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I got a bicycle that has this big tear in the chainstay.

Is it safe for city riding? What is the potential danger?

What could someone have done to cause this kind of damage?

Most importantly what is a good procedure and material to use to fix it?


  • 2
    If you mean you recently purchased this from someone, they may have knowingly sold you an unsafe bicycle. You nay be able to get the =m to take it back and refund you your money. Commented Dec 2, 2017 at 22:00
  • Is the frame aluminum or steel? It may have been damaged by a kinked chain or by something getting caught between frame and hub. Or possibly from an exceptionally hard landing from a jump. It's impossible to tell if a (reasonably) safe repair is possible without viewing it first-hand. Certainly a frame guy could fix it, but that would be expensive. Commented Dec 2, 2017 at 23:59
  • If you're the original owner, it may be worth checking your options at trekbikes.com/us/en_US/trek_bikes_warranty If you just bought it, you were sold a lemon.
    – Criggie
    Commented Dec 3, 2017 at 6:27
  • @Daniel R Hicks according to the website it's body using aluminum
    – Ezeewei
    Commented Dec 3, 2017 at 7:40
  • @Ezeewei you're in danger of falling into the "good money after bad" fallacy. You're considering throwing more money at a bad purchase to try and recover something from this lemon. Better to dump it and learn your lesson.
    – Criggie
    Commented Dec 3, 2017 at 22:31

3 Answers 3


The frame isn’t safe for using anywhere.

The potential danger is that the stay will break most likely locking up the back wheel. It might be merely inconvenient or it might be fatal taken in combination with speed or traffic.

A possible reason for the crack is inappropriate use of the frame e.g jumps, crash damage or it might just be an old cheap Walmart frame which has fatigued.

The frame seems to be alloy from the lack of rust in the fracture. It can’t be fixed within the scope of the bikes probable value. The presence of V brakes and a stamped metal derailleur indicates a low end bike.

Salvage parts if you want and transfer to a not broken frame or just purchase a whole new one. The cost balance on that task will depend on whether you have bike fixing skills and tools or whether you need to take it to a bike shop.

Seriously, don’t use this bike as it is right now.

  • its a Trek 4100 how much usually this kind of fix will cost? If I really want to get it fixed?
    – Ezeewei
    Commented Dec 2, 2017 at 22:08
  • 2
    @Ezeewei Repairing this kind of frame is theoretically possible, but it's virtually never done because it would require stripping the bike down, having a skilled bike framebuilder replace the tube, getting the frame re-heat-treated, repainted/coated, and then built back up. That's what it takes to do right and none of those steps can really be skipped, and it will cost far more than the bike is worth. Note it is possible to do it not right in any number of ways, i.e. have someone with a TIG welder go to town on it and never mind the heat treatment, but it probably won't end well. Commented Dec 3, 2017 at 7:58
  • 2
    @Ezeewei Have you welded aluminum before?
    – paparazzo
    Commented Dec 3, 2017 at 14:41
  • 2
    Welding aluminium is 10x harder than welding steel. Plus it could easily be a 7005 aluminium alloy which is pre-heat treated, so heating it again will destroy the tempering and make it more brittle or more soft, (can't remember which way it goes sorry.) Either way, the frame becomes even more dangerous "repaired"
    – Criggie
    Commented Dec 3, 2017 at 22:28
  • 1
    @Ezeewei Well, if you want to learn how to weld aluminium, you now have a big lump of the stuff to practise on! But please promise you won't ever use the result as a bike. Commented Dec 4, 2017 at 15:01

The potential danger is your death should the chainstay part at while being ridden, leaving you on an uncontrollable bike.

Calling it bluntly, you'd be foolhardy to ride a bike that you know is not structurally sound.

Salvage any good parts off it and junk the frame. You only get one life.


It's never a reasonable idea to ride a broken frame! Although in this case maybe it's not too bad. Since the crack is for sure not arose from the stress due to riding. The force will act in the direction of the chainstay and there you might still have enough strength. If you really want to take the risk, try gluing it with epoxy-metal I had good results with that.

BUT STILL, it will never be a save bike anymore and you should never think about selling it or so.

  • As you say, repairs won't be safe, so suggesting repairing it doesn't seem appropriate. Commented Dec 5, 2017 at 14:58
  • I repaired a broken frame and a break lever with epoxy-metal just by curiosity. By my surprise it worked. I mentioned multiple times that you should be aware and that it will never be save again. But I think it's okay to give a suggestion how it could work. or is it a common rule not to answer if it could be dangerous?
    – madn
    Commented Dec 5, 2017 at 15:51
  • I'm not sure if there's a site rule or policy against suggesting dangerous things. My own feeling is that good answers shouldn't do that but other people may very well disagree. You could ask on Bicycles Meta. Commented Dec 5, 2017 at 16:29

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