This section is UNDER REVISION.
The choice to take an overland trailer on an expedition is sometimes a difficult one, because on the one hand a trailer means more comfort, while on the other, it can mean more work and more frustrations. The decision is helped by the following pages.
Beware of tow bars fitted by independent fitment centres. Four-wheel drive vehicles often stress their tow bars in excess of what would be considered normal towing operations.
Ideas about tow-bars:
• Towing off-road stresses a tow bar far more than ordinary towing.
• Tow bars are sometimes used for vehicle recovery, although this is very unsafe and ill-advised.
• Off-road trailers are bigger and heavier than the average family man’s little ‘Venter’.
• When considering a tow bar; if the one being offered looks similar in strength to those fitted to a normal road car then it is not strong enough for your 4×4.
• Broken tow hitches occurring in the wilderness are not uncommon and depending where the breakage occurs, it can be difficult to repair without welding equipment.
• The standard 50mm tow ball is rated to pull a trailer with a mass of no more than 3500kg. This rating is calculated for towing on a paved surface not over rocks or in heavy sand.
• Maximum permissible weight on a standard 50mm tow ball is
150 kgs. This is often exceeded when towing off-road.
• When calculating how much weight the vehicle can carry after the trailer is hitched up, double the tow hitch weight and deduct that from the vehicle’s carrying capacity.
Ideal vehicle – trailer combinations
• Long wheelbase combined with short rear wheel-to-tow ball distance makes for a stable tow vehicle.
• Short rear wheel-to-tow ball distance with long trailer tow hitch to axle distance improves stability.
• Trailer’s vertical C of G must be less than 40% of the trailer’s tow hitch to axle distance.
• Short vehicle wheelbase or short rear wheel-to-tow ball distance combined with long trailer tow hitch to axle distance make for
• Vehicle tow ball height must equal trailer tow hitch height when trailer chassis is horizontal. Essential when towing twin-axle trailers.
• Mud flaps must be fitted to the rear of the tow vehicle to prevent damage caused by flying stones.
Off-road trailers must be designed and built to withstand extended outback travel: Sand, mud, rocks, ridges, troughs and axle-cracking corrugations.
Trailers – their advantages over roof racks are:
• Easier to load and unload, especially heavy items such as full Jerry cans.
• Payload is significantly higher than a roof rack.
• Trailers do not negatively effect the vehicle’s center of gravity.
• Extra equipment can be loaded onto the trailer lid which can also be used as a work surface.
• Packing and unpacking is often quicker than a vehicle. • Built-in kitchens help take the chore out of camp cooking.
• Heavy items carried in the trailer can be left at ‘base’ while game driving and exploring.
Their disadvantages over roof racks are:
• More difficult to tow over loose and uneven surfaces.
• Experience is required when driving off-road and reversing.
• Trailers can be a severe hindrance in any terrain where traction is a problem.
• A poorly loaded trailer can cause instability when cornering and braking.
• A trailer adds wear and tear to the towing vehicle, exaggerated when the trailer is badly loaded or when towed by inexperienced drivers.
The most important safety considerations when it comes to towing on-road is straight-line stability, oscillation or weave and stability in a turn. Factors which affect these are as obvious as trailer hitch weight and trailer weight to vehicle weight ratio as well as items which are seemingly inconsequential such as the spring rates of the towing vehicle and trailer center-of-gravity.
In this section we illustrate causes and effects of vehicles and trailers in an attempt to improve safety. I give credit here to Tom Sheppard’s outstanding book, ‘The Land Rover Experience’, published by Land Rover, from where much of this information and many of the analogies are taken.
Consider a trailer being towed on an undeviating course by a vehicle moving in a straight line. Here the only force acting on the trailer is via the tow hitch and as a result the trailer moves in a straight line. Now consider a gust of wind or undulations in the road surface (supposing that the vehicle is unaffected), the trailer now acts under a new force – sideways. The trailer’s tires will as a result be at an angle, albeit small, to the direction of motion. As a result an opposite side force is exerted by the tyres bringing the trailer behind the vehicle again. Understanding this simple principle is required as we go further.
Oscillation – decaying or increasing
Let us distinguish between decaying or increasing oscillation and how it relates to towing. Consider an ordinary school ruler with a hole in one end. With the ruler swinging on a pencil pushed through the hole, properties governing oscillation can be demonstrated. With the pencil stationary, the ruler hangs straight down by the force of gravity or in our scenario a vehicle moving on an undeviating course. Take the bottom of the ruler and pull it sideways and release it, keeping the pencil stationary. The ruler exhibits decaying oscillation as it swings back a few times quickly coming to rest, demonstrating straight line stability as described above. Now, take the pencil and simulate a vehicle moving over an uneven road surface by moving it sideways as the ruler is pulled sideways and released. If the frequency of the movement of the pencil matches the frequency of the swing, increasing oscillation takes place. This will happen as you instinctively try to match the phase of the ruler swing, trying to make the ruler swing as high as possible.
Again let us use the school ruler to demonstrate whiplash. This time hold the ruler in a horizontal plane with your forefinger and thumb over the hole. Flick your wrist to the right and left. As you do so, the ruler trails the wrist movement and then overshoots.
Applying what we have learned
With all these analogies it can be seen that an incorrect combination of hand and wrist action, ruler weight and thumb grip can produce varying effects. Getting these combination’s right, the action of the ruler is very much as described. This in turn demonstrates that the varied actions of the ruler (trailer) and the wrist (towing vehicle) result is varying degrees of decaying oscillation, increasing oscillation and whiplash. Understanding these principles will assist in improving performance and safety.
