This project was designed over the course of about 1.5 months. Using Autodesk Fusion 360, I was able to design and analyze the properties of several welded frame styles, rake and trail angles for the front suspension, and come up with a highly optimized design which reduces mechanical complexity, increased power, speed, and range, and allows for an extremely light and durable pit bike. 

All parts of the 3D modeling, structural analysis, and concept art creation were done by me. To start, I pulled inspiration from sources that inspire me, while also making an extensive list of system requirements. This includes existing pit bike styles, existing bare frame motorbikes like the Ducati Scrambler, and exploring powertrain options from companies selling street legal pit bikes. Some of my key system requirements for the project were focused around range, safety, and durability. I wanted to keep it simple, make the design water and weather proof, and get just enough speed to make it a viable and fun pit bike while not sacrificing speed for range. With my requirements set and solutions matched to each requirement, I began the design ideation process and dove into learning in great detail about the mechanics of a successful dirt/pit bike design.

Next I went into modeling 4 different frame styles to begin learning and understanding some proper geometries and why each works. The first was a much more elaborate design, using thin rod welded in a complex form to provide the structural strength needed for a slightly oversized mini-bike. This design leaned heavily away from the pit bike style and more towards a traditional motorcycle body, and was inspired by my interest in practicing stainless steel welding. While largely impractical, it was a cool modeling exercise and gave me some great starting points to begin correlating to BOM items from the existing market of parts and powertrain solutions.

The 2nd frame style was still designed for a much larger battery, and so was more of a hybrid between a larger bike style and a pit or dirt bike. 

The third iteration felt close, but the proportions were a little off and the chain driven motor still felt clunky and prone to failure over time. However the frame styling here felt good, and after some structural analysis I felt confident that the frame itself was going the right direction. By this time I had also created a physical BOM which corresponds to each modeled item, making sure my design was buildable and realistic. I did several micro-iterations here too, playing with other battery placements and weight distributions. Since those are mostly non consequential to the final process, I wont go into detail about those. 

By the time I reached the fourth iteration, my focus was on keeping speed and range while streamlining and reducing weight drastically. This was done by cutting out fat in the design, reconfiguring the battery box, and doing away with the chain driven motor powertrain. This was replaced with a 3000W hub motor, which takes care of space issues, reliability issues, and eliminates the risk of chain derailing. 

At this point the design felt much more refined, and had all of the handling, aesthetic, weight savings, structural, power / range, and durability requirements met that I had set out for in my initial system requirements. The powertrain was switched to a 60V 45Ah battery with a 60-72V rated 3kW hub motor. There is no chain maintenance, no wasteful weight, all steel DOM frame, and plenty of space for adding storage and range modifications in the future. In the future I will also be designing in a small 12v battery system which runs head and tail lights, charges phones and other devices, and has a small solar panel on it. Then comes fairing design, final fixturing design, and laser cut crash panel deisgn around the battery box and ESC. These final steps will leave me with a design I can fabricate and prototype to full capacity, iterate on, and refine further. An all aluminum frame will be made for the pit bike once I can acquire a TIG welder, for now I am limited to fabricating with steel only at home. 

To get a feel for the range, speed, and power of various versions of a powertrain for less money, I have converted an inexpensive mountain bike with solid brand name brakes and shifters to a 1.5kW hub motor ebike with a 48V 30Ah NMC battery pack. I welded up a custom battery and ESC fixture for the existing frame, The bike goes almost 35mph, climbs hills with ease, and so far has gone as far as 35-40 miles at full throttle 90% of that time. This system is an excellent parts test bed, allowing me to get a sense for the power and range tradeoffs and options I will incorporate into the final build.