Group 6 - Mini-Bike/CoPR

Component Summary

The questions for the component summary were given as follows.

• What forces are applied to the components,
  • If forces are applied estimate their magnitude
• Does the material choice affect the manufacturing process?
• Does the shape affect the manufacturing process?
• Why was each manufacturing process chosen for that component
• Do any components have a particular shape? Why?
• Is the component functional, cosmetic, or a combination of the two?
• Why do you think the manufacturing process was chosen for a given component?
• How complex is the component?

Component Analysis:
The folowing is a breakdown of where each of the questions in the gate 3 requirements for the components is answered. Some of the questions, specificly those about the manufacturing process, had a lot of overlap and did not necessarily apply to every component, so for the purposes of the summary they have been combined.

Name of Component
Materials
Manufacturing Process
Why was this process chosen

• Why was each manufacturing process chosen for that component
• Why do you think the manufacturing process was chosen for a given component?
• Do any components have a particular shape? Why?
• Does the shape affect the manufacturing process?
• Does the material choice affect the manufacturing process?

Forces Applied

• What forces are applied to the components
• If forces are applied estimate their magnitude

Complexity

• How complex is the component?
  • 1 - Single piece of one type of material with simple shape e.g. washer
  • 2 - One piece, one material, more complex shape e.g. dipstick
  • 3 - Multiple materials, no moving parts e.g. brake lever
  • 4 - Moving parts e.g. throttle assembly
  • 5 - Electronics e.g. timing mechanism

Functional or Cosmetic

• Is the component functional, cosmetic, or a combination of the two?

