One of the biggest challenges learning to build bikes in this day-and-age is the desire for short chainstays and fat-tires. Either one alone makes it more difficult, both together make for a lot of head scratching. Go figure that nobody wants skinny tires and long chainstays, except for maybe Rivendell.
Fatbikes make it somewhat easier because they use a 100mm wide BB shell but once you start introducing short chainstays into the mix all bets are off. You get into problems with chainrings and maybe even cranks hitting the chainstays and the chain rubbing on the tire in the low gears. There are just a lot of variables to keep straight.
I’ve only puttered along making bikes for this reason, among others – not whipping out 25 cross frames that fit a normal UCI legal 33mm tire, but by trying every tweak in the book. But that’s the beauty of CUSTOM bikes – you get what you pay for – something that is not available in the mass market and spec’d to your desires.
The latest adventure in chainline was with Adam’s 29+ frame where I attemped a ‘half-yoke’ to make room for Knard tires and also allow the use of an XX1 or a 3x crank using a 32t middle ring. Chainstay length was 440mm (434 effective). On my first 29+ i crimped the heck out of the Dedacaai 29er chainstays and got about 7mm of space on either side of the tire (see photo), but can only run a 26 tooth ring in the middle of a 3x crank and maybe a 34t on the outer, pushing the chainline out to about 54mm. Although stiff enough for me in the rear triangle, it’s questionable to crimp stays that much for longevity reasons. I wanted to find another way. I considered forking over the $185 for the Paragon Yoke (it’s very highly engineered and stiff) but that’s a lot of dough. I could figure something out myself.
I started out researching online and liked what i saw from AMPierce on his 29+ combo bike. Long story short, i took a 1/4″ thick x 1.25″ tall piece of 4130 and faced it down to 0.20″ on the mill. Drilled a few holes so take some heft out of it, slotted a previously ruined Deda chainstay (cut too short on a previous build) and brass brazed those two together. Then I dimpled the non-driveside chainstay, slotted both ends for the dropouts, loaded them up in the mitering fixture and coped away. All in all not too hard but it doesn’t LOOK that great from above since chainstay clearance of a flat plate looks different than a round/dimpled chainstay on the opposite side, the lack of symmetry bugs me.
After some fiddling in the fixture I tacked the stays to the BB, put in my rear wheel to check clearances, then welded it all in place. Lastly, I silver brazed a 5/8″ x 0.035″ bridge between the chainstays to offer more support. I thought of adding a horizontal plate to do that but i feel it’d be less torsionally stiff and end up being a mud-catcher, moreso than a bridge. Even though it worked and will likely last a long time, it’s not as elegant and has less tire clearance and frame rigidity as the Paragon Yoke. Unless someone comes out with another option I’m going to splurge on the Paragon yoke from now on.