As a mechanical engineer at CU Boulder, I have the great opportunity to take a class known as Component Design. This class is structured around a group project where teams build a drill-powered bike with the goal of competing in a race against one another. There are several rules that constrain the designs, such as no battery swapping during the race, a budget of $200, a weight limit of 50 lbs, and no modifications to the drill. Besides this drill, teams have to source their own materials and parts while staying under budget. Additionally, there are three race events to choose from: endurance (most laps in under 30 mins w/one battery), hill climb (fastest time to climb a designated incline with a certain amount of added weight), and maneuverability (fastest to make it through an obstacle course).
Our team decided to do the endurance race and we each got designated roles. I chose to be the CAD engineer but was heavily involved in the manufacturing process because of it. I designed our bike with inspiration from a Razor electric scooter because I had ridden it before with a ride-sharing service and felt like the proportions were very ergonomic.
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| Initial sketch |
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| Exploded view of final assembly |
I used an image of the Razor scooter I found online and scaled it up accordingly so that I could import it into SolidWorks. Then, I used the wheelbase and handlebar angles to create a rough model for the frame. Due to its price and easy weldability, our group decided to 0.065" wall square steel tubing for the frame members. The wheels, seat, and handlebar/steering assembly were sourced from a used bike parts bin that was provided to our class. With these components, I was able to integrate the steel tubing into CAD and design a frame with enough clearance for the drill, wheels, and steering tube. With the CAD being close to done, I generated a cut list for the frame so that our team could start the manufacturing process. The class requires drawings for almost every manufactured component so I learned how to properly tolerance items and what dimensions needed to be called out vs. suppressed.
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| Frame with drill plate mounted |
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| Frame drawing |
Besides the welded steel frame, the other main component of our bike was the drive system. It consisted of a water-jet cut drill plate with various holes and cutouts to mount our drill (with hose clamps) and pillow blocks (holds axle and sprocket) along with frame mounting points. We used this plate to make our frame more rigid as it was bolted to various members.
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| Drill plate drawing with revisions |
The drill plate was by far the most complex part of our bike due to the spacing and mounting considerations in order to get the sprocket on the drill axle to line up with our bike's wheel sprocket. In the end, we were happy with how it came out, only needing minor modifications to work properly.
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| Drill plate assembly |
For the most part, the build was pretty simple. The only thing that cost our team a lot of time was our coaster-brake wheel that the drill drove via a chain. We decided on using a coaster-brake hub because our wheel size was 18" and most kids' bikes did not have a normal gear cassette. Coaster brake hubs allow for the brakes to be applied to the rear wheel when pedaling backward. The internal mechanism is more complicated than you might think as it uses a spring, clutch, and threaded collar to accomplish this. The original rear wheel with the coaster brake hub in it that we used was taken apart when we found it in order to take the existing sprocket off. This was a big mistake because it seems like we assembled it incorrectly or the part was faulty to begin with and this caused a lot of headaches during the build process. In the end, our team just simply bought another coaster brake wheel from a local used bike shop and it worked as intended. It allowed our bike to coast when the power was off but then drive the wheel when the drill was turned on.
To achieve the gear ratio that we decided was adequate (about 4.33:1), we needed to replace the sprocket on the kids' bike wheel with something a lot bigger. We found a 39T bike spur gear in the spare parts bin and decided to use it with the 9T sprocket that was mounted on the drill plate. The only issue was attaching this larger spur gear to the smaller hub of the coaster brake assembly. We decided to use the existing spline that the smaller outgoing gear had used in order to mount a larger plate. After measuring the 3-bump spline profile dimensions, a member of our team put it into CAD and had this plate water-jet cut. It included five mounting holes that the larger spur gear would bolt into and would be secured onto the coaster brake with a snap ring. With a little bit of filing and sanding, we were able to persuade this adapter piece to properly fit onto the coaster brake hub.
Speaking of power, we used a standard bike brake cable with a hinge and rubber band to activate the drill's trigger. We originally wanted to use a metal spring but the one we had proved to be too weak to pull the hinge back after the user pressed the brake cable throttle. The rubber band worked well and allowed me, the designated driver, to have variable throttle control during the race. In hindsight, it might have been a lot easier to just use a set of brake calipers to engage the throttle rather than going with the hinge design.
