2021 Specialized Stumpjumper loses Horst-link FSR pivot in favour of flexstay

Geometry, frame, spec and model updates hope to maintain the Stumpy’s success

2021 Specialized Stumpjumper Expert trail mountain bike

Specialized’s brand-new 2021 Stumpjumper looks to strike the perfect balance between light weight climbing prowess and descending competence, thanks to a host of updates that, Specialized claims, should bring what it calls “a telepathic, magic carpet quality” to the bike’s ride. Quite a claim.

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The Big S hopes to do this by removing as much weight from the FACT 11M carbon fibre frame construction as possible, with the implementation of the now iconic Sidearm strut and its ‘Rider-First Engineered’ approach to ride feel.

The carbon frame construction is complemented by a 19 per cent progressive suspension kinematic, which now features a seatstay flexstay rather than a Horst-link pivot to help drive the 130mm of rear wheel travel. A more affordable aluminium frame is also available.

2021 Specialized Stumpjumper Expert trail mountain bike
The Expert model shares the same frame as the S-Works version.
Ian Lean / Specialized

Elsewhere, it’s got inbuilt chainstay chainslap protection and a down tube protector, along with Specialized’s SWAT on-board storage system.

Geometry has also been tweaked and each of the 2021 Stumpjumper’s six sizes have a high and low position. Like its Evo sibling, the new standard Stumpjumper is only available as a 29in-wheeled bike.

The new model promises to be a no-compromises affair.

2021 Specialized Stumpjumper frame details

2021 Specialized Stumpjumper Expert trail mountain bike
The Big S.
Ian Lean / Specialized

The 2021 Stumpjumper’s frame has been on a considerable diet and now weighs a meagre 2,240g (S4 including shock and all hardware claimed weight). Specialized has managed to reduce weight by doing three things.

First, it’s carefully analysed the frame’s shape, finding ways to reduce unnecessary and redundant carbon material. The shape of the Stumpy’s tubes have been crafted to provide the best, most optimised use of frame material in any one place.

Examples of this are seen on the front shock mount and Sidearm design, both designed to create the best balance between weight, durability and stiffness.

2021 Specialized Stumpjumper Expert trail mountain bike
The Sidearm strut braces the front end between top tube and seat tube.
Ian Lean / Specialized

Second, the carbon frame’s layup is area-specific.

Specialized says different types of carbon fibre have been used in different parts of the frame, enabling it to tune its structure to be seriously strong and light. The Big S boasts that it’s “the strongest, most expensive carbon [it’s] ever used in a mountain bike frame”.

Third, the carbon layup has been designed meticulously. Lab and trail testing of prototype versions allowed Specialized to blend numerical data and rider feedback to help define the perfect construction.

2021 Specialized Stumpjumper Expert trail mountain bike
The SWAT compartment is vast enough to store a day’s worth of supplies and other kit. Plus the door is a bottle cage.
Ian Lean / Specialized

Specialized hopes these techniques have culminated in the best trail bike it’s ever made.

And just like the 2021 Stumpjumper Evo, the standard Stumpy has been “Rider-First Engineered”. This means each of the model’s six sizes have been tuned to have the same ride-feel taking into account rider weight. For example, a 50kg rider on an S1 should experience the same ride as a 120kg rider on an S6 bike.

Unfortunately, this is nigh-on impossible to test unless one of us were to drastically lose or put on weight, or get taller or shorter, so we’ll just have to take Specialized’s word for it.

Elsewhere, the frame gets in-built down tube and chainstay protection – with ribbed sections to further suppress chainslap noise – along with internally routed cables from front to back with cable guides to help simplify installation and removal.

2021 Specialized Stumpjumper Expert trail mountain bike
A ribbed chainstay protector is claimed to eliminate noise from chainslap.
Ian Lean / Specialized

Specialized’s SWAT compartment features on the new bike, too. Like the old frame, its opening doubles up as a bottle cage that also has a multi-tool attached.

The frame has a BSA 73mm threaded bottom-bracket, uses a metric trunnion-mounted 190x45mm rear shock and has a 30.9mm seat tube diameter.

The aluminium version of the 2021 Stumpjumper is claimed to have the same ride quality as the carbon bikes, and although it features the same suspension kinematics, internal cable routing and geometry flip chip, it forgoes the SWAT compartment.

2021 Specialized Stumpjumper suspension

2021 Specialized Stumpjumper Expert trail mountain bike
The carbon bike’s rear end no longer has a Horst-link pivot, instead it uses in-built flex in the bike’s seatstay to permit the suspension’s action.
Ian Lean / Specialized

For the 2021 Stumpjumper, Specialized has moved away from its traditional Horst-link suspension setup, dubbed FSR. And although FSR has become synonymous with Horst-link designs, it actually stands for ‘Future Shock Rear’ and can encompass any of its bike’s suspension designs.

History lessons and technicalities aside, the carbon Stumpy now uses a flexible carbon seatstay – doing away with the Horst-link chainstay pivot altogether. This means the new Stumpjumper is a linkage-driven single-pivot design rather than a Horst-link bike.

