TRANSPORT AND ROAD RESEARCH LABORATORY

Department of the Environment

SUPPLEMENTARY REPORT II5UC

ACCIDENT RATES AND HANDLING CHARACTERISTICS OF THREE-WHEELED CARS CO~ARED WITH OTHER CARS

by

K E Holmes

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

Vehicle Safety and Accident Investigation Divisions Safety Department

Transport and Road Research Laboratory Crowthorne, Berkshire

1974

Abstract

I. Introduction

2. Accident analysis

3. Handling tests

3.1 Tilt platform tests

3.2 Cornering tests

3.3 Overtaking tests

3.4 Discussion of results

4. Conclusion

5. Acknowledgements

6. References

CONTENTS Page

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CRO~ COPYRIGHT 1974

Extracts from the text me~j be reproduced, except for commercial purposes, provided the source is ackn~aledged.

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.

ACCIDENT RATES AND HANDLING CHARACTERISTICS OF THREE-~EELED CARS COMPARED ~TH OTHER CARS

ABSTRACT

Analysis of the national injury accident data shows that single vehicle accidents involving three-wheeled cars occur at more than three times the rate for other cars. The overall injury accident rate is however, only about 30 per cent greater.

Handling tests representing cornering and overtaking ~noeuvres showed that on all except very slippery surfaces, one model of three wheeled car had a lower performance than one model of four-wheeled car because it tended to overturn before reaching limiting adhesion.

1. INTRODUCTION

About one car in a hundred of those registered is a three-wheeler and an analysis of the national accident data for three-wheelers has been made in comparison with data for four-wheeled cars. In addition an experimental investigation has been made of the handling performance of one model of three-wheeler, the Bond Bug, in comparison with the Hillman Imp.

2. ACCIDENT ANALYSIS

The aim of the accident analysis was to compare accident rates, and in particular frequency of loss of control, for three-wheeled cars with rates for four~heeled cars. But whilst the numbers of injury accidents in which different types of car are involved can be obtained by analysing the national road accident statistics (form Stats 19), loss of control is not directly reported, nor is there any corres- ponding record of the total travel of each type available. Therefore accident rates (numbers of accidents per hundred million kilometres) cannot be calculated directly.

Experience of detailed studies of accidents suggests that the type of accident in which the car under consideration is the only vehicle involved, and no pedestrian is struck, corresponds most closely to the idea of a "loss-of-control" accident (though it leaves out accidents in which control is lost of a vehicle which then strikes other vehicles). This type of accident will be referred to as type A.

Moreover the frequency of occurrence of the type of accident in which a car strikes another vehicle travelling on a different road at a junction of two roads is likely to depend mainly on exposure and not to any particular qualities of the vehicle. Thus the relative frequencies of such accidents for different types of car provide approximate estimates of the relative total travel by these types of car and may be used to make approximate estimates of their relative accident rates for other types of accident. Support for this hypothesis has been indicated by Jones. I This type of accident will be referred to as type B.

Accident data for type A and type B accidents, together with totals of all injury accidents (type C), have been examined for three-wheeled cars, two small four-wheeled models (Hillman Imp and British Leyland Minis), and all types of car. The data in Table I are numbers of vehicles involved in injury accidents of types A, B and C in 1969, together with the ratios A/B, C/B and (C-A)/B for the groups of cars indicated. The ratio A/B serves as an index of the relative loss-of-control accident rate and (C-A)/B as an index of the rate for all other types of injury accident.

The ratios A/B in Table 1 indicate that the "loss-of-control" injury accident rate for three-wheelers may be three or more times as great as for small four wheel cars and for cars in general, whilst the ratios (C-A)/B suggest that the rate for other types of injury accident may be no more than a few per cent greater than that for other cars. To judge from the ratios C/B the effect on the total injury accident rate for three-wheelers may be 30 per cent higher than that for other cars, this increase being largely due to the much higher single vehicle accident rate. The possibility that occupants of three-wheelers might be more susceptible to injury in an accident is not reflected in the level of severity of injury incurred in single vehicle accidents; 24 per cent of casualties in three-wheelers were fatal or serious, compared with 37 per cent of casualties in other cars. In multi vehicle accidents the corresponding percentages were 22 per cent for three-wheelers and 26 per cent for casualties in other cars. While the overall injury accident rate for three-wheelers is higher than that for other cars it is still much less than the rate for motorcycles

TABLE 1

Numbers of cars involved in injury accidents of different types in 1969

Type of injury accident

A - One car and no pedestrian

B - Car and one other vehicle on a different road

C - All injury accidents

Three wheel cars

486

233

1721

Hillman Imp

461

705

3915

BL Mini

1213

1713

10104

A/B

C/B

(C-A)/B

2.09

7.39

5.30

0.65

5.55

4.90

O.71

5.90

5.19

All c a r s

23925

43330

245035

0.55

5.66

5 .I0

Since loss of control in accidents is more prevalent among young drivers, a further breakdown by age of driver has been made for accidents of type A and B (see Table 2). Whilst this confirms the higher loss of control rate (ratio A/B) for younger drivers, it affords no explanation of the overall higher loss of control accident rate for three-wheelers by possible differences in age distribution for the different types of car. The ratio A/B is higher for three-wheelers than other cars for all age groups.

