TRANSPORT and ROAD RESEARCH LABORATORY …semi-elliptical leaf springs semi-elliptical leaf springs...

TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment Department of Transport

SUPPLEMENTARY REPORT 560

ROAD SURFACE IRREGULARITY AND VEHICLE RIDE PART 3 - RIDING COMFORT IN COACHES AND HEAVY GOODS VEHICLES

by

D R C Cooper M Phil and J C Young MIOA

Any views expressed in this Report are not necessarily those of the Department of the Environment or of the Department of Transport

Construction and Maintenance Division Highways Department

Transport and Road Research Laboratory Crowthorne, Berkshire

1980 ISSN 0305- 1315

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

Abstract

1.

2.

.

4.

.

CONTENTS

Introduction

Design of experiment

2.1 Test sites

2.2 Test sections

2.3 Measurements in coaches

2.3.1 Details of coaches

2.4 Measurements in lorries

2.4.1 Details of lorries

Experimental equipment

Procedure

4.1 Coach experiment

4.1.1 Objective measurements of ride

4.1.2 Subjective assessment of ride

4.1.3 Classifications of ride assessments

4.2 Lorry experiment

4.3 Analysis procedure

Results and discussion

5.1 Numbers of measurements

5.2 Coach results

5.3 Discussion of coach results

5.3.1 Response of coach occupants to ride

5.3.2 Travel characteristics of coach passengers

5.3.2.1 The difference between the public and TRRL assessment of ride

5.3.2.2 The difference between ride assessments in cars and coaches

5.3.3 Coach seat location and ride

5.3.4 Coach vibration levels and ISO standards

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5.3.5 The effect of load on coach vibration

5.3.6 The effect of speed on coach vibration

5.4 Lorry results

5.5 Discussion of lorry results

5.5.1 Response to ride o f lorry occupants

5.5.2 Lorry driver and passenger seats

5.5.3 Lorry vibration levels and ISO standards

5.5.4 The effect of load and speed on lorry vibration

5.6 Comparison of car, coach, and lorry vibration

Summary of results

Conclusions

Future work

Acknowledgements

References

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(C) CROWN COPYRIGHT 1980 Extracts from the text may be reproduced, except for

commercial purposes, provided the source is acknowledged

ROAD SURFACE IRREGULARITY AND VEHICLE RIDE PART 3 - RIDING COMFORT IN COACHES AND HEAVY GOODS VEHICLES

ABSTRACT

An investigation is described of the riding comfort in long-distance coaches and heavy goods vehicles, operating over typical trunk road surfaces. Ride, represented by the root-mean-square (rms) of the vertical acceleration at the seat-person interface, has been measured in both coaches and lorries and correlated with riding comfort assessments of drivers and passengers. Com- parisons are made with the ride in cars measured in previous experiments.

Results show that coach passengers and lorry drivers will tolerate higher acceleration levels more readily than will car occupants. Psychological factors, as well as vibration levels influence the subjective assessment of ride.

Evaluation of the highest measured acceleration levels against recommended International Standards of human response to whole-body vibration show a possibility that lorry drivers could, in certain cases, suffer some fatigue- decreased proficiency within their present legally permitted periods of continuous driving.

The ride in lorries could be improved by the more widespread use of suspension seats.

1. INTRODUCTION

Previous reports 1,2, discussed the importance of an even surface finish on new and reconstructed roads and described

experiments relating subjective impressions of ride to measurements of vertical acceleration in moving cars.

This Report describes a similar investigation of the riding comfort experienced in long-distance coaches and

heavy goods vehicles. Goods vehicles account for about 18 per cent of the total vehicle mileage travelled on roads

in Great Britain 3. About one-half of the total goods vehicle mileage is travelled by heavy goods vehicles. The

distance travelled by buses and coaches is about 2 per cent of the total for all vehicles, but where their capacity is

fully utilised, they provide a more efficient form of transport than private cars. These vehicles differ from cars both

in their geometric dimensions and in the types of suspension used. In addition, these vehicles, unlike the great

majority of cars, are driven by professional drivers who are exposed to vehicle vibrations during the greater part of

their working life. Evidence from recent research 4 suggests that prolonged exposure to vibration could contribute

significantly to driver fatigue. The recently introduced International Standard, ISO/DIS 26315 is a guide to the

evaluation of human exposure to whole-body vibration; levels experienced by the driver should be below the

recommended fatigue-decreased proficiency boundary for his permitted working hours. There is a need to examine

these levels more closely and to assess the possibility of the vibration having detrimental effects on driver performance.

This Report describes experiments in which vertical acceleration levels inside operational heavy goods vehicles

and long-distance coaches have been measured and related to the.subjective assessments of ride made by the vehicle

occupants. The results are compared and contrasted with those obtained in private cars 1,2

2. DESIGN OF EXPERIMENT

2.1 Test sites

The three sites, chosen to provide examples of poor, good and very good riding quality, were situated on the

A12 trunk road between Brentwood and Colchester in Essex and were among those used in the ride experiments

involving private cars described in the previous reports 1,2. Details of the sites are given in Table 1. In two of them

the pavement was in unreinforced concrete; and the third of hot-rolled asphalt.

2.2 Test sections

On each site, six consecutive 300m sections were used on each carriageway and were the same as those used in

the previous work.

2.3 Measurements in coaches

Recordings were made on in-service 'Anglian Express' coaches as they travelled over each of the three test

sites en-route between London and Great Yarmouth or Clacton and London. The team of four TRRL observers

boarded the coach at its Scheduled stop at Colchester Coach Station and carded out measurements on the west-

bound carriageway of each of the three sites, before alighting at or near Brentwood. The research team then

boarded the next scheduled coach for the return journey to Colchester and similar measurements were made on

the east-bound carriageway of the three test sites.

