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Our team has been given the task of redesigning the rowing bike which has been worked on by a few different groups in the past. We plan on constructing and analyzing our different designs based on their potential simplicity and functionality.
Since our project isn’t centered around the needs of a specific child, we have to design based on more general needs and accommodations. In doing so, we are focused on designing a bike that provides both a high level of reliability and stimulating exercise for its potential users.
The key inspiration for our first preliminary design is combining the driving and steering mechanisms into one comfortable motion. The core features of this concept include U-joints, a bevel gear, and numerous links to transmit motion from the crank and steering shaft. Expected issues from this design include fabrication due to time and experience constraints as well as the size and support structure of the bike frame.
The second idea controls similar to how a zero-turn lawn mower turns. You give power to each rear wheel separately and can brake each wheel separately to turn. Both of the front wheels would be on swivels which would let you turn easily. The power to the wheels would come from a rowing motion like a fan bike. Some expected issues would come from the bike steering too well which might make it harder to control, and the design has the chains and the rear wheels offset so we will probably have to work through a way to fix that issue.
Our third design has the steering and rowing controlled separately. The rowing will be controlled by the hands and the steering controlled by the feet using pedals. The row bar will be attached to a sprocket and chain with a freewheel system. The pedals will be on sliders and control the steering with pivots and links.
Our current selected concept is design number two. In terms of a complete bike redesign, it provides the most simplicity in both its fabrication and design. It also has the greatest potential in terms of steering ability thanks to its separately driven back wheels. Its biggest flaws are its potentially large size and weight which won’t be determined until later in the design process.
After spending time on our original concept design 2, we were able to further consider potential issues with the design. First, to address the weight of the vehicle, we settled on 1/4″ thick aluminum to build the frame. Also, rather than creating a gear box, we settled on two freewheel sprockets that will act like two bikes side-by-side.
Our final design is a simple welded aluminum frame that has two individual sprocket systems, each consisting of a crank shaft and drive shaft. The motion of this vehicle is activated by pulling on the crank shafts and then ratcheting it forward to repeat the pulling motion. To turn the vehicle, simply pull on one crank rather than both to rotate the frame in the opposite direction. The idea behind this was for it to act as a zero-turn lawn mower. A bench for seating will be constructed out of wood to build the aesthetic, but we did not think it was necessary to display this in the CAD model because it is not the core mechanism.
The first analysis we performed was a look at the drive shaft since it will be supporting most of the weight of the user. We referenced the material from McMaster-Carr, where we found the shaft, and found it to be 1045 Cast Iron Steel. Then we used SOLIDWORKS Simulation to analyze the von Mises stress and displacement. After fixing both ends and assuming a distributed load of 150lbf, we found a max displacement of only 0.0936 mm and no critical locations for the von Mises stress, thus, the material does not yield. This is expected because the load application of the shaft is much lower than what it was designed for.
Our second analysis was done on the gear set that drives the bike. We began by finding the gear ratio of the input (sprocket) to the output (freewheel). By dividing the number of teeth on the sprocket by the number of teeth on the freewheel (46, and 16 respectively), we found the gear ratio to be 2.875. We also used the gear ratio to calculate an estimated range of values for how much the back wheel would turn and translate the bike with each input on the crank. We used a lower bound of 1/8 turn of the sprocket and a higher bound of 1/4 turn. This led us to find that each input of the crank by the user would lead the back tires to translate the bike an estimated 1.88-3.76 ft.
Using the FEA in Solidworks, we were able to estimate the total stress on the frame that we plan to use. Looking at the scale on the first picture we see that the maximum stress is about 10x than that of the yield stress on our projected material, Aluminum alloy 6061. When looking at how the welds will hold up, welds have around 70% of the strength of the materiel welded leaving us with about 7x the maximum stress.
Using the same FEA, we see that the deflection is low enough that it will not be a problem.
There is some error involved because there are two points that are supposed to be held constant and the weight is also not going to be evenly distributed on the plate.
The fabrication process took about 6 weeks. Some of the main fabrication activities included construction and painting of the wooden frame, mounting the wooden frame onto the aluminum frame, and assembling the shafts with their corresponding parts. There was a bit of alteration needed from the original fabrication plan as we did have to machine some new parts such as the hub connecting plates on the back wheels, new crank arms, small connecting pieces for the front shafts, and new handles. Most of these parts were constructed out of aluminum and the rest were made with steel.
We began by putting together the wooden frame using plywood and 2×4’s. We used jigsaws, a miter saw, a power drill, screws, and bolts to assemble it. Once we did so we sanded it, painted it, and mounted it using bolts to our welded aluminum frame. Next, we mounted our inner and outer bearings to the frame as well as the front wheels. The front shafts connected to the crank arms and held the sprocket and bike chain which ran to the back shafts. The back shafts connected to the back wheels and held the freewheels and brakes whose lines ran up back up to the crank handles. Finally, we used set screws and shaft keys to lock all of the components in place before testing.
During the final stages of fabrication, the design was tested multiple times by Dr. Canfield and our group members. Almost all tests led to finding a problem in our design. In the first test, the original shafts bent and left us having to redesign the shaft to a thicker more durable shaft. Another test revealed that the original placement of the tensioner was on the wrong side of the chain causing excessive noise. Further testing revealed slippage of the sprocket on the front shaft. This was fixed by keying the front shafts. The final test before seemed flawless. However, a test by one of the users at the school sheared the front shaft at the threads. This issue would be fixed by getting a new shaft of constant diameter with no threads. Another test at the school revealed that the bike was too wide by a few inches.
While most moving parts of the bike are on the undercarriage, hands should not be placed near the exterior of the shafts. These points, while secured, can cause scratching or pinching during motion.
When entering or exiting the bike, use the brakes to keep the bike steady.
This year, the bike project was taken a completely different direction than those before it. The initial design stages gave us three paths to go down. After speaking with our coordinator, we decided that a path not taken before was probably the best idea. With this, it also meant new challenges that needed to be faced. Once the idea was confirmed we moved on to the design process. The parts list came together along with a few analyses on how things would hold up. Once the parts were in the fabrication started. The frame came first, water jetting the aluminum and having it welded together as well as the wooden frame being built. Once those were finished the axle assemblies came together and meshed with a bit of tinkering. Some troubles popped up in the testing stage but those were knocked out quickly. At the delivery, all were happy with the work even with the breaking of a part.
At the beginning the team had a few struggles and took a while to work well, but after getting situated and working through the initial stages, things started coming together. With the help and input of the shop team, Dr. Canfield and other interested parties, this project was able to get as far as it got. There were many learning experiences and rough times that made this project extremely helpful in learning to become an engineer.
