I got started on the chainstays this week; undoubtedly the trickiest part of a fat bike frame. I went with a chainstay yoke to provide clearance between the frame, cranks and tires. The long chainstays give me some extra wiggle room, but the yoke really saves the day and allows me to use the stock bend on the chainstays.
I'm hoping the yoke will be stiff enough to prevent any flex issues; the actual yoke is 1/4" thick 4130 so it should be good. I'm also hoping the brazing work is sufficiently strong... getting that thick piece of steel up to temperature was tough.
One of the advantages of this frame design is that it will fit Surly's new 29+ wheels (and still have loads of clearance). I don't have specific plans to buy a second pair of wheels for this bike, but it's kind of tempting since I'll have a bike that'll fit these new tall, fat wheels. Maybe someday. If you haven't seen the new fat 29er wheels from Surly, they consist of 50mm wide rims and 3" tires. Click here for some more info.
After the yoke was brazed, I was able to cut and fit the chainstays. The jig looks a bit jerry-rigged at this point but it should do the trick.
I also laid down some fillets on the front triangle this week. Once again the head tube suffered some distortion. I'm cautiously optimistic that it isn't too severe. It's good to know that I will never be heating the head tube again. Part of me thinks I should get it reamed and faced so that if it is junk, I don't put the effort into attaching the rear triangle to it. I did some quick filing of the fillets, but I don't think I'm going to put a bunch of time into them. At least not at this point. That might change once the frame is all one piece... it'll all depend on how nice it actually turns out.
I plan to have the yoke and chainstays brazed and attached to the front triangle this weekend. Once the yoke and bottom bracket shell are brazed, I can finish the last fillet of the front triangle (a.k.a. my final opportunity to warp the BB shell). The frame should be coming together fairly quickly now... my fingers are crossed that it'll come out OK.