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3. HANDLING TESTS

3.1 Tilt Platform Tests

A method universally used for comparing the static roll stability of vehicles (buses etc) is to subject them to a tilt test measuring the angle at which the load on the upper wheel(s) reduces to zero and are about to leave the platform. In Table 3 the results of tilt tests for the Bond Bug are compared with those for a 1971Hillman Imp. Chains were used to prevent roll over occurring. In all the cases quoted, the tilt was to the right (corresponding to a left hand turn in cornering) so that the effect of introducing a weight to simulate the driver was to reduce the angle at which roll over commenced, the reduction in angle of tilt corresponding to the reduction in the lateral acceleration for roll over during cornering.

TABLE 3

Comparative Results from the Tilt Platform Tests for 3 wheel and 4 ~eel vehicles (vehicle tilted to the right)

Vehicle

1971Hillman Imp

1970 Bond Bug

Condition

Empty With driver*

Angle at which rollover commenced - 8 (degrees)

48 47

Equivalent Lateral Acceleration (=tan

Empty With driver*

36 31

i. II 1.07

.73

.60

*Driver simulated with 66 kg (146 Ib) weights on the driver's seat.

3.2 Cornering Tests

Tests were carried out in both right and left handed directions on both a dry, high coefficient surface and a wet, slippery surface (a Bridport gravel carpet with an SRV* of 44).

Two different types of cornering test were used. In the first, the approach was straight, the vehicle entering a circular arc without any transition curve. Both the approach lane and the circular arc were marked by rubber cones. The approach lane width was set equal to one vehicle width plus 1/3 m and opened, on the outer line of markers, to give a 3 m lane width within the circular arc. In the second form of test, the car started from zero speed within a continuous circular track 3 m wide and accelerated gently until the limit was reached. Tests with the tangen- tial approach were performed with both 9 m and 50 m inside radii; the gradual acceleration test was carried out only with the 9 m radius.

On the tangential approach tests, the entry speeds were increased first at 5 mile/h (8 km/h) intervals and then at 2 mile/h (3.2 km/h) intervals near the limit on the 50 m radius. On the 9 m radius, increments of 3.2 km/h were used throughout. The maximum speed is defined as the speed above which the vehicle went onto emergency castors (see Plate I) or hit a marker indicating the limits of the lane.

* Skid Resistance Value, measured with the portable skid-resistance tester.

The results of the tests on the three wheeler are given in Table 4. Results which had been obtained in a previous series of tests for 1963 Hillman Imp on the tangential approach 50 m radius tests are compared with those for the three wheeler in Table 5.

The use of small wheels or castors in the form of a skid prevented rollover during the tests. A developed form of these would add little to the weight and probably prevents the majority of the rollovers occurring on public roads.

TABLE 4

Performance of the 3 ~eeler in Cornering Tests

Surface

We t

Dry

Inner Radius

50 m

9 m

50 m

9 m

Starting Condition

Direction

tangential

approach

tangential

approach

Right hand

Left

Right

Left

Maximum Speed (km/h)

49

54

3O

31

Cause of Failure

Rear wheels skidded & vehicle hit markers

Rear wheels skidded & vehicle hit markers

Front wheels skidded & vehicle hit markers

Onto emergency castors & vehicle hit markers

Right 24 wheelspln slow acceleration Left 22 wheelspin

tangential Right 60 onto emergency castors & vehicle hit markers

approach Left 55 onto emergency castors

tangential

approach

Right 29 onto emergency castors

Left 29 onto emergency castors

Right 25 wheelspin

Left 24 wheelspin slow acceleration

TABLE 5

Comparative Results from 50 m Righthand Curve Tests for a 3 and a 4 wheel vehicle Maximum speeds (km/h)

Vehicle Wet Dry

1963 Hillman Imp 48 84

1970 Bond Bug 49 60*

* limited by vehicle rolling onto emergency castors

3.3 Overtaking Tests

The overtaking tests (otherwise known as the accident avoidance or chicane test) is included because it consists of two opposite transients, one closely followed by the other and there is a possibility of the roll during the second being rein- forced by the roll momentum from the first.

