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aluminium alloy frame failure

Posted: 2 Jun 2019, 10:52am
by cdtb
Hi, any advice appreciated.....I suspect the frame is now useless (see uploaded images x2)?
A ~20 year old Birdy Blue frame. Bike used less than once a year for short journeys - so few stress-fatigue cycles. Bike stored inside the home so little chance of any galvanic corrosion.

I took the bike out for its annual ride, unfolded the bike and attempted to tighten the seat pillar as per normal (definitely no 'overtightening'). The 7005 aluminium alloy has fractured around the seat pillar clamp. Whilst this is a site of repeated stress fatigue cycles, the bike has been used rarely over 20 years. Does this look like a fault in the alloying process, leading to considerably reduced stress fatigue resistance?
Thanks

Re: aluminium alloy frame failure

Posted: 2 Jun 2019, 12:22pm
by NATURAL ANKLING
Hi,
I think that's why on aluminium frames there is a separate clamp that goes over the top of the seat tube.

Re: aluminium alloy frame failure

Posted: 2 Jun 2019, 12:43pm
by JohnW
Brucey's response to this will be interesting and educational.

Re: aluminium alloy frame failure

Posted: 2 Jun 2019, 12:59pm
by NATURAL ANKLING
Hi,
You can see from the other boss that the tear is around the profile of the weld to retain the boss.
Is not rocket science.
All the alu frames I have have external separate clamps.

Also because its a folder? then the clamp sees more than usual clamp release stress.
Also it will have a long seat pillar?
Bad design from the start.

Re: aluminium alloy frame failure

Posted: 2 Jun 2019, 1:09pm
by horizon
NATURAL ANKLING wrote:Hi,
You can see from the other boss that the tear is around the profile of the weld to retain the boss.
Is not rocket science.
All the alu frames I have have external separate clamps.

Also because its a folder? then the clamp sees more than usual clamp release stress.
Also it will have a long seat pillar?
Bad design from the start.


+1. My Tern has a top clamp as you describe. And yes, long seat post and repeated clamping. And aluminium. Folders are great but they do have serious flaws IMV.

Re: aluminium alloy frame failure

Posted: 2 Jun 2019, 1:33pm
by rjb
I would have a go at a diy solution. Depending upon the existing seat tube size if it takes a 28.6mm seat pin you could cut a steel seat tube from a scrap frame complete with the seat pin clamp. Cut this to size so it extends the full length of the existing seat tube dress it up where the seat stays are cut off then araldite it in situ but fit an external collar to the top of the aluminium frame before hand. :wink:

Re: aluminium alloy frame failure

Posted: 2 Jun 2019, 1:47pm
by Brucey
bad design; as others have said a separate collar would have been better.

If you weld any precipitation hardening material and then don't bother heat treating it properly (which is common practice with modern aluminium frames) then you will get an overaged region around the weld. The overaged material is quite a bit less ductile and is exactly the kind of material that you don't want near a seat binder. Coincidentally or not (I suspect 'not') the part that has broken off has failed in the exact shape I'd imagine the 'overaged' isotherm to follow.

The overaged region also has different corrosion resistance properties; I've seen 7005 frames suffer pitting corrosion in a neat line that follows a weld bead at a similar distance. If a crack starts in the brittle overaged region it will soon propagate along it.

cheers

Re: aluminium alloy frame failure

Posted: 3 Jun 2019, 8:44am
by francovendee
Every time I see a post about alluminium frame failures it compels me to have another look at mine. It's now into old age and I been told to expect it to fail. No cracks yet but I don't 'bomb' down rough tracks as fast as I did!

Re: aluminium alloy frame failure

Posted: 3 Jun 2019, 7:16pm
by Gearoidmuar
I just scrapped a bike 12y old, a Giant aluminium MTB, but with a seatpost clamp as recommended by Brucie. I scrapped it because, er, I scrapped it. It has a lot of miles on it and I used it as a tourer for some years with my then 220lb and maybe 35lb of luggage. I did Lejog on it, the first time I did it. Frame and fork were still perfect. Fork worked perfectly (air), Rockshox, and was never serviced.
Why did I scrap it? Because could've done with new BB, chainwheels etc. Not worth it.