This invention relates to a trailer, and in particular to a trailer
having at least two axles and of a type suitable for carrying heavy loads over
rough terrain. The invention is particularly applicable to four-wheeled close-coupled
trailers. The four-wheeled configuration has the advantage that the trailer can
be free-standing even when part loaded, and also facilitates hitching to a towing
Conventional trailers adapted to be towed by motor vehicles are generally
secured to the towing vehicle by means of a rigid tow bar, which may be an integral
extension of the trailer chassis. The attachment to the towing vehicle, which
is generally by means of a metal ring integral with the tow bar and secured by
a hook or pin to a bracket on the towing vehicle. This, in combination with the
designed length of the tow bar, allows a useful degree of articulation, especially
with a tracked towing vehicle such as the M109 SP Howitzer.
When a trailer is being towed over rough cross-country terrain, particularly
at speed, rolling and pitching movements of the trailer put considerable stresses
on the tow eye unit, causing deformations, cracks and, eventually, fractures.
This occurs because the tow bar cannot move up or down relative to
the trailer and the trailer and its load exert a considerable, if transitory, torque
on the tow bar.
DE-A-2,454,010 discloses a tow bar having forward and rear sections,
the forward section being pivotable upwards relative to the rear section, about
a horizontal axis, to accommodate sudden upward movements of the towing vehicle.
The front section has a rearward extension beyond the pivoting axis and above the
rear section, and a vertically extending compression spring, positioned between
the said rearward extension and the rear tow bar section, opposes the upward pivoting
The present invention consists in a tow bar assembly for a trailer
having two or more axles, comprising a forward tow bar section having means thereon
for attachment to a towing vehicle and a rear tow bar section, said forward and
rear tow bar sections being pivotable relative to one another, against a spring
bias, about a substantially horizontal axis, characterised in that the spring
bias is provided by an axially compressible spring assembly extending and acting
longitudinally of said tow bar sections and having a forward end engaging said
forward section and a rear end engaging said rear section, said spring mechanism
extending below and offset from the axis to oppose downward pivoting of the forward
tow bar section relative to the rear section, and to absorb bending stresses exerted
on said tow bar when in use.
It is possible by means of the present invention to provide a trailer
and a tow bar therefor which is of rugged construction and can withstand towing
at speed over rough terrain, and which absorbs smoothly the pitching movements
of the trailer relative to the towing vehicle, to reduce the stresses to which
the trailer tow bar is subjected. The axially compressible spring is preferably
in the form of a stack of Belleville springs, the number of which can be varied
to adjust the spring bias as may be required. The spring bias is preferably adjusted
so that when the trailer is not in use it balances the weight of the front tow
bar section and holds it in an approximately horizontal position convenient for
attachment of a towing vehicle.
The compression spring extends generally in the longitudinal direction
of the tow bar, preferably within an angle of 10° above or below the horizontal.
A preferred embodiment of the invention will now be described with
reference to the accompanying drawings wherein:
- FIG. 1 is a perspective view of a trailer incorporating a tow bar in accordance
with the present invention;
- FIG. 2 is a plan view of the trailer of Fig. 1;
- FIG. 3 is a side elevation of the trailer of Figs. 1 and 2, with a load in
- FIG. 4 is an enlarged view of a detail of the tow bar, as seen in the direction
of arrows IV in Fig. 2;
- FIG. 5 is an enlarged detail of the Belleville spring system used in accordance
with the invention; and
- FIG. 6 is a schematic side view of the trailer in use over rough terrain.
Referring to Figs. 1 to 3, a four-wheel trailer has a chassis generally
indicated by 10 made up of steel beams of rectangular box section welded together.
The chassis comprises a rectangular outer frame and a central pair of rectangular
section beams 12, running parallel to one another and extending from the rear end
of the chassis to its front end and beyond to form a first part of the tow bar.
In Fig. 1 the trailer is shown with a base frame 11 for supporting
a wide load which extends above the wheels. In Fig. 2 the trailer is shown without
the base frame but with brackets 13, 17 for attaching a load and in Fig. 3 the
trailer is shown with a load 19 in place.
A second part of the tow bar is formed by two parallel box section
beams 14 connected to one another by a section 15 and pivotally attached to the
front ends of sections 12 by a horizontal pivot spindle 16.
At the front end of the second section of the tow bar is a conventional
towing attachment 18 and a handbrake lever 20 which acts, via brake cables
22, 23, to actuate brakes on the front wheels 24 and rear wheels 25 of the trailer.
