Specialized Stumppumper Concept Bike - The Ultimate Pump Track Weapon

First Look

Specialized
Stumppumper
Concept Bike

WORDS Mike Levy

The popularity of pump tracks has skyrocketed over the last few years, with more and more of them popping up in not only backyards, but also at major events around the world. While banging out laps on a pump track is a great time, it doesn't take much for it to turn into a competition of sorts; inevitably the stop watches come out and riders are battling each other for the fast lap times. That, of course, quickly led to ever advancing pump tracks that grew is size and difficulty. What hasn't evolved much, though, is the bikes being used. Show up at any track, be it a local hidden gem or a world class monster like the one at last year's Kokanee Crankworx, and you'll likely see the majority of riders on their dirt jump bikes. But what if someone put some time and resources towards assembling a purpose built pump track weapon that is designed solely to rail berms and pump rollers? Specialized's Jason Chamberlain did just that when he set out to build the ultimate pump track bike. Behold the Stumppumper.


Focused intention: Chamberlain is well aware that the Stumppumper concept is a single minded approach to a niche aspect of our sport, but that didn't stop him from exploring the idea of a purpose built pump track bike. It's from exercises like these that he and Specialized learn about what works and what doesn't, and Chamberlain knows that what works on a pump track is the BMX approach. "It occurred to me that most pump track guys simply ride dirt jump bikes that are far from optimal," Chamberlain explains "If you want to go fast, then you need to look at BMX bikes, so that is where I drew inspiration. Rigid, stiff wheels, and light weight". The finished bike, weighing in at just 17.9lbs and using a rigid carbon fiber fork, reflects the single minded approach to looking for the fastest possible lap time. No, this is not going to be a bike for everyone.


Offset cranks: Carrying speed through a pump track's rollers and berms is key to a fast time, a feat that is always going to be easier to do on a bike with a low center of gravity. A ground scraping bottom bracket height is crucial to having a low CoG, but you'll never find an extremely low BB height on a bike that has been designed to excel at other types of riding. For example, a dirt jump bike will typically spin easier in the air if it uses a slightly higher BB, while a true mountain bike must also take pedalling clearance into consideration - neither fact making for an ideal, super low BB pump track setup. Chamberlain wanted to assemble the Stumppumper around a stock Specialized P.3 frame that uses 26'' wheels, but that would mean that the bike would use a rather mundane 301mm (11.8'') bottom bracket height.

The answer came to him in the form of installing the crank arms in an offset configuration, effectively lowering the bike's bottom bracket height by 22mm. The crank arms literally hanging down, or droop, from the BB spindle when at the 3 and 9 o'clock position. The result is an effective bottom bracket height of just 279mm (10.9''), nearly a full inch lower than the stock number. While the obvious benefit is the lower CoG, the offset configuration should also help to keep the arms from wanting to rotate as the rider makes their way around the track, likely relieving fatigue. Chamberlain claims that another advantage is the "balancing effect that the offset cranks have on your left and right legs, which stabilizes your body under high cornering Gs." While it may sound odd at first to run such a setup, one has to remember the Stumppumper is intended to spend its life on a pump track and nowhere else. "There is no drawback since tracks are smooth and you aren't turning your cranks - you don't risk clipping a pedal," Chamberlain explains when I questioned him as to if the Stumppumper's drooped cranks give it an odd sensation when pedalling. The drooped crank arms mean that they have to be set up for either left or right foot forward riders, otherwise the cranks will be angled up, defeating the purpose of the offset layout.

Specifications Release Date 2012 Price Fork White Brothers Rock Solid Carbon Fork Crankarms Specialized Carbon BB30 w/ Home Brewed Components custom 25t chainring Bottom Bracket Zipp Ceramic BB adapter cups Pedals Specialized Handlebar Specialized Carbon Enduro Grips ESI Silicone Foam Brakes Avid XX World Cup w/Shimano ICE 140mm rotor (rear only) Hubs DT road front hub (wide flanges), DT Single Speed rear hub (wide flanges) Spokes 32x 1.6/1.8 spokes with spoke head washers (preferred with 1.8 elbows) Rim DT EX500 Tires Specialized Renegade 2.3, Control Casing Compare to other DJ/Street/Park Bikes

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 aluminum 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.

After talking to Chamberlain about the project earlier in the season we had fully expected the finished bike to come complete with a carbon rimmed wheelset, but that isn't the case. The Specialized Renegade 2.3 Control Casing tires are inflated quite high for maximum rolling speed, so he chose to build the wheels with a set of DT's EX500 rims because they are more suitable for high pressures. A DT road hub is employed up front due to its wider spoke flanges that should 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".

In short, it's unlikely such a bike would ever be made available as a complete package. 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.

www.specialized.com
Photos by Ryan Cleek