Performed by: CSULA Mechanical Engineering Senior Design Students
Faculty Advisor: Professor Everardo Hernandez
Financial support and guidance by: Kinematic.bike
Abstract
The purpose of this project is to develop a Mountain trail bike. This trail bike will have a full-suspension frame design and geometry. It is important that this bicycle provides comfortability as well as high efficiency while navigating on different types of terrain. To maximize the pedaling and braking efficiency as well as keeping the overall weight of the bike as low as possible.
- Introduction
1.1 Background
Over the past decade there has been a steady increase in popularity in mountain and non-paved bicycling in the U.S (Statista 2021) [1]. With the increase in popularity there has been a demand in the market for full suspension bicycles, and to obtain the customer’s attention, performance and comfortability are key to achieving that, to achieve great performance and comfortability it all starts with the kinematics analysis for the full suspension.
The two main studies of kinematics that are done to a bicycle’s rear linkages are anti-rise and anti-squat. Both studies involve the performance of the shock absorber during the two most key moments during the usage of the bike, which are the breaking and the acceleration (pedaling).
Anti-Rise is when braking, the forces acting on the bicycle create a decelerating mass of the rider that causes the rear end to rise. Which has a tendency for the suspension to stay active under braking conditions. When braking weight transfer occurs in the opposite direction meaning the weight will shift from the rear wheel to the front wheel. For calculating the anti-rise, one must identify the instant center based on the linkages and pivot locations of the bicycle. Then identify the center of mass with a horizontal line across along with the front axle vertically. This will show the representation at which percentage the anti-rise will fall in. [2] This percentages represent how much the suspension compresses or extends by the breaking force. Having an anti-rise at 0% will have no influence on the suspension, in fact it will extend because of the mass transfer due to breaking. While having an anti-rise at 100% does have an influence on the suspension when there is mass transfer. With a 100% anti-rise the bicycle will be balanced during the mass transfer where the suspension will neither compress nor extend under braking conditions. [3]
Anti-Squat determines how much of the suspension mechanism itself is resisting the suspension compression. This is due to the mechanical resistance to compression from a person that applies forces towards the bicycle. Having an anti-squat over 100% means the suspension will extend under acceleration. It will require more effort to pedal uphill due to the pedaling effect that lifts rider weight. Also, it is much more difficult to tune suspension for downhill riding because of the extending forces of the anti-rise. With a suspension under 100% the anti-squat will tend to compress under acceleration. This factor helps with the tuning of suspension as it is easier to control the forces with shock tuning. With a 100% anti-squat the suspension won’t extend nor compress. This factor is ideal but impossible to always maintain, as the bike travels the anti-squat will change. The anti-squat could be determined based on the linkages and pivots that determine the instant center (IC). [4]
1.2 Project Scope
The purpose of this project is to develop a Mountain trail bike, with a full-suspension frame design and geometry. The bicycle shall provide comfortability and high efficiency on various types of trail terrain with the least amount of weight possible. Once all design requirements are met and manufacturing is completed, full testing of the frame will be run.
2.0 Approach
2.1 Design Approach
The design approach consisted of many steps that required significant alterations. The approach started with identifying the ideal results for anti-rise and anti-squat which coincided with the requirements. Once identified, analysis of existing frames was conducted to have a general norm of anti-rise and anti-squat in the market. With references already in hand, the creation of unique kinematic frame designs was completed with the system requirements in mind. Once a significant amount of unique kinematic designs where completed, one had to be chosen using trade studies. With a unique design on hand, there had to be some appealing visuals to the design which resulted in multiple industrial designs for both the front and rear frames. One had to be chosen so another trade study was conducted to narrow it down to one. CAD modeling was the next step for the design process. Having completed the CAD models for the rear and front frames, they were assembled with already chosen off the shelf hardware, multiple iteration was made to both frames to accommodate each other. The final step that is planned for the design process is the simulations which would lead to more iterations to the models.
