Bsi bs au 003 1970 (1999)

BRITISH STANDARD AUTOMOBILE SERIES

Specification for

Clearance dimensions
and fifth wheel
positions for tractors
and semi-trailers

UD C 62 9 . 1 1 4. 2 . 0 1 3 . 6

BS AU 3:1 970


BS AU 3 : 1 970

Fore word

This British Standard, prepared under the authority of the Automobile
Industry Standards Committee of the BSI, was first published in 1 963 and
later revised in 1 968 to assist interchangeability of tractors and semi- trailers
to the weights and dimensions permitted by the Motor Vehicles (Construction
and Use) Regulations No. 321 : 1 969. The present revision incorporates
additional data for articulated vehicles considered necessary as a result of
experience in the use of the standard.
Trailer manufacturers will note that the application of this standard depends
upon the knowledge of the dimensions of the towing vehicles. Users will note
that the physical interchangeability between the towing vehicle and the
semi- trailer does not itself ensure compatibility of load carrying, braking,
lighting and overall length.
It is important that the equipment be so designed that semi- trailers will couple

easily and satisfactorily with the towing vehicle, and not slope forward when
the fifth wheel is mounted in accordance with the dimensions given in this
standard.
NO TE

The values specified in imperial units are to be regarded as the standard. The metric units

are given as approximate equivalents.

A British Standard does not purport to include all the necessary provisions of
a contract. Users of British Standards are responsible for their correct
application.
C o mp liance with a British S tand ard doe s not o f itse lf co nfe r immunity
fro m le gal ob ligations.

S ummary o f p age s

This document comprises a front cover, an inside front cover, pages i and ii,
pages 1 to 4 and a back cover.
This standard has been updated (see copyright date) and may have had
amendments incorporated. This will be indicated in the amendment table on
This British Standard, having
been approved by the

the inside front cover.

Automobile Industry
Standards Committee, was
published under the
authority of the Executive

Ame ndme nts issue d since p ub lication

Board of the Institution on
20 March 1 970
© BSI 09- 1 999

The following BSI reference
relates to the work on this
standard:
Committee reference AUE/-

IS B N 5 80 0 5 9 6 7 7

Amd. No.

Date

Comments


BS AU 3:1970

Contents
Page
Foreword

Inside front cover

1

Scope

1

2

D imensions

1

3

Articulation

2

4

Heights

2

Figure 1 — D imensions of towing vehicle with single rear
axle and of semi- trailer

3

Figure 2 — Clearance dimensions between towing vehicle
and semi- trailer to permit specified articulation

4

Tab le 1 — Preferred dimensions for single axle semi- trailers
in articulated vehicles not exceeding 2 0 tons and tandem axle
semi- trailers in articulated vehicles not exceeding 2 4 tons
gross train weight

1

Tab le 2 — Preferred dimensions for single axle semi- trailers
in articulated vehicles exceeding 2 0 tons and tandem axle
semi- trailers in articulated vehicles exceeding 2 4 tons gross
vehicle weight

2

Tab le 3 — Towing vehicle with single rear axle height to face
of fifth wheel from ground vehicle unladen and uncoupled

© BSI 09- 1 999

3

i


ii

blank

BS AU 3:1 970

1 Scope
1 .1 This British Standard specifies clearance dimensions and the positions and heights of fifth wheel
attachments fitted to the tractive units of articulated vehicles for the purpose of coupling semi- trailers. It
relates to towing vehicles with single rear axles not exceeding 2 . 5 metres width and to semi- trailers fitted
with king pins complying with BS AU 1

1)

.

1 .2 The standard applies to vehicles in normal road use and excludes vehicles used under cross- country
conditions.

2 Dimensions
The basic dimensions shall b e as given in Table 1 , Tab le 2 and Tab le 3, with reference to Figure 1 and
Figure 2 . Table 1 and Table 2 give preferred dimensions for the tractors and semi- trailers as follows:
Table 1 : preferred dimensions for single axle semi- trailers in articulated vehicles not exceeding 2 0 tons
and tandem axle semi- trailers not exceeding 24 tons gross train weights.
Table 2 : preferred dimensions for single axle semi- trailers in articulated vehicles exceeding 2 0 tons and
tandem axle semi- trailers in articulated vehicles exceeding 2 4 tons gross train weights.
NO TE

Sub j ect to agreement between the user and the manufacturer, and where special conditions exist, the dimensions given in

either Tab le 1 or Tab le 2 may b e selected as alternatives.

Table 3: gives the heights of the fifth wheel face from the ground.

Table 1 — Preferred dimensions for single axle semi-trailers in articulated vehicles
not exceeding 20 tons and tandem axle semi-trailers in articulated vehicles
not exceeding 24 tons gross train weight
Detail

Symbol

Dimensions
in

mm

D min.

6 min.

1 50 min.

D max.

1 2 max.

305 max.

CA

64 min.

1 62 5 min.

CA-D

58 min.

1 475 min.

X1

60 max.

1 52 5 max.

Y

54 max.

1 370 max.

Z

63 min.

1 600 min.