2.3.1 Details o f coaches: Table 2 gives details of the coaches in which measurements were carried out.

2.4 Measurements in lorries

Arrangements were made with independent haulage contractors in the Chelmsford area for members of TRRL

staff to travel in the cabs of their vehicles to enable measurements of ride to be carded out. In this way, it was

possible to examine a wide range of rigid and articulated lorries and to sample them in both the laden and unladen

condition.

The TRRL operator boarded each lorry at its depot and travelled with it over as many as possible of the

three test sites that were on its scheduled route. Where convenient, arrangements were made to meet the same

lorry at a pre-arranged point on its return journey. Similar measurements were then made on the return carriageway,

the lorry being in a different load condition.

2.4.1 Details o f lorries: Table 3 gives details o f the lorries studied in the experiment.

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Types of lorries studied

Year of Manufacturer Type Manufacture

Albion Super Riever 1969

Albion Clydesdale 1971

Atkinson T32/46 L X B 1969

Atkinson T32/46 L X B 1971

Atkinson T32/46 C205 1971




Bedford KM 1969

British Boxer 1974 Leyland

British Leyland Super Comet 1974

British Marathon 1976

Leyland

Commer Chrysler Commando G11i 1976

Chrysler Dodge G16 1973

Scammell Handyman 1971

Scania LB 80 Super 1970



Unladen Weight

(kg)

6328

5176

9652

9652

9906

9906

Type of Suspension Remarks

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Tractor Unit 9906

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10918 Taper leaf springs Taper leaf springs Tractor & dampers & dampers Unit


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3. EXPERIMENTAL EQUIPMENT

Two sets of self-contained portable cassette recording systems, described in the previous report 1 , were used to

record vertical accelerations at the seat-person interface and other information relating to these measurements.

Checks on equipment functions were made and recordings monitored on a replay system mounted within a mobile

laboratory stationed at a convenient base depot. The cassette records were returned to TRRL for subsequent

processing and analysis.

4. PROCEDURE

4.1 Coach experiment

4.1.1 Objective measurements of ride: The two portable recording systems were used on each coach to record

vertical accelerations at four different seating positions, as shown in Figure 1. The first system was used to record

the input to the coach driver and also to the TRRL team member (A), normally seated at the front nearside of the

coach. At the same time, the second system recorded the vertical accelerations experienced by the TRRL members

at the mid-coach (B) and rear seat (C) positions. This arrangement enabled subsequent comparisons to be made of the

ride experienced throughout the coach.

4.1.2 Subjective assessment of ride: Coded information relating to the experiment, such as the type of coach,

identification number of the test site, direction of travel and measurement point, was recorded as the coach

approached the marked test lengths o f road. After passing over the test length, the subjective impressions of

riding comfort o f the driver and T R R L staff seated on accelerometers at the four measurement points in the coach

were recorded onto the tape cassette in a coded form. The TRRL observer (D) who also sat at the mid-point of

the coach,documented each run and also recorded the assessments of the passengers seated nearest to each measure-

ment position.

4.1.3 Classifications of ride assessments: Subjects were asked to classify the ride over a particular test length

into one of the four categories:-

1) Comfortable

2) Acceptable

3) Uncomfortable

4) Very uncomfortable

The subjects were also asked, where appropriate, to classify their reasons for rating a ride 'uncomfortable or

worse ' into one of the following three categories:-

1) Noise

2) Vibration

3) Bumpiness

In addition, the coach driver was asked for his opinion on a fourth category:-

4) Vehicle handling

The subjects were also asked how often they travelled by coach, and by car, and why they chose to travel by coach.

It was thought possible that some people travelling on coaches may never have ridden in a car and they might

assess the ride differently from those passengers whose normal means of transport was a car.

4.2 Lorry experiment

For each of the lorries included in the survey, a member of TRRL staff carried out the ride assessment sitting

in the passenger seat of the lorry cab as it travelled over the test lengths of road.

Vertical accelerations at the seat-driver and seat-passenger interfaces were recorded in each lorry as it passed

over the test sites. Coded information on lorry type, state of loading, etc was recorded as described in Section 4.1.2.

As in the case of the coach study, the driver's and the passenger's classification of the ride and comments on poor

ride assessments were recorded for subsequent analysis.

On a selected number of test runs, noise levels inside the vehicle cab were measured using a Bruel and Kjaer

Type 2203 Sound Level Meter.

The siting of the transport depots was such that it was not possible for the TRRL staff to monitor ride over

more than two test sites on one run with a particular lorry. Because the transport operators scheduled most of their

runs at very short notice, some lorries were missed. Although this reduced the size of the sample, a sufficient

number of lorries were included to give a representative cross-section of their ride levels.

In both the coach and lorry experiments, the drivers were asked to drive normally. They were free to choose

their own speed and traffic lane when travelling over the test sites. Any changes in lane were logged on a separate

data track of the tape recorder.

4.3 Analysis procedure

Completed cassettes were initially processed using a mini-computer to produce punched paper tapes which

were corrected for any obvious errors. Further analysis 1 of the paper tapes was then carried out on the TRRL

ICL 4-70 computer.

For each test section, a listing was obtained, of the vehicle average speed and the calculated root-mean-square

(rms) vertical acceleration within the frequency band of 0.2 Hz to 20 Hz experienced by each test subject. The

coded associated information for each test run was also listed.

5. RESULTS .AND DISCUSSION

5.1 Numbers o f measurements

Of the total o f 324 recordings made in coaches, only twelve were found unsuitable for analysis because of

operator error or equipment malfunction. Ten different coaches were sampled on 27 runs, resulting in 312 valid

recordings with corresponding assessments of ride from four TRRL staff, eight drivers, and over 300 different

fare-paying passengers.

The lorry experiment resulted in 126 valid recordings of vertical acceleration and corresponding assessments

of ride given by 12 drivers and four TRRL staff from a total of 67 runs using 18 different lorries. An equal number

of recordings were obtained for both driver and passenger. Of the total of 134 recordings, eight were unsuitable for

analysis.

5.2 Coach results

The coach results are summarised in Tables 4(a), 4 (b) and 4 (c). Tables 4(a) and 4(b) show combined results

from the three passenger-seating positions monitored in the coach experiment. Table 4(a) shows results given by

the fare-paying passengers, Table 4(b) gives those results obtained from the TRRL team. The total number of

assessments f rom the public given in Table 4(a) exceeds that of the TRRL team given in Table 4(b) because at

times more than one passenger seated on the long rear bench seat of the coach was asked for an assessment of

comfort . Table 4(c) shows the results from the coach driver's seat. In each of these three tables, values of the

calculated rms vertical acceleration are classified into bands of 10 x 10"3g. Each number entered in the Table

gives the number of times an acceleration level within a band was rated comfortable, acceptable, uncomfortable

or very uncomfortable.

Results given in Table 4(a) are for those members of the public seated nearest to the measurement position

(see Section 4.1.2). It is assumed that the same accelerations were experienced by both the fare-paying passenger

and the member o f TRRL staff. Normally, the passenger was seated next to the TRRL operator on the same

bench seat.

Results for the driver are considered separately in Table 4(c) because the driver's seat differed from the bench

seats fitted in the rest of the coach; also, the periods of exposure to the coach vibration of the driver were longer

than those o f the passengers.

The data given in Tables 4(a), 4(b) and 4(c) show trends similar to those in previous experiments 1,2, in that

increasing levels of acceleration increase the probability of a ride being assessed as uncomfortable, and there is a

large variation in levels of acceleration associated with any one of the four possible ratings of comfort. The

results have therefore again been interpreted in probabilistic terms as 'comfort characteristic' curves.

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5.3 Discussion of coach results

5.3.1 Response of coach occupants to ride: Results given in Table 4 are shown in the form of 'comfort

characteristic' curves in Figures 2 and 3. Figure 2 shows the probability of a member of the public seated in any of

the measurement positions rating the ride as acceptable or better. The 'comfort characteristic' curve obtained in the

previous work 2 for members of the car-driving public is also shown in Figure 2. Similarly, in Figure 3, the 'comfort

characteristic' curves giving the assessments of ride by the TRRL team in coaches and in private cars 1 are compared.

The corresponding curve for the coach driver is also plotted in Figure 3; this curve, because of the fewer observations

(71), is less reliable than the curves for the public (243 observations) and for the TRRL staff (210 observations).

Figure 2 shows that there is a 94 per cent probability that an acceleration level less than 60 x 10 -3 g rms

would be rated as acceptable by a member of the public travelling in the coach. However, it also shows that there is

only an 83 per cent probabili ty of this rating when the same level of acceleration was experienced by the public in

cars. Figure 3 shows that the corresponding probabilities of the TRRL team giving the same rating are 82 per cent

and 55 per cent respectively.

It is apparent f rom Figures 2 and 3 that factors other than the rms acceleration affect individual judgements

of ride. Although the overall characteristics remain the same, both the TRRL team and the public were less critical

of ride in coaches than in private cars. It is also clear that the TRRL team were generally more critical of ride than

the public. Possible reasons for these differences in 'comfort characteristic' curves are examined in the following

sections.

5.3.2 Travel characteristics o f coach passengers: Of over 300 passengers questioned on the coaches during the

experiment, about 40 per cent travelled by coach at least once per month and nearly 70 per cent used the coach at

least once every three months. Roughly half the sample never travelled by car. Half the total sample travelled by

coach because it cost less than other forms of public transport. Nearly 40 per cent preferred to travel by coach

because it was more convenient and 10 per cent thought it was more comfortable in coaches than in other modes

of public transport. It is possible that this feeling of comfort was helped by the courteous manner of the drivers,

especially when dealing with elderly passengers who made up over half the total carried.

In the light of this information the differences in assessments of ride, depicted in Figures 2 and 3, may now

be examined.

5.3.2.1 The differences between the public and TRRL assessment of ride: The TRRL team were probably more

critical o f the ride generally because they were aware of the purpose of the experiment and had carried out similar

work in private cars and so were more 'expert ' in assessing ride. During the course of the experiments they

travelled over each test length 28 times, and therefore had many more opportunities to judge the ride than the

public. Also, because almost half the sample o f the public never travelled by car, this section of the public sample

had no other standard by which to judge the ride and were much more likely to be satisfied by the ride in the coach.

An analysis o f assessments given by the two groups of coach passengers showed a slight, but significant, bias towards

a more critical judgement o f ride by those who did travel by car. Also some members of the public chose to travel

by coach because it was economic. Consequently, the psychological factors influencing the judgements of the public

and the TRRL team were quite different.

10

5.3.2.2 The difference between ride assessments in cars and coaches: The difference is probably due to the different

environments experienced when travelling by coach and by car. The coach is likely to produce more favourable

travelling conditions, providing more space, a better view and more distractions. These aspects of coach travel may

also serve to enhance feelings of safety in coaches as against cars. The traveller, therefore, may not notice, or may be

prepared to accept, a higher level of rms acceleration in coaches than in cars.

