The Effect of Bicycle Geometry, Rider Posture and Rider Anthropometrics
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The Effect of Bicycle Geometry, Rider Posture and Rider Anthropometrics on Pitch-Over Dynamics by Alex Moorhead B.S., University of Colorado at Colorado Springs, 2013 A thesis submitted to the Graduate Faculty of the University of Colorado at Colorado Springs in partial fulfillment of the requirements for the degree of Master of Science Department of Biology 2015 © Copyright by Alex Moorhead 2015 All Rights Reserved The thesis for the Master of Science degree by Alex Moorhead has been approved for the Department of Biology by Jeffrey Broker, Chair Jacqueline Berning Andrew Subudhi Jay Dawes 26 August, 2015 ii ABSTRACT Cycling is a popular activity that has been extensively studied. Unfortunately, within the research, there are sparse insights regarding pitch-over accidents. A pitch-over accident occurs when a bicycle is introduced to an abrupt deceleration, often from a front wheel impact or excessive front wheel braking. Pitch-overs due to front braking are avoided if deceleration levels do not exceed the longitudinal stability of the rider/bicycle system, which is defined by the location of the combined center of mass (COM) relative to the front tire contact point. The purpose of this study was to determine how bicycle designs and geometries, plus rider postures, effect rider/bicycle stability, and pitch-over propensity. This study began by presenting and validating the use of a new force plate method (FPM) for locating rider and bicycle COM locations, as an alternative to a traditional anthropometric method (AM) used by Winter et. al. (2009). COM location estimates developed from the FPM were then compared to estimates derived using the AM. Finally, the FPM was used to evaluate the effects of bicycle types and rider postures on COM locations, and thus deceleration thresholds at pitch-over. The FPM was found to be much faster and more accurate than the AM. Resultant error with the FPM was less than 1cm (7.8mm). Errors in the AM approached 135 mm. The FPM then exposed how both bicycle geometry and rider posture play large roles in effecting the deceleration thresholds of bicycles. Deceleration threshold differences between bicycles with similar rider positions were as large as .20 Gs, and these thresholds approached .21 Gs across different rider positions of the same rider on the same bicycle. The results derived from this study expose the effects of bicycle types and rider postures on pitch-over propensity, and provide an accurate method for examining these effects. iii TABLE OF CONTENTS CHAPTER I. INTRODUCTION……………………………………………………………………..……….1 II. LITERATURE REVIEW……………………………………………………………..…...…4 Mechanisms of Pitch-Over Accidents………………………………….…….4 Pitch-Over Trajectories across Deceleration Mechanisms…….….….5 Pitch-Overs Involving Fork Failure…………………………………….……..5 Pitch-Over Dynamics………………………………………………………………6 Optimal Braking……………………………………………………………………..7 III. MATERIALS AND METHODS……………………………………………………………9 Subjects…………………………………………………………………………………9 Materials……………………………………………………………………………….9 Protocols………………………………………………………………………………10 Analysis………………………………………………………………………………..17 IV. RESULTS……………………………………………………………………………………….24 V. DISCUSSION………………………………………………………………………….……...29 VI. CONCLUSIONS………………………………………………………………………………33 REFERENCES…………………………………………………………………………………………………..….35 iv TABLES Table 1. Comparison of FPM and AM locations………………….….……………………………....25 2. COM Position change due to saddle height changes….…………………………….….27 3. COM location differences due to posture and bicycle type………………………..…28 4. Average deceleration thresholds across bicycles, subjects and postures…….…28 v FIGURES Figure 1. Force plate validation; inclined position……………………………………….……………11 2. Rider with markers; “Hoods” position, flat and inclined……………………………..14 3. FBD of forces acting on system………………………..……………………………………….18 4. FBD with relevant dimensions………………………………………………………………….19 5. FBD, inclined positon………………………………………………………………………………19 6. Comparison of COM……………………………………………………………………………….20 7. Calculated COM vs. Measured COM………………………………………………………….24 8. Graphic representation of difference between FPM and AM……………………….26 vi CHAPTER 1 INTRODUCTION Bicycling is an activity enjoyed by many people for transportation, recreation and competition. Unfortunately, although bicycling is generally beneficial, it also carries inherent risks for its participants. According to Werner et al. (2001), there are hundreds of thousands of bicycling accidents every year. With such high occurrences, further investigation related to the possible causes of these accidents is valuable to all types of riders. Bicycle crashes can be initiated by many different factors. Collisions, component failures, rider error, hazardous