Offroad Trailers Part 1

TOWING Towing Off-Road
Tough Towing:
Part One
Off-road towing is a whole new dimension of driving difficulty, but in your low-range
Landie you have one of the best tools for the job.
Words & photographs: Robert Pepper
B
y ‘off-road’ what we mean
here is anywhere you’d need low
range – much tougher terrain than a
dirt road. Off-road trailer techniques
are basically the same as without a trailer, and
like any form of driving the key to success is
preparation. That starts with understanding
the forces involved in a vehicle/trailer (rig),
which translates into driving techniques.
Trailer Dynamics
Diagram 1 (not to scale) shows the centre
of gravity of the trailer, which is ahead of the
axle. This means the trailer exerts a downward
force on the tow hitch, known as the TBM or
Tow Ball Mass. Balancing that is a smaller, but
corresponding upwards force on the front axle.
A typical off-road camper trailer weighs between
700 and 1,300kg when loaded, and has a TBM
of around 80-150kg. As an example, an OH
(overhang) of 1,200mm, 130kg tow ball weight
measured on the scales, and wheelbase of
2,750mm gives us an actual weight increase of
187kg on the rear axle and a reduction in front
axle weight of 57kg.
Another important force is inertia. The trailer
will resist any turning force, wanting to go
straight on. In diagram 2 the tow car turns left,
producing a force at A. Because the centre of
gravity is ahead of the axle the force is resisted
by the moment (rotational force) between C and
B. The greater the distance between B and C,
the bigger the moment and the more stable the
trailer is in a straight line, which is why moving
38 LRW September 2011
Diagram 1
the load forwards stabilises a trailer at speed,
albeit at the cost of manoeuvrability. As the
distance between A and B is much greater than
between B and C, less force is needed at A to
overcome the moment of C and B, which is why
a longer drawbar also means improved stability,
but again at the cost of manoeuvrability.
Diagram 2
Side slopes are difficult with trailers.
Diagrams 3a and b shows the rig on a side
slope. Here the trailer is exerting a force trying
to move the back wheels of the tow car down
the slope even when the rig isn’t turning. This
is again because the centre of gravity is ahead
of the axle.
Diagram 3a
Diagram 3b
Trailer tyres need to be just as tough and grippy as that of the tow car.
When you tackle a hill more directly, trailers
both help and hinder with ascents and descents.
On a descent, the trailer pushes the tow car
forwards, unless the trailer is braked. In either
case the tow ball mass is increased, which is a
good thing as a tow car’s weight naturally shifts
weight to the front, so some weight is restored
to the rear axle. When completing the descent
(diagram 4) the descending trailer can help push
the tow car along the ground or up the next hill.
Conversely, when ascending a hill the tow ball
mass is reduced, but still adds to the natural
increase of weight over the rear axle caused by
the gradient, so expect to be lifting front wheels
much more often on ascents with trailers, and to
have reduced steering control. Keeping going, as
the hill ascent doesn’t end until the trailer, not just
the tow car, is on the flat (diagram 5).
At just about any point off-road the vehicle
will need to do a lot more work with a trailer.
The force required to move a 2,500kg 4x4
on flat bitumen is around three per cent of its
mass, so only 75kg. A 1,000kg trailer adds only
35kg or 40 per cent more effort. However, that
force rises dramatically with gradients, and thus
whenever the trailer needs to be dragged over
rocks, logs or anything other than the flat. For
example, on a 15-degree slope a 2,500kg 4x4
needs around 700kg of force to move off, but
with the trailer that rises 40 per cent to 980kg,
the same sort of force needed for the tow car
alone to overcome gravity on a 22-degree hill. In
short, the trailer may be easy to tow on a hard,
relatively level, high-traction bitumen surface,
but once you move away from the easy terrain
these forces start to make themselves felt.
Make sure all your chains and leads are well out of the way. Cross over chains, and
you often don’t need to use the full length. A good trick is to pull the chain tight to the
vehicle, back off one link, then put the shackle through that link. That should be tight
enough without extra drag. Tidy the other links out of the way.
Selecting a Trailer
Because driving a trailer exerts all sorts of
mostly unwanted extra forces on the tow
car, the setup and choice of both tow car
and trailer is critical. First, the trailer must be
strong enough for the job, which means it
must be specifically designed for off-roading.
Standard trailers do not last long in lowrange territory, and a good way to check the
manufacturer’s faith in their product is to ask
if there are any warranty restrictions based on
where the trailer can be towed.
It is important that the trailer follows exactly
in the tow car’s wheel marks in a straight line
so that rolling resistance is minimised in soft
ground, and it stays in the same ruts as the
tow car, which contributes to stability. This
means the trailer’s track (distance between
the centres of the tyres on an axle) must
be very close to that of the tow car – within
half a tyre width is a reasonable rule, as it is
not always possible to match tracks exactly.