3.0 System Overview
3.1 Project Concept Design
To understand what makes a difference between various mountain trail bicycles, it’s important to understand how the anti-rise and anti-squat of the system works. Looking at figure 5 will demonstrate how to be locating the anti-rise, and anti-squat of the system. Anti-Rise is the numerical value of the activation of the suspension while breaking. While Anti-Squat is the activation of the suspension during acceleration. These values will demonstrate a percentage of how the suspension will act during certain conditions. Having a percentage relatively close to 100% will result in less energy waste. Having a value above 100% will result in the suspension to be in tension, while having below 100% will be in compression. The anti-rise and anti-squat are based on the placements of the pivot points and location of the center of gravity of the overall bicycle. Using the pivot points to locate the intersection points it was made possible to determine numerical values of the anti-rise and anti-squat based on the location these values land on over the overall height of the location of the center of gravity.
Figure 5: Kinematic Analysis of bicycle on SolidWorks
With our kinematic design it demonstrated the anti-rise/squat to have relatively close numbers to 100%. Where the initial anti-rise at 0% travel demonstrates to be at 76.42% shock efficiency and at the final 100% travel to be 71.54%. There will be about 30% energy lose while the rider of the bicycle is accelerating. While the anti-squat at its initial travel begins at 105.79% then decreases to a final of 90.17%. For anti-squat since it is the activation of the suspension for the breaks this demonstrates that 100% travel only 90% of the suspension will be used. Looking at figure 6 will show how both anti-rise and anti-squat will react from 0-100% travel. Looking at this chart it shows a linear pattern as the suspension is compressed.
Figure 6: Anti-Rise and Anti-Squat of Kinematic Design
3.2 Industrial Design
With the Kinematic analysis an appropriate anti-rise and anti-squat was found using SolidWorks. With these given values an industrial design was conducted using the same concept of kinematic analysis. Two designs for each triangle of the bicycle were drawn as seen in figure 7 and figure 8, which then the best of each two was chosen. The industrial design consisted of a drawing of the rear triangle, and front triangle. Each were discus to evaluate which of the industrial designs were best in design, and application.
Figure 7: Industrial Designs of Rear Triangle
The rear triangle was selected based on these two industrial designs in figure 7. The rear triangle #1 was designed to have the suspension along the top tubing of the bicycle, while having a single pivot point. Along with having two separate rear triangles that will need pins to connect. The rear triangle #2 was design as a single rear triangle piece, while having the suspension along the seat tubing. While having two pivot points to increase the degree of freedom of the bicycle to fully enhance the usability of the suspension. Out of the two industrial designs of the rear triangle the second design was chosen to be the best out of the two designs.
Figure 8: Industrial Designs of Front Triangle
The front triangle was selected based on these two industrial designs in figure 8. using the same concept of the industrial design #2 of the rear triangle. The design process for the front triangle design was governed by multiple factors. One was to make a design that would fit the rear triangle design that was created by my fellow group member. The front triangle had to fit within the constraints of said back triangle. Another factor to be considered was the seat tube. The seat tube was to be in a straight line or not have any curves to it. The solution to this was to create a seat tube that had the same angle of 67 degrees as the head tube of the bicycle. This was done to have a cohesive flowing look to the bike. Second, was to create a design that was a) structurally sound and functional b) aesthetically pleasing. The intent of the design was to find the intersection where functionality meets aesthetics. The first design consisted of multiple angles on the frame with the intention to make it a unique design. The design is aesthetically pleasing but upon further review it was concluded that a more structurally sounds design was needed. The second design took into consideration the angle created by the top and bottom tube of the back triangle. The front triangle has the same angles incorporated into the design to create a flowing and aesthetic design. To create a more structurally stable frame, straight tubes were designed as they would better absorb impact from bumps, jumps when the bike is in use. The result was a frame design that is a balance of functionality and aesthetics.
3.3 Rear Triangle Design
In Figure 9 will show an exploded view of our design of the rear triangle. The rear triangle will consist of 3 major components the rear triangle, top linkage, and bottom linkages which determine the anti-rise and anti-squat of the bicycle by their placements of the frame. The three major components of the bicycle also harbor various hardware components to keep the linkages intact with the rear triangle/ frame. The use of pins secured by the screw on the sides of the component will keep the system intact. While having bushing, and bearings in between will provide freedom of motions which help the system move freely in an upwards and downwards motion. Using the top linkage as an anchor to secure the rear triangle from completely falling out of the system. The hardware components will be bought off the shelf, while the three main components of the rear triangle will be manufactured. The material that will be used will depend on the analysis of the bicycle through Finite Element Analysis on SolidWorks.