During the final assembly, a few washers were added to make sure the sprocket on the drill axle lined up with the spur gear on the rear wheel so that the chain didn't flex too much. This bike was working and now all we had to do was test it. After some testing, we realized that the drill-axle combo was moving around a bit due to vibration and thus causing the sprocket and spur gear to become misaligned. This often resulted in the chain coming off or the drill seizing up. To fix this, we salved some shaft collars from my roommates' drill-powered bike from the semester before. By placing these and some nylon washers between the sprocket and pillow blocks, the problem was fixed and the chain remained roughly in alignment.
Along with the frame, I also designed the dropouts for the rear axle. These were water-jet cut out of 1/8" steel and had a lot of room to adjust the rear axle location in order to get the chain tensioned properly. After adding the proper hardware, the dropouts held tension pretty well.
Our final testing after fixing all of the chain alignment issues included dialing in the correct drill torque setting. We needed our bike to be able to climb the inclines on the course without overdrawing current from the drill but also be fast enough to speed up on straightaways. It took a bit of trial and error but we pretty much went with the max torque setting in order to get up the hills and decided that I should try and coast as much as I can on the straightaways to save battery.
To achieve the gear ratio that we decided was adequate (about 4.33:1), we needed to replace the sprocket on the kids' bike wheel with something a lot bigger. We found a 39T bike spur gear in the spare parts bin and decided to use it with the 9T sprocket that was mounted on the drill plate. The only issue was attaching this larger spur gear to the smaller hub of the coaster brake assembly. We decided to use the existing spline that the smaller outgoing gear had used in order to mount a larger plate. After measuring the 3-bump spline profile dimensions, a member of our team put it into CAD and had this plate water-jet cut. It included five mounting holes that the larger spur gear would bolt into and would be secured onto the coaster brake with a snap ring. With a little bit of filing and sanding, we were able to persuade this adapter piece to properly fit onto the coaster brake hub.
Speaking of power, we used a standard bike brake cable with a hinge and rubber band to activate the drill's trigger. We originally wanted to use a metal spring but the one we had proved to be too weak to pull the hinge back after the user pressed the brake cable throttle. The rubber band worked well and allowed me, the designated driver, to have variable throttle control during the race. In hindsight, it might have been a lot easier to just use a set of brake calipers to engage the throttle rather than going with the hinge design.
During the final assembly, a few washers were added to make sure the sprocket on the drill axle lined up with the spur gear on the rear wheel so that the chain didn't flex too much. This bike was working and now all we had to do was test it. After some testing, we realized that the drill-axle combo was moving around a bit due to vibration and thus causing the sprocket and spur gear to become misaligned. This often resulted in the chain coming off or the drill seizing up. To fix this, we salved some shaft collars from my roommates' drill-powered bike from the semester before. By placing these and some nylon washers between the sprocket and pillow blocks, the problem was fixed and the chain remained roughly in alignment.
Along with the frame, I also designed the dropouts for the rear axle. These were water-jet cut out of 1/8" steel and had a lot of room to adjust the rear axle location in order to get the chain tensioned properly. After adding the proper hardware, the dropouts held tension pretty well.
Our final testing after fixing all of the chain alignment issues included dialing in the correct drill torque setting. We needed our bike to be able to climb the inclines on the course without overdrawing current from the drill but also be fast enough to speed up on straightaways. It took a bit of trial and error but we pretty much went with the max torque setting in order to get up the hills and decided that I should try and coast as much as I can on the straightaways to save battery.
On race day, I was a bit nervous that something small would go wrong and sabotage the event. However, our bike performed very well and had to stop due to a low battery (which is what we wanted to happen). Plus, we put a speaker on the handlebars so our bike was a crowd favorite. The one thing I think we should have done was integrate a drill setting change lever. Essentially like a gear shifter, changing the drill torque settings while racing would have been a lot more efficient. While our bike made it up the inclines with no issues, it was slow when it reached flat ground. We played around with more drill settings after the race and managed to make our bike go pretty fast on flat ground.
Me riding the beast during race day:
