Bsi bs au 210 2 1994 (1999)

BRITISH STANDARD

Drawbars for caravans
and light trailers —
Part 2: Specification for the design of
aluminium drawbars and chassis

UDC [629.11-43 + 728.76] :629.11.011.1

BS AU
21 0-2:1 994


BS AU 21 0-2:1 994

Committees responsible for this
British Standard

The preparation of this British Standard was entrusted by the Automobile
Standards Policy Committee (AUE/- ) to Technical Committee AUE/8, upon
which the following bodies were represented:
Association of Trailer Manufacturers
British Compressed Air Society
Caravan Club
Department of Transport
National Caravan Council Limited
Society of Motor Manufacturers and Traders Ltd.
Coopted members

This British Standard, having
been prepared under the

direction of the Automobile
Standards Policy Committee,
was published under the
authority of the Standards
Board and comes
into effect on
1 5 July 1 994
© BSI 1 1 - 1 999

The following BSI references
relate to the work on this
standard:
Committee reference AUE 8
Draft for comment 92 76306 DC

ISBN 0 5 80 223 24 8

Amendments issued since publication

Amd. No.

Date

Comments


BS AU 21 0-2:1 994

Contents

Page
Committees responsib le
Foreword

Inside front cover
ii

1

Scope

1

2

References

1

3

D efinitions

1

4

Symbols

1

5

Resistance calculation for drawbars

4

6

D eflection calculation

4

7

Fatigue

4

8

Mechanical design of chassis

7

9

C hassis strength

7

Joining of b ody to chassis

7

10

Figure 1 — D rawb ar reference dimensions

2

Figure 2 — Curves relating maximum stress and stress ratio
for 1 0

8

life cycles

Table 1 — Properties of aluminium alloys

3

Table 2 — Permissible stresses for aluminium alloys

5

List of references

© BSI 1 1 - 1 999

6

Inside back cover

i


BS AU 21 0-2:1 994

Foreword

This Part of BS AU 21 0 was prepared under the direction of the Automobile
Standards Policy Committee. Since the drawbar is only one part of the chassis of
a caravan or light trailer, and as the chassis can be made of materials other than
steel, or even be of composite construction, this Part of BS AU 21 0 aims to set out
requirements for the design of aluminium chassis for caravans and light trailers
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.
Compliance with a British Standard does not of itself confer immunity
from legal obligations.

Summary of pages

This document comprises a front cover, an inside front cover, pages i and ii,
pages 1 to 8, an inside back cover 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 the
inside front cover.

ii

© BSI 1 1 - 1 999


BS AU 21 0-2:1 994

1 Scope
This Part of BS AU 21 0 specifies:
a) a procedure for determining the mechanical
strength of drawbars for caravans and light
trailers where the vertical static force on the

4 Symbols
For the purposes of this Part of BS AU 21 0, the
following symbols are used.

l

is the distance (in mm) from the vertical
axis of the coupling device to the first point

coupling head is not less than 245 N and not
greater than 735 N;
b) requirements for the mechanical design of the

l

on the trailer frame (see Figure 1 ).
x

axis of the coupling device to the drawbar

chassis for caravans and light trailers.
This standard applies to aluminium chassis of
caravans and light trailers provided they are not of
welded construction. It also covers the aluminium

section corresponding to the maximum
strain rate (see Figure 1 ) .

e

is the distance (in mm) from the horizontal
axis of the coupling device to the drawbar

elements of chassis of composite construction, again

neutral axis at the first fixing point on the

provided the aluminium parts have not been
welded.
NOTE

The method for calculation of the mechanical strength of

is the distance (in mm) from the vertical

e

trailer frame (see Figure 1 ).

x

is the distance (in mm) from the horizontal

steel drawbars for caravans and light trailers is covered in

axis of the coupling device to the drawbar

BS AU 21 0- 1 .

neutral axis at the cross section

2 References

(see Figure 1 ).

