Journal of System Design and Dynamics, Vol.5, No.8, (2011), Pp.1572-1581

Journal of System Vol. 7, No. 3, 2013 Design and Dynamics Improvement of Bicycle Riding Comfort by Reduction of Seat Vibration∗ Junji YOSHIDA∗∗, Nobuyuki KAWAGOE∗∗ and Tomohiro KAWAMURA∗∗ **Osaka Institute of Technology, Dept. of Mechanical Engineering 5–16–1 Omiya, Asahi-ku, Osaka-shi, 535-8585, Japan E-mail: [email protected] Abstract Bicycles are popular with the general public because of their low price and easy main- tenance, and they will be an important vehicle in the future because of their low en- vironmental load. Improving the comfort of the ride is one of the important factors that will lead to increased popularity. We attempted to increase comfort by reducing vibration, and evaluated the results with a subjective test. We determined that low frequency vibration of the seat greatly affected the comfort of the ride. We then performed a transfer path analysis (TPA) and a hammering test to investigate how the vibration characteristics of the bicycle affected the vibration of the seat. Through TPA, the rear of the bicycle frame was found to have a high influence on the seat vibration, and the vibration behavior was obtained by modal analysis. In order to reduce seat vibration, a spring was inserted in the front of the seat and, to increase the stiffness, a steel plate and bolts were attached to the rear of the frame. As a result, the seat vibration while riding was decreased by about 10 dB, and the comfort of the ride was greatly improved. Key words : Bicycle, Riding Comfort, Transfer Path Analysis, Modal Analysis, Vibra- tion Control, Human Engineering, Vibration of Moving Body 1. Introduction Bicycles are popular vehicle alongside automobiles and motorcycles, and they are important in the daily life of many people because of their compact size, light weight, and low price. In addition, bicycles are an ecologically friendly vehicle because they are powered by the rider and do not require fossil fuels. The riding comfort is important for safety and easy handling, but also for good stability and good running efficiency. There have been many studies(1) – (12) that have quantified improvements in the comfort of various vehicles, including automobiles, motorcycles, and ships. In these previous studies, it was reported that comfort was related to low-frequency vibration of the seat. Studies investigating the improvement of comfort while riding bicycles, however, have rarely been performed(13), (14). In this study, we investigated the relationship between the comfort of the ride and the vibration of the seat. We measured the vibration and performed a subjective evaluation test of the comfort. We then tried to improve the comfort after obtaining the vibration characteristics from various measurements and analyses. We employed a utility bike, as shown in Fig. 1, as the improvement target because it is one of the most popular bicycles because of its low price. 2. Relationship between Riding Comfort and Seat Vibration *Received 18 Apr., 2013 (No.T2-2012-JCR-0221) 2.1. Subjective Evaluation Test for Riding Comfort Japanese Original:Trans.Jpn.Soc.Mech.Eng., Vol.78, No.792, C (2012), In this study, a subjective evaluation test was performed with several bicycles to investi- pp.2837-2847 (Received 26 Mar., 2012) gate the relationship between the perceived vibration of the seat and the riding comfort. For [DOI: 10.1299/jsdd.7.293] the evaluation, we used two utility bikes, two mountain bikes (one of which had a suspension Copyright © 2013 by JSME 293 Journal of System Vol. 7, No. 3, 2013 Design and Dynamics Fig. 1 Utility bike used for improvement of the comfort of the ride. system), and a road bike. We performed subjective evaluation tests of the perceived seat vibration and the riding comfort, and used a paired comparison method for the five bicycles. In the test, a participant first rode utility bike 1, which was set as the standard bicycle for the comparison. The participant then rode another of the bicycles. After riding the standard and comparison bicycles, the participants evaluated the seat vibration and the riding comfort of the target bicycle by comparing it with the standard bicycle. The riders were then asked to answer the following questions both orally and in writing: “how did the seat vibration of the target bicycle compare with that of the standard bicycle?”, and “how comfortable was the target bicycle compared with the standard bicycle?” In the evaluation of seat vibration, the participant chose from a total of 13 subcategories in which there were seven major categories very large, large, relatively large, same, relatively small, small, and very small and six middle categories alternating with the major categories. Numbers from +6 to −6 were applied to the major categories from very large to very