Note: Component names link to pictures

Name of Component	Materials Used	Manufacturing Process	Why this process was chosen	Forces Applied	Complexity	Functional or Cosmetic
Muffler	The outer shell is made of steel and the catalytic converter inside is made of a ceramic substrate and a catalyst such as platinum.	The outer shell is made from two pieces of rolled steel pressed into shape and then folded together around a catalytic converter and spot welded to give a good seal between the two halves.	The materials in the catalytic converter were chosen for their heat resistance and the way they react with exhaust gasses produced by the engine. Steel was chosen for the shell because it is strong and resistant to heat damage. Picking steel meant that a fairly thin sheet of metal could be used for the shell, and that meant rolling it rather than casting a thicker piece of aluminum which saves significantly on manufacturing cost.	The muffler is not subject to forces of any significant magnitude. The only forces present would be from the vibration of the bike while the engine’s running, inertial forces from slowing down, speeding up or turning, and the air flow through the catalytic converter. None of these pose any threat to the structural integrity of the muffler. In case of a collision or another force that the muffler was not designed to deal with, it is covered with a protective cage (which we’ve treated as a separate component).	3	functional
Metal Shield	The metal shield is a thin, curved plate made of aluminum	The shield would have been made from aluminum rolled into a thin sheet, then cut to a specific template and bent into its curved shape.	Aluminum was chosen for the metal shield because it is lightweight and cheap, but still strong enough to provide a layer of protection between the engine and the user. All in all it was chosen because it was cost efficient.	Under regular operating conditions, the metal shield is only subject to the forces from the vibration of the engine.	1.5	Both
Hand Break Lever	The metal hand break lever is made mainly of aluminum and plastic.	The aluminum parts of the handle would be cast and machined into shape, while the plastic parts were probably injection molded.	Aluminum and plastic were chosen for their lightweight strength at a relatively cheap price. Choosing a combination of plastic and aluminum means using two different manufacturing processes to make the part. Using all plastic might make it too weak, while using all aluminum would be more expensive, but would also save money by simplifying the manufacturing process.	It is subject to the force of a hand gripping it. In the engineering analysis section we looked at failure of the break system and determined that the average grip strength of a human is 67N, which is the force that would most commonly be applied to it.	3	Mostly functional
Muffler Cage	The muffler cage is made of wire, probably steel or aluminum, coated with latex.	The metal would be extruded to form it into wires, which would be cut, bent and welded into place. Then it would be coated with latex paint.	The metal would be chosen for support, since the cage is designed to protect the muffler. The latex would be to absorb vibration, and to insulate against the heat of the muffler so that if anyone were to touch the cage, they probably wouldn’t burn themselves.	It, like the muffler, is subject mainly to the force of vibration from the engine, although it is designed to absorb impacts to the muffler.	3	functional
Left Grip	The left grip is made from synthetic rubber.	The rubber would have been injection molded.	The rubber was chosen because it is compressible, and durable, and also makes for a comfortable handhold. This is not to mention that it is cheaper than some alternatives.	The grip would be subject to two major forces. It would be subject to the grip force of a human hand, which was shown earlier to be 67N. It would also have the weight of a person’s arm resting on the hand on the grip. We’ve estimated a human arm to weight about 3 or 4 kg. Multiplying the weight times gravity, we find that would be a force of approximately 30 to 39 N.	2	Mostly functional
Rear Axle	The rear axle is a long steel bolt.	Like any bolt, it would start off as stock which would be machined to specifications.	Steel was chosen because it is a very strong material. Since the bolt was threaded, the machining process was necessary to get the proper specifications correct. The bolt was probably chosen because it was easier/cheaper to order pre-made bolts than to design and manufacture an axle for that specific vehicle.	We estimated our minibike’s mass to be about 30kg. The force on the rear axle would be roughly half of the force generated by that mass, which would be 147 N.	2	Functional
Timing Mechanism	Our timing mechanism is made from plastic, aluminum and synthetic rubber.	There are a pretty wide variety of processes used because this is a complicated part. Much of the metal in it was machined for a precise finish and the rubber and other synthetics were injection molded.	These materials were mainly chosen for their electrical and magnetic properties. None of them were incredibly strong, but they did their job electronically. Because of the varied materials and complexity of the part, the manufacturing process would have to be fairly complicated and therefore pretty expensive.	It has to deal with vibrations from the engine and not much else, so the forces it is subjected to are small but steady.	5	Functional
Gas tank/Engine Interface	The interface is a thin plate made of aluminum.	The interface would have been made from aluminum rolled into a thin sheet, then cut to a specific template.	Aluminum was chosen for the interface because it is lightweight and cheap, but still strong and heat resistant enough to provide a layer of protection between the engine and the gas tank.	Under regular operating conditions, the interface is only subject to the forces from the vibration of the engine.	1.5	Functional
Right Handle	The right handle is made from synthetic rubber, plastic, steel.	The rubber and plastic are injection molded, while the steel is cut to shape.	The right handle has the same grip as the left, but is also the throttle control, so turning it forward causes acceleration. The plastic was chosen for being cheap and having a good strength to weight ratio. The steel was used as a spring to return the throttle control to the usual position, and the rubber was chosen for its comfort and durability.	It is susceptible to similar forces to the right handle as well as the twisting force used to activate the throttle. This is estimated to be less than 60N.	4	Functional
Throttle Cable	The throttle cable is made of steel surrounded by a plastic sheath.	The steel is extruded in thinner wires and then twisted to form the cable.	Twisting steel strand together yields a strong but flexible cable, which is ideal for transferring force over a distance	Forces on the throttle cable are significantly less than those on the break cable because it is only used to transfer a signal, rather than power from the hand to the brake pads, even though it is constructed from the same material, so it is more than able to take the strain of ordinary use.	2	Functional
Cutoff Switch	The cutoff switch is made of plastic, synthetic rubber, copper, and steel.	Any wires are extruded and coated in synthetic rubber, and then the switch itself is made from injection molded plastic	The components were chosen for their electrical properties, copper is a good conductor while plastic is a good insulator, and because they are lightweight and cheap, lessening the manufacturing cost	The only significant force applied to the switch would be from a finger pressing the button, which is not enough to cause any kind of failure.	4	Functional
Brake Cable	The break cable is made of steel surrounded by a plastic sheath.	The steel is extruded into thin wires and then twisted to from a cable.	Twisting steel strand together yields a strong but flexible cable, which is ideal for transferring force over a distance	In the Engineerig Analysis section, it was determined that the brake cable rarely experiences more than 140N of tension while it could withstand over ten times that amount	2	Functional or Cosmetic