terrain and loss of control are some of the most commonly identified factors associated with bicycling accidents (Broker, 2006). All of these factors can potentially result in a specific cycling accident known as a pitch-over. A pitch-over is characterized by a rider and his bicycle encountering a rapid deceleration, resulting in a forward somersault about the front wheel. Pitch-over accidents have been studied in situations where the event is unavoidable, such as front wheel impacts and simulated component failures (Werner 2001). However, very little research has focused on pitch-overs resulting from over-application of the front brake. According to the Consumer Product Safety Commission’s (CPSC) requirements for bicycles with hand brakes (front and rear), application of both brakes must effectively stop the bicycle from a velocity of 15 mph in 15 feet. The corresponding deceleration rate is equivalent to approximately 0.5 Gs (one G force is the acceleration due to earth’s gravity); a lower braking value than most bicycles are capable of achieving. Unfortunately, bicycle deceleration rates leading to pitch-over are often just slightly higher than the CPSC requirement in many bicycle type/rider posture combinations (Broker, 2006). 2 The disastrous pitch-over sequence is initiated, or avoided, based solely on the interaction between two factors; rider/bicycle center of mass (COM) location relative to the front wheel contact point and rate of bicycle deceleration. To better understand pitch-over crashes, this study focuses on the location of the system (bike plus rider) COM, how it is altered by various bicycle and rider position factors, and how these factors influence maximum deceleration threshold before pitch-over occurs. To determine the location of the COM of a rider and bicycle system, one must know the COM locations of both the bicycle and the rider. Locating the COM for a bicycle is easy, straight-forward and results in very low error. This is most easily performed using a double suspension method. The bicycle is suspended sequentially from two different sites (e.g., the handlebars and the seat), and the intersection of plumb bobs hung through the bicycle from the suspension locations establishes the location of the bicycle’s COM. Determining the location of a rider’s COM on a bicycle is not as simple. Suspending a bicycle and rider from two locations is impractical, dangerous, and replication of rider position in the suspended state is difficult. As such, the most popular method of determining COM for a rider seated on a bicycle incorporates an anthropometric method (Winter, 2009). This method, which derives an estimate of the rider’s COM based on the summation of estimated body segment COMs, is time intensive and subject to error. Although the anthropometric method has been used for many years, greater accuracy can be achieved with less computation time using a novel force plate-based method. The purpose of this study was to offer a new approach to determining bicycle rider COM, using force plates. After validation of such a method, its derivation of rider COM is compared to the anthropometric method and then applied to multiple bicycle/rider position combinations to quantify the effects of bicycle geometry, rider position and rider anthropometrics on the location of the rider/bicycle system COM. 3 Our hypotheses are, first, that the force plate method is more accurate than the anthropometric method, and the differences between the methods are relevant to accurate quantification of deceleration thresholds. Additionally, we hypothesize that COM location variations across rider positions, bicycle designs and rider anthropomorphics are large, and thus have a critical effect on the maximum deceleration rate a bicycle can achieve before pitch-over. Finally, it will be shown that some rider/bicycle combinations exhibit deceleration thresholds that are dangerously close to the braking requirements outlined in the CSPC guidelines. CHAPTER 2 LITERATURE REVIEW Mechanisms of Pitch-Over Accidents Pitch-over accidents are highly common occurrences in bicycling and often result in disastrous injuries (Bretting, 2010). There are a multitude of commonly misunderstood or disregarded variables that contribute to pitch-over accidents. Pitch-over accidents are essentially caused, or avoided by, the combination of two factors; bicycle deceleration and rider/bicycle center of mass position. Broker (2006) outlined many of the events that cause rapid bicycle deceleration – potentially leading to pitch-over. These events include the bicycle striking an object, such as a curb, the bicycle encountering a rapid elevation change, such as the uphill side of a deep gully or ditch, objects (including bicycle components) entering the front wheel spokes or locking up the front wheel, and excessive