Concurrent consideration of product usability and functionality: part II - Case Study: Development & Validation of design guidelines
This is the second part of the three part paper. The purpose of this part is to develop the integrated design guidelines for a specific consumer product. For our study, the mountain bike has been used as an example. The procedures and the generic guidelines described in part 1 have been applied in developing the guidelines for the above mentioned product. The rigorous analysis was carried out to determine and evaluate the importance of different but relevant design dimensions. A series of statistical tests were conducted in order to test the assumed hypothesis that the guidelines do ensure a successful development of a usability and functionality of the product.
Keywords: consumer product, product usability, product functionality, design guidelines, design dimensions, mountain bike, reliability & validity.
Biographical notes: Aniket Arora is a graduate student pursuing a Master's degree in Mechanical Engineering at University of Cincinnati. He holds a Bachelor of Engineering degree in Mechanical Engineering from Punjab Engineering College, India. He currently works with JP Morgan Chase as Business Analyst. His current research is focused on integrating design methodologies to incorporate ease of usability and functionality into consumer products.
Dr. Anil Mital is a Professor of Mechanical and Manufacturing Engineering at University of Cincinnati. He holds a BE in Mechanical Engineering with Gold Medal from Allahabad University, India, and an MS and PhD in Industrial Engineering from Kansas State University and Texas Tech University, respectively. He is a registered Professional Engineer and author/co-author of over 500 technical publications including over 200 journal papers and 22 books.
This part focuses towards developing design and manufacturing guidelines for a specific product, which in our case is the mountain bike. The objective of this part is to manifest that how the methodology and the generic guidelines developed earlier could be successfully applied to a certain consumer product. We selected mountain bike for this case study.
The generic guidelines developed earlier were modified for the specific product after the relevant inputs were obtained from the designers, users and the literature present. Not all dimensions present in generic guidelines are applicable to a case of a specific product. User interaction is important to plainly understand and incorporate the user requirements into the design of the product. The requirements are analyzed using usability-functionality transformation matrices. The information extracted is integrated to the customized checklists. As assumed, these customized guidelines prepared should enhance the usability-functionality of the product (Fig. ).
2. MOUNTAIN BIKES
There are different types of bikes available in market. These include road bikes, recumbent bikes, hybrid bikes and mountain bikes. The road bikes are designed for the use on paved roads, recumbent provide a more ergonomic design by having the rider in the laid back position rather than the more common upright position. Hybrid is a cross between road and the mountain bikes. Mountain bikes are specifically designed for the off road conditions including bumpy terrains or other rough trails. Though the recumbent bike is also available in its mountain bike variation and the hybrid bike too is good for some off road biking. But we will only focus towards mountain bike as a product for our study. For the enthusiast riding a mountain bike is purely an emotion and an inspiration. Mountain bikes are currently available in the rigid frame and the suspension frame design.
2.1 Components and Architecture of Mountain Bike
The most important components of the bike are frame, fork, brake, wheels, derailleur and crank sets. The brake, derailleur and crank set are bought from the vendor and assembled. For the purpose of brevity, a brief description of different parts used has been given below. The specifics are not included as this will be used as the concept design.
The most common frame design for the mountain bikes is the rigid frame and suspension frame design available in numerous variations. The members of the frame are down tube, head tube, top tube, seat tube, chainstay, seat stay, bottom bracket shell, shock (for suspension frames), main, and dropout pivot, and braze-ons like shock mount etc. There are 2 important suspensions in the full suspension frame design viz. the front (fork) and the rear suspension. The frame consists of the front triangle which includes fork, head tube, top tube, down, seat tube and bottom bracket shell. The rear assembly includes thee swing arm which consists of chainstays and the seat stays. For the suspension frame, the seat stays are connected to the subassembly of seat and down tube through the use of pivots. The shock is mounted onto the top tube using shock mount which is soldered to the top tube. Seat stays are then pivoted to chainstay using the dropout pivot and is pivoted to the lever at the upper end. Lever is mounted on the seat tube.
Fork connects the handlebar to the front wheel. The fork subassembly consisting of steering tube, fork crown, outer and inner fork legs, brake bosses, fork boots and the suspension comprising of spring and the dampening system. In this the inner legs are attached to the fork crown. The outer legs along with steering tube are attached to the front hub. The fork is connected to the frame through bearing mounted in the head tube.