small, respectively, for the perceived seat vibration. Hence, when the seat vibration was evaluated as small, the value became negative. In evaluating the riding comfort, the participant chose one of 13 subcategories, similar to the evaluation of seat vibration, but with the major categories of very comfortable, comfortable, relatively comfortable, same, relatively uncomfortable, uncomfortable, and very uncomfortable. As above, a number from +6 to −6 was applied to the major categories from very comfortable to very uncomfortable, respectively, so that the riding comfort could be analyzed quantitatively. Six males and two fe- males in their 20’s participated. In the test, each participant drove the bicycle at about 10 km/h and the air pressure of the bicycle tires were set at the standard pressure. For the test course, we used a road with a wooden deck so that the participants could more easily evaluate the vibration and comfort in a previous test. The values, averaged among all participants, of the perceived seat vibration and the riding comfort, respectively, are shown for each bicycle in Fig. 2 (a) and Fig. 2 (b). Utility bike 1 Utility bike 2 Mountain bike 1 Mountain bike 2 Road bike 6 543 2 1 0−1−2−3 −4−5 −6 −6 −5−4−3−2 −1 0 1 2 3 4 5 6 (a) Perceived seat vibration (b) Riding comfort High Low Bad Good Fig. 2 Relationship between the perceived seat vibration and the riding comfort. As shown in the figure, mountain bike 2, which has a suspension system, had the smallest perceived seat vibration and the best riding comfort among all the bicycles. Note that for the 294 Journal of System Vol. 7, No. 3, 2013 Design and Dynamics utility bicycle, both the perceived seat vibration and the riding comfort were zero because it was set as the standard bicycle against which the others were compared. The results also show that when the perceived seat vibration was smaller, the riding comfort was considered to be better, and vice versa. This tendency was identical to what was found in previous studies that considered the riding comfort of other vehicles(1) – (12). 2.2. Relationship between Vibration Acceleration Level of Seat and Riding Comfort The vibration acceleration of the seat of each bicycle was measured in order to investigate which frequency band had the highest influence on riding comfort. The measurement was performed on the same running course and under the same conditions as the subjective evaluation test, and measured vibration acceleration along the vertical axis at the seat. For mountain bike 2 (most comfortable) and utility bike 1 (standard bicycle), Fig.3 com- pares the acceleration level at each frequency (LVa: Eq. (1)). −6 2 2 LVa = 20log(a=a0); a0 = 10 [m=s ]; a : Vibration acceleration [m=s ] (1) 120 (dB) Va L 110 100 90 Mountain bike 2 Utility bike 1 80 Acceleration level: 0 5 10 15 20 25 30 35 40 45 50 Frequency (Hz) Fig. 3 Comparison of acceleration level at running condition between mountain bike 2 and utility bike 1. The horizontal and vertical axes show the frequency and vibration acceleration level(LVa), respectively, and the solid and dotted lines indicate the acceleration levels of mountain bike 2 and utility bike 1. As shown in the above figure, the vibration acceleration level of mountain bike 2, which was evaluated as the most comfortable, was not the smallest for all frequency bands, but it was smaller than that of the standard bicycle at frequencies of less than 30 Hz. This tendency was also observed with the other bicycles. From these results, it was found that the seat vibration is related to the riding comfort; in particular, when the seat vibration was small at low frequencies (less than 30 Hz), the bicycle was evaluated as more comfortable. Similar tendencies were reported by studies investigating the riding comfort of automobiles and motorcycles(1) – (12). 3. Vibration Characteristics of the Bicycles 3.1. Vibration Characteristics During Running Test As a first step to improve the riding comfort, we obtained the vibration characteristics of utility bike 1. We measured the vibration acceleration of the seat along the vertical axis while the bicycle was moving, and the results are shown in Fig. 4. The measurement conditions were identical to those for the vibration acceleration measurements in the previous section, and the vibration of utility bike 1 is shown as dotted line in Fig. 3. The results show that utility bike 1 has several vibration peaks at frequencies less than 30 Hz, and it is necessary to reduce these high vibration peaks in order to reduce the seat vibration. 3.2. Contribution Analysis Using Transfer Path Analysis In order to determine effective countermeasures for the seat vibrations, transfer path analysis (TPA)(15) – (18) was performed to obtain the contributions from the front and rear wheels.

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