Framebuilders - downtube thickness? (CroMo track frame)

i want to start modifying one of my track frames.
im just looking for some help for ordering my tubing for the project below

heres the scoop:
ive been cycling for maybe 2 years, and now that ive learned a little bit about frames and geometry i want to modify my beater fixed gear. i'm an engineering student, and ive been welding for about 4 years, so i know a little bit about structural integrity, ect

i want to cut out my downtube, then weld a new tube in that goes from about half way up the headtube to a few inches up the seat tube. then i will gusset the corners.
im doing this for a few reasons, 1. i want to be able to do barspins, and 2. because i like to work with metal and i take pride in my designs.
im a kid, i do tricks on a track bike. if you want to debate about it, please do it in a different thread.

heres my idea
-sorry the pictures are so crude, but they get the point across.
keep in mind i exaggerated the downtube position in the second picture to emphasize my idea. it wont be THAT high. only an inch or so higher

before:
http://i258.photobucket.com/albums/hh266/GRHebard/frame1.jpg

after:
http://i258.photobucket.com/albums/hh266/GRHebard/frame2.jpg


im not sure if the gussets are necessary, they wouldnt really add THAT much weight if i used 20g metal.
any advice/comments for keeping the frame stiff are appreciated
will i need to build a jig to hold the frame from warping once the downtube is out? or should it be alright? i plan on just using a demel tool, nothing crazy

my MAIN QUESTION with this is: what wall-thickness should i buy for my new downtube? i was thinking 1.25" OD

thanks,
Greg Hebard

How would your crearances be if you attached the DT at the HT to the TT. That is how Harleys are done and is considered stronger than the standard bike type joint that regularly falls appart on mortorcycles. Then you could run the tube direct to the BB. That would give you more clearance up high, and I assume that would be OK, since the cranks are going to be in the way low down anyway. That structure would be amply strong, I would guess.

With what you are proposing the gussets are certainly required, except I wouldn't join the lower tube that way, I would run it direct from the BB to a higher point on the HT, and get your clearance that way. Then gussets would only be required on the HT.

Check out the head tube on this diagram:

http://chopperhandbook.com/neatstuff/old_school_chopper_frame.htm

good idea. yeah, my main concern was that the downtube didnt meet with the BB.
i took a look at the frame and it looks like your idea will work, and seems like it would look pretty good as well.

i was considering taking rake out of the fork also, but bending it seems like it might be a bad idea.
any ideas there? :)

thanks,
Greg

the frame would be a lot stronger if you kept it triangulated and add the gussets on inside of the triangle. Although the stress on the downtube at the BB is actually lateral. So it'd be better to brace both the down and seat-tubes laterally with little gussets on their sides out to the width of the BB.

Another idea is to use a curved downtube that meets the other tubing at the same spot. Again, adding the gussets on the inside is the stronger way. Or even double-gussets like on the outside like BMX bikes.

Straight-tube fork are fine for strength too, and can be angled at the crown to give the correct rake/offset.

thanks. ill post something up when i start cutting it up

Whoops, never answered your original question. How heavy are you? What kind of riding? The downtube is the most important part of a bike as far as strength and stiffness goes. I'd go with double-butted 1.25" tubing with 1.25mm thickness down by the BB, 0.7-0.8mm in the middle and 1.0-1.1mm up by the head-tube. Even better to get it in a conical shape and ovalize it laterally at the BB, 1.38" wide by 1.25" tall. Then ovalize the head-tube end vertically. That puts more material in the plane of the stresses. I think Serrota did this on his Colorado models way back when.

I'm only like 140lbs

The bike it going to be for doing tricks. IE going off 4 or 5 stairs. Skatepark stuff

I think oval piping might be a little bit too expensive. I'm planning on just using 1.25" OD with 1/16th" wall thinkness.