5.3.3 Coach seat location and ride: Figure 1 shows the four seating positions in the coach at which acceleration

levels were measured:- driver seat; mid-coach; rear-seat; front-seat. From the 'comfort characteristic' curve for

the public in coaches, Figure 2, it can be seen that arms vertical acceleration of 70 x lO'3g corresponds approx-

imately to a 90 per cent probability of the ride being assessed as 'acceptable or better ' . The probability of

experiencing an acceleration exceeding this level at each of the four seating positions, is shown in Figure 4, which

indicates: a) the driver has a 50 per cent probability of exceeding 70 x lO'3g and therefore experiencing a standard

of ride which is less than acceptable, b) mid-coach, a 25 per cent probability, c) rear-seat, a 99 per cent probability

and d) front-seat, a 65 per cent probability. The ride experienced by the driver and the front-seat passenger are

similar to each other, the driver experiencing a slightly lower level of acceleration than the passenger, probably

because of the superior design of his bucket-type seat. The rear-seat, as would be expected, is the worst position of

all, because of the pitching motion of the coach and the fact that the seat is located on an overhang, behind the

rear wheels, which amplifies the vertical movement. The mid-coach seating position located at or near the centre

of mass of the vehicle provides the best standard of ride comfort least affected by any pitching motion.

5.3.4 Coach vibration levels and ISO standards: Figure 5 shows the proposed boundaries 5 for the effect o f whole-

body vibration on 'reduced comfort ' and 'fatigue-decreased proficiency'. Such limits are difficult to apply because

they are based upon continuous steady exposure to a single-frequency sinusoidal vibration. A coach driver seldom

experiences any particular level of vibration for a long period, except perhaps on motorways, and so comparison

with the ISO boundaries can only be made with the following assumptions:

(1) the spectral estimate, computed from a 90 second recording, remains constant over the

whole period specified in the ISO limits.

(2) for the narrow-band vibration, analysed with a 0.5 Hz bandwidth, the rms value of

acceleration within the bands is evaluated separately with respect to the appropriate

ISO limit at the centre frequency of that band.

Levels of rms acceleration measured at the site giving the worst ride, have been taken from frequency spectra

and plotted in Figure 5 at the 1/3-octave centre frequencies. When compared with the ISO boundaries these show

that a coach driver could expect to feel uncomfortable after about 2 hours driving, provided the measured level o f

acceleration remained constant throughout this period. In a similar manner, coach passengers, if continually

exposed to the measured level of acceleration, might experience discomfort in the mid-coach position after about

7 hours, and in the rear seat after about 1½ hours. Figure 5 also shows that car drivers would only experience

discomfort after a period of continuous exposure greater than 8 hours.

11

5.3.5 The effect of load on coach vibration: The effect of the degree of loading on ride was examined by plotting,

for each site, measured values of rms vertical acceleration against corresponding load factors for each seating position.

To eliminate the effect of speed, only those test runs where the measured speed was less than 10 per cent different

f rom the mean speed were used. The load factor L is detained as the ratio of the number of seats occupied to the

total seating capacity of each coach. Values of L encountered during the experiment ranged from 0.25 to 0.8.

The results showed little relationship between load and rms acceleration; there was a trend for the rms acceleration

to decrease slightly with load in the rear seating position, but linear and logarithmic least squares regression analysis

showed this trend to be significant only at the five per cent level.

5.3.6 The effect of speed on coach vibration: The variation of rms vertical acceleration with speed was examined

for each of the measured seating positions in the coach, on each of the three test sites. The best correlation was

found in the front seat position on Site 2 (with significance at the 1 per cent level) and the driver and front seat

positions on Site 3 (with significance at the 1 per cent and 5 per cent levels respectively). The regression lines

showed an increase of approximately 10 -2 g rms for each 10 km/h increase in coach speed over the range of speeds

considered.

5.4 Lorry results

Acceleration measurements are summarised in Tables 5(a) and 5(b) where values of rms vertical acceleration

are classified into band widths of 20 x 10 -3 g. There were a total of 63 recordings for the driver and the passenger

compared with over 200 for coach passengers and over 600 for the public in private cars 2. Estimation errors are

therefore larger for the lorries than for the coaches and cars, influencing any comparisons between the three

different types of vehicles. The lorry results, however, do contain a sufficiently large and representative sample to

permit an examination of the likely trends in the ride characteristics of operational lorries on typical British

trunk roads.

5.5 Discussion o f lorry results

5.5.1 Response to ride of lorry occupants: The 'comfort characteristic' curves plotted in Figure 6 show that the

lorry drivers were much less critical of the ride than the TRRL passenger; Figure 7 shows that drivers were more

likely to experience a lower level of acceleration than the passenger. The 'comfort characteristic' curve of Figure 6

shows that there was a 90 per cent probability of the driver rating the ride favourably if he experienced an

acceleration o f less than 75 x 10 -3 g rms ; the probability of a TRRL passenger rating the same level of acceleration

favourably was only 65 per cent in the lorry and 35 per cent in a car. Thus the TRRL team were prepared to accept

a higher level o f acceleration in the lorry before rating the ride uncomfortable than they were when travelling in

cars. The probable reasons for these differences in assessments are the same as those discussed in Section 5.3.1

for the coach experiment, in that the ride environment is very different in coaches and lorries from that experienced

in cars. I t is also possible that the judgement of ride in the lorry is influenced by noise. It has been shown 6 that

human tolerance to vibration in the presence of a high level of noise can be increased and a higher level of vibration

is more acceptable when combined with noise than without. Noise levels measured inside lorry cabs during test

runs were between 85 dbA and 95 dBA compared with levels o f about 75 dBA measured inside a medium-sized

saloon car. The predominant source of lorry noise was the engine. There was no perceptible difference in noise

levels measured inside the vehicle whilst travelling over the asphalt and concrete surfaces of the test roads.

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5.5.2 Lorry driver and passenger seats: Drivers of 66 per cent of the lorry sample were equipped with some form

of variable suspension seat, which could be adjusted to compensate for the driver's weight. Though this gave the

driver a ride that was different from that o f the passenger, spectra plotted in Figure 11 show the frequency content

to be basically the same for both seats.

