Cycle frame needs welding

[Shox Dr]

Coming back and re-reading,
If I've no cracks and no creaks on a really strong 7005 hardtail,

How do you know you have no cracks? Hotrodder has already said they could be hiding under the paint. As for "really strong" please quantify

Dave's got the cracked dropout on a road bike and Chunko is riding an old frame 100 miles a week, possibly older than mine.

Both have little to do with your frame

Why am I so confused about metal fatigue?

I don't know.

Thinking on, I am still on the original rims which were rim braked, but re-built with disc hubs and I presume disc spokes at the bike shop by a proper wheel builder on purchase.

"Proper" as in what, is it a type of qualification?. You asked him to rebuild some rims, while it would be nice to think he would point out any problems, that's not always the case

Would I be right in thinking the rims and spokes ought to be the first to suffer fatigue, the canary in the coalmine, as it were?

No.

 

[chunkolini]

Mine is a fancy pants heat treated classic, it is probably one of the best mass produced steel mountain bike frames ever.
Aluminium frames have been reckoned to have a finite life span, aluminium does not like bending and suffers badly from fatigue. A well designed steel frame has a degree of resilience built in and avoids metal fatigue.
Cheaper aluminium frames are built to be as rigid as possible and hence ride like planks. Ride a decent steel frame and compare the difference.
Remember babies, Steel is Real.
Dave I would say your frame is shagged. I would not risk riding it and I ride some real death traps around. if that went it would go CRACK! and fail horribly.
Rims and spokes? I am always amazed how long wheels last, I have done some hideous mountain biking and never killed a wheel. I have worn out loads of rims with rim brakes.

Note to owners of old nails, dont borrow your mates track pump to get a decent pressure in your old wheels retired for road use, at 100psi they make a hell of a bang, the kind of thing that gets talked about in pubs years after the event.

 

[inchpincher]

Dave - are they carbon seat stays, I can't enlarge the pics? I don't know about carbon but with steel you would unbond the dropout with heat and replace. Obviously with the frame stripped and jigged... I wouldn't trust a weld across the dropout at that point.

Edit: interesting pics... http://www.carbonframerepair.com/index.php/broken-dropout/

 

[hotrodder]

^^ this. Pics aren't big enough for detail but looks like the dropouts are bonded rather than welded. I'm assuming you bought it second hand or something as i would have thought that should be a fairly straightforward new frame under warrenty otherwise?

Like Shox i've been out of the bike trade for a good few years so things may have changed but there never used to be such a thing as 'disc spokes'. Silly lightweight spokes that were **** whoever made 'em yes but i wouldn't use them on anything let alone something with disc brakes. DT competition double butted (2.0/1.8/2.0mm) covered pretty much most things and anyone asking for something different or ally nipples was told to stop being daft

 

[c4tini]

what bike is it?
just have a go and see what happens.

 

[dannyp]

metal fatiuge in ali it a bitch

just be thankfull you saw a crack before it broke, usual story is you see a crack weld it and it snaps somewhere else shortly somewhere after

 

[citizen kane]

Unless the frame has some deep sentimental value I would say hang it on a wall and just remember the good times.

While its true to say that aluminium alloys are generally more sensitive to fatique than steel alloys a well designed frame will take this into account. A well made well designed aluminium alloy frame will have to all intents and perposes an infinite fatique life when operated within normal loads a bicycle is likely to see from "just riding along". All the evidence suggests that bicycle frames, be they a steel or aluminium alloy, fail from the accumulation of a relativly small number of high stress events such as pot holes, a rider sprinting, that sort of thing.

Without some way of recording loads its pretty much impossible to operate a safe life approach for a bicycle frame. Aluminium alloy aircraft have operated the safe life approach with full scale fatique tests, fleet leaders and tear downs used for life extension but increasingly there is a move to "retirment for cause", you know, its got a crack lets not fly it any more.

This is really the only practicle approach for a bicycle frame but requires an inspection regime. You know how every time you wash your car you find another scratch, dent or stone chip well when you wash your bike have a really good look, 99 times out of 100 if its going to crack it will be in the HAZ. The older it is the more you need to wash it.
Alluminium alloy is and has been the default material for bicycle frames for many many years. Catastrophic failures are rare, but so are lottery winners.