Wheel comprises of rims, spokes, tires, tubes, cassette and hubs. Rim, spoke, nipple and hub are required to assemble the wheel. The spoke is connected to the rim with the help of nipples which are connected to the rim through eyelets. The head of the spoke is connected to the hub shells. Hub is connected to the rim through spokes. The standard cup and cone hub contains the axle and bearings. Cassette hub and cogs are integral part of rare hub; it makes sure that the wheel move freely when the pedals are not turning. Eight or nine speed cassettes are more common these days. The wheel is mounted to the frame and the fork via dropouts.
Derailleurs (front and rear) are purchased from the vendor along with the shift cables and shifters. The rear derailleur is responsible for moving the chain from one cog to another or between the chain rings. It is bolted to the hanger on the rear dropout. The front derailleur move the chain from one chain ring to another. It is mounted either on the face of the bottom bracket shell or to braze on boss. Shifters are mounted on the handlebar and is the leverage device using which user can move the chain between the chain rings and the cassette cogs. The shifter cables connect the shifters to derailleurs. The chain is routed through chain rings, cog and the jockey wheel of derailleur.
Saddle is connected to the frame through seatpost. Saddle is connected to the seatpost using the clamping bolts. The seatpost is inserted inside the seat tube and is tightened using the seat binder bolt.
Another important subassembly for the drivetrain is crankset. It consists of crank arms, chain rings, bottom bracket, chain ring bolts and crank bolts. It is purchased from a vendor. The crank arm has spider arms protruding out to which the chain rings are bolted using the chain ring bolts. The crank arm is slipped onto the bottom bracket spindle; install the crank bolt to clamp it. Pedals are screwed to the crank arm at the end.
For the mountain bikes disc and the rim brakes are the most popular types of brakes. The brake assembly consists of brake caliper, brake lever, cables. The levers are mounted on the handlebar using the bolt. Disk brakes have the pads squeezed against the hub-mounted disc. Disc brakes have the rotor bolted to the hub of the wheel; the caliper is then mounted onto the fork (Zinn 2005). The cable or hose is then used to connect the lever to the caliper. In the rim brakes pad applies the force against the wheel rims. Rim brakes are mounted on the brake boss on the fork brace. The cable is tightened at the end of the lever using the barrel adjuster and with the cable-clamping bolts on the rear wheel side. For disc brakes the cable at the front end is terminated at the brake caliper and for V-brakes at the stop on the fork brace.
Steering subassembly consists of the handle bar, stem, shifters, grips and bar ends. The bar ends are connected to the handlebar using the bar end bolts. Grips are twisted onto the handlebar. The handlebar is connected to the frame through the stem to which it is bolted. The stem further is joined to the steering tube through either the clamp bolts or is slipped into it depending on the design of steering tube.
2.2 Bicycle Manufacturing Process
A brief description of the manufacturing process is given below for the rigid frame and for the parts which are manufactured in house and not purchased from the vendor, which includes frame, fork. The designer should nonetheless be completely familiar with other components design and functioning.
There are as many different frame materials as the variations in frame design. The materials which are most commonly used include, steel, Aluminum, carbon composites, metal matrix, titanium etc. The frame is made by welding together seamless tubes. These tubes manufactured through hot extrusion process can be joined together using TIG Welding, Brazing or Lugs. The joining procedure chosen is the matter of frame material and design variable selection. For steel, aluminum and Titanium TIG is the most efficient and cost effective method. Whereas, for the carbon fiber frames lugs are more commonly used. Lugs are manufactured through investment casting. After the joining of frame various braze-ons are added such as front derailleur hanger, brake noses, water-bottle bosses are added. The joints for them are produced using silver solder. The frame is then sandblasted to remove any flux remaining. The fluxes from the tough parts are removed using taps. This includes bottom brackets, derailleur hanger, bosses mounted etc.
For alignment, the frame is fixed to the jig and is inspected for the alignment. This is done at the room temperature and is called cold setting (Govindaraju 1999).
The fork assembly consists of steering tube, a fork crown, fork legs, brake bosses and fork ends. Fork legs and the steering column are manufacturing using the hot extrusion process. The fork crown is brazed to the steering tube on the jig. The fork ends are inserted in fork legs. The 2 subassemblies are assembled with each other using the same joining techniques used for frames. The fork thereafter bent cold using mandrel. The same finishing process is then applied to fork.
