Cracked bike frame??

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Hey guys,My neighbor stopped by and asked if I could fix a crack on this Mtn. Bike frame. Do I need to run special wire in my mig to weld chromo?
Reply:No, you don't. But if the frame were an expensive one, I would refer him to someone with a tig welder to minimize the HAZ from the mig. A bicycle frame that has cracked needs gusseting where the crack is, or it will crack again. Mountain bikes get more abuse than most offroad trucks, and have some thin wall tubing for weight savings. Try to get a picture or 2 of the frame, and the crack before you attack it, so someone here can give you some extra useful input. Make the pictures good ones, so we can see it easily. Paul.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:Thanks,It's just an older bike and not a great one. He was riding with his seat post way to high and put stress on the top 2" of the seat post portion of the frame. It cracked just above the welds.
Reply:tell him to get a new bike frame, one that fits him, next time the seat might break off, he could fall back wards into the tire, which can throw him him forward into the frame and lose a testicle, its happened.   anyway welding up cracks is not as straight forward as it seems, you have drill stop it, grind the crack out and reweld, then you need to find out why it cracked and reengineer it.
Reply:anyway welding up cracks is not as straight forward as it seems, you have drill stop it, grind the crack out and reweld, then you need to find out why it cracked and reengineer it.
Reply:Originally Posted by RojodiabloNo, you don't. But if the frame were an expensive one, I would refer him to someone with a tig welder to minimize the HAZ from the mig. A bicycle frame that has cracked needs gusseting where the crack is, or it will crack again. Mountain bikes get more abuse than most offroad trucks, and have some thin wall tubing for weight savings. Try to get a picture or 2 of the frame, and the crack before you attack it, so someone here can give you some extra useful input. Make the pictures good ones, so we can see it easily. Paul.
Reply:From many years experience in the bike industry and talking to several dozen frame manufacturers, scrap the frame. It is possible to repair it, but you will more than likley find or cause more cracks when grinding/welding the current one. Cromo frames have a given fatigue life, once they reach that point and start cracking in one place it's 100% garunteed that they will start to crack elsewhere.
Reply:Well, the only answer that is 100% right is that there are no absolute answers.In general though, I'd say toss the frame and get a new one.Objects can break for different reasons, fatigue or overload would be common reasons.  Or it could have been a problem with the original materials or workmanship or the design.  Etc, etc, etc.If, and I stress the IF, the problem was bad workmanship/materials at the original failure and you grind out or otherwise remove all the no-good metal and then reweld it properly, you could make it better.  But do you know the actual metal alloy used, and can you use the proper weld procedure for that alloy?  Including the proper filler metal, the proper preheat, the proper post-heat and cool-down, the proper post-weld stress-relief, etc, etc.  Lots of ifs and 'propers' to do there.As zap says, if you touch it then you are IT if something goes wrong down the road.  Do you want to be IT???And when you say "cracked just above the welds" for the seat post, do you mean cracked circumferentially around the seat post just above the frame top tube?  In that case, 100% toss it and get a new frame.  If you mean cracked the seat post longitudinally (length-wise split a seam) just above the frame top tube, then maybe you can fix that one.But how are you going to get 100% weld penetration through the seat post socket and then machine out the ID of that tube so the seat-post can fit back in???And some bike frames are brazed, or lugged and brazed, and not welded together.  Some may be welded together, many are not.I'd suggest not doing it and tell him to toss the frame and get a new one.
Reply:Bike frames have "working shelf lifes", and Mtn bikes are usually recommended to be changed out every 5 years of moderate use.  HARD uses like testicular impactions require nearly annual replacement.We've got some CRAZY riders here in the gorge (Hood River) who jump 30-45ft consistantly.  New frames are almost required every season.  They all limp like a pimp from the landings!Legally, I wouldnt touch that bike with a 10 ft tungsten...Last edited by DirtyLittleSecret; 06-11-2007 at 03:10 PM.ThermalArc 185MillerMatic 180 w/ AutosetVictor Cutskill Oxy/AcetyleneThermal Dynamics Cutmaster 38and spite!
Reply:Originally Posted by MoonRiseWell, the only answer that is 100% right is that there are no absolute answers.In general though, I'd say toss the frame and get a new one.Objects can break for different reasons, fatigue or overload would be common reasons.  Or it could have been a problem with the original materials or workmanship or the design.  Etc, etc, etc.If, and I stress the IF, the problem was bad workmanship/materials at the original failure and you grind out or otherwise remove all the no-good metal and then reweld it properly, you could make it better.  But do you know the actual metal alloy used, and can you use the proper weld procedure for that alloy?  Including the proper filler metal, the proper preheat, the proper post-heat and cool-down, the proper post-weld stress-relief, etc, etc.  Lots of ifs and 'propers' to do there.As zap says, if you touch it then you are IT if something goes wrong down the road.  Do you want to be IT???And when you say "cracked just above the welds" for the seat post, do you mean cracked circumferentially around the seat post just above the frame top tube?  In that case, 100% toss it and get a new frame.  If you mean cracked the seat post longitudinally (length-wise split a seam) just above the frame top tube, then maybe you can fix that one.But how are you going to get 100% weld penetration through the seat post socket and then machine out the ID of that tube so the seat-post can fit back in???And some bike frames are brazed, or lugged and brazed, and not welded together.  Some may be welded together, many are not.I'd suggest not doing it and tell him to toss the frame and get a new one.
