Specialized Stumppumper Concept - Lightweight Pump Track Bike

Specialized recently unveiled an interesting in-house concept, the Stumppumper, a lightweight Dirt Jump / Pump Track mountain bike.

Designed and built by Specialized employee Jason Chamberlain, who claims he was inspired by the minimalism of BMX bikes, it's a combination of in-house parts with some tasty pieces from SRAM, Home Brewed and White Brothers designed to be the precision bike for the pump track. Chamberlain says "The Stumppumper is intended for advanced riders only - it's fast, and twitchy like an F1 car. It's ultralight so it gets up to speed quicker, but may not hold momentum as well as a heavier bike".

Chamberlain wanted to assemble the Stumppumper around a stock Specialized P.3 frame and marry it with a "Light, stiff build". While the Stumppumper's components are, for the most part, readily available to any consumer, Chamberlain has carefully chosen the bike's spec with its intended purpose in mind. The most obvious difference between this machine and a common dirt jump bike is the lack of front suspension, with the front of the concept bike sporting a White Brothers Rock Solid Carbon fork. The sub-900 gram fork employs 34mm diameter carbon legs that are bonded to an aluminium crown and dropouts. White Bothers offers two axle to crown length options to preserve the bike's handling, a short 425mm or 445mm length that simulates a suspension fork under sag, with Jason opting for the longer choice to keep the Stumppumper handling as intended.

The Specialized Renegade 2.3 Control Casing tires are inflated quite high for maximum rolling speed and matched to  a set of DT's EX500 rim, suitable for high pressures. A DT road hub is employed up front due to its wider spoke flanges to improve wheel stiffness, while a wide flanged single speed DT hub is used out back. Chamberlain even took the extra time to use brass washers a the elbows of the 1.6/1.8 spokes, a recommended step when using the smaller size spokes in standard 2.0 flange holes.

The bike's drivetrain is about as exotic as a single single speed can get, especially considering that it isn't intended to be pedalled much. A set of Specialized's BB30 compatible S-Works Carbon crank arms are fitted up in the previously mentioned offset postion to imitate a lower bottom bracket height, and a custom made Homebrewed Components 25 tooth chain ring is slid onto the crank's spline in place of the stock spider. Light and simple, just like the rest of the bike.

Form follows function, and the function of the P.3 Stumppumper is to be simple, light, and fast as hell. That's why you'll find a set of ESI's 50 gram Silicone non-locking grips on the Specialized Carbon Enduro handlebar, a favorite choice of riders in the know. The pairing of Avid's top of the line XX World Cup brake with a 140mm Shimano ICE rotor (the DT 240 Single Speed hub uses Center Lock rotor mounting) may seem underwhelming at first, but remember that the brake is likely only going to be used to slow down at the end of your run.

Chamberlain and Specialized are well aware that the Stumppumper is about as a specific tool for the job as one can get. "It's a bit of a tough sell because it is so different. I am certain that it is faster in the hands of a rider who can capitalize on the design, though," Chamberlain continues "However, most guys are going to be more comfortable with what they are used to".

Lets be honest,  it's unlikely such a bike would ever be made available as a complete package, though I would not discount a limited production run, after all it's becoming a common practice now in the auto industry and can be quite lucrative, so start pestering your local Specialised stocking bike shop next time you are there.

But that's not to say that a dedicated pump track rider couldn't build up his own version, given that both the P.3 frame and all of the components are production items. The chances of you seeing sub-20lb pump track bikes that use offset crank arms at your local track are pretty slim, but I wouldn't bet against seeing Troy Brosnan or Sam Hill aboard such a machine come Crankworx time.

Images: Ryan Cleek @ Pink Bike

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