Chainline and fatter tires

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.

Russell's all mountain 29er. This seems easy now. 420mm chainstays with a 2.35 tire and XX1 cranks.

Russell’s all mountain 29er. This seems easy now. 420mm chainstays with a 2.35 tire and XX1 cranks.

When I started building, I didn’t fully understand the challenges and what to watch out for.  In all honesty, I didn’t even think about chainline on my first 10 frames and I just got lucky with the cranks and gearing i used.  But when you start trying to fit 40-45mm tires on a cross bike with a 68mm bottom bracket shell things can get tighter. When you talk about making a 29+ frame and fitting 3″ wide tires on a 73mm wide BB shell while maintaining a *normal* chainline then it gets even crazier. Word to new framebuilders, spend the time looking online for the manufacturer frame fit specifications of the cranks you intend to put on the frame (and get accurate tire width and diameter specs) and draw it out.
My fatbike. 4.8" tires, 430mm chainstays.

My fatbike. 4.8″ tires, 430mm chainstays.

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.

Joe's Mtn Cross frame that fits 40's, at least.

Joe’s Mtn Cross frame that fits 40’s, at least.

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.

That is CLOSE! 2mm on the 26t chainring, 7mm on the tire.

That is CLOSE! 2mm on the 26t chainring (MRP Bling Ring), 7mm on the tire.

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.

Checking for even tire clearance

Checking for even tire clearance

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.

half-yoke-done

6 Responses

  1. Hey Whit, thanks for another interesting post. It may not be symmetrical, but it’s pretty neat:

  2. But does it accomplish the tire and chainring clearances desired? If so, I prefer your solution to the Paragon yoke (unless it requires greater than $185 of your time).

    • The half-yoke definitely has the chainring clearance and chainline dialed but I’m unsure if it’ll rub the tire in the lowest gears. I was able to put a XX1 crank on there with a 32t ring and had tons of room. I’m pretty sure that a triple crank would fit too. Whether the chain would clear the tire in the lowest gears is another story. Tire clearance is just OK. I’d prefer more room so an adding an arc to the plate in a half-yoke would be a good call especially for a shorter chainstay frame.

      BTW, I’ve commandeered this frame for my own to test and am currently re-making Adam’s frame with the Paragon yoke. The yoke is beautiful but i have mixed feelings so far. First, the cost and the added amount of work for such an expensive piece. It comes in two pieces, needs to be milled for internal purging, and then fused together before anything else. Second, it’s very difficult to get chainstays shorter than 450mm because of the design. The two parts of the yoke that get welded to the BB shell are easier to weld when longer (fitting the torch in there is already going to be hard at 450mm). As comparison to show what I mean, Hunter Cycles’ yoke could be cut close to the BB and still be easy to weld. He says it’s a lot of pieces to set up as it is though.

      In the end i bet the Paragon yoke will be the same amount of setup time as the half-yoke but a much better (stiffer, lighter, stronger) design. But I’m still leaning towards using an 83mm BB shell for 29+ since you can use normal oval chainstays, get a short chainstay length, and get a good chainline (using a 150 rear hub of course).

      • EDIT/Correction on my comment:
        you can get shorter chainstays than 450 no problem with the Paragon yoke. I just welded up a 445mm chainstay and there’s about 18mm from a knard on a RH to the yoke with 20mm arms in front to weld to the BB. Depending on your tire clearance cutoff one could get down to 435 or so, if desired.

  3. And that comment about Rivendell is hilarious.

  4. […] but I think the only way will be to do a plate style yoke. I can try a different method than on my brown bike to make it stiffer laterally. This is always the crux of the frame and takes the longest to figure […]

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