The cumulative distribution of measured rms accelerations, plotted in Figure 7, shows that the input to the

driver is less than that to the passenger. A comparison is made in Figure 8 of rms levels of acceleration experienced

by driver and passenger. Driver rms accelerations with and without suspension seat are plotted against the correspond-

ing passenger rms accelerations. Figure 8(a) shows that, in the majority of test runs, the suspension seat reduced the

rms acceleration to the driver compared with that experienced by the passenger. The converse is true as shown in

Figure 8(b) when the driver is not using a suspension seat; in this case the passenger experiences a slightly better

ride than the driver possibly because, in general, the driving seat has had more wear than the passenger seat.

The use of a suspension seat effectively reduces the level of the vibration input to the driver and should

improve his riding comfort , although those used in the lorries sampled did not appreciably alter the vibration

frequency content.

5.5.3 Lorry vibration levels and ISO standards: Figure 9 shows points plotted at the 1/3-octave centre

frequencies for driver and passenger of a lorry, together with the ISO boundaries for exposure to whole-body vibration.

The points plotted represent the highest and lowest levels of acceleration recorded during the experiment. Under

the worst conditions, the lorry driver might be expected to reach the 'reduced comfort' boundary within his first

hour of continuous exposure as a result of the effect of vibration within the 3Hz to 4.5Hz frequency range. The

passenger would in fact reach these levels before the driver. The driver could reach the 'fatigue-decreased

proficiency' boundary after about 4 hours travel with possible impairment of driving performance. Current

legislation 7 permits continuous driving for up to 5 hours (4½ hours from 1 July 1979 and 4 hours from 1 January

1981 under EEC Regulations). It is interesting to note that the lorry passenger (possibly a co-driver) may reach

the limits slightly earlier, becoming 'vibration-fatigued' more quickly. It is therefore perhaps to be recommended

that the passenger should also be provided with the same type of adjustable-suspension seat as used by the driver.

For the I s o recommendation to be used as a guide in this manner, the same assumptions must be made as

those in Section 5.3.5 for the coaches. It is however apparent that, if the driver continuously experienced vibration

o f a magnitude corresponding to the worst case, he could exceed or approach the 'fatigue-decreased proficiency'

boundary during his legal continuous driving period. A relatively economic improvement in lorry ride can be

obtained by the more widespread adoption of suspension seats 8 in lorries. Research being carried out by several

lorry manufacturers on separate cab suspensions should also further reduce the vibration input to the driver and

passenger.

5.5.4 The effect of load and speed on lorry vibration: It was not possible to examine the effect of load and speed

on the vibration because the data was insufficient to provide a basis for analysis.

14

5.6 Comparison of car, coach, and lorry vibration

Figure 10 shows the cumulative distribution of rms vertical acceleration for the drivers of each of the two

types of vehicle examined in this Report, and of the cars reported in reference 2, based upon the combined data

from the three test sites. It can be seen that there are considerable differences in the ranges of rms acceleration

experienced in the three types of vehicle.

Frequency spectra for the car, coach and lorry are compared in Figures 11 (a) to (f). All spectra are of a

similar shape and show the differences in vehicle response when travelling over the total combined length of the six

test sections of Site 2 which had a good riding quality (see Table 1). The spectra were obtained using a 0.5Hz

constant bandwidth 'real-time' analyser and have an accuracy of + 11.3 per cent.

A comparison of coach, car and lorry spectra in Figure 11 shows that, of the three types of vehicle, least

energy is transmitted by the car seat and suspension system and that the coach suspension is better at attenuating

frequencies in the range 5Hz to 20Hz than the lorry. It can also be seen that the ride in the coach at certain seating

positions approaches that of a car.

The main suspension resonance of the vehicle body on its road springs ('heave') appears to be about 2.5Hz

for the coach, 1.5Hz to 2Hz for the car, and at about 3Hz for the lorry. A loaded articulated lorry produces a large

second resonant peak at about 5.5Hz which is absent in the spectrum of the unladen lorry (Figure 11 (c) (d)).

There is also a higher vibration energy content in the frequencies above 5Hz in the spectrum of the unladen lorry.

These differences in the frequency response for the two conditions of load must be due to interaction between

the trailer and the tractor unit. A loaded trailer does not bounce as much as an unloaded trailer, thereby reducing

the high frequency vibration. But when loaded, some flexing and pitching of the trailer may occur which can

influence the vibratory motion of the tractor unit.

Time histories of vertical acceleration at the seat-person interface produced on Site 2 for a typical medium-

sized car, a coach and a loaded articulated lorry are shown in Figure 12. These traces corroborate information given

in the frequency spectra in that waveforms of recordings from the coach and the lorry have a greater high frequency

content than those from the car. The waveforms shown indicate that peak-to-peak acceleration levels in the car

seldom exceed 0.25g at the seat-person interface; coach and lorry occupants frequently experience peak-to-peak

accelerations of around 0.5g.

Crest factors (the ratio of peak to rms acceleration) for the waveforms illustrated in Figure 12 are about 1.6

for the car and 2.0 for the coach and lorry. The exposure limits recommended by the ISO are not reliable for

vibrations of crest factor greater than 3.

6. SUMMARY OF RESULTS

From the sample of long-distance coaches examined, the levels of rms vertical acceleration were higher than those

previously recorded in private cars 1 travelling over the same sections of road. However, coach passengers appear

to tolerate higher levels more readily than car occupants. In coaches, the mid-coach position gave the most

acceptable ride, and the rear seat gave the least acceptable ride. Coach drivers were found to be less critical of ride

than car drivers for a similar level of vertical acceleration but more critical than coach passengers.