The rim is produced through the extrusion process by squeezing the aluminum through a dies. After which it is roll formed and cut into wheel rings. The end is closed using the resistance welding. The rim is hardened using the T3 hardening process(Govindaraju 1999). Anodization is used to improve the appearance. Before it is final it is important to tension the wheel, truing (laterally and radially) and dishing the wheel.
The fork and the frame are grinded (till it is completely free of flux) and cleaned using the emory paper. The frame is then sent for painting and coating.
3. CUSTOMIZED DESIGN & MANUFACTURING GUILDELINES
3.1 Development Procedure
Most of the customized guidelines for the bicycles will be prepared using the generic guidelines formulated in the previous part of this paper. As for example one of the points in ease of use guideline is "Provide means to leverage the force applied by a human". A corresponding suggestion in the customized checklists is "The lever on the left handlebar must operate the front brake and similarly the right handlebar lever must operate the rear brake". It is essential to understand that not all of the items in the generic guidelines could be transformed into the corresponding customized guidelines.
Consultation with the designers and users also played a crucial role in development of the guidelines in a more specific form. The recommendations by the designers and users were incorporated into the study during the process of developing design/manufacturing linkages. For example, the above mentioned guideline after consultation was refined further into an additional item viz. "The brake pads must contact the braking surface on the wheel if the force of 10 pound or less is applied 1 inch from the end of the lever. For brake cables, use frictionless cable sealing as it will reduce the force required". The usability-functionality requirements gathered from the customers were analyzed using the suitable techniques of the flow diagrams and usability-functionality matrices; this facilitated the development of design/manufacturing linkages.
3.1.1 User Requirements
The user requirements were obtained from the users of the mountain bike. A one on one interview was conducted to gather information. All the users had been riding mountain bike for more than 5 years and had knowledge of all the different parts. All the users had been servicing and repairing their bikes to a certain extent. The following are the usability-functionality requirements put in an elaborative format below:
- Adequate stiffness: The bike should be able to withstand the weight of the rider
- Should stay steady when the force is applied while pedaling.
- Comfortable: The saddle should be designed to provide the maximum comfort to the rider.
- Smoothness: Handlebar should have a smooth contact points and should not make a rider assume an awkward posture.
- Should provide a good traction over sand and slippery surfaces.
- Suspension: Protect the rider from shocks when riding on a bumpy roads or dirt trails.
- Adjustability: The seat height and the handle height should be easily adjustable. All different type of riders within a specified age range should be able to effectively operate the brake lever.
- Smooth Shifting: v The gears should fall into place smoothly in both the directions and at multiple speeds.
- Steady handling: The handling should be steady and responsive. It should stay stable at different speeds and tracks but should respond to the slightest of the flicks.
- Effective braking: The braking should be smooth and the stopping distance should be optimum. It should be effective in both the dry and wet conditions.
- The manufacturing and the afterlife of the bicycle should be environmental friendly.
- Suitable Material: The frame should be robust and sturdy but should be light.
- Accessibility: The critical components such as chain, crankset, cassette cogs etc. should be located in the accessible locations.
- Should require low maintenance.
- Removability: Seats and Handlebars should be easily removable.
- Should have good protection from mud or other substances which may splash onto the rider while riding.
- Appearance: The bike should have a trendy and a stylish look in terms of color and shape.
- It should have a streamlined shape.
- Safety measures for ex. provision of reflectors and chain guards etc.
- Should have a good vibration and impact resistance.
3.1.2 Mapping Design Dimensions
As also discussed in the earlier part, the usability-functionality requirements can be mapped into the following design dimensions (Table 1). Each of these dimensions is dependent on one or the other design, manufacturing and material variables. They need to be tightly controlled for the optimal product design.
3.1.3 Linkage Identification
This section uses transformation matrices to establish linkages between usability-functionality requirements and product & process requirements. It has been done using series of transformation matrices.
Technical Requirement Deployment
The various usability-functionality requirements are correlated to the technical requirements in this step (fig 2). The material properties and structural rigidity are vital. For example, the material should be strong enough so as to not yield under the load and should be adequately stiff. Also to facilitate easy removal and reducing the down time the bicycle design is the modular type. Other issues like transmission efficiency, friction characteristics etc. were carefully considered.
