Specialized Stumpjumper 15 is “most capable and versatile yet” thanks to new Fox GENIE shock

Specialized claims the new 15th iteration of the full-suspension Stumpjumper is the “one bike to rule all trails”.

The updated bike’s silhouette looks familiar, clearly taking design cues from the electrified Levo SL, which had a makeover in 2023. Like that bike, the new Stumpjumper does away with the sidearm in the front triangle, but there’s far more at play here.

Take a closer look at the in-line shock and you’ll see things are a little different. That’s because Specialized has co-developed a shock with Fox, dubbed the GENIE.

Specialized says this shock, along with other changes to the frame, enabled it to create the most versatile, capable Stumpjumper yet. In fact, it has gone as far as ditching the EVO version of the bike, which I’m sure we can all agree, is a bold move.

Robin Weaver has ridden the new Stumpjumper 15 and you can read his first ride review.

Five things you need to know about the new Specialized Stumpjumper 15

New GENIE shock uses two positive air-volume chambers, which are both tuneable, enabling you to dial in how the 145mm of rear-wheel travel is delivered
The latest Stumpjumper offers three headset cups, enabling you to alter the head angle from 63 to 65.5 degrees
A chip in the Horst-pivots enables you to alter the bottom bracket height by 7mm
All sizes have taller stack heights
Largest S5 and S6 size frames get 445mm chainstays

Specialized Stumpjumper 15 frame and suspension details

A revised SWAT compartment is apparently easier to use and offers better sealing against the elements. – Justin Sullivan / Our Media

Specialized is initially only offering the Stumpjumper 15 with its FACT 11M carbon fibre.

According to the spec sheet, it’s only the S-Works model that’s treated to a carbon link, though.

At the bike’s heart sits the new GENIE shock. This is a big part of what promises to make the new Stumpjumper more capable than its predecessor.

GENIE in a bottle

Specialized Stumpjumper 15 full suspension mountain bike — Two positive air chambers are connected to increase overall air volume. – Specialized

When Specialized set out to design a new Stumpjumper, one of the goals was to create a bike with a longer-travel feel, yet one that still feels poppy and lively on the trail.

To achieve this, the brand considered the level of progression designed into the frame and created by the rear shock. In a bid to limit compromise along the way, the ride dynamics team at the ‘Big S’ set about experimenting with rear shocks, ultimately creating a mutant design of its own.

The end result is the GENIE, which has a two-stage tuneable air spring that’s said to improve traction without sacrificing any of the big-hit control later into the travel.

Inside, you’ll find two connected positive air chambers, which increase overall spring volume.

The idea was to create a flatter, less progressive spring curve for the first 70 per cent of the Stumpjumper’s travel. This is said to provide a suppler, smoother coil-like shock feel, enabling the bike to find greater traction and provide a more comfortable ride.

However, as you close in on the final 30 per cent of the travel, air ports to the outer air chamber (the XL shock sleeve) are closed off (by the ‘GENIE band’, which is designed to slide over and blank them off).

Specialized Stumpjumper 15 Pro full suspension mountain bike — At the heart of the new bike is the new Fox GENIE shock. – Justin Sullivan / Our Media

This reduces the spring volume (you’re now relying on the smaller, inner positive air chamber) and effectively ups the spring rate, creating a smooth, progressive ramp-up as you close in on the final part of the Stumpjumper’s 145mm of travel.

Both the positive air chambers are tuneable. The outer sleeve accepts snap-together band spacers, similar to those that fit the Fox Float X2 shock, while the second chamber uses eyelet spacers as you’d find inside a Float X shock.

Both sets of spacers are unique to the Stumpjumper/Fox GENIE shock and a full complement will come with each bike.

That sounds good, but you’re expecting us to tell you the new GENIE shock is some quirky, proprietary size that’ll be a headache down the line, right?

Thankfully, Specialized has kept things as simple as possible, opting for a standard shock size (210x55mm), along with a standard Fox damper to help with ease of maintenance. Alternatively, you can easily replace the shock with an alternative of your choosing.