The two sections of the tow bar are pivotable relative to one another
about the horizontal axis formed by the pivot spindle 16, to take up some of the
vertical movement, particularly pitching movement, of the trailer relative to
the towing vehicle, particularly when travelling over uneven terrain. The pivoting
movement of the front section of the tow bar relative to the rear section is preferably
limited during use, typically to an angle between 15° upwards and 25° downwards.
The upward pivoting movement of the tow bar section is limited by an abutment
between a rearwardly projecting flange 28 on each of the beams 14 and a flange
32 on the front of the chassis.
Downward movement of the front section of the tow bar is opposed
by a spring mechanism generally indicated by 35. This is shown in more detail in
figs. 4 to 6. This spring mechanism can provide the required limitation of downward
pivoting, so that special stops are not needed for this purpose. The spring mechanism
comprises a stack of Belleville springs 26 arranged in opposing pairs on a cylindrical
shaft 38. This shaft extends generally in the longitudinal direction of the tow
bar between a bracket 30 on the underside of the front section of the tow bar,
in front of the pivoting axis, and a bracket 33, welded to the underside of flange
32, which is bolted to the chassis below the rear section 12 of the tow bar, behind
the pivoting axis.
At the front end of the shaft 38 is a block 40 providing a shoulder
against which bears the foremost of the Belleville springs 26. On the side opposite
the shoulder, the block has a channel 42 of semi-cylindrical cross section which
accommodates a cylindrical pin 44 mounted on the bracket 30. The block 40 simply
bears against this pin, allowing relative rotational movement between them. The
pin can simply lift out of the channel when the assembly is dismantled.
The opposite end of the shaft 40 is slidably engaged in a corresponding
cylindrical aperture through a block 46, which is mounted in the bracket 33 for
pivotal movement about a horizontal axis. The shaft 38 can slide axially through
the block 46. A tubular spacer 39 is provided between the stack of Belleville springs
26 and the block 46.
Since the shaft 38 is offset from the axis of the horizontal pivot
pin 16, pivotal movement of the front section 14 of the tow bar relative to the
rear section 12 will cause the pin 44, and hence the block 40, to move axially
towards or away from the block 46. Downward movement of the front section 14 will
move the block 40 towards the block 46, causing compression of the Belleville
springs 36 between the block 40 and the spacer 39 as the shaft 38 slides axially
through the block 46, which in turn pivots anti-clockwise as shown in Fig. 6. Thus,
downward movement of the tow bar is opposed by the stack of Belleville springs.
The spring system 35 is preferably axially preloaded such that the
front section 14 of the tow bar assumes an approximately horizontal position relative
to the trailer chassis when not attached to a towing vehicle. In this position
the shaft 38 is approximately horizontal. As the front tow bar pivots up and down
the shaft also pivots, through a smaller angle. When the trailer is standing on
level ground, the shaft is preferably not more than about 10° above or below the
horizontal at the limits of pivoting of the front tow bar section.
The spring mechanism 35 preferably comprises from 50 to 100 individual
Belleville springs, suitably about 80. The number of springs can be varied as necessary,
using spacers 39 of different lengths depending on the strength and preloading
required to the spring. Other types of spring mechanism may be used, for example
a coil spring or a hydraulic spring.
The tow bar construction in accordance with the invention greatly
reduces the stresses to which the rigid tow bar sections are subjected by movement
of the trailer relative to the towing vehicle when in use. The vertical movements
of the trailer relative to the towing vehicle are largely accommodated by the pivotal
connection between the front and rear sections of the tow bar.
Fig. 6 shows schematically how the front and rear sections 14, 12
of the tow bar pivot relative to one another when the trailer is towed over rough
ground by a towing vehicle 50, allowing all four wheels of the trailer to remain
in contact with the ground. In contrast, a trailer with a rigid tow bar 55, shown
in broken lines in Fig. 6, cannot pass at any speed over the terrain shown without
the rear wheels 25 leaving the ground, putting a considerable stress on the tow
bar and on the towing eye 18.
The value of the tow bar assembly of the invention is made singularly
apparent when a trailer is towed across severely undulating terrain where the pitch
of the undulations may be equal to, or less than, the track length of the towing
vehicle, but greater than the distance between the towing eye 18 and the rear wheels
25 of the trailer. Under these conditions the most severe loading on the tow eye
unit 18 will be when the tow eye location is at the bottom of a dip and the trailer
is being made to breast the retreating hump (see Fig. 6). In such a situation,
the cantilever effect of a rigid tow bar such as 55 would also induce a very unstable
situation on the trailer and a bending movement in the tow eye unit of something
equivalent to 2 to 3 times the cargo load of the trailer, thus inducing shear stresses
in the tow eye unit for which it was not designed.
Thanks to the articulation of the tow bar in this situation, the
tow eye unit remains subject only to those tensile stresses for which it was designed