Figure 9: Rear Triangle Design with Components designed on SolidWorks
3.4 Front Triangle Design
Based off the industrial design#2 of the front triangle a CAD model was simulated as seen in figure 10. The placements of the pivot points of the rear triangle were critical and could not be moved around. Therefore, the design was based around those pivot points. From this it was possible to move forward from the design where the seat tubing must have a linear tubing for the seating post to be properly inserted and be able to adjust to the rider’s height. The seat tubing was designed to have an angle of 67 degrees. The reason for this was researched as angles from 60-75 degrees provided a comfortable angle to sit on. While the head tube of the front frame was also designed to have 67 degrees to have a symmetrical look towards the design.
Figure 10: Front Triangle CAD Design on SolidWorks
3.5 Full Bicycle Design
With all components done from the frame the assembly processes for the full bicycle design began giving us a better understanding of how our design will work and areas that will require improvement. With this simulation of the frame, we can conduct various tests that will help us identify problems with our design as well as an estimation of the possible weight of the frame. Possible test that will be conducted are finite element analysis that will provide a stress analysis of the bicycle frame. While providing us a possibility of weight reduction.
Figure 11: Full Bicycle CAD Designed on SolidWorks
4.0 Progress
4.1 Mechanical System
Table 3. Frame Material Option Breakdown
|
Material |
Aluminum 6061-T6 |
AISI 4130 Steel, normalized at 870°C |
Ti6Al4V Titanium Alloy |
Carbon Fiber |
|
Material General Notes |
Relatively high strength Good workability High resistance to corrosion Widely available Yield Strength: 35,000 Psi |
Heat treatment allows it to obtain increased strength and wear resistance Increased toughness and ductility. Widely available Yield Strength: 63,000 Psi |
Widely used in the aerospace and automotive industries. Great strength to weight ratio Improved ductility and fracture resistance Yield Strength: 128,000 Psi |
Great strength to weight ratio Low thermal expansion Temperature tolerant to excessive heat Yield Strength: 200,000 Psi |
Once the frame was assembled it was important to test the initial structural integrity of the chassis. To do so, a series of static simulations which applied external loads at different areas on the frame was carried out to see how well the frame could withstand different stresses. It is important to note that this first iteration of the frame design was completely solid and had very little hollow areas throughout the frame’s tubing. The objective of this simulation was to see how well the frame could withstand multiple 200-pound downward forces that were applied at the seat tube, front fork and in the pedal area. In order to maximize the effort of these simulations, a material type had to be implemented onto the frame so that the results could be realistic to an extent. Table 1 above, highlighted four different material types along with general characteristics for each material [5]. It was important that different materials were tested in these simulations to see how their properties would withstand such loads and ultimately allow for the proper material to be selected for this frame. The four materials that were analyzed during these simulations were Aluminum 6061-T6, 4130 Steel Normalized at 870, Titanium Alloy Ti6Al4V, and Carbon Fiber.
The first material that was applied to the frame and tested through the simulation was Aluminum 6061-T6. Figure 12 is stress diagram that was generated after the simulation was completed. The diagram showed that the maximum stress experienced by the frame while under the multiple loads was approximately 15,000 Psi. It is important to note that the yield strength of Aluminum 6061-T6 is approximately 35,000 Psi. Since the maximum stress experienced was smaller than the overall yield strength of the material, that resulted in a Factor of Safety of 2.217 for the overall frame. This factor of safety is important because it showed that plastic deformation did not occur on the frame since the yield strength of this material was 2.217 times higher than the maximum stress experienced while under the 200-pound loads. This meant that the frame was stronger than the load that was applied to it. A large factor for these results is that the frame was almost completely a solid and so that made the frame heavier which meant that a larger force was required to actually show deformation on the frame. In addition, Figure 13 showed the displacement diagram that was generated for this particular material. The solid frame deflected about 1.54E-02 inches which is almost negligible. Since the frame was strong and heavy, the force that was applied showed minimal impact on the chassis.
The second material that was applied to the frame and tested through the simulation was AISI 4130 Steel Normalized at 870. Figure 14 is the stress diagram that was generated after the simulation was completed. The diagram showed that the maximum stress experienced by the frame while under the multiple loads was approximately 10,000 Psi. The yield strength for 4130 normalized steel is about 63,000 Psi. Since the maximum stress experienced was smaller than the overall yield strength of the material, that resulted in a Factor of Safety of 3.21 for the overall frame. The Factor of Safety showed that plastic deformation did not occur because the yield strength of this material was 3.21 times higher than the maximum stress experienced while under the load. Additionally, Figure 15 showed the displacement diagram that was generated for this particular material. The solid frame deflected about 5.649E-03 inches which is almost impossible to see and also showed that the overall frame was too heavy.