Washers	The washers were made of steel or aluminum	Washers are usually machined from stock.	Washers are machined from stock because it is the simplest way to make them. It also means that they can be made to very exact tolerances	Though there are a lot of washers on the bike, subject to different forces, generally washers are subject to compressive forces, and because of their shape they can withstand a great deal of force without failing	1	Functional
Chain Tensioner	The outside is made of Plastic while the inside has metal washers and bearings to allow it to spin.	The washers and bearings would be machined, and the plastic is injection molded.	The washers and bearings have to be able to slide past each other and fit to within exact tolerances, which means that machining is the best way to manufacture them.	The forces applied to the tensioner would be proportional to the tension in the chain itself. When we received the bike the tensioner was pretty loose, so the force on it was small.	3	Functional
Gaskets	Gaskets are made from a thin layer of aluminum or steel coated in adhesive and some kind of paper product	The gasket is cut out of rolled metal then coated.	Gaskets have to be very thin and flat, and the best way to achieve this is with rolled metal.	Gaskets, like washers, are designed to take incredible compressive force. In doing so, they deform slightly and form a seal.	3	Functional
Rubber Gasket	It was made from synthetic rubber	It is injection molded.	Rubber tends to be difficult to machine, and cannot be treated like most other materials used in manufacturing because it is so soft. Therefore it is almost always injection molded.	The rubber gasket is similar to the metal gaskets, although it isn’t able to take as much force. However, because it is softer, not as much force is needed for it to form a seal.	2	Functional or Cosmetic
Dipstick	Our dipstick was made from hard plastic.	It was injection molded and then the threads were machined	Molding is a cheap, fast way to shape plastic, but then the threads have to be machined in order to make user they fit with the right tolerance.	The only significant forces applied to the dipstick are when it is unscrewed by hand in order to check the oil level. This is why it could be made from lightweight, brittle plastic	2	Functional
Axle Bolt Cover	The axle bolt cover was made from hard plastic	It is injection molded.	Injection molding is a very cheap, fast way to manufacture plastic, and since the cover does not have to be exact, it was the most cost-efficient way to manufacture it.	Under everyday use, no significant force is applied to the cover. If the bike falls over, the cover cushions the harder axle, and will probably take some damage, but it is not an essential part	1	Both
Engine Spacer Plate	This was made from a thick piece of steel.	Cast from a dye (could have also been rolled and cut but the edges are too smooth)	The piece of steel could have been rolled, but it was very thick, which means that it would have been a lot harder to cut into shape, while casting it from a dye means that there is no need to cut through it.	The spacer plate is subject to compressive forces from the bolts holding the engine to the frame. It also takes the brunt of the engine vibration.	1	Functional
Chain Cover	The chain cover was made from plastic.	It was injection molded.	It is a plastic component that does not have to fit to exact tolerances, so injection molding was the fastest and cheapest way to manufacture it.	The chain cover is not subject to any significant force. It covers the moving chain to prevent clothes getting caught, but it does not support any weight	2	Both
Engine	Our engine used a lot of aluminum, as well as steel and synthetic rubbers and many more materials.	The casing for the engine was die cast (as evident from the tapered ridges) then machined where necessary until the desired result was achieved. Synthetic rubber was used to make the o-rings which were probably injection molded.	The engine has a very large surface area, and machining the entire thing would not be cost-effective. Instead, casting the engine, then machining any surfaces that have to fit to exact tolerances saves a lot of time and money in the manufacturing process.	The engine, pr rather the gas in the engine is what is applying most of the forces in the bike. The explosive forces of the gas, are turned by the pistons into rotational forces which are transmitted to the wheels by the chain	5	Functional
Throttle/Choke Assembly	The throttle assembly used a variety of metals.	Most parts were made from rolled and cut aluminum, or machined from stock, but there are many processes involved in making an assembly this complex	The goal in manufacturing is to make a part that does what it needs to do, as cheaply as possible. Therefore, rolling and cutting, or die casting is preferable to machining even though machining is more accurate. So when a more expensive method can be avoided, it is.	Much of the assembly is rather delicate. Thankfully the only forces applied to it come from the throttle cable, and are probably just a few Newtons	4	Functional
Gas Tank	The gas tank is aluminum.	It’s made in much the same way as the muffler in that it started out as rolled aluminum and was cut and then pressed into two parts which were joined to make the tank.	It is simpler than casting a tank and bolting it together, and insures that no more material is used than necessary.	The gas tank is subject only to the weight of the gas in the tank which given the materials involved is essentially negligible.	2	Functional
Frame	The frame is aluminum.	Much of the frame is made from tubes which are extruded, cut, bent into the proper shape and welded together. The base plate for the engine and a few other flat pieces are rolled and cut aluminum which is also welded on. Once the whole frame is together it is then painted red.	Extruded tubes of metal provide lightweight strength while the flat pieces served as better anchors for other components. The welding held the frame together and the paint made it look more professional and prevented against rust.	The frame is very strong because it is subject to most of the forces in the bike. It holds the load of the bike, and serves as an anchor for the engine which means that it is being acted on by a great deal of force.	2	Functional
Wheels	The wheels are alloy or aluminum with rubber tires.	The tires and wheels would be manufactured separately and then assembled together. The tires would be injection molded synthetic rubber around fiber. The wheels could be cast and then machined as necessary.	The wheels had to be machined to make sure they were round and well balanced, and the rubber for the tires had to be molded because of the nature of the material	The wheels take the force of gravity on the bike, which comes out to roughly 150N each	3	Functional
Starter Cover	Aluminum	was rolled and pressed into its current state.	The starter cover did not have to be very thick or have exact fits, so a simple rolling and stamping procedure produces a workable but cheap part.	The cover serves as an anchor to the pull start, so any force on the cord is transferred to the cover	3	Both
Foam Seat	The seat is made of plastic, foam, and steel nuts	The seat is made from injection molded plastic covered with foam and stitched leather stapled to the plastic frame. There are nuts glued to the inside the plastic frame to provide lightweight threaded holes to secure the seat to the metal frame.	The foam and plastic are chosen for lightness and comfort. The nuts are used to provide sturdy threads for the holes in the plastic so that the company can save money by not having to use a harder plastic or metal base for the seat.	The seat supports the person riding the mini-bike. The recommended maximum weight is 150lbs, which is roughly 68kg. That would yield a downward force of 667N	3	Both
Nuts, Bolts and Screws	Steel	Machined from stock	That is standard procedure for nuts bolts and screws as they have to fit to exact tolerances.	The forces applied vary, but the design generally spreads any forces over several connectors rather than just a single bolt.	1.5	Functional

