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Executive Summary

To optimize the design of a chain tensioning device (be it powered by human power or an external source), the first step was to dissect the use of an existing application. A single, fixed-gear bicycle was acquired and dissected; areas of improvement were addressed via DFMA, FMEA and DFE.

Results from the product usage and evaluation study had proven that the system was not only incredibly non-intuitive, but required far too many specific tools to work on in an emergency situation. Though the subject was technically inclined, results from an attempt to tighten the chain were incredibly poor and could easily result in injury. Even when this task was completed successfully, the tire was crooked, another opportunity for injury.

Our study of DFMA revealed the drive train system, while well made, was not designed with final assembly in mind. The manufacturing processes result in a durable, aesthetic product. However, there is still room to reduce production costs by a fair margin. Through minimization of part count as well as component and feature simplification it should be possible to reduce the production cost. Design for assembly leaves much to be desired. The design results in a system which requires specific installation orders and often obstructed access. The process is time consuming and requires a fair amount of training or experience to complete. Some of these shortcomings may be due to the fact that often the customer ultimately assembles the components.

FMEA results showed that most of the higher RPN failure modes were related to issues with lack of and improper maintenance on the drive assemblies. Specifically, improper lubrication and tension on the chain can cause slip/loss of drive likely resulting in harm to the user. These results suggest that a design solution that makes this maintenance easier or not required would greatly reduce the likelihood and severity of failures.

From the DFE analysis, we found that the amount of greenhouse gas emissions released in the manufacture and use of the product is fairly low. The largest contributor is bearing manufacture, but even if a $30/mtCO2eq tax went into effect, the total cost increase would be less than 2%. Considering how the product is used, it might be more environmentally friendly to make improvements that increase its appeal to consumers, instead of focusing on improving manufacturing processes. Having more people willing and able to use bikes could drastically reduce GHG emissions by reducing the use of cars, even if the GHGs released during manufacture do not change significantly.

Thus, the existing product was found to have quite a few issues. It is the goal of this team to improve the existing design as well as possible within the budgetary restrictions. Subsequent analysis will investigate these points further and determine possible design solutions.

Primary Stakeholders and Product Needs

The primary stakeholders in this supply chain are the Manufacturers, the bike shops who will retail this and the bike riding consumer.

Consumers:
Consumers are the target audience who purchases the product, these people would most likely own one or more single speed road bikes and may or may not be hipsters (but probably)

• Ease of use for both alignment and tensioning of the rear wheel
• Robust construction for reliable road use
• Low weight because this is a human powered vehicle
• Lowest possible cost for a quality product
• Ease of maintenance
• Compatible with many bike frame designs, IE not a custom part for each frame
• Easily installed on bikes not currently using it
• Weather resistance

Retailers:
These are the people and companies that would sell the product to the consumers Most likely this would be from specialized bike shops (IE iron city bikes)

• Small size for easy storage
• Universal compatibility use so they do not have to stock more than a couple sized/configurations
• Ease of maintenance
• Low price

Manufacturers:
Manufacturers create the product for sale and effects on time and pricing impact the entire supply chain.

• Time to manufacture is important and all custom machined parts slow down the process
• Cost to manufacture and of materials; exotic materials would not be good here.

Product Function and Evaluation

This application of a single-gear bicycle drivetrain offers a cost-efficient solution for transportation. It forgoes torque/power control for simplicity and low cost. The chain tensioning system is of particular interest as it is rudimentary and difficult to perfect.

Usage
If a user is riding a bike and senses that it can be loose, he or she should stop and check the chain visually. If it feels loose, flip the bike, and hope you carry two 15mm socket wrenches. Loosen the pair of nuts holding the rear hub assembly to the chassis and visually tighten the chain. Hoping that the tire is now in line with the bike, tighten the two nuts back into place and put your tools back in your pocket.

Usage Flow Chart

Mechanical Function
The pedal is pushed upon by the user's foot, which in turn imparts a moment on the drive sprocket via the crank lever arm. This moment is shared to the rear sprocket through the chain, which in turn imparts a moment to the rear wheel through the rear hub. The chain tensioner is a simple friction-compression nut solution that holds the tension in the chain by the friction against the threads.

User on bicycle

Full Drivetrain

Rear Chain Tensioner

Preliminary User Study: Chain Tensioner
The preliminary use study allowed for some very obvious conclusions, most of which were negative. The subject was quite technically inclined, but ended up with a system more unsafe than what he started with. This is largely due to the fact that the slop in the system allows for some tragic results that can climax in a serious injury to the user.

