Gate Three: Coordination review
Introduction
The main focus for gate two was to analyze our product, and as a team evaluate each component's design. In this section we also had to come up with things that could make the product better for the user, solid model a component from the product using one of the many solid modeling programs. We also had to do some technical work by selecting a component and performing an engineering analysis on an aspect of the part that could fail. Losing traction with the ground in our case, because having a good grip on the road is an important safety issue with most vehicles.
Component Summary
Component Complexity Scale 1 → 5
- Complexity = 1
- Non-moving component
- A single, simple function
- Example: Reflector
- Complexity = 2
- May move
- Has a single function
- Example: Wheels
- Complexity = 3
- Contains some sub-components
- May interact with the user
- Has one or possibly two functions
- Example: Brake pad
- Complexity = 4
- May move, interact with the user
- Multiple functions
- Example: Throttle System
- Complexity = 5
- Contains several sub-components that we were not able to remove
- Performs complex functions
- Example: Motor
Components
Horizontal Handlebar / Crossbar
Design Revisions
- Loop Attached to Frame
- Currently there is no way to lock the scooter. For most children who use the scooter solely for recreation this may not be a problem. If an adult or teenager is using the scooter for transportation, however, this becomes a great obstacle to the scooter’s use, as they cannot leave the scooter unattended. Welding or otherwise attaching a steel loop to the scooter’s frame transforms the scooter from a mainly recreational vehicle to one that is practical for transportation.
- Water/Salt Guard on wheels
- When taking the scooter apart, we noticed a large amount of salt and rust on the fasteners and other components near the wheels. From this information we can deduce two things: that the scooter was tested in the winter, and that winter weather, especially in buffalo, will have a big impact on the scooter. After a few years this could grow to be a very large problem. Already this problem made it difficult to remove some of the nuts. A simple solution would be a cover or guard that fits between the wheels and the other components so that the salt on the wheels is not transferred to the other components. Keeping the salt out could extend the life of the scooter and make it easier to service.
- Move wires into handlebar post
- Currently the wires connecting the brakes and throttle to the controller are outside the handlebar post, held there by a few thin loops. The handlebar post is hollow. There is no reason why the cables can’t be put inside the handlebar instead of hanging loosely on the outside. It would protect the cables better and improve the scooter aesthetically. The only problem it would create would be that the connection between the handlebars and the handlebar post would have to be changed in order to allow the cables to fit through.
- To implement, cut a hole at the connection of the vertical handlebar and the horizontal handlebar to insert the wires extruding from the throttle and the brake handle. Similarly, cut a hole in the semi circle frame-to-wheel connector to extract the wires from inside.
3D Solid Modeling
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Choice of Components
The motor was chosen as the main focus for the 3D modeling because it is where the rotational energy comes from to turn the rear wheel via a connecting chain. The motor is also heavy so it must be supported by the frame. It is attached to the frame by four screws. The interconnecting piece between the motor and the frame is the "motor base". This piece is glued onto the bottom of the motor and has the four holes drilled through to allow for easy connection to the scooter's main frame. This in turn allows easier maintenance of the motor than if it had been hidden internally.
Choice of CAD Package
The choice of SolidWorks for the modeling process was based purely on availability as well as ease of use. The modeler for the group needed a package that was quick to learn as well as completely versatile and functional. The ease of dimensioning and extrusions was a significant factor in the choice as well, because the motor was based heavily on those two functions.
Engineering Analysis
Purpose of Engineering Analysis
Engineering analysis can be used in the testing and design stages to help determine the most efficient and cost effective solution to the problem statement. The problem statement defines the scope of the problem faced. It helps focus the attention on the actual problem at hand. Making assumptions is vital in any engineering analysis to save time while not sacrificing accuracy. As long as the assumptions can be adequately justified, the calculations can be done without consequences. The diagram is used to obtain a visual representation of the problem at hand. It is important to list equations used in the engineering analysis to make it easier for others to decipher equations used in the calculations and decipher the concepts involved in the overall analysis. Calculations are needed to express quantitatively and qualitatively the results to solve the problem statement. The solution check verifies the correct usage of units and equations as well as defines reasonable boundaries for the result. The discussion section describes the quality of the assumptions made about the problem.