This shock, designed to limit compromise and be the best of both worlds, uses an in-line design and forgoes a piggyback reservoir.

In back-to-back testing, although the in-line GENIE shock got hotter than its piggyback equivalent, despite this additional heat build-up, the GENIE didn’t lose out in terms of performance, according to Specialized. This is why the brand chose to use it.

Compare and contrast

Specialized was keen to share some comparative stats with us to highlight why the new GENIE shock is better than the standard equivalent.

First up, it claims the GENIE shock provides 16.3 per cent better bump-force management. But what does that mean?

According to Specialized, if you smash the rear wheel of the old Stumpjumper EVO, equipped with a Fox Float X shock, and the new Stumpjumper 15 with the GENIE shock into the same-size bump with the same amount of force, the Stumpjumper 15 will use more of its travel and behave more like the long-travel Enduro.

You could achieve a similar result by running a lower pressure in the shock on the older bike, but it couldn’t then match the support or end-stroke ramp-up of the new bike in quite the same way.

Next, Specialized says when comparing the new GENIE shock to a standard shock designed to be as progressive, there’s 57 per cent less “traction loss time”. This is measured using the rotational speeds of the front and rear wheels under braking. That means the overall amount of time the rear wheel slips relative to the front, taken from 82 runs down the hill.

Then there’s “traction loss intensity”, which highlights the differences in wheel speed between the front and rear when a slip occurs. Specialized says the GENIE shock helps reduce traction loss intensity by 27 per cent.

In simple terms, this should mean the supple nature of the GENIE shock enables the rear wheel to move freely enough to keep the tyre in contact with the dirt more consistently through the bumps, preventing it from sliding out.

Another metric Specialized was keen to share was based around severe bottom-out events when you use all of the available travel during a big impact. With the GENIE and standard shock set to the same sag level using the same-sized volume spacer, Specialized claims the GENIE suffered 39 per cent fewer severe bottom-out events over 41 laps.

It also says there was 11.3 per cent less bottom-out event time, suggesting that when the GENIE does bottom out, it’s not as intense as it might be on a regular shock.

Of course, bottoming out a shock is common and not the end of the world, but do it time and time again aggressively and you’ll likely damage the shock, as well as throwing your control of the bike off when these events occur. Tighter control over this, while still having a sensitive initial part of the travel, is the goal here.

Other stats include 10.5 per cent less chassis pitching with the GENIE shock and 11 per cent less “uncomfortable vibration exposure”.

Goodbye EVO and sidearm

Male rider in orange top riding the Specialized 2019 Stumpy EVO — The Stumpy EVO (pictured) has been consigned to the history books. – Russell Burton / Our Media

At present, the Stumpjumper 15 is only available in Specialized’s FACT 11M carbon fibre.

There’s also no Stumpjumper EVO available. In short, the EVO has been consigned to the history books, largely down to how capable and versatile the new Stumpjumper 15 is designed to be.

Other frame news worth noting is Specialized has ditched the asymmetrical front-triangle design, which included a sidearm bridging between the top tube and seat tube on the driveside of the bike. Losing this freed up more space, buying the engineers valuable real estate to fit a chunkier shock and more space for a bottle, among other things.

Specialized says that although the sidearm has gone, the new Stumpjumper 15 is stiffer and just as strong as its predecessor.

The engineers have also worked hard to improve the amount of seatpost insertion available, so riders of all heights are able to run the longest-drop post possible. Depending on the frame size, the new Stumpjumper 15 features 50-70mm more post insertion, which is significant.

Of course, we can’t overlook the suspension platform. After trying a wide variety of suspension designs (we’re told just about every option was explored), Specialized settled on the tried-and-trusted FSR, four-bar Horst-link design. It opted for this approach because it’s simple, uses an efficient structure and could be tuned to the brand’s requirements.

