Specialized Stumpjumper Comp Alloy 6Fattie review

Plus size tires have benefits and limitations

Our rating 
3.5 out of 5 star rating 3.5
GBP £2,500.00 RRP | USD $2,800.00

Our review

With 27.5+ wheels and tires, the Stumpjumper Comp Alloy is a fun, confident trail bike as long as the aggressive riding is kept to a minimum
Buy if, You want a solid, adventure-ready trail bike and you’re sold on the big plus tires
Pros: Fun to ride, confident geometry, wall climbing ability, smooth rolling
Cons: Tires squirm when pushed, bouncy cornering, flexy fork
Skip to view product specifications

Forget the endless classification, there’s basically one niche Specialized’s Stumpjumper Comp Alloy 6Fattie fits into, and that’s simply ‘mountain bike’.


It has 135mm of travel out back, 150mm up front and troops around on big, squishy 27.5in plus size wheels and tires. Its confidence and stability will motivate you to try new lines both up and down, but it comes at a cost when speeds pick up. 

Specialized Stumpjumper Comp Alloy 6Fattie features

  • Frame: M5 aluminum, 135mm travel
  • Fork: RockShox Reba RL, 150mm travel
  • Drivetrain: SRAM GX 11-speed w/Race Face Aeffect cranks
  • Wheels/tires: Roval Traverse 27.5, Specialized Purgatory 3.0in (front), Slaughter 2.8in (rear)
  • Brakes: SRAM Guide R
  • Actual weight: 14.5kg / 32lbs
A 2.8in Slaughter takes care of the rear end. Its low center knobs keep rolling resistance low
Russell Eich / BikeRadar

Specialized parts for the win

Most bike companies have an in-house component brand. Typically, it’s to keep costs in check. Specialized does, too, but its own-label parts rival some of the very best out there. 

Specialized makes some of the best saddles. The Henge Comp is one I’d happily ride on other bikes
Russell Eich / BikeRadar

Take for instance the Henge Comp saddle, with Specialized’s Body Geometry. It’s so comfortable I’d buy one to swap onto my other bikes. 

Another perk emblazoned with the Specialized logo is the Zee bottle cage with an integrated mini tool attached to the bottom. This is smart execution, it’s something I’d gladly add to other bikes. 

Other touch points were spot on, too. The 780mm wide bars had a shred-ready rise and bend to them and the Sip grips felt nice with good texture. Plus, they were not an overly hard, thin rubber, thankfully.

The Command Post dropper remote is superb. It’s roughly the same size, shape and location as a SRAM shifter paddle, and it’s MatchMaker compatible
Russell Eich / BikeRadar

I got along well with the Command Post dropper post, even though it gets chided for its ball-bustlingly fast return rate. I appreciated its 12 pre-set height indents and rock-solid, mechanical-feeling function. And the under bar remote lever is one of the best out there, mimicking a SRAM shifter lever in size, shape and placement almost exactly.

Surprisingly quick

Maybe it’s the bulbous tires tricking the mind, maybe it’s the wheel weight when lofting the bike, but the Stumpy 6Fattie looked like it was going to be slow. But within a few pedal strokes I realized it’s not. It almost felt quick.

The Race Face Aeffect crank spins a little 28-tooth chainring
Russell Eich / BikeRadar

That urgency was the result of the relatively fast-rolling tires with short knobs and the 28-tooth chainring. The former keeps rolling efficiency high and the latter fooled me into dropping the chain down the cassette, which lead me to believe I was pushing a higher gear than I really was. 

That Specialized feel

Specialized’s mountain bikes have a certain feel to them. They’re low, have short rear ends and encourage steering from the back wheel. For boosting over rocks and squaring off corners it’s a fun set up. They’re playful and easy to hit bonus lines and jumps with.

Most, but not all, of that raucous attitude was present with the 6Fattie. Having the big wheels and tires slowed things like jumping and slashing corners. It could still be done, it just required a bit more pop and strength to get there. 

Good if you’re going straight

When rolling straight, there seemed little that could get in the way of this plus-size tire Stumpjumper. On smoother trails and at mellow speeds, the Stumpjumper Comp 6Fattie’s ride quality is impressive. Smooth is the best way to describe the ride.

With under 20psi in the tires front and rear, vibrations and small bumps are erased yet it still manages to feel quick when on the pedals.

