F2008-04-022

AN ERGONOMIC INVESTIGATION FOR THE DISCOMFORT OF THE LOWER EXTREMITIES DURING INGRESS/EGRESS OF SUV

Lim, Sung-Hyun, Choi, Woong-Ryul, Lim, Hyoung-Uk Hyundai Motor Company, South Korea

KEYWORDS – Discomfort, Ingress/Egress, Side Sill, SUV, Ergonomics

ABSTRACT - The purpose of the study is to investigate the discomfort of the lower

extremities during ingress/egress of sports utility vehicles (SUV). Ingress/egress is one of the most important issues for ergonomic evaluation of vehicles. Nowadays the width of the side sill of SUV tends to increase in order to strengthen ‘tough’ feeling of SUV, which could make the customers hard to ingress and/or egress and increase the discomfort of the lower

extremities of the customers during ingress/egress. It means that, in order to determine the optimal side sill design of SUV, we have to consider not only the ‘tough’ feeling of SUV but also the ease to ingress/egress. So we investigated the subjective discomfort of the lower extremities for several design alternatives of the side sill, especially for SUV.

A three-factor within subject design was adopted for the experiment. As independent variables, the step over width, the step height and the H-pt height were selected. The

independent variables mean the distance from the seated hip point (H-pt) to the most outward point of the side sill, the distance from the ground to the highest point of the side sill, and the distance from the ground to the seated hip point (H-pt), respectively. For the statistical

analyses, the subjective discomfort during ingress and/or egress was selected as a dependent variable.

The results showed that 1) the step over width and the step height had significant effects on the discomfort during ingress/egress of SUV 2) an equation which can predict the discomfort level of the lower extremities during ingress/egress was developed.

INTRODUCTION

Ingress/egress is one of the most important issues for ergonomic evaluation of vehicles. Nowadays the width of the side sill of SUV tends to increase in order to strengthen ‘tough’ feeling of SUV, which could make the customers hard to ingress and/or egress and increase the discomfort of the lower extremities of the customers during ingress/egress, as shown in Figure 1. It means that, in order to determine the optimal side sill design of SUV, we have to consider not only the ‘tough’ feeling of SUV but also the ease to ingress/egress. So we investigated the subjective discomfort of the lower extremities during ingress/egress for several design alternatives of the side sill, especially for SUV.

The purpose of the study is to investigate the discomfort of the lower extremities during ingress/egress of SUV.

(a) Discomfort of the foot (b) Discomfort of the calf Figure 1. Examples of Discomfort during Ingress/Egress of SUV

METHODS

Subjects

A total of 10 subjects volunteered for participation in the experiments. The subjects included two 153cm tall women, the 5th percentile Korean women based on the stature, and a 180cm tall man, the 90th percentile Korean man based on the stature.

Experimental Procedure

The subjects were asked to stand at a specified position in front of the front door of a SUV which was modified in order to be able to change design parameters of the side sill with a specified value, then ingress the vehicle with their own pace, take their preferred driving posture, and finally egress out of the vehicle. After the second trials of ingress/egress the vehicle, they evaluated the subjective discomfort of the lower extremities during

ingress/egress with 9-point scales (1: most discomfort, 9: least discomfort). Motions were recorded using video cameras.

Experimental Design

A three-factor within subject design was adopted for the experiment [1]. As independent variables, the step over width, the step height and the H-pt height were selected, summarized in Table 1. As illustrated in Figure 2, the independent variables mean the distance from the seated hip point (H-pt) to the most outward point of the side sill, the distance from the ground to the highest point of the side sill, and the distance from the ground to the seated hip point (H-pt), respectively. For the statistical analyses, the subjective discomfort of the lower extremities during ingress/egress of SUV was selected as a dependent variable.

Table 1. Independent Variables

Independent Variables Step Over Width Step Height H-pt Height Level 3 3 3

Definition 690, 720, 750 460, 490, 520 460, 495, 505

A

B CFigure 2. Independent Variables

RESULTS

Results of the Experiment

The effects of step over width, step height and H-pt height on the discomfort of the lower extremities during ingress/egress of SUV were evaluated using an analysis of variance (ANOVA). The results are summarized in Table 2.