The product features identified previous could only be implemented through the right use of manufacturing process, machines, and fixtures. The product of highest quality can only be obtained if the tight controls are established for the process and machines. The manufacturing processes will give the optimum results only if the manufacturing variables are strictly controlled. This step involves the process deployment (fig and manufacturing deployment. The knowledge of the different process variables to be controlled for improving the manufacturing processes was obtained from the manufacturing handbooks.
3.2 Checklist Development
Based on the information garnered using the above steps the design and manufacturing guidelines are developed. The guidelines are prepared in the form of checklists. Due to the restriction in space only the guidelines for Ease of Use and Performance are shown over here (Table 2 & Table 3). The items in the questionnaire could be evaluated by the designers on a 1-5 scale, with 1 being the least important and 5 being the most important.
4. SURVEY DEPLOYMENT & TESTING
4.1 Data Collection and Data Analysis
In order to test the effectiveness of the guidelines a survey was undertaken and the subsequent analysis was performed. The objective of the study was to evaluate whether if the guidelines ensures that the user's usability-functionality requirements are met or not.
The test was administered to 18 people. The population included the users of the product who maintain their bike themselves, mechanics and few designers. Each participant was given the overview of the study and were told about the design and the features of the bike under consideration.
Reliability and Validity tests were conducted on the data collected through surveys. The reliability test was conducted to see if the test yielded similar results on different people. The internal consistency of the test was determined through the reliability coefficient which in this case is the value of Cronbach Alpha. The cronbach alpha values of .4 or above was accepted as the reliable measure.
Validity establishes that how well the instrument measures what it is intended to measure. The validity of the survey questionnaires is tested by comparing it to the questionnaire which is regarded as a standard. With the absence of the standard in the case, it became essential to compare the average score of items with the overall score of the questionnaire. To find out the degree of relationship between these 2 variables correlation analysis has been carried out. The Pearson's r is most common measure to determine the association between two variables in terms of the direction and strength of relationship.
After the reliability and validity tests it was essential to determine which all design dimensions identified for this product are most influential or most important for the customers. The objective is to quantify the importance of these dimensions. The knowledge of this will help the designer to concentrate their efforts more effectively towards more important design dimensions. This will reduce the subjectivity involved in the selection of design variables. It should be noted that these dimensions are highly correlated to each other which renders the normal statistical techniques ineffective. Therefore, in order to overcome the drawback the Principal Component Regression (PCR) was used for analysis. The variables for only 2 design criteria viz. Ease of Use and Performance are screened. Those dimensions which were found to be relevant after considering the user requirements were evaluated (Table1).
According to (Dunteman 1989), when variables are highly correlated Principal Component analysis replace the correlated ones with independent ones. Principal Component is the linear combination of the original correlated variables. PCR is the regression model using Principal Components as the variables (Han and Kim 2003). The degree of influence of any variable in PCR is describes as the ratio of the variance explained by the variable to the total variance (see eq. (i)). Larger the ratio greater is the influence of the variable.
Value of 0.4 or above for the Cronbach alpha is considered to be acceptable. All the sections are considered reliable according to the rule.
As discussed earlier, the validity of the questionnaires is measured by comparing the average score of the questionnaire items with the overall score of questionnaire. And on analyzing the scores we conclude that all the design criteria are considered to be valid. The correlation coefficient for the Performance, Safety, Ecological Affinity, Reliability & Ease of Use are modestly valid as their values lie between the range 0.4 to 0.69. All the sections were found to be significant at 0.05 level.
(ii) Variable Screening
The next step was to screen the dimensions for their importance using the PCR. Based on the MINEIGEN (which was set to be equal to unity) criteria only 3 and 4 Principal Components were selected for the two criteria. The cumulative variance explained by them is 66.14% and 88.46% respectively. The questionnaire items for Ease of Use were named from V1-V8 and for Performance P1-P7. Table 5-6 represent the variance explained by each Principal Component (PC) and the loading of each variable (design dimension) on each of these PC's.
Almost all the dimensions represent equal degree of importance. Though for Ease of Use it could be observed that user found Ergonomics to be more important than others. The results come as no surprise as only the relevant dimensions were studied.
The design guidelines if followed properly are a roadmap to the development of a successful consumer product. Based on the outcome of various test conducted, it can be safely said that the guidelines developed simultaneously ensures the usability and functionality of the bicycle.
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