Good off-road trailers permit you to change
parameters such as the track, wheel offset or
stub axle length to line the trailer wheels up
exactly with the tow car. It is also a sound
practice to use exactly the same tyre and
wheels on tow car and trailer so you have
more spare tyre options.
The hitch must be an off-road coupling not a
standard ball hitch, so the trailer can be at any
angle relative to the tow car, and if the trailer or
tow car rolls it won’t necessarily take the other
part of the rig with it. ➤
September 2011 LRW 39
TOWING Towing Off-Road
Suspension is much overlooked on trailers,
but makes a big difference to ease of towing.
The more the trailer bounces around the greater
the energy and traction required to tow it, and
the more it destabilises the tow car, which
again has an effect on traction. The tougher
the terrain, the more noticeable the effect. The
trailer should also follow the tow car exactly to
minimise the corrective force the tow car needs
to exert on the trailer.
The best solution is a properly set up
independent coil suspension with shock
absorbers. The advantages of coils for trailers
are exactly the same as for vehicles; the coils
compress and extend with virtually no built-in
damping, so all the damping can be precisely
controlled by the finely tuned shock absorbers,
which offer much finer control of suspension
movement than crude leaf springs.
Independent suspension obviously permits
one wheel to move independently of the other.
This is a bonus even with a trailer that is a tripod
and so nominally has both wheels on the ground
at the same time, in the same way a road car
nominally has all wheels on the ground all the
time but benefits from independent suspension.
Another obvious advantage of independent
suspension is the large ground clearance, and
for trailers this is important as the tow car may
choose the best line for itself and sacrifice the
trailer’s clearance.
The ‘properly set up’ part needs explanation,
as not all independent systems are designed
equally well. One not so apparent advantage
of independent trailer suspension is the ability
to set the wheels up with camber, caster and
toe. These are wheel alignment settings that
help keep the trailer tracking behind the tow
car, and the principle is the same as the wheel
alignment on cars. Live-axled (non-independent)
trailers do not have this flexibility. In the case of
some trailers, such as the Tvan we used for
testing, the wheel alignment principle has been
developed further. When the trailer hits a bump
on one wheel, say the left, that wheel will drag
and cause the trailer towbar to move to the left,
exerting an unwanted force on the tow car. The
suspension also compresses on the left, and as
it does so it ‘toes in’ which means the wheel
turns slightly inwards. If the wheel on the right is
in extension it toes out. This has the combined
effect of the trailer self-steering back into line,
and is a further improvement on fixed wheel
alignment settings. The concept is known as
passive wheel steering and has been around for
many years, but is not often used on trailers.
This is a pity, because if more trailers had it
there would be less need for sticking-plaster
Here is robustly built independent suspension with excellent ground clearance,
particularly in the centre. The shocks are behind the trailing arms for protection, and
each wheel has camber, caster and toe settings to improve stability
Two recovery points are important so the trailer can be pulled backwards, or sideways
solutions such as trailer sway control systems.
The design is also beneficial when cornering.
The off-road trailer must also have recovery
points. Sooner or later you’ll need to pull it
backwards, perhaps even with the tow car
attached, so there must be at least one point to
do that. Two are preferred so one can be used
to impart a rotating effect to the trailer, or both
used with a bridle for a straight-back pull.
Trailer Setup
Any trailer used off-road must be braked,
regardless of whether brakes are legally
required on-road. You may be legally allowed to
tow 750kg without brakes, but reconsider that
decision when you’re sliding down a hill. And
they have to be electric brakes, not overruns.
Electric brakes allow you to vary when the
brake force comes in, by how much
relative to the tow car brakes, and to
apply the trailer brakes independently
of the tow car – all useful features in the rough.
Overrun trailer brakes have none of that
flexibility; even worse, as soon as you reverse
with any significant resistance, for example over
mud, sand, or a rock, the overrun triggers and
the brake activates.
The trailer tyres are also very important. While
they may not be driven, they will need to grip
laterally and provide braking traction. They must
also be tough enough to air down and withstand
the usual off-road abuse.
For on-road use you’d typically set the
trailer’s centre of gravity forwards to promote
stability and reduce the chance of trailer sway.
Diagram 4
40 LRW September 2011
The trailer tyres are also very important. While they may not be
driven, they will need to grip laterally and provide braking traction
A blind-spot mirror proving its worth – the
driver can see the trailer is going to miss
the cut log, and the main mirror shows
the van will just clear the tree
A good off-road hitch permits
360-degrees of lateral rotation, and 90 up
and down. It is also important that it is
very easy to hook up, as when off-road
lining things up is difficult
Diagram 5
spade/axe carrier. That is because it would get
in the way at extreme angles; in fact the spare
wheel itself came quite close to hitting at some
points. Therefore, take care to remove any such
accessories from the rear of the vehicle, or even
from the front of the trailer.
