Meet T’s Fatbike

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Frame #11 is completed, painted, ridden, tested, muddy, snowy, and all is good. No doubt, with this weird design there are a couple of minor issues, but nothing to worry about.  I’m pretty happy how it turned out and that it rides quite nicely, if I do say so myself.  To sum up, the bike is a Moonlander-sized fatbike that fits the Big Fat Larrys (4.7″ tires) on the Surly Clownshoe 100mm wide rim.  The bike was built for a friend in Durango and it was almost entirely his brainchild, I just fabricated it as best I could.  The geometry is 70 deg HT angle, 72 deg ST angle, 24.6″ effective top tube, and long 467mm chainstays (18.4″). It has a huge 170mm head tube on it so that almost no headset spacers are needed.  The wheels were built by Mike at lacemine29.com with Fatback hubs with the big through-axles.  The fork is a White Brothers Snowpack fork with a 468mm axle to crown and 42mm of offset making for a rather large 94mm trail figure.  This geometry is quite similar to the 9:zero:7 frames and since this is a ‘test’ bike he wanted to mimic what’s out there. He had a Truvativ 100mm-spindle ISIS bottom bracket and the wrong cranks. Don’t even get me started on the whold BB/Crank compatibility and “standards” thing.  After some futzing with cranks and BB’s, T found an old bike (his wife’s singlespeed…) that had the right cranks on it. (FYI, none of the 7 shops in Durango carry ISIS cranks anymore.)  The frame weighed in at 6lbs, 2oz. OUCH! But fully built it was a reasonable 34.5lbs – only a half-pound more than mine.

We did two rides on the bikes – one on the foot-packed Colorado Trail at Junction Creek, and one atop Molas Pass on snowmobile trails. We traded off from the Choad (T’s brown fatbike), my smaller fatbike with a Surly Nate/Endomorph combo, and a 29″-wheeled bike with 50mm wide Speedway rims and 2.4″ Ardents, just to see how a 29er would fare on the same terrain (granted, the 29er has 50mm wide rims which is over DOUBLE the size of the widest normal XC rim).  Here are some of the things I thought were interesting:

  • The 29er with those wide rims and 2.4 tires floated surprisingly well even though the contact patch was much smaller than the fatbike tires (see photo).  The wide rims are key for this though and i wish there was something bigger than a 2.4″ 29er tire to try out.  We were able to run this tire down to 10lbs of pressure with this rim on soft snow and that helped a lot.
  • The Choad out-floated my Rotundsicle for the most part, and overall just felt better at low tire pressure.  I was only able to ride my tires with any confidence at 4PSI in the front, 5 in the rear tire; whereas the same pressure in the Big Fat Larrys felt much more solid.  We were able to actually goto 3PSI in the front and 4 in the rear and it felt like more pressure and a more stable ride than in the smaller width Nate/Endomorph tires on my bike. Lesson learned – go with the widest rims you can get!
  • The Big Fat Larry tires are not good rear tires. I hope there’s another option next season for a big 4.5″-ish tire. (I don’t think it’s a great front tire either.)  The Surly Nate as a 4.5″ tire for front and rear would be…amazing.  I haven’t got my hands on the 45North Husker Du tire but I am thinking that would be a great tire too…and that seems to be the general consensus on the web.
  • This may be obvious, but wider and fatter is better on soft snow, but…not as much as I thought it’d be.  I was able to ride almost all the same parts of the trail as T was on his BFL’s.  The Endomorph is not a good soft-snow tire, or a good hardpack tire IMO. There are no knobs to grip once you get down to more packed snow so unless you’re on flat terrain, I’d go with another tire.  I can see this tire quietly disappearing in the next couple years since more ‘treaded’ options are coming out.
  • The rear shifting works just fine even with the droputs tilted up/counterclockwise! Shimano ‘recommends’ having the derailleur hanger at an angle of ~25-30 degrees from a line coming straight down from the axle to the ground.  The Choad has something around the exact opposite, negative 25 degrees…but it shifts just fine with 10spd SRAM.  The biggest issue is not the angle of the dropout and derailleur hanger but instead from the fat tire and chainline of the non-offset rear triangle of the frame.  This is due to the fact that you must either get a Surly Mr. Whirly Offset Double crankset and chainring setup or run only one ring up front and place the chainring on the outer position of the spider.  This limits what size ring you can use and makes for a pretty bad chainline (lots of crossover when you’re in your lower gears in back).
  • We both thought that the weird seat tube ‘legs’, and the short spliced tube for the BB/DT did stiffen up the BB. But with 16mm round & undimpled chainstays, the rear triangle still flexed but not as much as my first fatbike build with regular chainstays. Next time, I’ll use 19mm tubes instead.  I wasn’t sure there’d be enough clearance, but there’s plenty of room to even use 22mm round tubes for the chainstays.

Now for the issues:

  • The seat stay above the disc caliper on the dropout is way too close because of the angle of the dropout.  I had to adjust the brake with the disc caliper not completely bolted into the frame.  The rear adjusting screw cannot be used with the seat stay that close…bummer. The dropout is this angle because of the elevated chainstays.  Had I bent the chainstays vertically, or the seatstays, near the dropout, I could have avoided this problem. Live and learn.
  • The chain slightly touches the drive-side chainstay when in the smallest cog. It’ll just rub the paint off, but still a bummer.  The easy solution is to use hooded/Wright/Breezer style dropouts that would have pushed the chainstay more to the outside, or slot the chainstays so there’s no chainstay on the inside of the dropout.
  • The chainline. Like i said above, there’s a lot of crossover when in the lower gears in back (higher tooth cogs).  It’s why the Moonlander has an offset frame and offset wheel in back – those two things push the cogset out so that there is less chain crossover.  It’s nothing to worry too much about because it shifts and rides fine, but with any mud or grit in the chain (especially a 10speed chain and cogset) you will experience some skipping in the lowest 2 cogs.

I have some sweet GoPro footage of the rides so I’ll make a lil video and post in the next couple days…

2 Responses

  1. looked like some snow fun on the vid!!!! the drop in section around minute 8 looked dicey!!! frame is sweet!!

    TK

  2. […] LINK for the “wallpaper” of T on Molas Pass with the fabike I made.  Here’s the link if you’re interested in more pictures of the build and the “why”.  It’s […]

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