2.1 Normative references

corresponding to the maximum strain rate

d

is the maximum deflection (in mm) at the

This Part of BS AU 21 0 incorporates, by reference,

coupling device relative to the first fixing

provisions from specific editions of other

point (see Figure 1 ) .

publications. These normative references are cited
at the appropriate points in the text and the
publications are listed on the inside back cover.
Subsequent amendments to, or revisions of, any of

P
D

these publications apply to this Part of BS AU 21 0

This Part of BS AU 21 0 refers to other publications

E

standard are listed on the inside back cover, but
reference should be made to the latest editions.

3 Definitions

(see ISO 3853).
is the modulus of elasticity (in N/mm 2 ); its
value varies according to the alloy

that provide information or guidance. Editions of
these publications current at the time of issue of this

is the value determined by calculation for
the longitudinal force (in N) occurring

between the tractor vehicle and the trailer

only when incorporated in it by updating or revision.
2.2 Informative references

is the manufacturer’s total mass (in kg) of
the caravan or light trailer (see ISO 7237) .

I
g

(see Table 1 ) .
is the moment of inertia (in mm4) ) of the
section.
is the acceleration due to gravity (in m/s 2 )
(see ISO 3853).

For the purposes of this Part of BS AU 21 0, the
definitions given in BS 81 1 8 apply, together with
the following.
3.1
chassis

the element, including the drawbar, between the
axle assembly and the body of the caravan or light
trailer
3.2
drawbar

that part of the chassis which enables the

connection of the caravan or light trailer to the
towing vehicle

© BSI 1 1 - 1 999

1


BS AU 21 0-2:1 994

Figure 1 — Drawbar reference dimensions

2

© BSI 1 1 - 1 999


© BSI 11-1999

TF

H3
H2
TB

TF

O
TF
TE

TF
TB

M

TF
drawn tube
extrusions
sheet
extrusions
sheet and plate
extrusions
extrusions
extrusions
extrusions
extrusions
clad sheet
clad plate
sheet
sheet
extrusions
sheet and plate
extrusions
sheet and plate

sheet and plate

extrusions

Form

20
150d
3.0
25
6.0
10
150d
25
150d
25
150d
25
150d
20
20
12.5
25
6.0
6.0
25
25
25
25

mm

mm

—

20
0.2
3.0
—
6.0
—
3.0
—
0.2
—
—
—
—
—
3.0
12.5
0.2
0.2
—
0.2
—
0.2

Max.

Min.

Thickness

255

270
255
240
255
240
130
—
125
160
110
240
230
370
365
380
130
165
190
170
280
270

295
310
295
295
310
310
280
—

275
185
150
280
370
435
425
440
200
245
300
280
340
320
10

8
8
—
—
7
—
12
—
14
14
8
8
8
11

7
—
6
—
—
12
8

7
—
8
8
7
9
11
—
13
12-16
7
7
7
10
6
8
—
4-8
5-8
10
same as
tensile

115
150

same as
tensile

same as
tensile
tensile
206

118

same as
tensile

same as
tensile
556
525
556
386
324
556
664
772
726
463
494

600
560
660
630

618

587

618

71 700

68 900

68 900

65 500
68 900
72 400

68 900

68 900

2 800 B

2 710 B
2 710 B
C

2 800
Ae
2 690 A
2 670

2 660 A

2 710 B

Elongation
0.2 %
0.2 %
Bearing Modulus
tensile Tensile
of
Density Durability
proof strength 5.65 Ỉ S0 50 mm compressive
strength
proof stress
elasticityb
stress
ratingc
%
%
N/mm2
N/mm2
N/mm2
kg/m3
N/mm2 N/mm2

a See BS 1470.
b For modulus of rigidity multiply by 0.38.
c See BS 8118-1:1991 clause 2.4 .
d For round tube and hollow sections these properties do not apply above 75 mm.
e C for immersion in fresh or sea water.

