Thursday, January 31, 2008

With a couple mountain bike races coming up that I want to race in, Truman's Big Love, SS March 2 Spoke Pony Showdown 3hr/6hr March 15th I guess it’s a good idea that get a bike together. And I mean together. I used to have a couple 29r’s (Titus Racer-X 29, & Surly Karate Monkey)

This was my Titus Ti-Racer-X, I had it Custom made.

I’m going to build a 29r frame with a twist. They will internal 3-speed hub specific . . . "Just say NO to Chain Slap" . . . a problem with 29’r is the down-tube/ fork crown clearance. To remedy this I’m going to borrow an idea from an old BMX bike the GJS-A Frame.

Only with a slight modification by using two ½" tubes going from the head tube to the down tube instead of one like the GJS has. Either way this will give clearance for a suspension fork’s fork crown. Be it a Pop-Loc, or whatever the case may be.

I’m going to build this frame from straight gauge 4130 from airparts, aircraft supply. Sure it’s not the lightest tubing, but for my first few frames I’ll have nothing to lose but a few bucks. I am going to order in the bottom bracket shell, dropouts, cable stops, and guide. Maybe when I get a lathe I’ll make my own bottom bracket shell, but it’s far easier and cheaper in the long run to just buy them. Some people do make their own dropouts, I still might, but I’ll probably just couple that with the purchase of my small parts.

The specs for the front triangle look like this.

Down tube: 1-1/4"

Tope Tube: 1-1/8"

Bottom Bracket: 68mm (Remember its 3 speed specific, so I have 120mm rear spacing)

Head Tube: 1-3/8" and 1-1/2"
This is a tough one. For 1" it’s simple. 1-1/4" give an ideal I.D. of 29.9mm for a 1" headset since the bore needs to be 30.1. Plus the DIA matches well with the down tube.

For 1-1/8" is a bit harder. The key is getting a bore of 33.9mm. The next size tube up from 1-1/4" is 1-3/8"O.D., or 34.9mm O.D. at .035 thickness. Boring this to 33.9 will leave .5mm wall thickness, not thick enough and going to the next size up 1-1/2" to get the correct bore will require getting a tube thickness of .095, or 2.41mm. At 1.4Lbs.per foot that a heavy pipe.

My solution is to use the 1-3/8"x.035" and make reinforcement rings out off 1-1/2" x .058" tubing for the top and bottom of the head-tube. The I.D. of 1-1/2" x .058" is 35.15mm, and the O.D. of the 1-3/8"x.035" is 34.9mm. Leaving .125mm of gap. Fill with braze and go.

Since I’m building a fork with 450mm axel to crown this idea is more of an experiment than applicable. (The details of my fork design I’ll discuss later, but basically I want to get my bars 2"-3" below my seat.)

Seat Tube: 1-1/8" (at .035"/9mm wall makes it compatible for a 26.8 post), and 5/8" tubing
Another idea I’m going to employ is the idea of the split seat tube. I want to give my bike 16-1/2" chain stays. The problem is if I use a conventional full length tube, the back tire will hit the seat-tube. This will give the tire clearance, and to some point the tire will actually track between the tubes with a WTB 2.3. Exi-Wolf. Will it survive out of the saddle crank mashing? I guess I’ll find out.

I'm you look at my post from yesterday, click on the picture to see what I have in the works. Right now it looks like this.

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