Specialized is confident that removing the Horst-link – in the carbon Stumpy’s case – is a good thing and it’s claimed to save 55g while improving stiffness and increasing maintenance intervals, thanks to the pivotless one-piece swing arm

The 2021 Stumpjumper’s suspension is roughly 19 per cent progressive, with most of that progression happening in the first two-thirds of the travel. This should equate to a soft beginning stroke and relatively firm mid-stroke, without making the end of the bike’s travel too firm. The end stroke can be tuned, and made firmer, with air spring volume spacers, though.

Specialized Stumpjumper suspension leverage ratio
The leverage ratio of the Stumpy has 19 per cent of progression.
Specialized

Specialized says it’s tuned the suspension to work best with spring rates created by high volume air shocks. The majority of an air shock’s spring curve is fairly flat, but ramps up quickly towards the end of its stroke because the air is harder to compress.

The air shock’s spring ramp-up provides the bottom-out resistance while the flattening of the kinematics’ progression allows riders to use that last bit of travel.

Specialized Stumpjumper spring curve
This spring curve looks similar to most high-volume air shocks.
Specialized

To complement these in-built characteristics, Specialized has opted for a light compression tune on the rear shock.

The digressive tune – where the force required to overcome the damping rises quickly on slower hits as the shock’s compression speed increases – provides resistance against pedalling forces and platform stability. Then, because the damping doesn’t significantly increase after the initial spike, as the shock compresses quicker, the rear end should remain supple on faster hits to absorb bumps and provide comfort.

The progressive rebound tune has been customised to provide more damping on high-speed hits while being light on slower bumps. This, Specialized claims, means the rear shock is quick to rebound on smaller hits, while providing controlled rebound on bigger ones.

Interestingly, this progressive rebound tune is the opposite to RockShox’ digressive Rapid Recovery rebound, which has a fast return speed – with less damping – on big hits and a slower return speed – with more damping – on smaller bumps.

These traits combine to create what Spesh hopes is the perfect recipe for the ultimate trail bike.

2021 Specialized Stumpjumper geometry and sizing

2021 Specialized Stumpjumper Expert trail mountain bike
A flip-chip changes between high and low geometry settings.
Ian Lean / Specialized

Available in six sizes, the 2021 Stumpjumper uses style-focused sizing. Although each of Specialized’s S1 to S6 size names have a corresponding traditional name – S1 is XS, while S6 is XXL – the brand says a rider could opt for one size smaller or bigger than recommended to accommodate different tastes in riding style.

Standover heights across the sizes only increase by 27mm from 730mm to 757mm. What’s equally impressive is that seat-tube lengths start at 385mm and only increase to 465mm.

There’s a geometry flip chip at the rear shock mount that swaps between a high and low setting. Flipping the chip changes the head angle between 65 and 65.5 degrees, seat-tube angle between 76 and 76.5 degrees and raises or lowers the bottom bracket by 7mm.

High / low S1 S2 S3 S4 S5 S6
Stack (mm) 610 / 614 609 / 613 618 / 622 627 / 632 636 / 641 646 / 650
Reach (mm) 415 / 410 435 / 430 455 / 450 480 / 475 505 / 500 535 / 530
Head tube angle (degrees) 65.5 / 65 65.5 / 65 65.5 / 65 65.5 / 65 65.5 / 65 65.5 / 65
Bottom bracket height (mm) 335 / 238 340 / 333 340 / 333 340 / 333 340 / 333 340 / 333
Front centre (mm) 724 746 770 800 829 836
Chainstay length (mm) 432 432 432 432 442 442
Wheelbase (mm) 1,152 1,175 1,200 1,228 1,268 1,302
Top tube length (mm) 563 583 605 632 660 692
Standover height (mm) 730 734 744 749 757 757
Seat tube length (mm) 385 385 405 425 445 465
Seat tube angle (degrees) 76.5 / 76 76.5 / 76 76.5 / 76 76.5 / 76 76.5 / 76 76.5 / 76

Reach figures start at 410mm for the S1 in the low setting up to a lengthy 535mm S6 bike in the high setting. The wheelbase for a S1 starts at 1,152mm and extends to 1,302mm for the S6.

The chainstay lengths aren’t adjustable – unlike the Stumpjumper Evo – but the S1 to S4 bike’s chainstays are 432mm, while the S5 and S6 bikes are 442mm.

2021 Specialized Stumpjumper models and specifications

2021 Specialized Stumpjumper Expert trail mountain bike
It’s a shame Fox’s 36 isn’t offered on the standard Stumpjumper like it was on the outgoing model.
Ian Lean / Specialized

The 2021 Stumpjumper has six variants, four of which are made from the same FACT 11M carbon fibre main frame and seatstays as the range-topping S-Works, while the other two are alloy chassis bikes.

At the bottom end – retailing for £1,900 – is the Stumpjumper Alloy that’s fitted with SRAM’s SX Eagle drivetrain, a RockShox 35 Silver fork and X-Fusion 02 Pro RL rear shock, and TransX dropper post.