15

A comparison of measured levels with ISO recommended limits showed that, even at the highest measured

acceleration level, the coach driver would not exceed the recommended limits within the legal period of continuous

driving. However, some reduced comfort could be expected after about 2_hours driving. For the coach passengers,

reduced comfort at the mid-coach position would not be expected for some 7 hours of continuous exposure at this

level although at the rear-seat position some discomfort could be expected after only 1½ hours or so.

The vertical acceleration levels in lorry cabs were found to be higher than those in both coaches and cars,

although the drivers were less critical o f the ride than the car and coach drivers for similar levels of acceleration.

At the highest level of acceleration, the lorry drivers could be expected to suffer some decreased proficiency after

about 4 hours continuous exposure, this period being just permissible under current driving hours regulations.

Reduced comfort could be expected within the first hour of exposure. In many of the lorries examined, lorry

passengers, possibly co-drivers, could reach the ISO limits in a shorter time than the drivers, because of the

difference in design of the passenger and driver seats.

Vertical acceleration levels in cars, coaches and lorries ranged up to 0.12 g rms for the car driver, 0.13 g rms

for the coach driver and 0.22 g rms for the lorry driver. The 'comfort characteristic' curves for cars, coaches and

lorries suggest that environmental and psychological factors as well as vibration levels influence the subjective

assessment of ride. For example, vehicle size, spaciousness and noise level together with the expectation and

motivation of the vehicle occupants may be of importance.

The poorer standard of ride in coaches and lorries can be influenced by such factors as vehicle wheelbase

length, and suspension characteristics, as well as the geometrical characteristics of the road profile, in terms of the

amplitude and wavelengths of road surface irregularities. The effect of the latter, with increasing vehicle speed is

generally an increase in rms acceleration level as found in the car experiment 1 , but for the coaches and lorries

insufficient data and a limited range of speeds did not permit any significant correlation to be established.

7. CONCLUSIONS

An investigation has been made of the riding comfort in long-distance coaches and heavy goods vehicles operating

over a variety of typical trunk road surfaces. Ride, represented by the rms vertical acceleration at the seat-person

interface, has been measured in bo th coaches and lorries and correlated with riding comfort assessments of drivers

and passengers. The main conclusions o f the investigation are : -

. The largest acceleration levels measured were nearly twice as high in lorries and on the rear seat of coaches

as those measured in cars.

. Coach passengers and lorry drivers will tolerate higher levels of vertical acceleration more readily than

car occupants.

. The most acceptable ride in a coach is at the mid-point of its wheelbase, where it can be similar to that

experienced in a car. The least acceptable ride is in the rear seat of a coach, where it can be worse than

that experienced by a lorry driver.

16

4. Psychological factors as well as vibration levels influence the subjective assessment of ride.

. There is an increase of approximately 10 -2 g rms vertical acceleration for each 10 km/h increase in coach

speed.

. From comparisons with ISO recommendations, lorry drivers and passengers could suffer some decreased

proficiency after about four hours if continually exposed to the highest level of vibration found in the

investigation. Current legislation permits periods of up to five hours of continuous driving, but this

period is to be reduced to four hours by 1981.

7. Lorry-passenger vibration levels measured were higher than those of the driver. The difference is attributable

to the better seating provided for the driver.

8. FUTURE WORK

The surface condition of urban roads, where frequent 'patching' and ' trenching' occurs, is generally worse than that

of rural major roads; although speeds are much lower on urban roads, vehicle occupants, particularly those of urban

buses which are restricted to these areas, may be exposed to relatively high levels of vibration. Many of the urban

passengers are standing, and their response to vibration inputs is different from that of seated vehicle occupants.

However, because exposure times are comparatively short, passengers may be prepared to tolerate higher levels

of vibration.

Little is known of the effect of road surface irregularities on the riders of two-wheeled vehicles. Although

75 per cent of accidents to motor cyclists occurred in built up areas in 19769 , it is not known what effect road

surface unevenness has on the accident rate.

The road surface/ride relationship of buses and motorised two-wheeled vehicles on urban roads will be

examined in future work.

9. ACKNOWLEDGEMENTS

The work described in this Report forms part of the research programme of the Construction and Maintenance

Division (Division Head: Mr P D Thompson) of the Highways Department of TRRL. Mr P G Jordan, Section Leader,

provided guidance with the analysis of results.

The authors would like to thank Mr A Gurley of the National Bus Company and Mr D P Mockford of

Grey-Green Coaches Ltd, for their assistance in the organisation of the work on coaches.

The cooperation of the following haulage contractors and their drivers is also gratefully acknowledged:

Clements Transport Ltd

Welch's Transport Ltd

J A Wilkinson and Sons Ltd

17

Mrs M H BurtweU of Construction and Maintenance Division, and Mrs J J Webb formerly of Highway Traffic

Division, TRRL, assisted in the collection of data.

10. REFERENCES

1. COOPER, D R C, P G JORDAN and J C YOUNG. Road surface irregularity and vehicle ride, Part 1 -

Variation and interpretation of ride measurements. Department of the Environment Department o f Transport,

TRRL Report SR 341. Crowthorne, 1978 (Transport and Road Research Laboratory).

. COOPER, D R C and J C YOUNG. Road surface irregularity and vehicle ride, Part 2 - Riding comfort in cars

driven by the public. Department of the Environment Department of Transport, TRRL Report SR 400.

Crowthorne, 1978 (Transport and Road Research Laboratory).

. DUNN, J B. Traffic census results for 1972. Department of the Environment, TRRL Report LR 618.

Crowthorne, 1974 (Transport and Road Research Laboratory).

4. HARRIS, W, R R MAKIE et al. A study of the relationships among fatigue, hours of service and safety of

operation of truck and bus drivers. Bureau o f Motor Carrier Safety Report No. BMCS RD-71-2. Washington,

DC 1972. Federal Highway Administration, US Department of Transportation.