29-inch wheels and tires fit into this frame as well. Even with the plus-size tire there’s a ton of room between the stays
Russell Eich / BikeRadar

On the ups, it was pretty amazing easily levitating its way up even jagged, technical climbs. Leaving the rear shock wide open posed no detriment. That said, even when flipping the climb lever, the big, soft tires still kept the traction high. 

Send it into some twisty bits of trail though and things felt different.

The plus-size tires made the bike feel loose and flexy. On the front end, I kept checking the headset because of the feeling and I repeatedly checked the axles and even the linkage bolts. Everything was snug though, the tires were just bouncing me around. 

Blame also sits with the Reba RL fork. With its slender 32mm stanchions, 29er wheel length and 150mm travel, it’s not stiff. Between the axle and crown there’s a lot of length there. It reminded me of a fat bike fork in the way it wobbled with the big, heavy wheel and tire at its end.

Stop the bounce

I experimented quite a bit with tire pressures. Initially, I set the rear 2.8in Slaughter at 20psi and the front Purgatory 3.0in at 16psi. From there I kept bleeding pressure until I got down to 16psi in the rear and 13psi up front. 

That was way too low. At around 84kg, the tire sidewalls were flexing and deforming during even modest inputs of weighting a corner or hitting the brakes. 

Up front a 27..5 x 3.0 Specialized Purgatory leads the way. Even with the 38mm wide Roval rim, it’s a rounded casing
Russell Eich / BikeRadar

To combat this, I pumped the tires up to 18psi rear and 16psi front. Even with those pressures, the dreaded sound of the tire unbeading still happened when getting fully zesty, like when landing sideways or when slamming hard into a berm. 

Therein lies the rub. Too much pressure and the plus-size tires have limited traction as they skim over the top and push and slide instead of bite and find purchase. Too little pressure and the sidewalls deform and twist about at the worst of times. 

‘Find the sweet spot’ is what you’re probably thinking. But that’s the thing, there isn’t really one to be found. It’s always a compromise. To keep the tire weight respectable, the tire’s sidewalls are thin, which makes the magical, perfect psi a unicorn. 

Stumpjumper Comp Alloy 6Fattie vs the competition

It’s a crowded field in the $2.500 to $3,000 range, however most of the bikes fall into the trail bike genre. The outliers to this are Commencal’s enduro-ready Meta AM V4.2 and Trek’s XC-leaning Fuel EX 7 29er

Its return is fast and it’s not infinitely adjustable in its stroke, but I enjoyed the mechanical, no-nonsense feel of the Command Post IRcc
Russell Eich / BikeRadar

If a 27.5in bike is your preference, Giant’s Trance 2 is also a solid contender for riders who like to push their limits while having a large margin of error.

Canyon’s Spectral AL 6.0 with its 2.6in tires is a close competitor to this Stumpjumper although a bit more aggressive in its ride quality, but it lacks a dealer network for test rides and servicing. 

Specialized Stumpjumper Comp Alloy 6Fattie bottom line

So who’s this bike for? It’s for a trail rider who wants to get more aggressive than a pure XC bike, but has no aspirations of enduro-style charging.

It’s ideal for a rider who wants to be able to try, and probably conquer, every climb and tricky descent. At slow to moderate speeds, it’s a confident machine because the big tires mute the little bumps, rocks and roots. They shave that layer of vibration right off. 

On the flip side, aggressive riders will likely struggle to find the traction and predictability they demand. Luckily, Specialized offers this same build with 29in or standard 27.5in wheels for the same price of £2,500 / $2,800 / AU$TBD. 


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Product Specifications


Name Stumpjumper Comp Alloy 6Fattie
Brand Specialized

Bottom Bracket Race Face PF30
Rear Shock RockShox Monarch RT
Brake Levers SRAM Guide R
Chainring Size (No of Teeth) 28
Year 2018
Stem Specialized Trail
Shifters SRAM GX
Seatpost Specialized Command Post IRcc dropper, 125mm
Saddle Specialized Henge Comp
Rims Roval Traverse, 38mm internal
Rear Tyre Specialized Slaughter Grid, 27.5 x 2.8
Rear Hub Specialized
Brakes SRAM Guide R discs
Rear Derailleur SRAM GX
Handlebar Specialized, 780mm width
Grips/Tape Specialized Sip Grip
Front Tyre Specialized Purgatory Grid, 27.5 x 3.0
Frame Material M5 Aluminum
Fork Offset 51
Fork RockShox Reba RL, 150mm
Cranks Race Face Aeffect
Chain KMC X11
Cassette SRAM XG 1150, 11-speed: 10-42
Frame size tested L