The range-topping S-Works Stumpjumper will set you back £8,750 and features SRAM’s XX1 Eagle AXS drivetrain and Fox Factory suspension along with Roval carbon wheels on DT Swiss hubs.

2021 Specialized S-Works Stumpjumper

2021 Specialized S-Works Stumpjumper
2021 Specialized S-Works Stumpjumper.
Specialized
  • Frame: FACT 11M carbon frame, SWAT integration, BB height adjustment, 130mm travel
  • Shock: Fox Float DPS Factory EVOL, Kashima, three-position adjustment with open mode adjustment
  • Fork: Fox Float 34 Factory GRIP2, Kashima, 44mm offset, 140mm travel
  • Groupset: SRAM XX1 Eagle AXS
  • Brakes: SRAM Guide G2 Ultimate
  • Wheels: Roval Traverse Carbon rims on DT Swiss 240 hubs
  • Bar/stem: Specialized Trail FACT Carbon / Deity Copperhead
  • Seatpost/saddle: RockShox Reverb AXS / Specialized Bridge
  • Cost: £8,750

2021 Specialized Stumpjumper Pro

2021 Specialized Stumpjumper Pro
2021 Specialized Stumpjumper Pro.
Specialized
  • Frame: FACT 11M carbon frame, SWAT integration, BB height adjustment, 130mm travel
  • Shock: Fox Float DPS Factory EVOL, Kashima, three-position adjustment with open mode adjustment
  • Fork: Fox Float 34 Factory GRIP2, Kashima, 44mm offset, 140mm travel
  • Groupset: SRAM X01 Eagle
  • Brakes: SRAM Guide G2 RSC
  • Wheels: Roval Traverse Carbon rims on DT Swiss 350 hubs
  • Bar/stem: Specialized Trail FACT Carbon / Deity Copperhead
  • Seatpost/saddle: Fox Transfer Factory / Specialized Bridge
  • Cost: £6,500

2021 Specialized Stumpjumper Expert

2021 Specialized Stumpjumper Expert
2021 Specialized Stumpjumper Expert.
Specialized
  • Frame: FACT 11M carbon frame, SWAT integration, BB height adjustment, 130mm travel
  • Shock: Fox Float DPS Performance Elite EVOL, three-position adjustment with open mode adjustment
  • Fork: Fox Float 34 Performance Elite GRIP2, 44mm offset, 140mm travel
  • Groupset: SRAM X01/GX Eagle mix
  • Brakes: SRAM Guide G2 RSC
  • Wheels: Roval Traverse Alloy on Roval hubs
  • Bar/stem: Specialized Trail alloy / Specialized Alloy Trail
  • Seatpost/saddle: X-Fusion Manic / Specialized Bridge
  • Cost: £4,750

2021 Specialized Stumpjumper Comp

2021 Specialized Stumpjumper Comp
2021 Specialized Stumpjumper Comp.
Specialized
  • Frame: FACT 11M carbon frame, SWAT integration, BB height adjustment, 130mm travel
  • Shock: Fox Float DPS Performance, rebound and three-position compression adjust
  • Fork: Fox Float 34 Rhythm GRIP, two-position sweep adjustment, 44mm offset, 140mm travel
  • Groupset: Shimano SLX 12-speed
  • Brakes: Shimano SLX M7120
  • Wheels: Roval 29 rims on Shimano hubs
  • Bar/stem: Specialized alloy / Specialized Alloy Trail
  • Seatpost/saddle: X-Fusion Manic / Specialized Bridge
  • Cost: £3,500

2021 Specialized Stumpjumper Comp Alloy

2021 Specialized Stumpjumper Comp Alloy
2021 Specialized Stumpjumper Comp Alloy.
Specialized
  • Frame: Alloy frame, BB height adjustment, 130mm travel
  • Shock: Fox Float DPS Performance, rebound and three-position compression adjust
  • Fork: Fox Float 34 Rhythm GRIP, two-position sweep adjustment, 44mm offset, 140mm travel
  • Groupset: SRAM NX Eagle
  • Brakes: SRAM G2 R
  • Wheels: Roval 29 rims on Specialized hubs
  • Bar/stem: Specialized alloy / Specialized Alloy Trail
  • Seatpost/saddle: X-Fusion Manic / Specialized Bridge
  • Cost: £2,500

2021 Specialized Stumpjumper Alloy

2021 Specialized Stumpjumper Alloy
2021 Specialized Stumpjumper Alloy.
Specialized
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  • Frame: Alloy frame, BB height adjustment, 130mm travel
  • Shock: X-Fusion 02 Pro RL, rebound adjust, lockout
  • Fork: RockShox 35 Silver, Solo Air, 44mm offset, 140mm travel
  • Groupset: SRAM SX Eagle
  • Brakes: Tektro Gemini Comp
  • Wheels: Specialized
  • Bar/stem: Specialized alloy / Specialized Alloy Trail
  • Seatpost/saddle: TranzX / Specialized Bridge
  • Cost: £1,900