Problem Statement
Find the maximum velocity of the scooter around a turn of constant radius so that the tires will remain in contact with the ground without sliding
Assumptions
- Mass of the rider = 68 kg
- Mass of the scooter = 20 kg
- To obtain the μs,asphalt,dry, take the average of the range of μ values
- To obtain the μs,asphalt,wet, take the average of the range of μ values
- To obtain the μs,concrete,dry, take the average of the range of μ values
- To obtain the μs,concrete,wet, take the average of the range of μ values
- Radius of the turn is constant, thus a circular path
- Radius of the turn = 3 m
- Force of gravity is 9.8 m/s^2
Diagrams
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Governing Equations
- Fc = mv2/r
- Fc = Centripetal force [kg*m/s^2]
- m = Mass [kg]
- v = Velocity [m/s]
- r = radius [m]
- fs = μsN
- fs = Force of static friction [N]
- μs = Static friction coefficient []
- N = Normal force [N]
- W = mg
- W = Weight [N]
- m = Mass [kg]
- g = Force of gravity [m/s^2]
Calculations
Rubber -> Asphalt (Dry) μs in range of .5 - .8
Average μs = (.5 + .8)/2
Average μs = .65
Rubber ->Asphalt (Wet) μs in range of .25 - .75
Average μs = (.25 + .75)/2
Average μs = .5
Rubber -> Concrete (Dry) μs in range of .6 - .85
Average μs = (.6 + .85)/2
Average μs = .725
Rubber -> Concrete (Wet) μs in range of .45 - .75
Average μs = (.45 + .75)/2
Average μs = .6
N = W
W = (m,rider + m,scooter)*g
N = (m,rider + m,scooter)*g
Fr = μs*(m,rider + m,scooter)*g
((m,rider + m,scooter)*v2)/r = μs*(m,rider + m,scooter)*g
v = sqrt(r* μs*g)
Rubber -> Asphalt (Dry)
v = sqrt((.3 m)*(.65)*(9.8 m/s^2))
v = sqrt(1.911 m^2/s^2)
v = 1.38 m/s
Rubber -> Asphalt (Wet)
v = sqrt((.3 m)*(.5)*(9.8 m/s^2))
v = sqrt(1.47 m^2/s^2)
v = 1.21 m/s
Rubber -> Concrete (Dry)
v = sqrt((.3 m)*(.725)*(9.8 m/s^2))
v = sqrt(2.1315 m^2/s^2)
v = 1.46 m/s
Rubber -> Concrete (Wet)
v = sqrt((.3 m)*(.6)*(9.8 m/s^2))
v = sqrt(1.764 m^2/s^2)
v = 1.33 m/s
Solution Check
- The units cancel to the correct units for the final answers
- The velocities seem reasonable given the weight of the rider, the coefficients of friction, and general knowledge
Discuss and Interpret
From data found on various websites, the values of μ were tabulated. The values of μ for each scenario (i.e. concrete/dry, asphalt/wet) had a range. Instead of using the range of μ, the average was calculated by adding the two limits and dividing by two. By looking at the free body diagram of the scooter, it is clear that the normal force N is equal to the weight W of the scooter. This equation would change had turn been banked at an angle. The weight of the system is equal to the mass of the rider added to the mass of the scooter and multiplied by the force of gravity. Because the weight is equal to the normal force, the normal is set equal to the value of W. Using the equation f = μ*N and using the new normal force N that was calculated, the force of friction can be obtained. By using the equation v = sqrt(r* μs*g), the velocity can be calculated for each coefficient of friction. After plotting the values of velocity versus the values of the coefficient of friction, it is seen that the two are linearly related; when fitting the linear curve, there is an R2 value of .9984. The value of R2 should be as close to 1 as possible and it is clear that this is a valid curve fit.
Using these calculated velocities, the rider can determine what the maximum riding velocity is for various weather conditions and surfaces. These velocities can be calculated for any surface where there exists data for a coefficient of friction between that surface and rubber.
Works Cited
- "Coefficient of Friction Reference Table - Engineer's Handbook." Mechanical Engineering Design Guide - Engineer's Handbook. Web. 10 Nov. 2009. <http://www.engineershandbook.com/Tables/frictioncoefficients.htm>.
