Frame failure: Idle speculation on root cause

[Emanresu]

Used to work at a tube manufacturer as basically that’s what you have albeit assembled in a certain way. There’s a lot can go wrong such as incorrect drawings sequence, poor batch of billet, and wrong annealing times. It’s a skill and not an art.

Think @wafter is on the money with stress. Just put it down to bad luck and get a new one. Would be amazed if the producer would be interested after getting the original sale.

 

[roubaixtuesday]

Used to work at a tube manufacturer as basically that’s what you have albeit assembled in a certain way. There’s a lot can go wrong such as incorrect drawings sequence, poor batch of billet, and wrong annealing times. It’s a skill and not an art.

Think @wafter is on the money with stress. Just put it down to bad luck and get a new one. Would be amazed if the producer would be interested after getting the original sale.

Yeah, as per title it's literally just idle speculation, no intent to go to producer or anything.

 

[Emanresu]

Basically there’s a huge cost in terms of energy in making a tube. Power to reduce the billet over a number of passes and then the intermediate annealing. We made parts for aerospace and nuclear so could afford the metallurgists. Making bikes is cut, weld and pray.

 

[Milzy]

At a guess I would say that maybe you had too much seat post showing, even if it was not above the limit mark. Over time the seat post has rocked back and forth and has caused the failure.
Of course I could be totally wrong.

You are right. Seen it a few times.

 

[Reynard]

Looks like whoever built that tandem didn't do any basic maths on the expected (cyclic) loading for that particular geometry.

Bike frames are the shape they are, because the triangle is the most stable shape for bearing and transmitting loads. But the seat post acts like a lever when you pedal - and the taller the rider, the greater the force (torque) applied.

So a force that's too great for the geometry will shorten the expected lifespan of the metal. Metals are, by their nature, elastic, and can deform and return to their original shape many, many times. But eventually, that does harden the metal since this cyclical loading eventually changes the internal crystalline structure, and then that metal becomes more brittle. The seat clamp and the juncture with the top tube are stress raisers, and the perfect places for work-hardened metal to fail.

 

[andrew_s]

I'd agree with Paulus, Ian H, & Milzy - not enough seatpost within the frame.
I'd want the bottom end of the seatpost at least 3" below the underside of the top tube.

Bear in mind that the minimum insertion marking on a seatpost will be positioned for a regular solo, with no allowances made for the extended seat tube or for stoker bars

 

[TheDoctor]

I read the OP, and thought 'I bet @Reynard will know'!

 

[Reynard]

I read the OP, and thought 'I bet @Reynard will know'!

 

[Gillstay]

I think work hardened, possibly poor manufacturing as Emanresu says. Very interesting fail. Good timing as well with bike prices being so low.

 

[roubaixtuesday]

Good timing as well with bike prices being so low.

https://www.cyclechat.net/threads/frame-failure-idle-speculation-on-root-cause.300835/#post-7334170

 

[Gillstay]

https://www.cyclechat.net/threads/frame-failure-idle-speculation-on-root-cause.300835/#post-7334170

Yep, bike prices were low in 2024, not sure what I am missing ?