When Karl Groetzinger set out to design his first fully built frame design, he had one goal in mind - make it different from everything out there. Karl first began designing bikes 10 years ago, but didn't have the facilities to build them. Now, as a 26-year-old, he's finished his degree in mechanical engineering at the University of Stuttgart, taught himself aluminum welding, and has been able to make his visions come to life.
All MountainIntended Travel
140-150mm forkWheel Size:
27.5" front/rearFrame Size:
One-off frame, not for sale.More info: @karlgroetzinger
While most manufacturers use metal tubes for the chainstays and join these to a gearbox housing with a weld, Karl uses folded sheet metal all the way along the chainstay and into the BB/mount for the gearbox.
The first question anyone will surely ask is, "Won't that break?" Well, that was the big challenge for Karl; if he was going to make something unique he was going to make sure it worked too. By using sheet metal to integrate the Pinion box, he was able to spread the loads and not have a weak join between the stays and the BB area. Alongside this unique construction method, he's also making sure the area around the gearbox and the chainstays have plenty of metal keeping everything pointing the way it should be. The folded metal cuboids he has used for the stays are 50x20mm in profile plus have a 3mm wall thickness thick, which is about twice the thickness of conventional aluminum tubes used on mountain bike chainstays.
To prove his design, Karl has been using a frame-mounted strain gauge and a 3D scanner to measure how the lack of a seat stay affects the bike. He has concluded that there are "only slight plastic deformations" but this is something he intends to remedy in the next model by using an even stronger 7000 series aluminum alloy, which has an almost 2 times higher yield stress. Also on the next version will be a belt drive instead of a chain and potentially a gusset between the seat tube and top tube.
The attention to detail is incredible, including machining these brake mounts after welding to ensure they were perfectly aligned.
Karl is using a strain gauge with measuring sensors on the cantilever to learn more about his design and how it can be improved for version 2.
So what will the disadvantages of this design? Well, the extra material needed to reinforce the chainstays doesn't come without a weight penalty. The frame currently weighs 2.5 kg (5.5 lb), which puts it about 200 grams (0.5 lb) heavier than, for example, Nicolai's Argon AM Pinion. The full прототип build Karl has selected here tips the scales at around 15kg (33lb). We're also not yet sure how the design will react to torsional forces as the bike is cornered hard. Really though, none of that matters. This wasn't a performance-driven design - Karl set out to create a bike, unlike any that had been seen before and it only takes one look to know that he has succeeded.
Karl is gaining further insight on the physics of the bike from a GOM 3D scanner.
The bike is currently a one-off design exercise although Karl has been surprised at the amount of interest and could consider making a production run if it continues. He currently has designed the frame around his own large proportions with the intention of fitting it with a 140-150mm fork and using it as an all-mountain bike. No geometry info is currently available. The best way to keep up with the project is to follow Karl on Instagram