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I cycle every day because I live in a city and I find it a fun, fit and convenient mode of transport. I am not a cycling hobbyist, am only averagely fit and know pretty much nothing about bike engineering. So every year or so I take my bike in to the shop for servicing.

The bike in question is a Dahon Jack - it's a full-sized frame folding model. So the frame design is a bit unusual. It looks like this:

enter image description here

The last time it was serviced, a couple of weeks ago, the man in the shop drew my attention to a small crack in the frame. It's on the very top bar and has originated in the gap in the frame where the bike folds. Here's a photo:

enter image description here

The two white lines on the tip of the crack are scratches I put there to see if it's getting any bigger. It hasn't in the two weeks since it was discovered.

The guy in the shop said that while any frame crack is serious, this probably wasn't a big deal. Because it's a folding bike, the load is carried on the thick diagonal strut instead of the top bar. So even if that top bar does suddenly split, it's not going to cause the bike to collapse or throw me off.

He advised me not to take it off-road (which I never do anyway) but that otherwise I should be safe to ride for the time being, unless the fracture grew significantly.

Is he right? Should I get this bike replaced right away? If not, how long do you think it might be safe for me to continue to ride, and what warning signs should I look for to say it's no longer safe?

  • 3
    A check of Dahons web site states they have a 5 year frame warranty. Do you qualify? – mikes Sep 29 '15 at 21:39
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    That part is in compression when it's used, so the crack isn't too critical. I would look very carefully inside the open end of the part that connects to this and make sure there's no damage or a chunk of grit that's caused the crack on the other side by producing a point load at the start of the crack. – Móż Sep 30 '15 at 0:30
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Someone has to say live dangerously. I would ride it. Don't ride it hard but you should not ride a Dahon hard in the first place. I looked at how the bike folds and unfolds and that is not a high strength connection in the first place.

Bikes don't typically explode when they fail. You get a bend and sometimes you don't even crash.

If it does die if you don't need a folder you can find decent used bikes for $400.

  • 2
    You can find a decent bike for a $100 depending on what your needs are. :-) – RoboKaren Sep 8 '16 at 0:37
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That looks like a failure from hoop stress, which is (likely) caused by over-tightened locking lever. You can see the crack originate from the lock ring and propagating in axial direction.

I would not say it's safe. Just make sure you cycle slowly, as the folding bicycle was designed to do just the same.

Please bear in mind that it is an unnecessary, albeit small, risk. You lost a safety measure, just like riding a bike with only one brake as an analogy. It looks as though the frame is aluminium and that material fails catastrophically once the crack is initiated.

  • It is an aluminum frame, but that's not a lock ring. It's a sleeve over the end of the frame tube where the bike folds. – Matt Thrower Sep 29 '15 at 15:09
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    Is there any insert that giving a hoop stress? Then it's either the sleeve is too loose, or the insert is too tight, or the combination of both – Nhân Lê Sep 29 '15 at 15:16
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No frame fracture is safe. You wouldn't know how stress in the frame will be redistributed and which point become the next one. Especially if defect is located in frame's triangles.

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I'd reduce riding as much as possible, and either replace it or go talk to a competent frame builder for a repair. The more you ride on it, the worse it will get over time, even if that crack doesn't appear to change.

Less frame repairing means it'll cost less sooner, leaving it later may mean more work which will increase costs. If you like the frame then its worth looking after and repairing.... you're unlikely to find another identical frame, and it may have the same problems over again.

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    repairing aluminium frames is expensive and risky, so that' unlikely to be cheaper than buying a new bike. – Móż Sep 30 '15 at 0:29
  • @Mσᶎ agreed - but OP says he really likes the frame, so a matching repalcement could be hard to find. Risk can be mitigated by talking to a competent frame builder, not Uncle Tony with his harbour freight stick fizzer. Someone earlier says that this maker has a 5 year warranty on frames-that is the best fix. – Criggie Sep 30 '15 at 0:39

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