Tuesday, December 2, 2008

The 1-1/8" Head Tube Trick

I decided to go with a 1-1/8" head tube. 1-1/8" really refers to the steered tube of the fork, and kinda contributed to my problem.
A frame that is refered to as having a 1" head tube probably has a 1-1/4", or 1.25" diameter tube with a .035" wall thinkness, leaving 1.18", or 29.97mm. Whichs work perfect because bearing cups for 1" head tubes are 30.1mm.
A frame using a 1-1/8" head tube requires bearing cups that measure 33.9mm in diameter, so the inside diameter of you tube has to be around 33.9. So what are your options? not many if you going with straight gauge 4130. Oh sure you can buy tubing that is cycling specific and save yourself the headache, but what fun would that be? . . . I mean this is almost like bending your own chainstays . . . who would want to do that? . . . oh yea me . . . crap!
OK back to choices. 1-3/8" with .035 leave you with a wall thinkness of 33.15. If you ream that out, it will leave you with about 0.35mm of thickness. Since almost al head tubes need to be .9, 1.0, or 1.1 . . . 0.35 might be a little thin.
you can go 1-1/2", but the wall thickness needs to be .095" or 3.05mm thick tube, about x3 the weight. too bad nobody make 1-7/16" diameter, .o35" tubing . . . oh yea, cycling specific tubing.
OK I have a plan. 10mm reinforcment rings made from 1-1/2", .058" thick tubing at the top and bottom of the tube. the outside diameter of 1-3/8" tubing is 34.925, and the inside diameter of 1-1/2", .058, is 35.15. This leaves a gap of .1125mm, or about 1/10th of a millimeter. That oughta do it.

The Rings . . . There can only be two!

A little clean up, a little Flux . . .

and there you have it. I guess the technical term for what I did was sweat them on. I appplied the brass to the top of the rings, and then flowed the brass behind the rings, then it came out the bottom. It looks like a brazed both sides of each ring, but I didn't. This is how I know brass is behind each ring. It the same process for joining a bicycle frame with lugs.

1 comment:

dmar836 said...

For one-offs, I could always bore the HT ID for you - but the head tube obviously would have to be cut to it's final length first.
I like the creative process as it is.