The third material that was applied to the frame and tested through the simulation was the Titanium Alloy, Ti6Al4V. Figure 16 is the result for the stress diagram that was generated after the simulation was completed. The stress diagram showed that the maximum stress experienced by the frame while under the multiple loads was approximately 13,000 Psi. The yield strength of this titanium alloy is about 128,000 Psi. Since the maximum stress experienced was smaller than the overall yield strength of the material, that resulted in a Factor of Safety of 3.58 for the overall frame. The Factor of Safety showed that plastic deformation did not occur because the yield strength of this material was 3.58 times higher than the maximum stress experienced while under the load. This meant that the bike was stronger than the load that was applied onto the three different areas. Figure 17 showed the displacement diagram that was generated for this particular material. The solid frame deflected about 9.795E-03 inches which is very small and once again showed how a heavy frame can withstand large loads without suffering from major stresses.
the load. This meant that the bike was stronger than the load that was applied onto the three different areas. Figure 17 showed the displacement diagram that was generated for this particular material. The solid frame deflected about 9.795E-03 inches which is very small and once again showed how a heavy frame can withstand large loads without suffering from major stresses.
The fourth material that was applied to the frame and tested through the simulation was Carbon Fiber. Figure 18 is the result for the stress diagram that was generated after the simulation was completed. The diagram showed that the maximum stress experienced by the frame while under the multiple loads was approximately 8,000 Psi. The yield strength of carbon fiber is about 200,000 Psi. Since the maximum stress experienced was smaller than the overall yield strength of the material, that resulted in a Factor of Safety of 2.217 for the overall frame. This Factor of Safety showed that plastic deformation did not occur because the yield strength of this material was 4.14 times higher than the maximum stress experienced while under the load. Figure 19 showed the displacement diagram that was generated for this particular material. The diagram showed that the solid frame deflected about 3.779E-03 inches which is very small and once again proved that a heavy frame can withstand large loads with very little deflection occurring.
The results were not surprising and showed that the frame was very sturdy and did not deflect in any major areas. This provided important feedback and was a clear indication that in order to meet the system requirements, the frame had to be hollowed out to remove significant weight throughout the frame. Through these iterations that improve the design, the simulations will be able to better produce more accurate results and will ultimately eliminate any remaining flaws from the frame which will ensure the creation of a well-designed mountain bike.
4.2 Electrical System
For the electrical side of the project, we will work with a pinion gear box.
Figure 20: Pinion Gearbox (Image Provided by the Client)[6]
The pinion gear box is limited to a grip shift due to the high amount of force needed to create a shift. A positive thing about this gear box is that it has a 600% [d][7] gear range meaning that it makes it possible for riders to go up hill without making any effort or using the highest gear. It has two cables coming out of the gearbox that help the gear go from low to high speeds (low gear to high gear). The downside to this gearbox is that it is prone to failure due to the increased forces for the shifting. The objective of this project is to create an electronic shifter mechanism that would be able to shift the gears of a pinion gear box. The main goal for this is to remove the need for the grip shifter.
4.3 What lies inside a pinion gear box
The image above shows the pinion gear box, but what we are working with is the inside of it. As shown on the image below, given by the client
Figure 21: The Inside of the Pinion Gearbox (Image Provided by the Client)
If we open the cap of the gear box this is what we see. For this project, we need to find a motor and use that motor to rotate the gear box either clockwise or counterclockwise. From the image, there is a small opening in the middle of the gearbox where the motor will be located. A 3mm hex key can fit in that opening, meaning that the shaft of the motor must be that same size. The gearbox will not just get help from the motor but also from the rider since it is easier to shift gears while pedaling. The plan for the motor is to be controlled by two pushbuttons, one to go counterclockwise and the other to go clockwise.
Before we did anything a motor had to be chosen, there were three types of motors which were stepper motors, servo motors and dc motors. After all the calculations, research and the decision made by the client we decided on a DC motor.
