Fat Bike Build Log

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Specs and some lessons learned from a recent fat bike build. This was frame #3 for me, so very much still learning how to do this.

The numbers:
HA 68
SA 74
FC 780
RC 430
WB 1210
ETT 670
BB Height 315
BB Drop 69
Axle Height 384
BB to Bar 835
Stack (to c of bar) 995

I designed it to have a 67° HA, but ended up with 68°. I’m assuming the discrepancy a mix of deformation and incorrect spec on the fork. I rechecked the jig setup, and it’s correct.
I’m wondering if I should do all the brazing on the down tube before I cut the miters? The DT has two sets of bosses and three cable guides. The last frame I made I cut the miters for the DT, then brazed the bosses, after which the miters were no longer tight. Maybe I’m using too much brazing heat?

The bike rides how I hoped, and feels great. I’ve only had it out in ideal firm snow and dirt so far, we’ll see how it does in squirrelly soft snow.

Because the geometry isn’t dynamic on the full rigid, I’m guessing the 68° HA rides similar to a +/-66° on a bike with a suspension fork, so the 1° steeper may not be a bad thing.

I designed the dropouts, using send-cut-send and a friend with a CNC for fabrication work. They use a SRAM UDH. I hand cut my miters, and the flat landing plate makes it easy to get a tight joint. Not the lightest dropouts, but cheap and they work great. Integrated ISO mount saves some steps and potential alignment issues.

The yoke is a variation of the $10 yoke designed by Benjamin Land. All the pieces were cut and bent by send-cut-send.

Lessons:
My welding skills are very much still developing.

My heel can hit the seat stay if I’m twisting my foot a bit. I should have landed the seat stays higher on the ST, or given them an S-bend (though I don’t have a tube bender). I’m going to measure my heel locations and add that to my frame 3D models to be sure I have clearance on future builds.

The SRAM FAT5 chainline is too wide. The cranks come with a -4mm offset, which was way too much cross-chain in the granny gear. I swapped for a 0mm offset and it’s much better on both ends of the cassette. Overall I’d say the SRAM q-factor is a bit tight for a 197 rear end.

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Thanks for sharing! Looks like a killer build. I love the SRAM UDH dropouts, especially the built-in ISO mount. That’s gotta be hard to design both in one shot. Also, the nice use of laser-cut parts. Simple but effective

I am curious, what standards did you design to?

  • Tire clearance?
  • Chainline?
  • Rear Axle Width: 197?
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Thanks!

I wanted to fit a 27.5x4.5 tire, and shortish RC. I went with 197 because it seems like it’s the most common standard. I don’t think that tire size would work with a 170 rear end, but I didn’t model it to confirm.

For the chainline I was leaning on SRAM recommendations, which is a 76.5mm chainline for a 197 rear end. I ended up with a 72.5 chainline using the 0mm offset, which gives a much better chain angle for the larger cogs. I only ride single speed otherwise, so I’m not sure how that compares to standard drivetrains.

Send-cut-send is great for someone like me without machine tools. I’m lucky to have a friend with CNC access to help me, but those dropouts could also be fabricated with only laser cut parts and minimal hand and brazing work.

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What size tire are you able to fit with that setup?

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Really cool build. I’ve only built one fat bike and it was also frame #3 for me.

Similar to you, I built it with a short RC. I think our numbers are similar, but I have:

STA 75 degrees,
HTA 66 degrees,
FC 770mm,
RC 420-440mm
Tire size up to 4.8 w/ sliders all the way back (440mm RC)

Some things I learned:

~ Where I’m riding ~ all my fat biking is essentially our local enduro trails but covered in snow. Ride up, ride down. I built this bike around the riding setting.

  • Having a ton of trail / wheel flop on a fat bike isn’t always great, but it depends where you ride.
    ** Where I’m riding, where our snow berms are 3-5 feet high, having lots of trail is beneficial because I can put my weight back and the front wheel figures itself out.
    ***Contrasting this with my friend’s Surly Wednesday, she can’t ride the same stuff because the HTA is quite steep, and the FC is short as well, she always feels she’s gonna pitch over the bars.

  • Having a longer RC is preferable for me, without the handlebars low the front wheel wants to lift on steep climbs.
    ** When riding flat turns / more chill trail the bike isn’t as responsive.

  • Need a taller HT on fat bikes
    ** I had just finished an enduro hard tail and decided to make this an ‘enduro fat bike’, but a bit dialed back.
    *** I found out that I don’t want my bars low like I do on my Enduro hard tails, I like them low to drive weight into the front wheel when using a short RC, but when you’re driving all that weight into the front wheel and the snow berm isn’t super hard yet, you get a squirrelly front end.

I used a 197 rear end, and the SRAM 5" cranks like you. I also find the Q narrow and it was really hard to get the chainstays to fit properly. It would’ve been 100% easier if I wasn’t dead set on a short RC.

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This was a big part of my design intent- to be able to ride in soft snow with most of my weight on the rear with the front acting more like a floating rudder (if that makes sense). I find when the snow is soft with too much weight on the front it just digs in and washes out. I think climbing would benefit from a longer RC to spread the load for better float, but I can use my body position to help with that. I’ve done one soft snow ride so far, and it performed really well all around.

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