. INTERNATIONAL STANDARD, ISO/DIS 2631. A guide for the evaluation of human exposure to whole-

body vibration. International Organisation for Standardisation 1972.

. SOMER, H C and C S HARRIS. Combined effect of noise and vibration on human tracking performance

and response time. Aerospace Med. March, 1973, pp 276-280.

7. MINISTRY OF TRANSPORT. Transport Act 1968, (Part VI), London, 1968 (H M Stationery Office).

. AUTOMOBILE ENGINEER. Ride simulation for suspension seats, May 1971, pp 40 -43 , Iliffe and Sons Ltd,

London.

. DEPARTMENT OF THE ENVIRONMENT. Road Accidents, Great Britain 1974. London, 1976 (H M

Stationery Office).

18

A a

B I ~.~, ~ F~re~io~ ~] C Team c passengers

D d

Acceleration measurement point

Driver b

a

L_>4 D

d

X C

Fig . 1 C O A C H M E A S U R E M E N T P O S I T I O N S

1.0

0.9

0.8 .;-

_~ 0.7

0.6 ¢o

E 8 0.5 U

"~ 0 .4 - -

23 0 . 3 - -

£ a_ 0 .2 - -

0.1--

0

0

~ = ~ ~ " = ~ = = ~ = -- Coach passenger

- o - . - _ ~ o / - - - " % % 0 ~ •

% •

---- Car occup t % O %

• Coach passenger

O Car occupant

I I I I I I I I I I I I I

10 20 30 40 50 60 70 80 90 100 110 120 130

rms vertical acceleration (g x 10 -3 )

I I

Fig. 2 COMFORT CHARACTERISTIC CURVES -- MEMBERS OF THE PUBLIC

1.0' ~ . ~ k ~ ~ ~ O ~ O Coach driver

-~ 0 . 9 - ~ • 0 ~ k 0 Coach (TRRL)

., o.8- \ , , ,%, \

0.7

o.6

0.5 8

.5 o.4

"- 0.3

~ 0.2 m 0.1 0

• Cars (TRRL)

~'\ ~ o

• ~ • ~ . Coach > ~ A O ~ % ~ (TRRL team)

Cars (TRR L team) " ~

Coach driver

I I I I I I I I I I I I I I I I

0 i0 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170


Fig. 3 COMFORT CHARACTERISTIC CURVES -- TRRL STAFF AND COACH DRIVERS

A 00

v 1.0 t - ¢o

0.9

0.8 O ° ~

*~ 0.7

u 0.6 ¢.)

m

r -

0.5 e--

~- 0.4 o )

x 0.3 Q )

0.2 o _

.~ o.~

Q _

\ \ ~ X - - ~Z~ Rear seat

\

Mid-coach

\ z~

% \

%

0 ! t t t t ! I I "'r "~. ¢ i I IA"I "/~-

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 rms acceleration (a) (g x 10 -3)

F ig . 4 D I S T R I B U T I O N O F V E R T I C A L A C C E L E R A T I O N S I N C O A C H E S

E"

E ¢ -

O

¢.) ( . }

co

10

4

3 m

1.0

0.4

0.3

0.2

0.1

0.04:

0.03 ]

A Rear-seat passenger

O Mid-coach passenger

• Coach driver

• Car driver

I + 11.3 per cent confidence limits of spectral analysis

0~,"~'/ , f

.,,~,,4" ~.~°t, I /

• /¢ \~ / I J,°~'"

() /

~-_2. /

k

= = •

t

c)

0.02

0.01

0.2 1.0 1.6 2.5 4.0 6.3 1.25 2.0 3.15 5.0

Frequency (Hz)

Ji. ,~k 1 10.0 16.0

8.0 12.5 20.0

Fig. 5 ISO BOUNDARIES AND VERTICAL ACCELERATION LEVELS IN COACHES

1.0

0.9

"c. 0.8

_8 0.7

0.6

~ 0.5

~ 0.4

~ 0.3

m 0.2

0.1

0 0

. I . ~ , ~ • , 0 % ~ Lorry driver

- \ ".._

X X x ~ TRRL lorry passenger

\ \ o \ \ o

I I

10

I t t t I I I I I I I I I

20 30 40 50 60 70 80 90 100 110 120 130 140 150


Fig. 6 COMFORT CHARACTERISTIC CURVES FOR LORRIES

A ¢O

v

e" oO

¢ b

t -

O

O.}

¢J

c-

¢-. (M

Q . X

..Q

o

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

°iF, 0,

40 50

\ ~o

Lorry driver

TR R L lorry passenger

~O,, "O

~o "o~

1 l 100 150

rms vertical acceleration (a) (g x 10 -3 )

" o ~

200

Fig. 7 DISTRIBUTION OF VERTICAL ACCELERATIONS MEASURED IN LORRY CABS

0 0

O

• =

E

\

0

I 0

\ .\

\ -

• .\ • \

I o o

\

O O O O

\

o o

0 0

A

b X

E

- 0

° o = o

O~

U U

0

\ +

•

e ° o ~

• •

"k O O O O

o ° ° \ • \

• • • 0 ~ • \

I I o o o o L.O 0

O O

I \ o

rn

b ,e-

X

t "

E o o o ,~

cJ

I--

LIJ

z O

Z W

D

D 0 "I"

LLI

IT" 123

I--

U.I

Z O

z w

Z O rr" uJ >

123

I--

1,1,1

Z 0

Z U.I e l .

D

>- ne nr" 0 .,.I

1.1.

0

I - (J UJ LI . LI .

