Someone tried to steal this bike - they damaged the top tube. The frame is designed with a wide flat top tube. Is this still safe to use?
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18The biggest problem with this one may not be long-term cracking, but rather a catastrophic failure (the top tube just folds in half) if you exceed its now greatly reduced buckling strength.– MaplePandaJul 27, 2022 at 23:55
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3I can confirm from personal experience that aluminum frames are prone to catastrophic failure; it is advisable to not be riding the bike when this happens.– Alexander NiedJul 28, 2022 at 4:58
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11Grab an empty coke can by the end caps, see how much effort is required to bend or crush it. Now poke a dent in one with a finger, and try again.– Grimm The OpinerJul 28, 2022 at 8:13
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3Just wanted to say, sorry for what happened. What an unfortunate way to go for the bike.– undercatJul 30, 2022 at 5:31
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@GrimmTheOpiner Even more fun: put an intact aluminum can on the ground, upright. Put your foot on top and put a good deal of weight on it - the can won't collapse. Now carefully bend over and tap the side of the can hard enough to cause the can's wall to deform just a little. The can is likely to collapse immediately - it was strong enough to support that weight as long as it wasn't distorted. The moment the tap distorted the side of the can, it failed.– Andrew HenleJul 31, 2022 at 12:16
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3 Answers
My opinion, which is very much my own, is that this is safe to ride home and may provide some level of “emergency use” where no other bike is available and you need to get around for a short while, but realistically the frame is at End Of Life now, the structural integrity is severely compromised and the long term reliability is deeply uncertain.
Replace the frame as soon as funds allow.
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5While I've given +1, it's also possible that the top tube will buckle on that very first ride home when hitting a bump that's ordinarily unnoticed by a rider on an undamaged frame. i.e. one might get several 100Km out of the frame, one is equally likely to get <1Km out of the frame. When that top tube goes, you probably don't want to actually be riding it. Especially not in traffic.– FreeManJul 28, 2022 at 17:37
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@FreeMan that is absolutely true, though the frame is still supported by the most important weld to the downtube. The important aspect is that the frame can no longer be depended on.– NoiseJul 28, 2022 at 20:23
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@FreeMan Just for the sake of pedantry, I'd say its increasingly likely the bike will fail the more you use it, not equally, because the fatigue on it from repeated stress. I still totally agree with your point though, that it failing right away could easily be a possibility. It just gets a bit more likely to fail the more you use it.– JMacJul 29, 2022 at 12:14
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1Fair point, @JMac, my point was that there could already be enough damage that it might fail on the very next ride and that there's no real way of knowing without expensive diagnostic testing. (The kind of expensive that would make buying a nice, new, high-end frame look cheap.)– FreeManJul 29, 2022 at 12:25
I’m not an engineer. With that caveat, I think this frame is likely to fail. It may depend on how thick the tubing is, and if you see little cracks anywhere on the tube, I would stop immediately.
Anyway, you now have a serious dent in the tube. All the forces on the frame, like hitting a bump, riding out of the saddle, etc, are now flexing that area. Chances are there are already some micro-fractures there, and they will propagate with load. That will eventually crack the tube through.
Personally, I would replace the frame. With steel, in principle, dents may be rolled out if the tube isn’t cracked, although that’s a big dent. You’d consult a framebuilder for that. Aluminum doesn’t repair in that fashion due to metallurgical properties.
answered Jul 27, 2022 at 18:34
I would ride that bike as little as possible before replacing it. Being aluminium, it will fail rapidly once the metal tears properly. If it were steel with the same damage, it could still outlast us with that dent.
Can you claim insurance on it? Get your LBS to state it is unsafe to ride as a result of attempted theft, and make a claim.
There is no repair for aluminium bikes, because welding would require heat-treating to restore the temper. This is unfeasable for a complete frame because of costs.
Your best bet is to treat it as a donor bike of parts, and try to source a replacement frame or a complete bike that has wear then combine the two.
Finally, consider how you can protect your bike better in the future. That level of locking in that location was insufficient to deter thieves, so you need to make your bike less-attractive. Add more locks, make it less appealing, or even park it elsewhere (inside). Good luck !
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1Thanks, opinion is fairly unanimous- this is a friend's bike and the attempted theft was several years ago, so insurance not poss. My feeling was that it was best to stop riding it but wanted to get some other opinions as I'm no expert. we'll look for a similar used frame and swap the parts over!– dtcooperJul 27, 2022 at 20:15
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1I don‘t think the material plays a big role here. A dent like that will simply reduce the buckling strength severely. Even if it had been done at proper temperatures without harming the microscopic structure of the material.– MichaelJul 28, 2022 at 6:35
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4@Michael You should post that as an answer then. I know aluminium cannot cope with bending in the way steel can, so yes, material plays a big role here.– Criggie ♦Jul 28, 2022 at 7:43