Reply:Jeezus. Looks like most of these guys never repair stuff, just toss everything out and build new stuff. Just weld it, add a well thought out and close fitting gusset, preferrably one that will wrap the area where you repair, and tell the guy to try it out. I will bet if his seat post is longer and stronger to spread the load properly, he will not have any problems. We fix frames every now& then. When I used to race bmx and jump bikes, we fixed them all the time. 90% of them were fine after the fix. I have repaired literally 30 or so bikes, some forks, and the break you describe is pretty common on certain bikes.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:If it's a cheap bike frame, let me know and I will take it!
Reply:Its not that it cant be fixed, but rather its not worth enough the risk/hassle to repair.  Mtn bike frames are not exactly cheap, but they arent exactly so expensive to warrant the risk of failure.  $400-600 for a new frame (super nice can run in excess of $5k).  SUPER cheap insurance compared to my umbrella coverage.ThermalArc 185MillerMatic 180 w/ AutosetVictor Cutskill Oxy/AcetyleneThermal Dynamics Cutmaster 38and spite!
Reply:Originally Posted by DirtyLittleSecretHARD uses like testicular impactions require nearly annual replacement.
Reply:the HAZ with TIG will be larger than for MIG and minimizing the HAZ is not always good!a smaller HAZ will effectivly let the weld cool faster which will result in a harder (and thus more brittle) weld- makes no odds with mild steel but does with Cr-Mo. this is the same reasoning behind normalizing welds in thicker Cr-Mo- more mass to the part = larger quenching effect. also why thin wall tubing is almost always welded with TIG or oxy-fuelanyways to the bike frame. general fatigue could certainly be an issue but from the description it sounds as though the failure was initiated from insufficient support for the seatpost- typically 70mm minimum insertion into the frame is required or the post should extend past the seat/top tube junction.heat treatment isn't an issue. VERY BASICALLY, most alloy steel bicycle specific tubing is supplied already heat treated (for higher end sets) and is butted- the tube has a thicker wall at each end to compensate for the changes that occur from welding.IMO though this is one those situations where if you have to ask how to do it you don't have enough experience to. the consequences of a catastrophic failure don't bear thinking about.bear in mind that bike s/posts are sized in 0.2mm increments and MUST be a very good fit to prevent frame failure. the weld MUST completly remove the crack which means the frame will need to be machined afterwoods. the cost of the necessary reamer alone would make this uneconomic. if it's a quality frame send your neighbour to a framebuilder. oh, and tell him to buy a longer seatpost while he's there .Last edited by hotrodder; 06-12-2007 at 10:37 PM.
Reply:Originally Posted by RojodiabloJeezus. Looks like most of these guys never repair stuff, just toss everything out and build new stuff. Just weld it, add a well thought out and close fitting gusset, preferrably one that will wrap the area where you repair, and tell the guy to try it out. I will bet if his seat post is longer and stronger to spread the load properly, he will not have any problems. We fix frames every now& then. When I used to race bmx and jump bikes, we fixed them all the time. 90% of them were fine after the fix. I have repaired literally 30 or so bikes, some forks, and the break you describe is pretty common on certain bikes.
Reply:Yeah if it was me, depending on the stress the frame would see, I´d weld it over for sure....My Babies: HF Drill pressHF Pipe Bender3   4.5" Black and Decker angle grindersLincoln Electric PROMIG 175that´s it!
Reply:almost all bicycle manufacturers will replace a broken or cracked frame for free even if it is an outdated model.
Reply:Wow, thanks for all the info guys. He's already tossed it.
Reply:Just a thought, if he rides it and it breaks and he does an impersonation of a toffee apple with the shaft stuck up his butt, is he likely to1. Remain a good neighbour2. See the funny side3. Sue you for every penny you've got.Just a thought.
Reply:Originally Posted by RojodiabloJeezus. Looks like most of these guys never repair stuff, just toss everything out and build new stuff. Just weld it, add a well thought out and close fitting gusset, preferrably one that will wrap the area where you repair, and tell the guy to try it out. I will bet if his seat post is longer and stronger to spread the load properly, he will not have any problems. We fix frames every now& then. When I used to race bmx and jump bikes, we fixed them all the time. 90% of them were fine after the fix. I have repaired literally 30 or so bikes, some forks, and the break you describe is pretty common on certain bikes.
Reply:Originally Posted by elvergonI thought MIG would produce less HAZ than TIG...are you sure about that?Sure TIG is the way to go for chromemoly no doubt bout that...