Specialized StumpPumper lightweight dirt jump mountain bike concept
Specialized StumpPumper concept pump track bike. Photo ©Ryan Cleek/Specialized

Spotted over at PinkBike, Specialized’s Jason Chamberlain has been working on a lightweight, dedicated pump track machine.

Dubbed StumpPumper and inspired by BMX bikes, it’s built specifically for the small jumps, rollers and berms of pump tracks. That means it has a lower center of gravity, no suspension and tight, twitchy handling meant for experienced riders. Beyond the geometry, the most interesting part is the offset crank installation. Rather than put them in perfectly opposite each other, they’re installed such that the rear one is angled down slightly, giving them about a 170° angle. This means you’ve gotta decide which leg will run in the front and commit to it, but if you’ve ever ridden a pump track, you’ll know pretty quickly which way you feel more comfortable. Even without offset pedals, it usually feels more natural one way or the other, but this bike’s set up does mean you won’t want to switch it up. The benefit? An effective BB height of just 10.9″ and a bike that comes in at 17.9lbs.

It’s not likely you’ll see the StumpPumper on a showroom floor, but if this gets you excited, check PB’s full post for spec list and details.

UPDATED! Pump on past the break for comments from Jason Chamberlain

“The thing we might want to emphasize is that this is a one-off. It’s a “concept”. There are no plans, nor were there ever plans, to build a production version for sale. It is far too niche and far too expensive. It is just an experiment. If anything, some version of this bike will show up under our top athletes at some high profile pump track events.

“Why didn’t I build a custom frame too? There were not any more gains in the frame to be had. The P geo is perfect and 17 lbs is plenty light. The things I really wanted to explore was the slammed offset cranks, rigid but light wheels and rigid fork. There is a lot of skepticism over the fork, which I expected. On a smooth pump track, you really don’t need a suspension fork. Any small bumps can be handled with tire pressure and keep the fork rigid for better response and less energy lost when pumping. Damping in a fork is designed to deliberately absorb and dissipate the energy of bumps. This is good a good thing if you are riding in the rough, but it’s the opposite of what you want on a smooth pumptrack were all your speed in generated by pressing and pumping. Hence the BMX analogy – suspension forks scrub speed and energy in this application.

“However, a person will be fastest on what they are comfortable on. Even if they would be faster on paper and in theory with this setup, they may have mental reservations and that translates into reality. It’s not a bike for everyone, but it’s a bike I recommend trying if your goal is to go really fast on a pump track.”


  1. Putting different parts on something isn’t creating a new bike, or news worthy IMO. This is just like that fat tire trials bike a little while ago…it was just a pugsley with low gearing. this is just a dirt jump bike with a carbon fork and cranks installed wrong.

  2. if the engineer wasn’t so lazy, he could have just made a frame with an actually lower bottom bracket and installed the cranks normally. but that wouldnt be very “newsworthy”.

  3. you call can knock Jason, but keep this in mind…your job probably isn’t as cool, he gets paid to go to tinker at Specialized everyday, what do you do?…

  4. If the BB were lowered it would not be the same effect as the cranks “hanging” from the BB shell. Imagine if both crank arms were at 6:00 at the bottom of the stroke. There would be no forces trying to rotate the crank. Offsetting the arms starts to approach this effect and reduces inequalities of force between your left and right legs, reducing fatigue and making it more stable. This is particularly beneficial under the high G’s. Ever notice how quickly you get tired on a pump track? Do you notice one legs burning sooner than the other? This helps.

  5. Just because engineering software says offsetting the cranks 10 degrees shows a reduction of inequity in force, doesn’t mean it is better. It’s a pump track! You are supposed to be tired after a session.

  6. Besides, if you were going to build a true pumptrack specific bike…why even have a drivetrain? You don’t need to pedal at the pump track. It would save even more weight.

  7. Aside from Ti pedal spindles, anyone could build pretty much the same bike.

    The drivetrain is sort of required to get started, though there are some rumblings that certain comps might mandate chainless to eliminate cheater pedaling.

  8. It’s amazing to me how many people feel the need to bag on something they don’t understand. I used to work with Jason Chamberlain and he is one of the most creative and talented people I have ever met.

    He has tons of projects like this that allow him to test out his theories in the real world. Pinkbike probably just noticed this bike sitting in a corner and started to ask questions about it. Jason is the kind of guy that will explain something for hours until you understand his concepts. He’ll even build a model out of Lego to illustrate his ideas to you.

    All the man did was take some off the shelf readily available parts and built a concept bike to test an idea.

    I don’t understand why the majority of people on these blogs and forums feel the need to critique and sh*t on everything that they don’t like.Get over yourselves, none of us are that important and our opinions are just opinions that don’t mean jack.

  9. I know right? Though I do find I take some sick pleasure in reading all these silly vitriolic options. I also find myself wondering what I could slip into the comment thread to really get folks going. Maybe something about Mike Sinyard sourcing carbon frames from a secret lunar colony or Obama pushing legislation allowing poor people to use food stamps to buy bikes…

  10. @Al – I couldn’t agree with you more BUT, some of these are just too damned good. (“What’s the name of the woman’s version…….”? and “Volagi’s FlowHumper”……?) I hate it when I see comments in newspapers concerning an interesting article that are just insensitive and vitriolic dribble…. people who are spouting off just to see themselves in print, but we’re talking about bikes here…. let’s have some fun. And a lot of the points made are justified. I was tempted to rip the Raleigh in another article…. but I couldn’t think of anything really clever to say. – C

  11. I love this. Not something I would likely ride but I love to see new concepts and tinkering like this. The crank orientation is a great idea for this application!

  12. The negative comments are natural to any internet forum. Look, slapping off-the-shelf parts together and tweaking them as a concept is how we get new ideas and new designs. Yes, they are available, but did any of you guys put them together and come up with it? By slapping drop bars on a Transition started the process of developing the Venge.

    All manufacturers do this. If it’s worth pursuing, they’d move on to further development and begin to manufacture/machine new parts and possibly move it into production.

  13. Oh look what we have here. Another pumptrack bike from one of the biketaiwan.com clan of industries. Punched out of a cookie cutter mold, slap on some brand name stickers and stick the customer for whatever you think you can get for it.

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