just wondering ?
 

how much less would a chassis weigh if built in 2mm wall ali box section ?

have a search but i think the general concencus was ali is a different material and has different properties so wouldnt be suitable

I'd expect Aluminium to show signs of fatigue stress a lot sooner than steel with worse results, based on when I was looking for a new push bike.

That said, it is used in aircraft structures, so I guess it's feasible? Damn expensive though I'd imagine!

Aircraft get stripped down and thoroughly inspected at regular intervals - and they tend not to get the same pounding as a road-going car.

This has been discussed many times in several forums - the general opinion is that aluminium will suffer from fatigue cracking very quickly. It also age-hardens, which wouldn't help. So - OK for race cars, but not for road-going ones.

It would most probably way more to retain the require strength. The design does not lend itself to the material.

TT

Aircraft actually take more of a hammering than a road car, its the buid philosiphy that makes it work. Steel is more practical than ali for a road car (that does not have to fly!)

planes are not welded for a start so there is no haz, they are made with screws rivets and glue. if you look back a bit into the history of planes there has been plenty of issues with ali fatigue look at the de Havilland Comet, they had a habit of falling apart in the air as did a fair few other planes that were conceived in the 50's. a lot of the new planes manufactured these days are made out of composite materials which are much stronger and lighter.

ali is a fantastic material but its really not suited to use on a chassis unless you can model it and do some fea to work out the issues due to the shortcomings of the material

Steel weigh about 8750kg/cubic meter and aluminum about 2600kg/cubic meter - thus ally is about 3.37 times lighter than steel. Sounds good, but to get to the same tensile strength as steel, you have to up the thickness to be on par with steel.
When I built/ designed ocean going yachts for a living before retiring, many designs were built either in steel or aluminum. But overall the ally went 30% in size, thickness etc to comply to the same scantling as steel, but still these yachts turned out much lighter than their steel counter parts....

Back to the subject at hand; since we use 25 x 25 x 1.6mm tube for the frame, a meter length will weigh about 1.256kg. The equivalent in ally to be used should be 25 x 25 x 2.0mm and a meter length will weigh about 0.52 kg.
But to be absolutely safe you can up the ally to 25 x 25 x 3.0mm and a meter length will weigh only 0.78kg and a frame built of this over sized grade will be much stronger than the std steel frame and still be 38% lighter!

I was thinking 2 to 3 mm myself...........wow 38% lighter:D ..........looks like a plan for my trackday car then

It might be a good idea to listen to leroybrown he knows what he is talking about when it comes to aircraft and the hammering they take

Arfon