Predicting ride comfort with reclined seats

Abstract

Reclined seats in transport suggest luxury and comfort, but a review of the literature revealed little study of how backrest inclination influences the discomfort caused by vibration of a seat or a backrest. This thesis seeks to advance understanding of the influence of backrest inclination on vibration discomfort and provides a model for evaluating vibration discomfort and metrics for optimising seats with different backrest inclinations.

Vibration discomfort depends on the direction and location of vibration input to the body. Subjects used magnitude estimation to judge vibration magnitudes from thresholds of perception up to 2 ms-2 r.m.s. at the 11 preferred 1/3-octave centre frequencies from 2.5 to 25 Hz. The first two experiments determined absolute thresholds and discomfort with x-axis backrest vibration (Experiment 1) and z-axis backrest vibration (Experiment 2) with four backrest inclinations (0°, 30°, 60°, and 90° from vertical). The third experiment investigated discomfort with vertical seat pan vibration and five backrest conditions (no backrest and backrest inclined to 0°, 30°, 60°, and 90°). With x-axis vibration of the back, inclining the backrest had similar effects on thresholds and equivalent comfort contours. Thresholds increased at frequencies from 4 to 8 Hz with increasing inclination of the backrest. With inclined backrests, 40% greater magnitudes of vibration were required from 4 to 8 Hz, to cause discomfort equivalent to that with the upright backrest. Frequency weighting Wc in current standards predicted discomfort and perception of x-axis vibration of the upright backrest (0°) but weighting Wb was more appropriate for inclined backrests. Frequency weighting Wd was appropriate for both discomfort and perception of z-axis vibration of the back at all backrest inclinations. With vertical seat acceleration, the frequency of greatest sensitivity decreased with increasing vibration magnitude. Compared to an upright backrest, around the main resonance of the body the vibration magnitudes required to cause similar discomfort were 100% greater with 60° and 90° backrest inclinations and 50% greater with a 30° backrest inclination.

The fourth experiment investigated whole-body vertical vibration on a rigid seat with no backrest and with four backrest inclinations. With an inclined backrest, discomfort caused by high frequency vibration increased at the head or neck but discomfort at the head or neck caused by low frequencies (5 and 6.3 Hz) reduced. With inclined backrests, the procedures in current standards overestimate overall discomfort at frequencies around 5 and 6.3 Hz but underestimate discomfort caused by frequencies greater than about 8 Hz.

The final experiment investigated a model for predicting vibration discomfort with three compliant reclined seats. At each frequency, the measured seat dynamic discomfort, MSDD (the ratio of the vibration acceleration required to cause similar discomfort with a compliant seat and a rigid reference seat), was compared with seat effective amplitude transmissibility, SEAT value (the ratio of overall ride values with a compliant seat and a rigid reference seat using the weightings in current standards). The compliant seats increased vibration discomfort at frequencies around the 4-Hz resonance but reduced vibration discomfort at frequencies greater than about 6.3 Hz. The SEAT values provided appropriate indications of how the foam increased vibration discomfort at some frequencies but decreased vibration discomfort at other frequencies. Differences between the SEAT values and the measured seat dynamic discomfort are consistent with the need for different frequency weightings when the body is supported by an inclined backrest.

An empirical model was evolved from the experiments for predicting vibration discomfort with reclined seats. It is concluded that reclining a backrest will tend to be detrimental at frequencies greater than about 10 Hz with greater discomfort in the head or neck induced by vibration of the backrest. At frequencies around 5 and 6.3 Hz, reclining a backrest can reduce discomfort.

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Identifiers

ORCID for M.J. Griffin: orcid.org/0000-0003-0743-9502

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