The anti-squat has also been tweaked, leaving it lower than that of the Stumpjumper EVO and higher than the old Stumpjumper, all in a bid to balance suspension performance with efficiency. That means at around the sag point, anti-squat will be just over 100 per cent, dropping off steadily as the bike makes its way through its travel.

And, if you’re a fan of wedging kit, tools, clothing or even food inside your frame, the SWAT 4.0 down tube storage is designed to make your life easier. It’s now apparently sleeker, using a short lever to open and lock it, as well as being better sealed against the elements.

Specialized Stumpjumper 15 geometry details

While the EVO might now be officially dead, the latest Stumpy inherits a lot of its adjustment – as well as its angles. So, while the EVO isn’t around anymore, it still sort of is.

There are three different-angled headset cups to choose from, each offering one degree of adjustment (the middle setting is 64.5 degrees). Flip chips in the Horst pivot enable you to alter bottom-bracket height by 7mm (it’s 337mm in the high position). In the low bottom bracket setting with the slackest headset cup, the head angle is a highly relaxed 63 degrees.

The inspiration from the outgoing EVO doesn’t end there, though. While steeper than the current Stumpjumper, the latest bike’s seat tube angle is pretty much identical to that of the EVO, at 77 degrees. This should help promote an efficient seated position from which to attack the climbs.

Specialized Stumpjumper 15 — A flip chip at the Horst-link pivot enables you to alter the bottom bracket height by 7mm. – Justin Sullivan / Our Media

Specialized will only offer the S1 and S2 frames with mixed-wheel builds. Sizes S3-S6 use 29in wheels at the front and rear, but should you wish to downsize the back wheel, Specialized will have aftermarket links for sale that will enable this without disrupting the geometry.

With the two smaller sizes rolling on a smaller rear wheel, chainstay lengths are a little more compact, at 430mm and 432mm. The S3 and S4 sizes (both on 29in wheels front and rear) use a 435mm chainstay, while the S5 and S6 boast longer 445mm stays to better help position the rider between the wheels.

Compared to the regular Stumpjumper and the Stumpjumper EVO, the new bike has taller stack heights, with the biggest jump up coming on the larger frame sizes.

	S1	S2	S3	S4	S5	S6
Seat tube angle (degrees)	77	76.5	77	76.9	77.3	77.6
Head tube angle (degrees) mid-pos	64.5	64.5	64.5	64.5	64.5	64.5
Chainstay (mm)	430	432	435	435	445	445
Front centre (mm)	720	751	780	812	843	879
Seat tube (mm)	385	385	405	425	445	465
Top tube (mm)	541	573	595	624	647	677
Head tube (mm)	95	100	110	125	140	155
Fork offset (mm)	44	44	44	44	44	44
Trail (mm)	129	130	130	130	130	130
Bottom bracket drop (mm)	41	38	38	38	38	38
Bottom bracket height (mm) – high	334	337	337	337	337	337
Wheelbase (mm)	1149	1181	1213	1244	1285	1322
Standover (mm)	738	751	745	745	745	751
Stack (mm)	608	618	627	640	654	667
Reach (mm)	400	425	450	475	500	530
Fork length (mm)	551	561	561	561	561	561
Seat post max insertion (mm)	245	245	255	255	285	285
Seat post min insertion (mm)	80	80	80	80	80	80

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Specialized Stumpjumper 15 range overview

While Specialized has seemingly killed off the Stumpjumper EVO, there will be a coil-shock equipped version with a longer-travel 160mm fork up front. This uses a smaller 27.5in rear wheel across all frame sizes.

All Stumpjumper 15 bikes will come with the Fox GENIE rear shock, along with SRAM Maven brakes (which certainly gives an indication of how capable the Stumpjumper is), with the exception of the Öhlins-equipped bike. This gets TRP DH-R EVO brakes instead.

Key models also sport the new Roval Traverse SL II wheels. The new rims are reported to provide 21 per cent more vertical compliance thanks to their revised shape.

Other stats include a 15 and 25 per cent jump in front and rear-wheel strength respectively, as well as being said to better resist pinch flats thanks to the new bead profile.