How to Design a Human-Powered Vehicle

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Enthusiasts love to mod cars, but try making a slick human-powered vehicle. It’s a full-blown engineering effort.
 
You need to account for speed, safety, ability to repair and endurance. Based on guidelines, the ease and time of manufacturing could also be a major consideration.
 
It can cost upwards of $3,000 to design, test, and manufacture a competitive human-powered vehicle, which in many cases are offshoots of bicycles or tricycles. Many teams rose to the challenge of building these street burners for ASME’s E-Fest Human-Powered Vehicle Challenge (HPVC) contests.
  
South Dakota State University (SDSU) has won many HPVC speed and endurance awards at E-Fests over the last two years. This year’s vehicle, called FlapJack, was a result of observations made over years. They took a design of last year’s bike and improved on it, adding a new drivetrain system and flaps for cooling and safety.
 
“The key to the entire bike was simple, basic, and reliable. Everything’s simple steel, no crazy parts,” said Alex Gray, a mechanical engineering at SDSU. “We put a lot of emphasis on drivetrain, as drivetrain failure seemed to plague most of the teams here.”
 
Innovation, not speed, was a priority for students of the University of Hawaii at Manoa. Their unique folding bike had two chains, with one chain flowing from the front to the middle, and another one from the middle to the back, and simulating as if they are one chain. “That pretty much dictated how the rest of the bike would be built,” said Dayton Lee, a student at the university.
 
Both universities designed models in SolidWorks, which was also used for simulation. In both cases, the conception and design took roughly four months, and manufacturing took three months and wrapped up just a few weeks before the competition.
 
SDSU designed FlapJack as a single piece, starting off with the frame design as its basis. Human-powered vehicles can break easily, so the students chose simple steel as it would be easy to find parts for repairs.
 
FlapJack had two wheels in the front for braking, and one wheel in the back for propulsion. The initial focus was on the drive system including the steering and the fairing system, which work side to side, and to ensure the fairing was narrow enough so the vehicle would be more aerodynamic. The drivetrain was the final piece, and finally came the external flaps, which served two functions.
 
The flaps opened up at slow speeds to provide cooling to the driver inside a vehicle, and that was added from previous lessons learned. In the 2017 E-Fest West competition in Las Vegas, many riders were overheating as it was over 100 degrees. The flap draws in cool air, and the hot air dissipates from a hole at the back of the driver. The flap also opened up on hard braking, providing better safety. “We found that with hard braking, we had an 18 percent reduction in stopping distance,” Gray said.
 
The University of Hawaii at Manoa students took a modular approach, creating different blocks of the vehicle separately and patching them together. The development was broken up into the fairing, the frame and the drivetrain, and three SolidWorks models were enjoined to create the vehicle. Besides a reversible two-chain system, the big change from last year’s model was the adoption of chromoly (chromium and molybdenum) material, which was tougher compared to aluminum that was responsible for many breakdowns in last year’s vehicle.
 
Given the tight deadlines, the Hawaii students couldn’t prototype the bike using 3-D printing. “Even though you want to make prototypes, you may not have time. We are always pressed. The SolidWorks models worked fine,” Lee said.
 
For the students, getting hands-on experience with engineering a vehicle is a valuable skill that will help them in professional modeling and simulation. The students also picked up fiscal lessons by making vehicles within budget, and team members were assigned to find sponsors and acquire materials.
 
Agam Shah is associate editor at Mechanical Engineering magazine.
 
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