The user was asked to assume that the chain was not tensioned enough. From the usage flow chart, we see that the first step was to flip the bike upside-down and produce tools. We had to provide the tools as they were esoteric and not readily available. The user then loosened the rear hub (though it should be noted it took some trial and error to figure out what to act on), and proceeded to tighten the chain. Without knowing how tight to make the chain, the user went with what he thought was good enough, but from the pictures below, it was obviously incorrect. The user also did not align the wheel with the chassis, which could end in catastrophe under load.

Thus, there's a lot of room to improve the system. This usage study was a very important tool for future decision choices.

Drivetrain in use

Flipped Bike

Loosening the rear hub

Readjusting the wheel

Chain is worse than it was

Wheel was also misaligned

List of Parts

The following details the list of parts of a fixed gear bicycle tensioner and drivetrain.

Drivetrain Assembly

Pedal Assembly

Rear Hub Assembly

Spindle Assembly

Parts list table

Part Number	Name	Quantity	Mass (g)	Subassembly	Function	Material	Manufacturing Process/ Purchased Component	Image
1	Dustcaps	2	<1	Crank Assembly	Keep dirt out of bottom bracket	Plastic	Injection Molding
2	Lock nuts	2	6	Crank Assembly	Retainment/Assembly	Steel	Catalog Purchase
3	Nut w/ integrated lock washer	2	16	Crank Assembly	Retainment/Assembly	Steel	Catalog Purchase
4	Toe strap holders	2	6	Pedals	Ability to secure toe	Plastic	Injection Molding
5	Flange nut	4	<1	Pedals	Attach toe strap holder to foot grip	Steel	Catalog Purchase
6	Screw w/ integrated washer	4	<1	Pedals	Attach toe strap holder to foot grip	Steel	Catalog Purchase
7	Screw	8	<1	Pedals	Attach foot grip to pedal	Steel	Catalog Purchase
8	Pedal cap	2	<1	Pedals	Keep dirt out of pedals	Plastic	Injection Molding
9	Foot grip	2	27	Pedals	Prevent foot from slipping off pedal	Aluminum	Die Casting
10	Outer jam nut	2	9	Sprocket Assembly	Retainment/Assembly	Steel	Catalog Purchase
11	Washer	2	<1	Sprocket Assembly	Retainment/Assembly	Steel	Catalog Purchase
12	Inner jam nut	2	7	Sprocket Assembly	Retainment/Assembly	Steel	Catalog Purchase
13	Pedal	2	54	Pedals	Central Component in Pedals	Aluminum	Die Casting
14	Ball bearings	24	<1	Pedals	Friction Prevention	Steel	Catalog Purchase
15	Nut	2	3	Pedals	Retainment/Assembly	Steel	Catalog Purchase
16	Double D Washer	2	<1	Pedals	Catalog Purchase	Steel	Catalog Purchase
17	Nut w/ seat for ball bearings	2	5	Pedals	Retainment/Assembly	Steel	Catalog Purchase
18	O-ring	2	<1	Pedals	Keeping lubricants inside pedal	Rubber	Catalog Purchase
19	Chain	1	304	Drivetrain	Drive rear sprocket using rotation of front sprocket	Steel	Stamping and Pressing
20	Crank	1	260	Spindle/Crank Assembly	Lever arm for force from foot	Steel, aluminum	Die Casting
21	Crank w/ sprocket	1	626	Spindle/Crank Assembly	Transfers force to chain	Steel	Assembly - Stamping and Die Casting
22	Rear sprocket	1	82	Rear Hub Assembly	Sees force from chain and sends it to the tire	Steel	Stamping with Post Machining
23	Bearing	2	19	Rear Hub Assembly	Reduces system friction	Steel, Aluminum	Catalog Purchase
24	Rear wheel	1	?	Rear Hub Assembly	Interface between rear sprocket and ground	Steel, Rubber, Aluminum	Assembly - Catalog Components Mated with a CNC lathed hub
25	Rear axle	1	101	Rear Hub Assembly	Allows wheel to move	Steel	CNC Lathing
26	Sprocket retaining ring	1	15	Rear Hub Assembly	Holds sprocket components together	Steel	Catalog Purchase
27	Ball bearings	2	14	Spindle/Crank Assembly	Countering Friction in Pedals	Steel	Catalog Purchase
28	Left threaded cap	1	54	Spindle/Crank Assembly	Retainment/Assembly	Steel	Catalog Purchase
29	Spindle	1	209	Spindle/Crank Assembly	Allow wheel to move	Steel	CNC Lathing
30	Right threaded cap	1	47	Spindle/Crank Assembly	Retainment/Assembly	Steel	Catalog Purchase
31	Crank Spacer	1	47	Spindle/Crank Assembly	Retainment/Assembly	Steel	Stamping
32	Frame	1	2272	Frame	Holds all other subassemblies	Steel	Fabricated/Welded

