Try and break a piece of 1/8" thick aluminum bar and see how much it can handle before it buckles, and do so with the other materials you're thinking of building a rack out of. I priced out some bar stock in stainless and aluminum and they were both around 15$ for 6ft. worth of 1/2 wide 1/8 thick stuff. I was thinking of making a few racks for fun for the local community bike shop. I will update this thread should I ever get around to it.

Metal Supermarkets has stores in Canada, the US, and the UK: http://www.metalsupermarkets.com/MSC...gion=UK&Map=UK

They sell in small quantities so it's quite convenient.