Short-travel trail bikes have long cornered the market on lightweight and snappy climbing, but also often fall short when it comes to suspension performance and big terrain capability. The new Stumpjumper tears up that old script — we’ve never made an all-round trail bike this light, efficient, AND capable. The new Stumpy brings a telepathic, magic carpet quality to your ride. It’s planted and weightless at the same time. Stable, yet nimble.
To make the Stumpy the ultimate singletrack flow finder, we combined the lightweight, efficient DNA from our Epic EVO with the benchmark handling and capability of the Stumpjumper EVO. The result is a well-balanced performer that climbs and accelerates like it has a motor, descends with conviction, and flat out tears through every conceivable trail condition.
THE BACKBONE OF THE RIDE
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 2,240 grams; that’s damn light! Over 100 grams lighter than the previous Stumpjumper chassis.
To get to this weight — while improving handling characteristics, pedalling 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 optimize the layup.
*Including paint, shock, all assembly hardware, link, carbon extension, molded downtube and chainstay protectors, axle, and seat clamp.
HEY LAZY CARBON, BEAT IT!
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, our engineers optimized the dimensions and radii of the Stumpy’s frame tubes and their intersections, eliminating “lazy” material that would otherwise hide in unoptimized 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 optimizes 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.
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, our 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.
THE MISSING LINK
The pivotless seatstay/chainstay of the Stumpjumper leverages everything we’ve learned about building lightweight, full suspension trail bikes (something we’ve been doing since ‘93). A closer look at the rear dropout area revealed that there was no mechanical pivot. By applying our expertise in composites, we were able to ditch 55-grams of hardware, enhance lateral chassis stiffness, reduce maintenance, and precisely match suspension kinematics and chassis stiffness. In short, it’s the lightest, most efficient, control enhancing suspension solution for a mid-travel trail bike we’ve ever made. The elegance of this solution is fully realized and appreciated on the trail.
Through precise shaping of the seatstays and chainstays, along with utilization of appropriate composites, we created a flex zone along the seatstay, above the brake mount, that serves up 130mm of travel without any rear end pivot hardware or redundant material required for bonding pivot parts in the carbon.
GOODBYE PIVOT, HELLO EFFICIENCY
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.