Design For Manufacturing and Assembly [DFMA]

Design For Manufacturing [DFM]

Notice the extra lip on the inside diameter of the sprocket required for the retaining ring to seat against. This feature may be designed away.

Dissection of the fixed gear drive train revealed several areas for improvement. The majority of the drive train consists of cast or stamped steel and aluminum components which allows for a high volume of relatively cheap parts to be made. High tolerance components such as the rear axle, hub and spindle are likely CNC lathed in order to achieve required tolerances for bearing surfaces. Unfortunately this drastically increases the time and money spent to produce such parts. However, considering the cost of machining the rear hub it is multifunctional: it locates the bearings, provides torque transfer and mounting for the sprocket and secures the wheel. It should be noted some components of the pedal assembly (dust cap, toe strap holders) are injection molded plastic, allowing for very cheap manufacture after initial mold investment. Interestingly, it seems the use of injection molded plastic for other applications such as the pedal and pedal grip could be a viable option at the expense of some durability. Furthermore, standardizing bearings between the pedal set, crank assembly and hub could reduce costs and increase the feasibility of injection molding the pedal set. Other notable components include the rear hub, sprocket and retaining ring and their method of interface. The hub uses a stepped threaded area in order to accommodate the larger diameter sprocket and smaller diameter retaining ring. This setup increases manufacture time and cost by requiring two separate sized threads to be machined onto the hub as well as a lip machined into the sprocket for the retaining ring to seat against. Instead a single diameter thread may be used along with a locking nut. A similar oversight occurs in the pedals where ball bearings are retained by a special nut and jam nut instead of a single bearing set and lock nut assembly.

Overall the materials and production methods provide a durable, well finished system but certainly leave room for improvement. By minimizing part count, standardizing components and design features and simplifying parts it should be possible to reduce the production cost by a fair margin.

DFM Summary

• The drive train is made of durable, aesthetic pieces at the expense of production cost.
• Casting and stamped processes are used where applicable to reduce high-volume cost.
• The drive train uses many non-standardized parts across the system.
• Some components are excessively complicated or superfluous for their function

Design for Assembly [DFA]

The pedal bearing set consists of many separate parts which must be installed in a specific order and orientation. Also note the obstructed access to pedal removal when pedal grip is in place.

Dissection of the fixed gear drive train show inconveniences that negatively affect assembly time, assembly worker training and labor cost. It is likely several of these inconveniences are a result of the fact that assembly is completed by the customer as well as standard bike specific tooling. An experienced user is required to install the drive train assembly correctly as there are many components with no clear assembly order, especially in the bearings for the pedals. Additionally parts must be installed from several directions and access is often obstructed if the system is assembled in the incorrect order. For example, there is no feature in the pedal grip to allow for pedal removal. Instead, it is necessary to remove the pedal grip to gain access to the nut for pedal removal. This may be easily remedied through a clearance hole in the pedal grip. Additionally, the use of unnecessarily thin retaining rings and sprocket requires 2 special bike wrenches which are significantly thinner than standard wrenches. While this is relatively standard in the biking industry for enthusiasts or shops, it certainly inconveniences an average user looking to conduct maintenance or repair. Also, special care is required when installing the pedal bearing set as it is made up of many individual parts and requires specific assembly order. Tens of ball bearings must be installed, followed by a custom nut requiring specific orientation, a washer and lastly a jam nut. This is a mistake prone assembly and also requires quite some time to install due to the many separate components. There are, however, some well thought out components which ease assembly. For example, the pedal grip uses slots for mounting which maximizes installation compliance, even in the event of a warped or damaged grip assembly. Also, integrated washer/nuts are used to hold the wheel to the frame, minimizing the number of fasteners on a part which is likely removed often.

While there are some positive points for the drive train assembly there are still numerous ways to improve upon installation simplicity and time. We believe there may have been a lesser focus on design for assembly due to the fact that these components are often assembled by the customer. As such the manufacturing company realizes no lost revenue (other than customer backlash) due to the relatively inefficient design.