Table 4: Electrical System Deliverables
This table shows what is going to be delivered at the end of the year. It lists all the specifications that must be done for the project to be successful. For the project to be successful we must complete all these specifications which include the size, weight, performance, and durability of the motor. As we can see every objective has its own method of verification which could either be testing, designing, or bill of materials (bought).
Figure 22: Greartisan DC Gearbox Motor Used for the Pinion Gearbox, bottom picture is the DC motor recreated using SolidWorks. [f]
This motor is a 12V with a reduction ratio of 236 and a motor speed of 16000 (RPM) [8]. For now, our team is using a 5V battery that performs 53 rotations per minute. The goal is to be able to use the 12V battery to power the DC motor successfully. The dimensions of the DC motor are length is 1.13 in, height 0.47 in, and the length of the shaft is 0.31in. As we can see from this image the motor is connected to another gear box, and the shaft is different than the entrance to the pinion gearbox. There was a modification that was needed to be able to turn the pinion gearbox. Once it is assembled it will be able to turn the pinion gear box.
Figure 23: Adaptor Connected to the Greartisan DC Motor
This adaptor has the shaft in the form of a hexagon, the same shape and size as the entrance of the pinion gearbox. The dimensions of the adapter are the length is 0.3937 in, length of the shaft is 0.1250 in, width of the shaft is 0.12 in, and the diameter is 0.3125 in.
4.4 Pinion Gearbox Design
The objective was to design a cover for the pinion gearbox. There were restrictions when designing it such as needing to leave the curve design on the bottom to leave clearance of 0.45in, so that the crank does not make contact with the cover. Another restriction was keeping the circular shape at the front of the pinion gearbox the same to match and to be able to fit the backside of the Pinion gearbox that is bolted into the bicycle.
Figure 26: Front View of the Pinion Gearbox Cover on SolidWorks
On the front part of the pinion gearbox, the logo of our client was placed in the center. For aesthetic reasons, rectangles were placed around it to make it stand out. We have not had a chance to see the pinion gearbox in person yet, so we are still not sure how the cover is removed. If it is twisted to remove and place back into place, or if the cover clicks when it is placed back. We wanted to create smooth curves on the sides of the pinion gearbox just like the one's computer mouse has, but when we tried it on SolidWorks the sides of the covers were not thick enough for the design we wanted. Instead, small dents were made on the sides of the cover, enough for a fingertip to be placed and assist when removing the cover. This design will also be helpful to the rider if it happens to be dark outside and are not able to see the cover, they can just feel the texture and it will be easier to remove. The length of the front cover is 2.37 inches, and the height is 0.93 inches.
Figure 27: Back View of the Pinion Gearbox Cover on SolidWorks Assembled with the DC Motor and the Adapter into the Cover
When the motor that was going to be used for the bike was chosen, the dimensions were taken to replicate what we ordered into SolidWorks. The length of the rectangle made on the back of the cover to hold the DC motor and the adapter is 1.21 inches, the height is 1.18 inches from the left side and 0.86 inches from the right side. A part of this rectangle had to be removed to make room for the DC motor where the cable that powers it up will be attached to. The shaft of the DC motor is connected to an adapter to go into the center of the pinion gearbox to rotate the mechanism and shift the gears.
Improvements that need to be made in the design are how to cover the side where the DC motor is popping out. We checked if it was in the center by asking the TA if he could see if it was and he said yes according to the dimensions we took from the bolts. We might be wrong and may need to ask the advisor for assistance. We might have to make a small rectangle slit going vertically over the DC motor where this slit can be easily moved with a push to keep the DC motor from slipping. We also must figure out how exactly the cover is going to remain in place without slipping now that it has additional weight placed on it. The plan is to get a hold of a gear box during the winter break to analyze how a real one looks and works.
5.0 Conclusion
Completion of the initial designs of the rear and front frame completed for the mechanical team, the assembly between the two parts was made alongside any modifications to the original models that included any dimension alterations or small edits to the joints that removed any interferences with the two bodies. Major task that is being worked on is the simulations and that would bring more future alterations to the completed frame that would be justified by the simulations. Alongside the simulations a manufacturing process would be chosen to begin discussing physical prototypes. On the electrical side, majority of the electrical designs and circuits have been decided on and, a major part of the code block has been created to drive the DC motor, future tasks include physical testing of the motor design, debugging any issues that arise with the code for the motor, alongside manufacturing the motor cover.