While the paint job is more muted than its 650b cousin, the 6Fattie Stumpjumper is a head-turner in its own right. The big tires are immediately noticeable, yet don’t look ridiculous – make no mistake, this isn’t a fat bike.
While the rubber is big and bulky, the carbon mainframe complements it with smooth lines and an impeccable finish.
Costing the same as the 650b Stumpy we rode alongside it in the Alps during testing, the 6Fattie gains a carbon frame to keep its weight in check at 14.02kg. With more money spent here, it’s no surprise to see lower-grade bits hanging off the frame.
Shimano Deore brakes and a SRAM GX groupset perform virtually as well as their pricier counterparts, but the Performance-level 34 fork and Float shock from Fox lack the subtle control levels of the Pike and factory shock on the 650b. While the Roval rims are shared, the 6Fattie also has slightly cheaper, but Boost-width, Specialized hubs.
Wide tires, narrow rim:
Wide tires, narrow rim
Specialized’s carbon Stumpys now have the SWAT down tube storage port – under the bottle cage is a door in the tube, allowing you to stash items such as tubes, tools and, if you’re Spesh rider Mitch Ropelato at least, hot dogs. Some see it as a gimmick, but for lovers of pack-less riding, it’s a nice feature.
The 6Fattie’s carbon front end is shared with the 29er Stumpy, coming in a touch shorter than the 650b version, with a reach figure of 431mm (L). Despite this, the 6Fattie actually felt a better shape than the 650b version.
This is thanks to the BB drop – the distance below a line between the wheel axles that the BB sits. The 650b has an 18mm drop, whereas the 6Fattie sits 33mm below the axles, leaving you feeling like you’re sat much more ‘in’ the bike, rather than over it, making it confident when leaned over into a corner. At the back, the 148 Boost alloy rear end is a touch longer at 437mm, giving a little extra high-speed stability with the subsequent longer wheelbase.
The 6fattie stumpy’s head angle isn’t particularly slack, but the wide rubber helps it hold lines excellently at speed:
The 6Fattie Stumpy’s head angle isn’t particularly slack, but the wide rubber helps it hold lines at speed
The big story, though, is the chunky 6Fattie Purgatory and Ground Control tires. At three inches wide they offer far more volume than regular tires, and also stand taller over the rim, giving an outside wheel diameter similar to that of a 29er. The result is a bike that gobbles up rough terrain and spits it out the back far more proficiently than you might imagine, holding momentum, but still managing to maintain plenty of agility.
The tires are mounted on 29mm internal width rims – of all the plus bikes we’ve ridden, these are the narrowest-mating we’ve experienced. While we worried about excessive tire roll, the Control casing did a reasonable job, although in big compressions, or where we needed to load the front wheel to hop the rear round a switchback, there was a definite element of roll. On most trails though, our testers rarely noticed any issues.
A bike that rips on most trails
The large volume and low pressures (around 14psi) means the tires conform well to rough surfaces, giving a lot of grip – something that’s especially noticeable on technical climbs. Where grip wasn’t as we expected was on mud: the lack of a particularly aggressive tread meant the tires didn’t dig in as well as we’d have liked.
On heavily front wheel loaded, slow speed technical sections, the large air volume also gives a sense of disconnect between the tire and bike, while hitting roots at speed can result in a slightly pingier rebound than we’re used to – something which we found ourselves adjusting to.
The 6fattie is a point-and-shoot ripper on most trails you put in front of it:
The 6Fattie is a point-and-shoot ripper on most trails you put in front of it
On the majority of trails though, the 6Fattie rips. It’s definitely a point-and-shoot bike, rather than threading like a needle through techier sections, but the big wheels bound over terrain like a collie dog on speed. You’ve just got to let the brakes off, hold on and scrub your speed at the end of the section.
The masses of grip up front means braking points can be later, while the shoulder tread of the Purgatory tire and the low BB means it’ll hang on in fast corners, allowing you to carry heaps of speed. The 67-degree head angle might not be slack, but the big tires mean it holds a line with absolute steadfastness.
While there’s definitely a lag between putting a sprint effort in and the bike accelerating up a hill, if you’re happy to sit and spin the 28t chainring, the 6Fattie doesn’t feel too bad on longer smooth climbs either. The tires are heavier, but the low tread rolls nicely on tarmac.
Rocky Mountain Sherpa
Built for bikepacking deep in the backcountry, the carbon framed Sherpa was one of the first full-suspension plus-size bikes. See our full Rocky Mountain Sherpa review.