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I saw this little dentation on the welding between the down tube and the head tube, looked suspicious to me, is it safe to ride?

enter image description here

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    The photo is a bit blurry. Are there any visible gaps or bad spots in the weld? Does it look like the tube walls got melted through? It’s normal for welds to have a kind of wave-like appearance. I think failure on the opposite side of the tube (i.e. the underside of the downtube where it’s welded to the headtube) is much more common since forces there are much greater. So I’d pay special attention to that. Since a lot of surface rust is visible: Pay special attention to rust and corrosion. Take out the seatpost and bottom bracket and check for rust inside the frame.
    – Michael
    Aug 10, 2021 at 10:43

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Safe to ride? Yes. If a (metal) frame fails, it won't kill you, as diamond frames have lots of inherent redundancy. Most likely you are able to limp home with your bike. So based on that nearly every frame is safe. A non-diamond frame might be a different matter since they can fail suddenly and cause you to crash, hurting yourself badly. But a diamond frame is practically always safe.

Such weakness would however be very dangerous on a fork, and finding a compatible replacement fork might not be easy. On stem, seatpost or handlebar it would also be very dangerous but fortunately stems, seatposts and handlebars are cheap to replace. If a fork, stem, seatpost or handlebar fails, you will hurt yourself badly.

However, the big question you should be asking: will the frame be durable? Welds are always weak areas and any dent could weaken the weld there. If your frame fails for 1 years after buying it and it won't kill or hurt you and you can limp home, you are still left with a bicycle that's broken, i.e. you lose whatever money you put into that bicycle.

Personally I wouldn't buy the bicycle because the frame not only has a dent at a weld, but also has deteriorating paint and also the headset lower cup is so rusty it calls into question how rusty the bearing itself is. Even if it's just cosmetic on the headset, replacing it won't be cheap because a decent set of headset tools costs over 100 dollars/euros/pounds/whatever, and a bicycle shop charges probably half that to swap a headset. I suspect the bicycle is so old that it has a threaded headset -- those require a new one with compatible stack height to replace (if the stack height of the new unit is lower you can always try to see if you can find suitable spacers; if the stack height of the new unit is higher then you're out of luck).

Also the brake hardware is completely rusted out. It's not only ugly but I also suspect similar rust can be seen in every part of the bike. The covered picture shows only a very small percentage of the bike.

I wouldn't pay any money for this bike.

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    “If a (metal) frame fails, it won't kill you, as diamond frames have lots of inherent redundancy.” If the downtube suddenly separates from the headtube as you hit a pothole you can totally have a fatal crash. Doesn’t matter if it’s due to a bad weld, fatigue crack or badly bonded carbon fiber.
    – Michael
    Aug 10, 2021 at 10:48
  • If the downtube separates from the headtube, a decent metal frame should have strong enough weld at the head tube to top tube junction. It's less than ideal if the top tube is the tube carrying all of the load, and it's certainly not long-term-durable that way, but it should prevent a major accident. Of course if both downtube-headtube and toptube-headtube welds are damaged then it's possible both of them fail at the same time, causing a dangerous crash.
    – juhist
    Aug 10, 2021 at 13:39
  • @juhist Seeing MTB fail videos, the usual outcome after a DT/HT separation is the top tube folding. The weld holds on the entire time, but that doesn’t matter if the tube has failed…
    – MaplePanda
    Aug 10, 2021 at 19:43

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