Aluminium alloys

[Alembicbassman]

My background is in aviation. The service life of aluminium alloy aircraft is about 30 years. The aircraft, on short haul routes, fly up to 10 sectors a day. That's a lot of cycles over 30 years !!! The alloys experience the stresses of presurization, aerodynamic forces, extremes of hot and cold weather, chemical de-icer etc...

An aluminium bike frame pootles around the streets on Earth and is only guaranteed for 5 years.

My frame is 10 years old, no problems yet.

Are manufacturers being a little over cautious?

 

[Alan Frame]

I suppose the difference is that aircraft are rigorously maintained throughout their lives.

If manufacturers could be sure that bikes were equally well looked after, then maybe the guarantee period would be longer.

 

[yenrod]

I'm sure an aircraft goes thru' plenty more stresses compared to a bike.

 

[Baggy]

Coincidentally my brother has just texted me to say he is retiring his duralinox Peugeot frame as he's found some cracks in the lugs. It's 22 years old

 

[Globalti]

Interesting post.

Surely the stresses on a bike frame are much sharper and concentrated in a much smaller area? Also I doubt if bike aluminium is of a comparable quality to aircraft ali.

My great uncle was Chief Designer at BAC at Hurn, he designed the tail of the VC10 amongst other things, as well as being involved with the BAC Super 1-11. I always thought of Uncle Eddie when I was in one of those rugged old 1-11s owned by some dodgy African airline as the pilot smashed it down onto a runway or reversed the thrust to back it across the apron.

BY the way what are your thoughts on the suitability of ali for bike frames? Having owned a couple of ali frames I would say it has the wrong characteristics and is the least suitable metal.

 

[Over The Hill]

I cannot see why a well made alloy bike frame should not last for ever.

I see it as a good material because it is much lighter than Steel. So what is the problem with it?

Bikes are not made to the same manufacturing quality as a jumbo jet so I would guess that if it gets a crack in it then it most likely started with a crack in it.

 

[LLB]

As I understand it, it is where they rivet which causes the main problems in planes, not the welds.

I'd have said that the best material for a bike would be GLARE which would be bonded together like the Lotus elise chassis as opposed to welded.

 

[Chris James]

As I understand it, it is where they rivet which causes the main problems in planes, not the welds.

I would have thought quite the opposite. Rivetting is used to allow expansion preventing stresses building up due to pressurisation and depressurisation cycling.

This was a big problem in the 2nd World war with the so called Liberty Ships unexpectedly sinking in the Atlantic. The problem was tracked down to brittle cracks starting at hatches and the cracks could spread rapidly due to the ships being welded (for speed - whole ships were built in hours!) as opposed to the traditional rivetting.

http://en.wikipedia.org/wiki/Liberty_ship

Aluminium alloys are okay materials to build frames from. But they don't have a fatigue limit and are not particularly strong. Hence they require greater diameter tubes and thicker walls (also as al alloys are difficult to weld) than steel, for example, to prevent early fatigue failure. This reduces the weight advantage that it has over steel due to it's lower density (approx a third that of steel).

A side effect of the large tube diamters is that the frame will be very stiff. Possibly jarringly so. So good for racing but possibly less comfortable. Having said that, I notice that many modern steel frames are going with oversized tubes and ovalized downtubes so a stiff frame is often desirable.

 

[PpPete]

There is very little welding of structural aluminium components in Aerospace - the first aircraft that uses it extensively is the Eclipse ( a small 6 seater ) and that uses something very special called Friction Stir Welding - NOTHING like the kind of welding that I've seen on most aluminium bikes.

 

[itisaboutthebike]

I cannot see why a well made alloy bike frame should not last for ever.

I can - Alu has a finite fatigue life !

 

[TheDoctor]

Yes...but if that finite life is longer than my finite life, that's forever as far as I'm concerned!

 

[02GF74]

Are manufacturers being a little over cautious?

no, they don;t want to be giving you free frames for ever.

A toaster or LCD TV is subjected to much lower stresses yet I doubt you can get a 5 year warranty from the manufacturer for nothing.

 

[domtyler]

Yes...but if that finite life is longer than my finite life, that's forever as far as I'm concerned!

You'll never die doctor, you're The Doctor!