then......sounds much simpler
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Old 07-05-14, 06:20 PM
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Found a temporary solution till I can swing a brand new bike for her. Passed a huge lawn sale & I stumbled across a small framed '95 Trek 820, 26" MTB complete but with 2 flats, needs a saddle & looks like it was left outside for past 4/5 months. Minimal rust, everything works, & tires are nearly new Treks, not dry-rotted. Paid a third of the quote for the TIG welding. Solid, lighter platform to start with for the time being.

Thanks again for all the input and responses. I am amazed at the speed and variety of help I've sen here. It takes me much longer to get the number & variety of responses when I'm using the automotive Forums I am usually on over the past ten years. Very happy to be a new member of your community. Thanks!
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