7020

5251
5154A

2014A

6063
6061

5083

6082

Alloy BS or
international
Condition
no. a

Table 1 — Properties of aluminium alloys

BS AU 210-2:1994

3

BS AU 21 0-2:1 994

5 Resistance calculation for drawbars

6 Deflection calculation

5 .1 The calculation for resistance for drawbars shall

6.1 The deflections

be carried out over the whole length

at the same time as the resistance calculation in

l of the
drawbar, taking into account distances l and e
x

x

in

order to determine the maximum strain rate. The

strain rate shall be checked at the position where
the maximum bending moment occurs when
and

e=e.
x

d

fR

shall be calculated,

x

f

x

x

— For braked trailers:
f

and

— For braked trailers:

d

Only flexure is taken into account for the calculation.

M = 0. 36 Pgl

fD

clause 5 , as follows.

be calculated as follows.
NOTE

f

M (in N·mm)
e/l < 0. 1 5 and e /l < 0. 1 5 shall

5 .2 The maximum bending moment

for the drawbar with

l=l

d,d

fR

d +d

= 0. 75 (

f

fD )

— For unbraked trailers:

x

— For unbraked trailers:

M = 0. 24 Pgl
f

x

5 .3 The maximum bending moment (in N·mm) for

the drawbar with

e/l > 0. 1 5 and e /l
x

x

> 0. 1 5 shall be

d

calculated as follows.
NOTE

Only flexure is taken into account for the calculation.

Three bending moments

M, M
f

fD

and

M

fR

shall be

calculated and the largest value of the three shall be

fR

d +d

= 0. 75 (

f

fD )

The value shall not exceed that defined in 6.2
NOTE

Because of the increased flexibility of aluminium

compared to steel, it should not be taken for granted that the

used for the calculation of the maximum admissible

drawbar design is sufficiently stiff without performing a

stress

deflection calculation.

M

fmax .

6.2 The permissible deflection

— For braked trailers:

M = 0. 36 Pgl
M = 0. 8 De
M = 0. 75 (M + M
f

device shall not exceed:

x

fD

d

x

fR

f

M = 0. 24 Pgl
M = 1 . 0 De
M = 0. 75 (M + M

Because of the constraints on manufacture, all
drawbars shall be considered to have type 1

x

fR

f

classification as defined in BS 81 1 8- 1 : 1 991 .

fD )

5 .4 The permissible stress rates

B, calculated for the

whole length of the drawbar, shall not exceed the

allowed stress

B

bc

to a maximum of 25 mm.

7 Fatigue

x

fD

l

70

fD )

— For unbraked trailers:
f

=

d at the coupling

(see Table 2).

Using the curves in Figure 2, the fatigue life shall be

predicted.
NOTE

Bmin

is the minimum stress, which is the lower numerical
value of the stress in a stress cycle.

Bmax

I/V

3
is the section modulus (in mm ), of the

drawbar cross section corresponding to the
maximum bending moment.
NOTE

is the maximum stress, which is the higher numerical
value of the stress in a stress cycle.

where

It is assumed in this standard that aluminium sheet,

plate or strip conforming to BS 1 470 or bars, extruded round tube

Ömin
Ömax

is the stress ratio, which is the ratio of the minimum
to maximum stress in a stress cycle, tensile stress
being considered positive and compressive stress
negative.

When these points are plotted on Figure 2a) and Figure 2b) ,
they always occur on the curve or in the area defined as
“acceptable”.

or sections conforming to BS 1 474 are used. This does not prevent
other, suitable and fit for purpose, aluminium alloys being used.
The properties of some aluminium alloys are shown in Table 1
and Table 2.

4

© BSI 1 1 - 1 999


BS AU 21 0-2:1 994

Table 2 — Permissible stresses for aluminium alloys
Thickness
Alloy BS or
international
no.

C ondition

Form

Min.