DFA Summary

• The designers likely did not prioritize installation simplicity or speed.
• Special tools are required for proper assembly.
• The installation process is mistake prone and requires a fair amount of experience.

Failure Mode and Effect Analysis [FMEA]

Failure Mode and Effect Analysis was performed on the drivetrain of the fixed gear bicycle. This was used to identify the cause, effect and ease of detection of mechanical failure in the system. This is done by scoring major components in factors of (S)everity, (O)currence and (D)etectibility. These scores are multiplied to determine a Risk Priority Number, which allows us to best identify areas that need improvement.

The FMEA results point to the chain as the largest source of failure. Failure for this system is sudden and possibly harmful to the user if they suddenly lose balance because the pedal no longer is connected to the drivetrain. The chain skip/failure would make the bike lose functionality until normal chain positioning is restored. Also of note, most of the high RPN failure modes stem from a lack of proper maintenance and tension in the chain/sprocket/hub assembly. It seems then that a process which makes maintenance easier and a lack of proper maintenance more noticeable would lower the overall risk of failure in the entire drivetrain.

Design for Environment [DFE]

Designing for the environment involves considering the environmental impacts of manufacturing, use, and disposal of the product. In this case, the manufacturing process appears to be the most harmful, but the use phase also generates some greenhouse gas emissions (GHGs), mostly because of the need to replace tires and tubes. Using the EIO-LCA model, the amount of GHGs produced is small enough that the cost would not increase dramatically if a $30/mtCO2eq tax went into effect.

We used the known quantities of bearings and aluminum, and assumed that the drivetrain would have an expected lifetime of 10 years. We estimated that the average user would require a new rear tire every 2.5 years and a new tube every 1.5 years. The alternative calculation uses the Bicycle Parts sector, and assumes that all purchases (drivetrain and maintenance) are included in that sector. It has a result similar to adding components separately, and both calculations show that an added tax would not significantly increase the cost of the drivetrain.

EIO-LCA results

GHG contributors and relevant sectors

Motorcycle, Bicycle, and Parts Manufacturing Sector

Despite the relatively small amount of GHGs released, there are no clear signs that the product was designed with the environment in mind. It uses materials that can be recycled (aluminum, steel), but the coatings used to prevent corrosion are difficult to remove, and recycling is impossible with the coatings in place. Crucial components like sprockets and cranks seem durable, which could mean that it will not require the parts to be replaced. Unfortunately, if another part of the system breaks, it is likely that the entire bike would end up in a landfill. Using safe, removable coatings and making it clear which parts can be recycled would make the disposal process less harmful to the environment. Easy maintenance will increase the product's life and make it more appealing to consumers.

As shown in the EIO-LCA results, the main contributor to GHGs is bearing manufacturing. While there's no easy way to avoid using bearings in the assembly, it might be possible to buy bearings from manufacturers that use lean manufacturing processes, or that get energy from renewable sources. After bearings, tubes and tires are the largest contributor to GHGs. It is possible to reduce the number of tubes and tires used by improving quality and by ensuring that consumers know how to prevent damage, like keeping tires properly inflated and avoiding potholes.

Since the cost increase due to the possible $30/mtCO2eq tax is only a few dollars, and less than 2% of the pre-tax cost, it is possible that the tax would actually increase sales. Cars would be much more expensive to drive, so people would be encouraged to ride bikes instead. Wear and tear would increase with increased use, but replacing a few more tires would still cost less than other forms of transportation.

Team Members [TM]

Michael Ornstein - Team Leader

Rob Wojno - Product Function, Evaluation and Usage Study

Anne Dirkes - DFE

Brian Langone - DFMA

Adam Brecher - FMEA

Each member of the team expressed interest in their assigned field; work is done on a regular basis with Mr. Ornstein overlooking the direction and making sure it's in line with expectations.

References [REF]

Bureau of Labor Statistics. Inflation Calculator [Internet], Available from:<http://http://www.bls.gov/data/inflation_calculator.htm> Accessed 23 September, 2012.

Carnegie Mellon University Green Design Institute. (2008) Economic Input-Output Life Cycle Assessment (EIO-LCA), US 1997 Industry Benchmark model [Internet], Available from:<http://www.eiolca.net> Accessed 23 September, 2012.

Dieter, George E., and Linda C. Schmidt. Engineering Design. 4th Edition. New York, NY: McGraw-Hill, 2009. 707-715. Print.