D istance from fifth wheel face to point C on clearance plane

h

4

1 00

Angle of forward inclination of clearance plane from point C

µ

8°

8°

W

61 . 5 max.

1 560 max.

¶

1 0 ° min.

1 0 ° min.

Position of fifth wheel from centre of rear axle

D istance from point C on clearance plane at back of cab to
centre line of rear axle
D istance from centre line of king- pin to point C on clearance
plane at back of cab or other chassis equipment

D istance from fifth wheel to extreme rearwards proj ection of
vehicle
D istance from king- pin to extreme forward proj ection of
semi- trailer
D istance from king- pin rearwards to any proj ection likely to
foul towing vehicle

Maximum flat width of b ack of cab to permit 1 00 ° articulation
of semi- trailer
Clearance angle at sides of rear of cab to permit 1 00 °
articulation of semi- trailer

1)

+

BS AU 1 , “Dimensions of 2 – 2

© BSI 09- 1 999

in diameter fifth wheel king pin for use with semi-trailers ”.

1


BS AU 3:1 970

Table 2 — Preferred dimensions for single axle semi-trailers in articulated
vehicles exceeding 20 tons and tandem axle semi-trailers in articulated vehicles
exceeding 24 tons gross vehicle weight

Detail

Symbol

Dimensions

in

Position of fifth wheel from centre of rear axle

mm

D min.

1 2 min.

305 min.

D max.

27 max.

685 max.

CA

76 min.

1 930 min.

CA-D

64 min.

1 625 min.

X2

78 max.

1 980 max.

Y

60 max.

1 525 max.

Z

81 min.

2 060 min.

Distance from fifth wheel face to point C on clearance plane

h

4

1 00

Angle of forward inclination of clearance plane from point C

µ

8°

8°

W

98. 5 max.

2 500 max.

¶

Zero

Zero

Distance from point C on clearance plane at back of cab to centre line
of rear axle
Distance from centre line of king- pin to point C on clearance plane at
back of cab or other chassis equipment
Distance from fifth wheel to extreme rearwards proj ection of towing
vehicle
Distance from king- pin to extreme forward proj ection of semi- trailer
Distance from king- pin rearwards to any proj ection likely to foul

towing vehicle

Maximum flat width of back of cab to permit 1 00 ° articulation of
semi- trailer
Clearance angle at sides of rear of cab to permit 1 00 ° articulation of
semi- trailer

3 Articulation
Care shall be taken to ensure that when working to the positions and heights shown in the tables there
shall be no fouling between the rear of the towing vehicle and its mudguards and the adj acent parts of the
semi- trailer at a fore and aft articulation of up to plus and minus 8 ° from the designed static position when
the combination is in a straight line.
The clearance plane passing through point C (see Figure 2) at the back of the cab is a flat transverse plane
sloping forward from point C at an angle of 8 ° as shown and is intended to provide clearance for the 8 ° fore
and aft articulation requirement, and for semi- trailer clearance over chassis equipment mounted at the
back of the tractor cab.
The cab profile shown in Figure 2 permits a maximum semi- trailer articulation of 1 00 ° . The clearances of
vehicles built to the dimensions in Table 2 are such that the flat back of cab may extend to the full legal
width of 2. 5 metres.

4 Heights
The heights of the fifth wheel face from the ground, with the vehicle unladen and uncoupled, shall be as
given in Table 3 when the fifth wheel is set at a minimum of 6 in (1 50 mm) and a maximum
of 27 in (685 mm) forward of the rear axle centre line. The heights of intermediate positions shall be
determined on a sloping line between the position 6 in (1 50 mm) forward of the rear axle centre- line and
the position 27 in (685 mm) forward of the rear axle centre- line.
NOTE 1

These heights are based on the towing vehicle rear springs having a deflection of approximately 3 in (75 mm) laden to

unladen.
NOTE 2

It is emphasized that advantage should be taken of the height tolerances specified only in extreme cases where coupling

could not be otherwise effected.

2

© BSI 09- 1 999


BS AU 3:1 970

Table 3 — Towing vehicle with single rear axle height to face of fifth wheel from
ground vehicle unladen and uncoupled (see Figure 1 ) (United Kingdom and overseas)
Dimensions
Tyre size

at

H

D = 6 in

at

(1 50 mm)
in

D = 27 in

(685 mm)
mm

in

mm

7. 50 – 20
8. 25 – 20

47

1 1 95

50

1 270

48. 5

1 230

51 . 5

1 31 0

50

1 270

53

1 350

51 . 5

1 31 0

54. 5

1 385

or tyres of equivalent diameter
9. 00 – 20
or tyres of equivalent diameter
1 0. 0 – 20
or tyres of equivalent diameter
1 1 . 0 – 20
or tyres of equivalent diameter
NOTE

The dimensions for

H in this table are subj ect to a tolerance of plus and minus 1 in (25 mm) .

Figure 1 — Dimensions of towing vehicle with single rear axle and of semi-trailer

© BSI 09- 1 999

3


BS AU 3:1 970

Figure 2 — C learance dimensions between towing vehicle and semi-trailer
to permit specified articulation

4

© BSI 09- 1 999


blank

5


BS AU 3:1970

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