Max.

mm

mm

a

Axial

B

Bending

B

t

N/mm

b

B

c

2

N/mm

2

B

bt

N/mm

2

Shear

B

bc

N/mm

N/mm

TF

1 39

1 39

1 54

1 54

83

20

1 50

1 47

1 47

1 62

1 62

88

1 39

1 39

1 54

1 54

83

1 32

1 32

1 47

1 47

79

1 40

1 40

1 56

1 56

84

1 34

1 34

1 49

1 49

80

82

82

96

96

49

2 01

80

77

94

90

48

2 01

80

77

94

90

48

2 01

87

87

96

96

52

1 39

62

62

69

69

37

117

1 31

1 31

1 45

1 45

78

2 01

0. 2

plate

3. 0

3. 0

21 2

—

25
6. 0

222
6. 0

sheet and

2

222

drawn tube

M

c

b

N/mm

20

sheet and

extrusions

2

—
extrusions

6082

B

q

2

Bearing

—
3. 0

10
1 50

d

25

plate
5083
extrusions
O

sheet and

—
0. 2

1 50

d

25

plate
TF

extrusions

—

1 50

TE

extrusions

—

25

d

6063

6061

d

TF

extrusions

—

1 50

TB

extrusions

—

20

1 35

124

1 53

1 42

81

2 39

extrusions

—

20

1 54

2 02

1 54

224

1 08

2 78

1 2. 5

1 47

1 99

1 47

220
1 00

2 61

25

1 47

2 07

1 47

229

2 01 4A
TF

clad sheet

3. 0

clad plate

1 2. 5

52 51

H3

sheet

0. 2

6. 0

75

69

85

79

45

1 67

51 54A

H2

sheet

0. 2

6. 0

95

88

1 07

1 00

57

1 78

25

111

111

126

1 26

67

216

25

1 00

1 00

114

114

60

2 02

25

1 54

1 54

1 71

1 71

92

2 38

25

1 48

1 48

1 64

1 64

89

227

extrusions
TB

sheet and

—
0. 2

plate
702 0
extrusions
TF

sheet and

—
0. 2

plate
a
b
c
d

See BS 1 470.
Applies only when buckling is not the criterion; see BS 81 1 8- 1 .
Joints in single shear.
For round tub e and hollow sections the permissible stresses do not apply ab ove 75 mm.

© BSI 1 1 - 1 999

5

Figure 2 — C urves relating maximum stress and stress ratio and 1 0

8

life cycles

BS AU 21 0-2:1 994

6
© BSI 1 1 - 1 999


BS AU 21 0-2:1 994

8 Mechanical design of chassis
8.1 Bolted j oints

Minimum spacing between bolt holes
The spacing between the centres of the bolts shall
not be less than 2.5 times the bolt diameter.
8.1 .2 Edge distance
With extruded, rolled or machined edges, the
distance to the edge, measured from the centre of
the bolt hole shall not be less than 1.5 times the bolt
diameter. If, on the beating side, it is less than 2
times the bolt diameter, the permissible beating
strength shall be reduced as follows:
8.1 .1

where

is the modified bearing strength;
is the bearing strength given in Table 2;
is the actual ratio of edge distance to bolt
diameter;
r
is the ratio of edge distance to bolt
diameter.
8.1 .3 Web stiffness
The web stiffness shall be demonstrated by
calculation as detailed in BS 8118-1:1991, by test or
both.
8.1 .4 Bolt material
Steel bolts conforming to BS EN 24014:1992 and
nuts to BS EN 24032:1992 shall be used. Both the
bolts and nuts used shall be rendered corrosion
resistant by zinc plating to BS 3382-2:1961.
Washers shall be provided under all bolt heads and
nuts and these washers shall be zinc plated to
BS 1706:1990 Fe/Zn 8 and passivated. The length of
the unthreaded part of the bolt shall be such that as
far as possible no part of the thread is within the
thickness of the member The thread shall project
beyond the nut by a minimum of two turns. A form
of locking device shall be used.
PBM
PB
ra

c) painted with priming coat with an inhibiting
pigment containing not less than 20 % by weight
of zinc chromate or other approved chromate in a
suitable water resisting vehicle. The priming coat
shall not contain any copper or mercury
compounds, graphite or carbonaceous materials,
or lead; or
d) separated by the use of a continuous
membrane of non-reactive material e.g. plastic.

