Frame #9 – Fatbike skooling

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This build has been a big learning experience. I’ve had my best welds, and some of my worst.  More of my best though.  One pass on the head-tube and it was all good.  The BB cluster was..challenging.  The 100mm BB shell makes it more difficult.  Here are some of the ‘highlights’ of the lessons learned while building the fatbike which is almost painted and ready to ride! Now we just need some more snow…

  • With a 170mm rear wheel-spaced Fatbike, nothing fits on my current fixtures…whether it’s the chainstay fixture or my alignment table.  With some simple ‘fabrication’ I’ve gotten it all to work but each step forward results in a slight pause to re-tool.  It’s actually fun, but something to think about when you build a different genre of bike.  If you’re using 170mm rear axle spacing, your current dummy axle or the Henry James Universal Axle (UA) won’t work.  I think Anvil makes a 170mm dummy axle but Hank does not as far as I know.  Everything else works great on the Acccess 65 like the BB-post…there is plenty of room to have a 100mm BB shell with room to spare. But as for the rear axle spacing,  I made it work by going to the outermost extent of the UA (about 140ish mm), putting longer 5mm bolts in the ends instead of the ones attached by chains to the UA, and then filling the extra space with two sets of rear dropouts.  It looks junky, but it works just fine. This way you can use the rear BB drop post on the Access 65 instead of measuring drop manually and using another dummy axle. You can even fine-tune the UA to 172mm or so to allow for some inward movement in the chainstays after welding.
  • Fatbike tires are MUCH bigger and have a different profile than a regular MTB tire (duh).  But seriously, the profile is much LONGER and wide than a regular tire so the accompanying chainstay crimp may have to be longer as well to allow for tire clearance (depends on the types of chainstays you are using though).  I kinda over-did it with my True Temper s-bend chainstays but at least there’s plenty of clearance so no snow/ice buildup to occur! The photo in the slideshow shows that once the tire bead leaves the rim, it’s pretty much straight up to the top of the tire almost 4″ later.  So the chainstays need to have a wide crimp or a good gap for about 4″…!
  • Welding the BB-cluster of a 100mm wide shell is more challenging than I’d have thought.  Most everything is the same but even when using a 31.8mm seat tube, there is a huge amount of space from the edge of the BB shell to the side of the seat tube.  I found it difficult to get the filler rod in there at the right angle while running the TIG bead.  It’s the place of the worst welds on the frame because of this junction.  The problem area is just the junction between the downtube/seat-tube/BB.  I usually weld from the middle of that junction down to the BB shell but if you do that it’ll be harder to get the right angle on the filler rod on both sides.  If you go in one direction (which I was taught is the ‘accepted’ way to TIG) then you will only have one side that is a problem.  Of course I thought of this after the fact (I am trying to say with the same weld sequence on each bike).
  • My C-channel Bringheli alignment table doesn’t work as-is for a bike with a 100mm BB shell.  It is a quick fix though with a trip to the hardware store.  The BB post is a standard 3/4″ coarse thread bolt and with a 3/4″ coupling nut and a short length of 3/4″ threaded rod I was able to extend the stock post to whatever length desired.
  • The frame/fork alignment tool that I got from Alex Wetmore only goes to an axle spacing of 140mm.  So you have to use or make some spacers to check if your rear axle spacing is correct.  I love this alignment tool, it’s hugely useful for quick & accurate alignment checks so I’ll need to make some 15mm spacers I can clamp onto it for the next fatbike build.
  • The Park dropout alignment tool doesn’t work for axles this wide either.  But using a couple of dropouts for spacers did the trick.  Otherwise, the extension cups are too wobbly and don’t tell you jack since they’re extended all the way out and about to fall off without using spacers.
  • Hmm, i think that’s it!…? I’ll update later if i think of more things.

4 thoughts on “Frame #9 – Fatbike skooling

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    1. That was my first attempt using the Harbor Freight tubing roller I got awhile back but have yet to master. I rolled some 7/8″ 4130 x 0.035 in the stock 1″ dies and so that’s why there’s some definite flattening of the wishbone crown piece. I think it turned out looking kinda cool though so that’s why I kept it. The bend you can get on the roller is not sharp enough for a regular bike’s wishbone stay, so another method/bender would need to be used in that case.

    2. Craig,
      I only have used the Harris wire from Henry James. For the chainstay bridge, or where more of a fillet is needed, I use the 50N which appears the same as BAg-24. (50% Ag, 1% nickel), but for the braze-ons i just use the 56% Ag wire.

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