To enjoy mountain biking’s golden age you need a bike that thrives in varied terrain and unlocks your potential on the trail. In the forty years since Specialized first introduced the Stumpjumper, They have ridden with all kinds of trail riders from all around the world in every terrain imaginable. And they have learned that “trail riding” can mean a lot of different things to a lot of different people.
To unleash the full potential of the full spectrum of trail riding, they created two Stumpjumper siblings: Stumpjumper and Stumpjumper EVO. Some of us find our flow railing endless ribbons of backcountry singletrack in a haze of endorphins. This is Stumjumper’s native habitat. For those who crave ever bigger hits and sketchier terrain, they created the Stumpjumper EVO.
The frame is the backbone of the bike. It’s the origin of control and efficiency and is the centerpiece of any serious weight saving effort.
Ready to build with all the fixin’s*, an S-4 Stumpjumper chassis is 2,420 grams; that’s damn light! Over 100 grams lighter than the previous Stumpjumper chassis.
To get to this weight — while improving handling characteristics, pedaling efficiency, and toughness — required the development team to pull out all the stops. The process started with frame shape, proceeded to composite selection, and finished with hundreds of iterations to optimise the layup.
The shape of a composite frame determines the vast majority of that frame’s weight, stiffness, and durability. After crunching countless Finite Element Analysis calculations, Specialized’s engineers optimised the dimensions and radii of the Stumpy’s frame tubes and their intersections, eliminating “lazy” material that would otherwise hide in unoptimised corners. This ensures that every fibre is delivering the most strength and stiffness it can.
Small adjustments in shape make big differences in how the material performs. A great example of this is the shock mount at the top tube. By keeping the top tube “intact” (rather than piercing and bending it to shroud the shock, like the previous Stumpjumper), it’s more structurally sound and therefore more effective at bearing a load. The flat sections of the shock mount likewise carries those loads more effectively than the curved shock mounts of the previous Stumpy. Together, this top tube/shock mount section is lighter AND stiffer than its predecessor, without sacrificing durability.
Another example of how shape optimises material use is the Sidearm design. The asymmetric Sidearm strut supports the shock link pivot, boosting stiffness by connecting rear end with front for a direct and integrated feel. The Sidearm also prevents torsional loads from entering the top tube, which helps the bike track and handle with precision.
The top tube is structurally optimised by retaining its tubular integrity, while the flat sections of the shock mount plates intelligently transfer loads to the robust sides of the top tube.
The Sidearm strut supports the shock link pivot, boosting stiffness by connecting rear end with front for a direct and integrated feel.
Shit happens out on the trail. Flats, unplanned dismounts, rainstorms. Just ask Murphy, he wrote the law. Luckily the Stumpjumper’s SWAT compartment will faithfully carry all the stuff you need to get out of most trailside tight spots. And even with that high-volume cavity in the downtube, Specialized engineers figured out how to distribute all the loads around the opening. The Stumpy still delivers the stiffness required for telepathic handling on the trail, while also providing heavy-duty strength at an unprecedented light weight.
Specialized’s Rider-First Engineered™ process delivers the ideal balance of tuned stiffness, weight, and ride quality from all six sizes. This ensures that every rider experiences the same ultimate trail ride characteristics, regardless of their size. This is achieved through extensive data acquisition of forces through every conceivable part of the frame in every frame size, they then validated that number crunching with countless hours of hard riding.
Through precise shaping of the seatstays and chainstays, along with utilization of appropriate composites, Specialized created a flex zone along the seatstay, above the brake mount, that serves up 130 mm of travel without any rear end pivot hardware or redundant material required for bonding pivot parts in the carbon.
The one-piece design improves torsional and lateral stiffness, which in turn enhances responsiveness and pedalling efficiency. These lateral and torsional stiffness gains are achieved by eliminating the flex intrinsic in mechanical pivots.
Specialized’s trail obsessed team of engineers stay awake at night thinking of how to make suspension work better. For the Stumpjumper, they tailored every aspect that contributes to final suspension characteristics — kinematics, leverage curves, damping, and spring rates — to work together in harmony to deliver a highly controlled, responsive, and precise ride that handles bump forces like a bike with far more travel.
The Stumpjumper has a very progressive leverage rate with a steep slope. This helps achieve a lively, snappy ride with generous utilisation of travel and a good platform to pedal against through the mid-stroke, as well as great bottoming resistance at the end of the stroke. These are exactly the characteristics you want for a mid-travel trail bike designed to see full-size trail duty.
To work in concert with the steep leverage ratio, they used a larger volume spring to provide a more gradual spring ramp. This spring rate enables more usable travel in the most common trail conditions, while still maintaining progression for good bottoming resistance.
The goal was excellent pedalling performance and mid-stroke pedaling support without sacrificing bump performance. To achieve this, they used Fox’s Digressive piston to add low-speed compression without overproducing on high shaft speeds normally associated with square edge bumps. They tuned the piston very light in order to give just enough support while remaining active and excelling in rough terrain.
The new Stumpjumper geometry contributes to a level of control and efficiency never before found from a mid-travel trail bike. How? The generous cockpit gives you the room you need — centering you on the bike and improving traction and corner control. The low bottom bracket, slack headtube angle and reduced fork offset keep things stable in the rough. A steep seat tube optimises power output and puts your weight over the front while climbing. And short chainstays keep things flickable and nimble.
To hone handling to your style and terrain, the FlipChip at the rear shock eye allows you to change bottom bracket height by 7mm and headtube angle by one-half degree.
The Stumpjumper’s S-Sizing is based on what matters: rider size and style, not inseam. Six sizes, all with similar headtube lengths and standover, allow you to choose the size that best suits your individual style. Smaller S-Size numbers are more nimble, thanks to their shorter reach and front center measurement, while bigger S-Sizes deliver more stability and a roomier ride.
Determining your S-Size is easy. If you rode a Medium before, then S-3 will be your equivalent size. But if you want a more nimble ride, you should drop down to an S2. Want a bike that’s more stable at speed? Bump up to an S4. Following are examples of three riders, all 5’8” (1.73m), who chose three different sizes.
Creating the ultimate trail bike out of costly materials and utilising processes typically reserved for space exploration was pretty hard. Bringing that unprecedented performance to as many riders as possible was even tougher. But Specialized did exactly that with our their alloy Stumpjumpers, delivering the same great geo, Rx-Tuned suspension, ride quality, and S-sizing as their carbon siblings, but at much more attainable price points.
Being tasked with creating the rippingest trail bike we’ve ever made is no small feat. Steve Saletnik, Matthias Segerer, Chance Ferro, Kayla Clarot, Luke Beemer, Fred Chiu, and more than a dozen others dug into every aspect of handling, stiffness, and suspension kinematics to do exactly that.
For more information and specs visit: https://www.specialized.com/za/en