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| a fatbike for everyday |
I wasn't really paying attention when fatbikes first became a thing, but anyway somewhere around the start of it all, there was the Pugsley from Surly. It could manage 4-inch-wide tires while using mostly standard MTB stuff. In particular, it used a standard 135mm MTB hub in back. This may seem kind of unsurprising, the tire of course is only around 100mm wide, so a 135mm hub seems like plenty. You need to remember though that there needed to be room for a derailleur setup. And not just a modern 1-by setup, you're looking at maybe 3 chainrings up front with 9 sprockets in back, and the chain needed to clear the tire. They solved this by building the frame crooked: the hub was shoved towards the drive side so it stuck out sufficiently, and the not-drive-side frame curved inwards to catch that end of the hub. To absorb some of the offset, the rear rim has the holes drilled off-center. After a while, special fatbike hubs came along that were 170mm and 190mm wide, making things symmetric again, and stronger, no doubt. So Pugsley was a curiousity, some bike-geek trivia that we could all tuck away in the back of our minds. It went out of production in 2021.
Well years later, along came me, myself and I, and "we" got two reasonably-priced modern fatbikes with 190mm hubs. They're kind of fun, sometimes a lot of fun, and the bikes easily showed their usefulness on snow-ice-pavement. Then a winter commute situation arose that seemed like it might fit a fatbike. Being experienced in the ways of gear hubs (and also usually annoyed with derailleurs), it seemed like a gear-hubbed winter commuter fatbike was just the ticket. Getting a 190mm hub is expensive. I remembered the Pugsley. I discovered a couple of used examples online, but the price made me hesitate, and then I discovered something else. Something cheaper.
My eyes somehow picked out, in a random used fatbike posting, the telltale sign: the rear spokes ending in one line, offset from center of the rim. Apparently, Surly wasn't the only brand that made these twisted offset frames for fatbikes using 135mm rear hubs. (Forgive my ignorance.) A local-ish Norwegian-ish brand Diamant also made them, but in aluminum, and with their less-prestigious brand name stuck all over it. So, Diamant Mammut, model year ~2014. (They sold the same thing in a different color as Nakamura BigBob, and apparently in the USA as Motobecane Fantom FB4). This bike also had the requisite mounts for fenders and racks, very much a drop-in Pugsley replacement if you ask me. I obtained one, at a good price and not too far away. (Nice thing about used fatbikes is that there seems to be an over-supply of them, and probably the owner didn't ride them enough to wear much out.)
OK, the next part was a bit tricky: how to prepare it for a life of thankless toil. What was the frame offset, anyway? What kind of hub is going to work well, and what length of spokes? For a Pugsley, this will be possible to look up on the net, no such luck for a Mammut.
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| one plausible version of the wheel geometry |
Well, the rear rim was drilled 10mm to the drive side, all the spoke holes in one line. Its 80mm wide. Using a flat stick, it looked like the non-drive-side end of the hub stuck out 3mm or so beyond the rim. A bit of math and it seems the hub offset should be 24-25mm to the drive side, more than a Pugsley (17.5mm they say). I also disassembled the old wheel and examined the spokes, did what measurements I could, and used https://www.kstoerz.com/freespoke/ to make some estimates. A bit of trial-and-error, and I found that 250mm and 252mm spokes were sufficient to build the wheel on an Alfine 8, although the 250mm ones were a touch short. Putting things back into the spoke calculator suggests that the hub offset was more like 20mm, or even 17.5mm. The smaller offset appears to be necessary to explain how 250mm non-drive-side was short while 252mm drive-side was OK. However in that case, the hub end should have stuck out more beyond the rim. I guess that remains a mystery.
Despite all the unknowns, the new wheel fit fine, the brake disc slotted in perfectly, and the chain tensioner lined up where it needed to. Spoke tension meets my standards, which aren't professional, but thus far I have never had spoke break on any wheel I've built. (Has to be said, this was my first non-symmetric wheel build.)
I ordered huge fatbike fenders from https://classic-cycle.com/ and they ended up fitting well, although the top of the front fork was a bit of a pinch. I had to file, cut, and heat-form the fender to sit well... but after that, it sure did sit well. Those fenders are huge, and quite stiff. There was also a little messing around to get the fender stays past obstructions, mostly the disc brakes.
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| front | back |
I had to replace the rear hydraulic brake line (in part because a new handlebar required a longer line) and somehow failed to get it tight enough, which was messy. The oil is non-toxic I guess. The rear disc got oil seepage on it, but actually that was OK riding on snow and ice. I just kept riding it until until the oil film gave up. Hydraulic discs are OK, but IMO too much work for any commuter bike that can avoid them. Go with Shimano's roller brakes and/or a foot/back-pedal/coaster brake for minimal maintenance. I did test-fit a wheel with a roller brake to the Mammut frame, but found the unusual shape of the frame on the non-drive-side put it out of reach of the brake arm. A roller would have been a nice brake for my purposes, but then I would have missed out on the Alfine 8. (I really like the Alfine 8 combined with the two chain wheels up front.)
Anyway, I rode around my Mammut all winter with the regular summer fatbike tire in back (optimistically labeled as 26x4.5, more like 26x4.2), and a Dillenger 5 (26x4.6) in front. I was taking it shopping and riding it into the city for meetings. This was a huge success, I really liked riding it. It wasn't super fast, and conditions didn't require such a bike, but I liked it. Those tires made the randomly-distributed crappiness of winter paths minimally bothersome. A person might look at the tires and assume they float on snow like a small rabbit, but no, not at all. In fresh snow, you just end up squishing a path twice as wide as anyone else. They are merely more stable, able to roll on/over more crap without it bothering the rider. The tires were wide enough to reliably find gravel laying on ice, and also to give some warning when it was time to slide. A few times the front tire (studded) even slid on ice (in a friendly way), which I can't recall having done, without falling, with a thinner tire. When the conditions warmed and I first hopped onto a regular bike, it felt small, bouncy and unsafe. Clear paths eventually fixed that feeling. I can't deny its good to zip around on high-pressure tires and smooth clean pavement.
Returning to the winter mindset one moment longer, the Mammut worked out so well that we tried a FitNord Rumble 300 for the lady, and that worked out so well that we decided it would be good to build yet another commuter fatbike for a role that involved sitting overnight at a train station. Spooky. I found another Mammut, basically identical to the first one, but even cheaper than before. The guy selling it was pointing out how it still had 31 not-broken spokes in the back wheel. Perfect, sold! I wonder if I can make a chain tensioner work with a 27t sprocket on a Nexus Inter-5E (a SG-C7000-5D). That'll be a project for after summer.
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