Mmm. Steel.

I work with several types of tubing. I'll briefly describe each- remember that most bikes have at least a little bit of each.

True Temper OX Platinum - this is a very hard (195,000 psi) but still ductile steel that can be used with fairly thin walls (down to 0.7mm at the butts). I typically use OX Platinum for head tubes and down tubes.

True Temper Verus HT - this is a heat treated cromoly steel which is still quite hard (150-180,000 psi) and lightweight. Butts are as thin as 0.8mm. I use Verus HT for seat tubes, toptubes, chainstays, and seatstays.

4130 cromoly - this is the standard steel used for many bicycles and aircraft. It's not as hard as other steels (110,000 psi) but is useful in some specific places. I mainly use 4130 in bottom bracket shells (manufactured by Paragon Machine Works in California) and sometimes for seatstays and/or downtubes for REALLY large bikes. Butts never get thinner than 0.9mm, so this is the heaviest of the steels that I use, simply because it must be a bit thicker to be strong enough.

I do not use super thin (Foco, S3, etc - anything with butts thinner than 0.7mm) steels, simply because I don't feel I can build an abuse-friendly frame that will last with them.

My 29er forks are built with Verus HT and 4130 cromoly tubing custom machined by Zen Machine of Lyons Colorado. Unicrown forks are built with True Temper unicrown blades.