Bike Check: SC-2

The first full frame build for the "Short Crank" series was a fat bike frame but being the second project in the series it gets the designation of SC-2. The choice of a fat bike frame was because winter was coming and I felt like my previous fat bike wasn't cutting it around here. It was built around 27.5 x 4" tires because I thought it would be "good enough" for the minor snow we get here in the desert on the eastern side of the Sierra, but what I didn't realize is that our snow conditions sort of suck for fat biking. The temperatures are too inconsistent and lower snow amounts means it's open season for trail runners to posthole up all the trails.

If I lived well up north where the snow builds up and gets packs down thanks to lots of snowmobiles and other fatbikers maybe the first fatbike would have done better - it was actually a great trail bike in the summer with 29 x 3" wheels. It had very progressive geometry and did great on firm snow, but in anything soft the short rear end sank down and stalled. You couldn't ride anything steep enough to take advantage of the long front center. Instead it worked against you, making it too hard to weight the front wheel to get traction.

There was also the problem that due to COVID supply issues, the only tubing I could buy in the right diameters and lengths was some super thinwall stuff. A summer of trail riding on the wide cranks caused cracks in the down and seat tubes, and though I was able to repair and reinforce things the bike's days were probably numbered. But since the steep seat tube angle no longer felt right as it was, and felt even worse with shorter cranks, it was time to start over.

First thing to design around was new wider wheels. Any advantage of rollover with the 27.5" wheel wasn't going to make any difference without traction to get you moving in the first place as well as keep you on you tiny packed line of packed snow. So I went wide and knobby, with 26 x 5" Johnny 5s on Blizzerk 90 rims. Hubs were "Pub" branded Bitex 150/197 from eBay. To fit the big tire in front I needed a new fork too and got lucky on a closeout Salsa carbon fork.

For the short cranks I found a pair of Race Fact Aeffects in 175mm that I planned to redrill as 152s, but I hadn't worked out the exact process in time so I put some cheapo Fomtor 152s with a 30t 104 BCD offset ring mounted to the outside position. I moved the old brakes and drivetrain across. The TRP G-Spec Trails don't have the full piston power of their Quadiem siblings, but the carbon levers are nice in the cold. Shifting was handled by my standard min-max drivetrain of an XT 8000 11s shifter, Deore 5100 12s rear derailleur, and Microshift Advent 11-48 10s cassette. Finishing off was some 11 speed SRAM chain that was likely on sale. Add a dropper and my favorite Soma Dream Riser handlebars and the build kit was complete.

For the frame, I steepened the head angle slightly to 68 to get a little more weight on the front end. The stays were lengthened at both ends. The dropouts went back relative to my center of gravity to balance out the bike even more, and the bottom bracket moved forwards to compensate for the shorter cranks. This means the reach on paper shortened from 534 to 490 and the chainstays were extended from 420 to 485.

There's a big debate in the fat bike world whether fat bikes should have shorter stays to get more weight on the back wheel or not. The argument is that if you don't have traction to pedal forwards then steering doesn't matter. But it seems the opposite is also true, there's no point pedaling forwards if you can't control your bike. Then since snow surfaces vary so much I don't know if anything really applies universally. Since I had already tried super short it was worth trying longer.

Given the long stays, there was enough room to offset the seat tube backwards. This gives you a slacker effective seat tube (about 71 degress) without the seat tube looking as slack. The angle of seat tube visually is 72 degrees. The big bonus is that when you hang the seat tube off the back of the bottom bracket shell you have a convenient entry for a dropper cable, and an excellent path for any water that seeps down the seat tube to drain right out the back. Since the water exits that way, you can then skip a vent hole in the bottom bracket shell which keeps that water from having to drain through the bottom bracket.

The roomy back end also meant not having to cram the chainstays between the tire and chainring, so I was able to use large 3/4" and 5/8" square tubing to build the chainstays. It's all 0.035" wall except for the right side 3/4" segment which used 0.049" for a little durability insurance. For the seat stays I stepped down from the 5/8" square to 1/2" square.

There is a catch though - while there's more room for the tire, chainstay, and ring, as your chainstays get longer the tire gets closer to the lower run of chain when you're in your low gear! Luckily, there was still enough clearance to not have any chain rub on the tire.

The frame build was uneventful for the most part, except I finally made the rookie mistake of brazing in the bottom bracket backwards so the threading goes in the wrong direction. This would seem like a problem mostly in theory, but after a few months the drive side square taper bottom bracket "cup" started walking out! Loctite fixed that one. I have since figured out how to shorten cranks so this next winter it will get the Aeffects and external Hollowtech cups which might be less likely to unscrew. Or maybe that's wishful thinking. I'll loctite it just to be sure.

On snow the bike does well. It's centered and when the wheels slip sideways the bike feels like it's pivoting around your center of gravity instead of the wheels shooting out sideways from under you. It also doesn't feel like the rear wheel sinks in as much. I didn't get as many opportunities to ride it in the snow, but I did take it on our annual fatbike camping overnighter and it performed well loaded. This year we rode mostly on groomed snowmobile trails so the extra traction and floatation did not get put to the test of previous years much tougher conditions. We'll see what this year brings.

After the snow melted it also became my trail bike given the retirement or cannibalization of my previous trail bikes.The long stays are a little odd - the front end doesn't want to come up without a lot of body english, and then the delay until your rear wheel runs into whatever you're manualing over is really weird, like there was a little glitch in the matrix. With 5" tires you can't expect it to ride like a regular bike anyway, but what it lacked in singletrack prowess in made up in just the dumb fun of buzzing around on those ridiculous tires.

I'll ride it more this winter but I don't think I have any plans to update it. I've thought about using the old 135/177 wheels and maybe an 83mm bottom bracket to make something more desert touring oriented but I'm not sure I really need something like that taking up space on the rack. Maybe I'll just relace them to 150/197 and call it good. But for now I need more time on this frame and its 5" tires to better understand fat bike design.