Northwest Arkansas brothers build the world’s lightest dirt jumper.

Story and Photography by Kai Caddy

W hen you drop a couple of young men with a strong background in engineering, design and building into the cycling paradise that is Northwest Arkansas, you get the world’s lightest BMX dirt jumper.

Brothers Karl (20) and Paul (18) Zeunges built a titanium BMX dirt jumper from scratch that weighs in at just 16.7 pounds. The frame itself is 3.9 pounds. They can’t find any proof that anyone has built a lighter dirt jumper.

“For a while, we’ve been obsessed with building pretty crazy projects,” Karl said. “We’ve done some others in the past. An electric monowheel is one of our bigger ones. And we were part of flugtag.”

The dirt jumper wasn’t their original idea.

“We originally started with wanting to make almost like a mini downhill bike,” Paul said. “And then we realized that full suspension would not be a good first build option.”

The brothers enjoy riding at places like the Railyard Bike Park in Rogers. The Railyard features curved walls, dirt rollers, berms, jumps, bridges, a rail car – the perfect playground for a BMX dirt jumper.

But, initially, lightweight wasn’t even the goal. As the project started and the brothers started looking at parts, they realized they could try and make it as light as possible.

“It was mostly just the idea of what would a superlight bike ride like,” Paul said. “Because there was no one out there able to explain what it’s like.”

The Zeunges are no strangers to the engineering world. Their father is a math teacher who runs a STEM program and their grandfather is a retired machinist.

Design inspiration came when they saw a video of U.K. rider Matt Jones building a lightweight Marin Alcatraz. Later they incorporated some design ideas from the Santa Cruz Jackal as well. Armed with sketches and the idea to build the lightest bike possible, they attended the Bentonville Bike Fest to see what riders thought of the idea.

“Everybody was kind of shooting us down,” Karl said. “They said it would never work. Our target was 15 pounds. Because it seemed so unrealistic. A lot of the BMX guys thought that it wouldn’t work and the mountain bikers didn’t really know what to think. They didn’t like the idea. They said it would be too light and everything. And we just wanted to try it.”

Paul explained that the riders thought the bike would move around too much in the air.

“But we thought it would be more agile,” Karl said. “We asked them, ‘Well, have you ridden a bike that light’ and the answer was always no.”

Paul said, “We’ve had people riding the bike who are a lot better than us. And they can whip the bike past 90 degrees – really far – almost effortless. A big problem with it is it’s so light that you kind of just go up off the ground before you usually do. And most people aren’t used to that. So it takes a few minutes of riding to get used to the bike.”

After the sketches, the brothers took the design into CAD, and 3D printed parts to get the frame’s angles and then hand-mitered all the tubes.

The brothers did most of the work themselves. They only needed help from a welder; their grandfather; local frame builder Ryan Johnson did some reaming for them; a tube bender; and a machinist at 5DEV mountain bike components.

“We combined three hard things,” Karl said. “We have a tapered head tube, we’ve got a bent down tube. And then we’ve got a top tube that meets the down tube. Not only is the mitering difficult, but the weld is actually very difficult to get in that very tight angle.”

The bike’s seat post is also part of the frame and the saddle is permanently affixed to the post.

“We thought that seat post designs are so universal, that they become very clumsy and kind of clunky,” Karl said. “They have too many parts. I don’t even know how many parts it takes to put a seat on. And we just have one piece. We wanted to keep it simple and elegant. And we knew we could save a lot of weight there. And some of that came down to money, too, because we could save money by doing that.”

Karl said the project ended up costing around $3 per gram. And the pair was eager to save weight without having to spend money to do it. They shaved off the brake mounts on their front fork, drilled holes in parts and even 3D printed a top cap for their fork because they could save a few grams over the stock cap.

“It’s a lot easier than it sounds,” Karl said. “It’s just a matter of getting parts that are light. A lot of it is just shopping, really.”