W ..r

i -

oo

._~

(E_O L x B) a:)e~Ja:~u! leas-Jal~uassEd ~.R uo! : leJala0ae swJ

10

4 m

3

1.0

A o~

E c - o 0.4

--~ 0.3 ¢J

._ 0.2

0.1

0.04

0.03

0.02

0.01

0.2

c~ lit

I I I I

Z~ ,Passenger

0 Driver (worst condition)

• Driver (best condition) o d S

I + 11.3 per cent confidence l imits _ • of spectral analysis .S SS~

; ~ ,~'

, /

'--" . . . . / ~ ' 1 I / - - ~,o?) /

I<,~)o,°;I '

<'>W' a %-___~ 0 •

I t

I i tl

, , 2 8

1.0 1.6 2.5 4.0 6.3 10.0

1.25 2.0 3.15 5.0 8.0 12.5

Frequency (Hz)

I I 16.0

20.0

Fig. 9 ISO B O U N D A R I E S A N D V E R T I C A L A C C E L E R A T I O N LEVELS IN LORRIES

0

/

/ !

! I . / I

t

0

/ l

l

l /

l l

/

/

/

1 /

l . ¢

/ /

/

' 1 "t3

o / - J

4' 1

1 1

w-

° / c"

8 / o, /

:>

.-Q

( J

7

I

.I I

I - -

/ 4 I

I I I I I I I I I {5 (5 d d d d d c5 d

(e) ueq~ Ja]eaJ6 uo!;eJalaoae ue 6u!aua!Jadxa J.O A~!l!qeqoJ d

I /

/ -

0

0 0

0 CO

0 ( .0

0

A

b X

0 v

c- O . _

(1) I

g ~ . I

0 O0

0

0

0

r ,n I L l

r r

0 .--I

a z <

( n

o <~ 0 o

r~ <C 0 I J .

0 ( n r r

I.U > n-

z

o UJ U . U . .<

( n z 0 I - <~ n- I L l ...J I L l

o <~

i i < o I - r r

u J > I t .

0 z 0 I -

r r

u )

0

._~ IJ .

A

b X

N -1-

t -

O

¢.) co

4-a

>

4 (a) Coach (Speed = 93 km/h)

Load factor L = 0.69

3 Front-seat passenger

I

2 ~ C~ach-driver

0 I ~ ~

0 5 10 15 20

4

/

0

(b) Coach (Speed = 93 km/h) Load factor L = 0.69

Rear-seat passenger

Mid-coach passenger

5 10 15 20

A

E~ X

N I

t -

O

co ¢.)

>

6 (c) Laden articulated lorry

(75 km/h)

5 - It! ~ Passenger

4 -

Driver

0 I = I

0 5 10 15 20

6 (d) Unladen articulated lorry

(78 kin/h)

, ~ Driver #1

II II iI

/ d~--Passenger

s ~* " ~ ' % % o j

5 10 15 20

t - ._o

x

¢o

> 0

0

(e) Small-car (driver)

i , I I I - ~ ' l " - - Y - l"

5 10 15 20

Frequency (Hz)

0

0

(f) Medium-car (driver)

5 10 15 20

Frequency (Hz)

Fig. 11 COACH, LORRY AND CAR VIBRATION SPECTRA (SITE 2)

Time (s)

(a) CAR DRIVER (95 km/h)

~:[ I I

0 2 4 6 8 10

Time (s)

- i <

(b) LADEN ARTICULATED LORRY DRIVER (75 km/h)

~ Driver < : I I I I I

0 2 4 6 8 10

0

LR. O

l

I I I I I 2 4 6 8 10

t - O

¢J o <

0 2 4 6 8 10

Time (s)

< I I I I

0 2 4 6 8 10

(c) COACH (82 km/h)

Fig. 12 VARIATION OF VERTICAL ACCELERATION WITH TIME (SITE 2)

(549) D d 0 5 3 6 3 6 1 1 ,400 4 / 8 0 H P L t d S o ' t o n G 1 9 1 5 P R I N T E D IN E N G L A N D

ABSTRACT

ROAD SURFACE IRREGULARITY AND VEHICLE RIDE. PART 3 - RIDING COMFORT IN COACHES AND HEAVY GOODS VEHICLES: D R C Cooper M Phil and J C Young MIOA: Department of the Environment Department of Transport, TRRL Supplementary Report 560: Crowthorne, 1980 (Transport and Road Research Laboratory). An investigation is described of the riding comfort in long-distance coaches and heavy goods vehicles, operating over typical trunk road surfaces. Ride, represented by the root-mean-square (rms) of the vertical acceleration at the seat-person interface, has been measured in both coaches and lorries and correlated with riding comfort assessments of drivers and passengers. Comparisons are made with the ride in cars measured in previous experiments.


Evaluation of the highest measured acceleration levels against recommended International Standards of human response to whole-body vibration show a possibility that lorry drivers could, in certain cases, suffer some fatigue- decreased proficiency within their present legally permitted periods of continuous driving.


ISSN 0305- 1315

ABSTRACT

ROAD SURFACE IRREGULARITY AND VEHICLE RIDE. PART 3 - RIDING COMFORT IN COACHES AND HEAVY GOODS VEHICLES: D R C Cooper M Phil and J C Young MIOA: Department of the Environment Department of Transport, TRRL Supplementary Report 560: Crowthorne, 1980 (Transport and Road Research Laboratory). An investigation is described of the riding comfort in long-distance coaches and heavy goods vehicles, operating over typical trunk road surfaces. Ride, represented by the root-mean-square (rms) of the vertical acceleration at the seat-person interface, has been measured in both coaches and lorries and correlated with riding comfort assessments of drivers and passengers. Comparisons are made with the ride in cars measured in previous experiments.


Evaluation of the highest measured acceleration levels against recommended International Standards of human response to whole-body vibration show a possibility that lorry drivers could, in certain cases, suffer some fatigue- decreased proficiency within their present legally permitted periods of continuous driving.


ISSN 0305- 1315

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