Table 2. Summary of ANOVA results for discomfort during ingress/egress

Source DF SS F 2.40 P-value 0.133

HP 2 7.26 SH 2 53.01 12.41 0.001* SW 2 33.91 15.55 < 0.001* HP*SH 4 HP*SW 4 0.71 0.93 0.14 0.54 0.968 0.711

SH*SW 4 2.69 1.17 0.350

Note: ‘HP’: H-pt Height, ‘SH’: Step Height, ‘SW’: Step Over Width, * : statistically significant result (p<0.05).

The step over width and step height had significant influences (p < 0.05) on the discomfort during ingress/egress. Figure 3 shows that as the step over width increased, the discomfort of the lower extremities during ingress/egress increased. A post hoc Newman-Keuls test [2] indicates that when the step over width was 460, the discomfort level was significantly

different from those when the step over width was more than 495mm. As shown in Figure 4, as the step height increased, the discomfort during ingress/egress also increased. A post hoc Newman-Keuls test [2] indicates that when the step over width was 520mm, the discomfort level was significantly different from those when the step over width was less than 490mm.

Figure 3. The effects of the step over width on the discomfort during ingress/egress. The different letters show a significant difference at α=0.05 level. (1: most discomfort, 9: least discomfort).

Figure 4. The effects of the step height on the discomfort during ingress/egress. The different letters show a significant difference at α=0.05 level. (1: most discomfort, 9: least discomfort).

Figure 5a indicates that there was a tendency that when the step height was more than 490mm and the step over width was more than 495mm, the discomfort was less than 4.0 which was likely to be ‘bad’ discomfort level. In these cases, the calf of small female subjects hit the side sill of SUV and/or the heel of them did not contact the floor, as illustrated in Figure 1. Figure 5b represents that the step height and the step over width of ten current SUVs in world wide market. The A, B, C models were likely to be ‘bad’ discomfort level and the other models were likely to be ‘moderate’ discomfort level.

(a) Experimental results (b) Discomfort level of SUVs

Figure 5. The tendency of discomfort according to the step over width and the step height.

Figure 6a also indicates that there was a tendency that when the step over width was more than 500mm and the H-pt height was about 750mm, the discomfort was less than 4.0 which was likely to be ‘bad’ discomfort level. In these cases, the calf of small female subjects hit the side sill of SUV or the heel of them did not contact the floor, as illustrated in Figure 1. Figure 6b represents that the step over width and the H-pt height of ten current SUVs in world wide market. The A, B, C models were likely to be ‘bad’ discomfort level and the other models were likely to be ‘moderate’ discomfort level.

(a) Experimental results (b) Discomfort level of SUVs

Figure 6. The tendency of discomfort according to the step over width and the H-pt height.

Discomfort Prediction Model of the Lower Extremities during Ingress/Egress of SUV

Linear regression analysis was conducted in order to determine the functional relationship between the design parameters of SUV and the discomfort of the lower extremities during ingress/egress. The regression models made it possible to predict the discomfort level of the lower extremities during ingress/egress for an anonymous design of SUV. The input variables of the regression models were the step over width (SW), the step height (SH), the H-pt height (HP) and the stature of the target customer (ST). The dependent variables were the discomfort level of the lower extremities during ingress/egress of SUV with 9 point scales. The regression equation for the discomfort during ingress/egress was Discomfort=25.8−β1(SW)−β2(SH)−β3(HP)+β4(ST).

CONCLUSION

It is reasonably concluded that the discomfort of the lower extremities during ingress/egress of SUV was strongly influenced by the step over width and the step height. As the step over width and the step height increased, the discomfort of the lower extremities during

ingress/egress increased. In addition, there was a tendency that the small female subjects were likely to feel ‘bad’ discomfort when the step over width was more than 500mm and the H-pt height was 750mm. The results can be applied to develop a new SUV vehicle design in which drivers will be able to ingress/egress with less discomfort of the lower extremities.

REFERENCES

[1] Keppel, G. “Design and Analysis: a researcher’s handbook”, Prentice-Hall, 3rd edition,

Englewood Cliffs, NJ, 1991. [2] Montgomery, D. G. “Design and Analysis of Experiments”, Wiley, 5th edition, 2000.