Framebuilders - Hi! & a couple questions

First of all I wanted to introduce myself & say Hi. I've been lurking/searching around here most of the winter & finally decided to make my own frame.

I've been riding bikes for years, but really got back into it last summer when I bought my first mountain bike(I'd primarily been into observed trials before). After spending plenty of time on it, I realized it just didn't have the geometry I wanted, and after an exhaustive search I didn't find any frames that were really that close to my targets. I'm no pro metalworker by a long-shot, but I have built a tube-frame car chassis in the past, so building my own bike frame seemed like a natural challenge!

I took the advice I'd read on here and found a couple old bikes to chop up & practice with, so I made a jig, setup the geometry I wanted & got to work. My main goal was to test out the geometry I *though* I wanted & see how I liked it, and also see if I could keep the frame straight & square. I wasn't concerned about details, or even longevety with this frame, so I welded it quickly with my flux-mig(I'll be borrowing either a gas-mig, or possibly a Tig for the *real* frame).

Anyway, I'm happy with the geometry so far, but I have a couple questions:

1.)All the tubing I used was mild steel, and the frame is *very* heavy - close to 8lbs. I'll use 4130 for the final frame(not sure what exact tubing yet), but I'm hoping to keep the weight down around 5lbs, is that reasonable?

2.)There's quite a bit of torsional flex in the frame - if I lean the handlebars against the wall & push on the seat, the flex is very noticable. Will 4130 tubing cause much improvement, or do I need to look into adding gussets and/or bracing?

Thanks!

Here's a pic - don't laugh too much though, the welds look horrid & I only painted it to help me find any cracks easier. :D

http://img155.imageshack.us/img155/7255/frame017xz9.jpg

I can't answer any of your questions, but you picture caught my eye. Maybe it's just the angle at which the shot was taken, but it looks to me like your knees are going to be up at your chest riding that. And you'll be sitting perfectly upright too. There might be a reason you can't find a frame with this geometry...

Lol, no it's not too bad. Then again, I designed it as a trials-inspired street/dirt jump frame, so it doesn't have the riding position of a traditional mtn bike either.

Can you resize that photo it's huge and hard to view. In theory the stronger tubing in a 4130 is not stiffer, but it's yield point will be higher.

Your top tube looks really small, and you welds look spotty so perhaps they aren't fully stabilizing the tube. The wider you spread the tubes at the head the better.

Weight wise the weights of various steels are similar. so just look at the amount of material you will save. Say your tubes were 1/16th inch that would mean standard chromo would be 56% the weight. If you are experimenting with 1/8th inch, then you will be at 28 % with chromo. This is just the tubes, doesn't count parts like heads.

Sorry about the pic, I downsized the it.

Well yes, the top tube is too small - it was from an old bed frame, as the frames I cut apart wouldn't work. It was just what I had around to work with. And yes, the welds are pretty horrible, lol, but I think the penetration is decent.

Thanks for the advice on the steel! Sounds like a 15%-20% reduction in frame weight is possible. That would put me in the 6-pound range, which is pretty reasonable.

Here's a couple more questions:

1.)Does double- and/or triple-butted tubing offer any stiffening over standard straight gauge, or is it simply for weight savings? I realize formed tubing(ie: non-round)could possibly help.

2.)In browsing the 4130 offerings on Nova, I see some offerings as "Road" and others as "MTB". Is there a rule-of-thumb for what wall thickness is suitable for a mountain bike? A couple factors to consider are that I'll be jumping off things 5' or more high - often on pavement. On the other hand, I'm only ~145lb, and a pretty smooth rider - I've only ever cracked one rigid fork(I ride a supension fork now), and never any frames.

Thanks again for all the advice & help!