The tow car should be rated to tow
considerably more than the trailer weight as the
towing ratings are for on-road use, not off-road.
For the off-road rating, dividing by about three
is a good rule: for example, low-range Land
Rovers can usually tow 3,500kg on-road but
are recommended for 1,000kg off-road.
A vehicle with a short wheelbase is in general
better for towing off-road than a long wheelbase.
The shortie will have a tighter turning circle, and
is of course physically shorter, and the overhang
(distance between the rear axle and tow hitch)
is also typically shorter, which is important for
minimising the forces described earlier. Shorties
are more prone to trailer sway, but sway isn’t a
problem at very low speeds.
Fuel consumption will be dramatically
increased in rough terrain. Drivers may be used
to small increases when towing below motorway
speeds on flattish dirt roads, but off-road it is
all hills, stopping and starting, and that means
energy required to move several hundred kilos
of trailer up, down and around, whereas on
the flat often all you need is enough energy to
maintain speed. We found our fuel consumption
increased by between 50 and 70 per cent for a
diesel compared to without a tow car, far more
than the on-road increase of 10-30 per cent.
What to Take
That comes with the disadvantage of reducing
manoeuvrability as the trailer is more reluctant
to change direction, and increasing tow ball
mass, which means more forces for the tow
car to stabilise. Therefore, for low-speed offroad work a centre of gravity further back is
preferable, as speeds are such that trailer sway
will not be an issue. Changing the centre of
gravity isn’t easy or in some cases possible, but
relocating jerrycans is one way to shift quite a
bit of weight around. As ever with trailers, try
to locate the weight as centrally as possible.
Achieving a low tow ball mass by balancing a
heavy weight at one extremity of the trailer with
a weight at the other end simply makes the
trailer balanced but unwieldy.
Tyre pressures are, as ever, critical. Trailer
pressures can go a long way below that of the
tow car, assuming identical tyres. For example,
if we have a tow car weighing 3,000kg we
might assume for the sake of argument that the
weight is split equally over each axle, 1,500kg
apiece, or 750kg per tyre. We then add a trailer
of 1,000kg. Each of the tyres on the trailer
would appear to support 500kg, but it’s actually
less because the tow ball mass is say 100kg,
so it’s more like 450kg a tyre, and the rear tyres
are now up beyond 800kg. The pressure a tyre
requires is proportional to the weight borne, so
less pressure in the trailer tyres. The way to set
pressures is simply to keep lowering the trailer
tyres until they reach the same contact patch
length as the tow car. For example, during the
state forest run with my Defender (2,700kg)
and the semi-loaded Tvan (1,000kg) we ran
16, 18 and 12psi across the three axles, which
used Cooper light-truck construction tyres on
16-inch rims, and that worked fine.
Tow Car Setup
The car itself needs some setup work, and
anything that improves its off-road capability
is to be welcomed. Low range is a must,
given the slow-speed work and extra torque
required. Mud tyres and locking differentials are
two easy improvements, and even stock Land
Rover suspension could do with an upgrade
unless it is self-levelling air suspension. A
small but valuable addition is clean mirrors
with blind-spot sub-mirrors on each so you
can see what the trailer’s wheels are doing.
The ideal tow car will be torquey with a tight
turning circle, although the Defender TD5’s
90kw engine never ran out of puff.
Regular readers may note that in most of
the photos my car is not fitted with its usual
All your recovery gear is a starting point as
trailer recoveries are invariably more complex
than single vehicles, so you need the options
a wide range of kit provides. In particular, take
something on which to rest the jockey wheel
of the trailer should you need to unhook
and manoeuvre the trailer independently of
the tow car. That may involve some terrain
remodelling, for which you need a spade.
Moving the trailer on uneven ground may be
difficult, so you’ll need a winch, even just a
small Tirfor-style hand winch. A power winch
on the tow car is no bad idea, but it may not
be pointing in the right direction. Traction
ramps such as Maxtrax never go amiss and
are easily carried on most trailers.
A good spotter is essential as it is not
possible to see exactly what the back-end of
the trailer is doing when backing up, or the
offside when turning, and no camera system
can yet replace a vigilant human. Handheld
radios work better than shouting.
Other vehicles are invaluable for trailer
recoveries, whether they are towing a trailer
or not. If there are non-trailer vehicles, put one
ahead of and one behind the towing vehicle.
Basically, go prepared with the gear, people
and knowledge. Next month we’ll add driving
techniques to that list. LRW
Part two of this feature continues in the
next issue of LRW and looks at off-road
driving techniques while towing
September 2011 LRW 41