NOTE A joint made by any other means not detailed in this
standard may be used provided that its load carrying capacity
and life expectancy can be shown to be satisfactory.
8.3 Contact with timber

Aluminium surfaces in contact with timber, unless
the timber is fully seasoned and the environment
dry and unpolluted, shall be painted. Timber in
contact with aluminium shall not be treated with
preservatives containing copper sulfate, zinc
chloride or mercuric salts.

NOTE 1 Other preservatives may be used provided that it can
be shown that the timber so treated is not harmful to the
aluminium.
NOTE 2 Oak, chestnut and western red cedar are likely to be
harmful to aluminium particularly where there are through
fastenings, unless the timber is well seasoned.
9 Chassis strength

The chassis manufacturer shall demonstrate, either
by test or by calculation, or both, that all elements
of the chassis are fit for their declared purpose with
reference to BS 8118:1991.
1 0 Joining of body to chassis

The chassis manufacturer shall provide, in writing,
instructions in respect of strength and fixing
arrangements for the attachment of the caravan or
light trailer body to the chassis. There shall be no
fixing directly or indirectly through the top or
bottom flanges of the drawbar at the foremost body
fixing position.

NOTE Other suitable anti-corrosion coatings may be used that
adequately protect the jointed parts against electrolytic
corrosion.

(see also 8.3 )
For such composite joints, the steel or cast iron
elements shall be either:
a) zinc plated to BS 1706:1990 Fe/Zn 8; or
b) hot dip galvanized to BS 729:1971; or

8.2 Joints with steel or cast iron

© BSI 11-1999

7

8

blank


BS AU 21 0-2:1 994

List of references

(see clause 2 )

Normative references
BSI publications

BRITISH STANDARDS INSTITUTION, London

BS 729:1971,
BS 1706:1990,
BS 3382,
BS 3382-1 & BS 3382-2:1961,
BS 8118,
BS 8118-1:1991,
BS 8118-2:1991,
BS EN 24014:1992,
BS EN 24032:1992,

Sp ecification fo r ho t dip galv anized coatings o n iron and steel articles .

Method for sp ecifying electrop lated coatings of zinc and cadm ium ,

on iron and steel.

Sp ecificatio n fo r electrop lated coatings on threaded com p onents .

Cadm ium o n s teel com p onents.

Zinc on steel com p o nents.

Structural us e of alum inium .

Code of p ractice fo r design.

Sp ecification fo r m aterials,

w o rkm anship and p rotectio n.

Hexago n head b olts — Product grades A and B.

Hexagon nuts,

style 1

— Product grades A and B.

Informative references
BSI publications

BRITISH STANDARDS INSTITUTION, London

BS AU 210,
BS AU 210-1:1987,
BS 1470:1987,

Draw b ars fo r carav ans and light trailers.

Method o f calculation for the m echanical s trength of steel draw b ars .

Sp ecification fo r w rought alum inium and alum inium alloys for general engineering

p urp oses: p late,

BS 1474:1987,

p urp o ses: b ars,

Sp ecificatio n fo r w rought alum inium and alum inium ,

ISO 7237:1981,
ISO 3853:1977,
Strength test.

© BSI 11-1999

sheet and strip .

alloys fo r general engineering

extruded ro und tub es and sections.

Road v ehicles — Mass es and dim ensions o f carav ans — Term s and definitio ns.

Road vehicles — Carav ans and light trailers — Tow ing b rackets and co up ling b alls —


BS AU
21 0-2:1 994

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