Design Revisions

1) The mini bike has a chain tensioner that is clearly designed to be a pulley and yet rather than rotate with the chain, it lets the chain run over it, causing wear and tear to both it and the chain. Giving the tensioner better fitting bearings could solve this problem and eliminate all the unnecessary wear and tear and wasted energy. It would mean a slight increase in the cost of the bike however, and though it might be more fuel efficient, consumers might not notice the small increase in efficiency when compared to an increase in price.

2) Our mini bike is somewhat underpowered. One way to deal with this would be decrease the diameter of the wheels. This would give the bike a little more torque, although its top speed would be lower. The target consumers for the mini-bike are not people that want a lot of speed, because the mini-bike was never fast to begin with. However, we would be giving them a better functioning product that was less likely to stall as you try to start it up. Also, smaller diameter wheels would contain less material and therefore be less costly to produce.

3) The electronic timing mechanism that detects the position of the flywheel and controls the firing of the piston could be replaced with a timing chain. The timing chain might not be as precise, but it will be much more cost effective, and there seems no point in having an expensive electronic timing mechanism on an otherwise practical and simple vehicle. The only difference customers would notice would be the drop in price.

Solid Modeling

The chosen assembly was the rear wheel assembly. It was chosen because of its central nature to the propulsion of the mini-bike. Propulsion is the key feature of the mini-bike product.



Wheel:

• The rim is aluminum
• The tire is rubber and is tubeless, which means that the tire is pressed against the rim due to the air pressure in the tire

Sprocket:

• The sprocket is made of aluminum
• There are 70 teeth on the sprocket
• It is mounted by 6 bolts to the rim of the wheel

Disc brake rotor:

• Made of aluminum
• Has holes on the contact surface to dissipate the heat of braking
• It is mounted to the rim by means of 6 bolts

Flywheel:

• Is mounted to the motor
• Transfers the energy of the motor to the wheel and sprocket

Engineering Analysis

Problem Statement: Find out the stress on the brake cable of the mini-bike for group 6 and evaluate it with respect to the ultimate tensile strength.

Diagram:



Assumptions: There are multiple assumptions to be made to check for failure of this system. The perpendicular distance between the forces and the hinge remain constant. The cable does not deform under stress in such a manner as to significantly reduce its cross-sectional area. The distributed load exerted by the hand is evenly distributed for the distance that it is applied. The force applied by the hand to the cable is applied over enough time so that it acts as a loading situation (gradual increase in force over time) as opposed to an impact (sudden application of force) situation.

Assumed Values

Name	Value
diameter of steel wire	0.001588 m
ultimate tensile strength of steel wire	760 MPa
force applied by hand	67 N
distributed force minimum perpendicular distance to fulcrum	0.0254 m
distributed force maximum perpendicular distance to fulcrum	0.1016 m
cable fulcrum perpendicular distance	0.03048 m

Sources: http://en.wikipedia.org/wiki/Tensile_strength Steel, high strength alloy ASTM A514 for the ultimate tensile strength of the wire

Governing Equations:



Calculations:

Solution Check:
Brake cables on bikes are not known to fail under ordinary operating conditions so the large difference between the applied force and the maximum force the cable can take is as expected.

Discussion of Results:
The scenario for failure of the group 6 mini-bike was the failure of the brake cable. As shown in the calculations the ultimate tensile force that the steel cable can withstand is 1505.2 N while the ultimate tensile force the cable will experience is only 140 N. The safety factor calculated for this problem of 10.75 means that under the assumed conditions there will be no failure of the specified component in the specified way. The limitations to this solution are numerous. The assumed hand force applied is only a rough guess given some slight experimentation meaning the maximum force could be higher especially since humans have a wide range of strengths. The distance and position over which the hand force is applied in reality would vary widely for different sized hands and grips though the distance and position assumed in this problem is a pretty common one for a standard brake grip (determined though life experience on bikes with friends and family). The exact material of the cable is unknown however the steel ultimate tensile strength used is a relatively low and common one so a reasonable maximum tensile force should have been obtained. This evaluation of the ultimate tensile force applied vs the ultimate tensile force that the cable can withstand would be improved by better knowledge of the geometry of the cable at its ends.

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