MTBMX Frame: World's Shortest Chainstays

  • Finished product
  • MTBMX frame with world's shortest chainstays designed and produced over a summer funded by an $800 grant I was awarded by my highschool. 2012
     
    My high school has a summer grant program, where they will sponsor a personal project if approved by the Parents Association. Summer of 2012 I applied, and was awarded $800 to create a custom bike frame. I designed an innovative bike frame and worked with a welder in Texas to translate my designs into applicable metal work. The bike frame came out extremely well and is highly regarded within the small MTBMX biking community, as it has the shortest chain-stays in the world, and a unique low stand-over with tight geometry. There is an 80 page forum online where people all around the world commented on the design and build process, expressing their mostly favorable opinions. MTBMX is a progressive and creative sport that allows the rider to express themselves through their style, and line choice. The stunts and tricks preformed require a lot of patience, perseverance and bravery to learn and execute. I enjoy the sport because it allows me to be creative in the ways I ride, and offers me a place to vent and get some exercise. The frames are designed to be lightweight, strong and nimble to allow complex tricks/maneuvers to be performed.
     
    As I became a better rider, I began to develop my own concept of the optimal features and dimensions of a bike frame. I wanted to design a frame that was more purpose-built with inspiration from BMX and trials bikes. I don't mean to say that other frames are inadequate, but I felt the need to push the boundaries and actually make something different. I wanted a frame with the tightest geometry physically allowed and still keep a 26-inch wheel. I researched bike geometry, ergonomics and leverage ratios to teach myself the necessary information to create my dream frame. I studied the evolution of bike frames (designed for MTBMX/BMX) and discovered a trend. The chain-stay measurement, the distance from the rear axel to the pivot point of the cranks, was getting shorter and shorter. Put simply, that measurement, in conjunction with others, dictates how nimble the bike can feel. A common MTBMX frame has chain-stays that are roughly 15in long, whereas my frame’s chain stays are 13.6in. My frame’s rear end is as short as (or shorter than) most BMX bikes. It maintains all of the advantages of a 26in wheel (rolling resistance, roll speed, comfort, component options etc.), but with the agility of a 20in wheeled BMX bike.
     
    The design of the frame is rather unconventional. In order to get the chain stays as short as I wanted them to be, the rear tire would have to physically enter the bottom bracket shell, where the bottom bracket bearings and crank spindle are seated. To achieve this, I designed a bottom bracket shell with a sealed cutout, with Spanish press in bearings to get rid of traditional deep-set threaded cup-inserts. Along with the custom bottom bracket shell, the entire rear end had to be reengineered to maintain stiffness and strength due to the mandatory deletion of the chain stay brace, which normally is located between the bottom bracket and the rear tire. The end product is very successful. It’s like nothing I have ever ridden before. With nearly slammed (the wheel is pulled back slightly to maintain proper chain tension) 13.7in chain stays and a short seat tube, the back end is extremely maneuverable. That, coupled with a high bottom bracket, and a lightweight rear wheel, allows the back to lift up extremely high with minimal effort. I mainly attribute this to the extremely low seat-tube. I can suck the bike up higher, without the physical limitation of a high seat. The bottom bracket height helps me to pop off the rear end and level out in the air, while the short chain-stays keep the rear wheel close to help pick up quicker. User effort for nearly every maneuver (except sitting) is dramatically reduced. It’s at a level where I can go at the same line for longer, with less fatigue and enjoy myself more.
     
    All hands on metal work and paint was hand done by custom frame builder and metalsmith Matt Boeckman. 

     
  • "BikeCAD" Technical drawing 
  • Scale linedrawing for custom bottom-braket interface
  • Custom lathed and scalloped double-thick 83mm wide spanish bb shell
  • Shortest chainstays ever featured on a 26in wheeled bike
  • Tire clearence with average mountain bike tire
  • 3/4 rear view before paint
  • Relative comparison