The approach lane was again one vehicle width plus 1/3 m and the transition length was 12 m plus one vehicle length. The overtake lane was 3 m wide and the offside markers for the approach lane were in line with the nearslde markers of the overtaking lane. The overtaking lane was 36 m long with markers of 4 m inter- vals. Again tests were carried out on a dry and wet slippery surface, by increasing the approach speed in steps of 5 mile/h initially and 2 mile/h in the vicinity of the maximum speed. Results are shown in Table 6 ~ere they are compared with those obtained from a 1963 Hillman Imp used in a previous series of handling tests.

TABLE 6

Comparative Results from Overt~ing Tests for a 3 and 4 Wheel Vehicle Maximum speeds (km/h) through the hazard

Vehicle

1963 Hillman Imp

1970 Bond Bug

Wet Dry

66 80

63* 64**

wheel lift occurred above this speed

rolled onto emergency castors above this speed

3.4 Discussion of Results

It may be seen from Tables 5 and 6 that the maximum speeds achieved on the dry surface with the three wheeler were considerably lower than with the four wheeler in the 50 m curve test and in the overtaking test. In both tests the limit for the three wheeler was set by rollover whereas in the case of the four wheeler it was set by loss of adhesion. The maximum speed of 60 km/h for the three wheeler on the 50 m curve corresponds to a lateral acceleration only a little less than 0.6 g indicated as a likely rollover limit by the results of the tilt table test

(Table 3).

On the wet slippery surface the maximum speed for the three wheeler was approxi- mately the same as that for the four wheeler in the 50 m curve test, the limit being set by loss of adhesion (Table 5). In the overtaking test on the wet surface, however, the three wheeler was on the point of rolling over when adhesion was lost (Table 61. Also in one of the tests on the 9 m curve on the wet slippery surface the limit was set by rollover (Table 3). Since most real road surfaces would provide better grip in the wet than the Bridport gravel carpet used in the tests, it seems likely that on the road the maximum speeds for tight corners and overtaking manoeuvres will usually be set by liability to roll over rather than loss of grip even when the surface is wet. Furthermore even when loss of grip results, in practice the vehicle will strike a kerb in a proportion of cases and in these cases rollover is probably more likely to occur with a three wheeler than a four wheeler (though no direct test of this was included in these present investigations).

The results of the tests described show that small three wheel cars of the type tested are more liable to roll over than typical small four wheel cars. This is consistent with evidence from the national accident data which shows that control is much more frequently lost of small three wheel cars than of small four wheel

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cars, and that, as a result, the total injury accident rate is about 30 per cent higher for the three wheelers than for the four wheelers.

4. CONCLUSIONS

i. The injury accident rate for three wheeler cars is about 30 per cent greater than that for other cars. The principle reason lles in the large difference in "loss-of-control" rate as represented by the single vehicle accident involvement rate which is between three and four times larger for the three wheeler.

2. The test results for one model of three wheeler confirmed that it is more likely to roll over than four wheelers and will do so in an extreme manoeuvre unless the friction between tyre and road is unusually low.

3. Three wheelers would be more roll resistant if some simple form of skid could be used, similar to those fitted in these tests.

5. ACKNOWLEDGEMENTS

The work described in this report was carried out in the Vehicles Safety and Accident ' Investigation Divisions of the Safety Department of TRRL. The test driving was performed by Mr J K Meades.

I.

.

6. REFERENCE S

JONES, I S. Road accident studies with particular reference to the handling and stability characteristics of cars. Ph. D Thesis, University College, London 1973.

ROAD ACCIDENTS 1970 (In Great Britain). Department of the Environment, Scottish Development Department, and Welsh Office. 1972 (H M Stationery Office).

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ABSTRACT

ACCIDENT RATES AND HANDLING CHARACTERISTICS OF THREE-WHEELED CARS COMPARED WITH OTHER CARS: K E HOL~S: Department of the Environment, Supplementary Report IISUC: Crowthorne, 1974 (Transport and Road Research Laboratory). Analysis of the national injury accident data shows that single vehicle accidents involving three-wheeled cars occur at more than three times the rate for other cars. The overall injury accident rate is however, only about 30 per cent greater.

Handling tests representing cornering and overtaking manoeuvres showed that on all except very slippery surfaces, one model of three wheeled car had a lower performance than one model of four-wheeled car because it tended to overturn before reaching limiting adhesion.

ABSTRACT

ACCIDENT RATES AND II&NDLING CIIAP~CTERISTICS OF TIIREE-~IEELED CARS COMPARED WI~I OTHER CARS: K E HOL~S: Department of the Environment, Supplementary Report II5UC: Crowthorne, 1974 (Transport and Road Research Laboratory). Analysis of the national injury accident data shows that single vehicle accidents involving three-wheeled cars occur at more than three times the rate for other cars. The overall injury accident rate is however, only about 30 per cent greater.

Handling tests representing cornering and overtaking manoeuvres showed that on all except very slippery surfaces, one model of three wheeled car had a lower performance than one model of four-wheeled car because it tended to overturn before reaching limiting adhesion.