Centre of gravity (C of G)
A trailer’s C of G exists in the horizontal and vertical plane. Both have an effect on the trailer’s stability. To demonstrate C of G in the horizontal plane consider a round bottle (trailer) laid on its side and spun. It will spin around its center of gravity. Grab the bottle by one end while it is spinning. Instead of spinning around its C of G it will now spin around the end where it is held (trailer hitch). Energy is transferred to this end and will exert a reactive lateral force (the bottle will attempt to rotate and spin simultaneously) on your hand (tow hitch). It is necessary therefore to consider the position on the trailer’s C of G and its associated lateral force acting on the vehicle thereby affecting stability. An operator can control this C of G by packing sensibly and making sure that the trailer’s C of G is at an optimum – which lies 10 – 20 cms in front of the axle.
C of G in front of the axle
With the trailer’s C of G ahead of the axle, as the towing vehicle swings the trailer exerts a force on the vehicle that reduces yaw and as a result the oscillation begins to decay. On the other hand, trailer’s C of G in front of the axle degrades cornering stability. When the C of G is too far forward it can provoke a slide and roll-over in a turn as a result of the same forces in place in our rotating bottle example.
C of G behind axle
With the trailer’s C of G behind the axle the result of vehicle swing creates a force that amplifies yaw and begins what can become increasing oscillation.
“>Towing on-road – Summary
• All stability problems are amplified as the trailer gets heavier. Once the trailer’s weight exceeds 70% of that of the towing vehicle you are entering the critical zone with regard to stability and safety.
• Estimate the trailer’s C of G at 10 – 20 cms ahead of the axle.
• Concentrate heavier articles over the axle thereby reducing inertia.
• Remember to keep the tow ball greased and replace the grease in very dusty conditions because grease + dust = grinding paste.
• Take extra care in tight bends. The forward trailer C of G tries
to push the vehicle’s tail around the corner faster.
A vehicle’s ability on slippery ground is degraded significantly when pulling a trailer. The forces exerted by a trailer off-road are similar to those on-road but are amplified. Instability and control problems can manifest themselves at very low speeds.
Off-road towing points to be considered
• Disconnect the jockey wheel and stow it before the ground
• Disengage the over-run brakes by sliding the lock over the towing arm, preventing it from retracting and engaging the brakes. When driving, allowance rarely needs be made for the trailer’s ground clearance as in most cases it exceeds the vehicle’s. However, a trailer will reduce maneuverability off-road and off-road trailers are frequently damaged by trees, stumps and bushes that the tow vehicle runs past.
• Make allowances for the additional weight when braking and descending steep slopes by braking gently and changing to low gears timorously.
Descending steep slippery slopes with a heavy trailer behind you is one of the most difficult off-road manoeuvres and should be done with extreme caution. The loss of steering control experienced in such conditions is exacerbated by the trailer. As the driver tries to direct the vehicle from the front, the trailer acts on the tail, steering it anyway it feels. In this predicament jack-knifing can occur and no amount of expert action on the part of the driver seems to be able to stop it. If gentle acceleration fails to pull the trailer straight or if acceleration is too risky in terms of losing control of the vehicle, then the only way out may be to disconnect the trailer and to manhandle it down the slope on the end of a winch cable.
During the years I have received a great deal of conflicting feedback from many trailer manufacturers over what I have written. Nevertheless I must report it as I see it. These are my opinions and are based on what I have heard through the bush telegraph about trailers; what makes them good, and what breaks when they break. My own experience of trailers amounts to towing large yachts, horse boxes and traveling with people who dote on their trailers. I do not own an off-road trailer and have no desire to. I would rather drive a bigger vehicle or carry less.
Suspension and chassis
The type of suspension is critical to the success of an off-road trailer. Trailers tend to bounce around a great deal so it must be built to withstand severe punishment. Suspension types range from torsion bar to leaf spring. Rubber torsion bar suspension is not an option, no matter how “heavy-duty” the axles is claimed to be. They all eventually fail in off-road use.
Leaf springs fitted with shock absorbers are the strongest and appear to work the best. Check where the springs mount onto the chassis – this is where breakage occurs and it must be reinforced. The chassis should be a rigid steel frame, steel tube or heavy channeling, extending all the way to the rear spring shackles. Springs mounted directly onto a stiffened load box are rarely strong enough. Breakages also occur at the joint between the A-frame and the load box and this should be reinforced.
Axle breakages often occur not because the axle is not strong enough but because the mounts aren’t. Should the mount slip or break the axles shifts and the wheels no longer run true. This
creates a build up of heat in the bearing which eventually ceases. When this occurs the hub shaft snaps from the overload. The failure is due to inadequate strength in the mount and not the axle’s
ability to carry a load.
Shock absorbers need to be fitted to leaf spring-sprung trailers. I recommend their use for two reasons; shock absorbers reduce bounce and reduce shock loads to the axle. Vertical wheel travel is of little importance in trailer design but the ability of the axle to absorb shock and not bounce is.
The tow hitch is a critical component and badly designed ones break! The trailer’s tow hitch must be attached with high-tensile steel bolts. Mild steel bolts are not strong enough. Check your trailer’s bolts and change them if they are mild steel. Light-duty tow hitches welded onto the chassis draw-bar, which is the cheap way to build them, are not strong enough and many have failed in the bush.
Wheels and tyres
To improve stability on bush tracks the wheel track should closely match that of the towing vehicle’s. To avoid having to carry spares specifically for the trailer and to allow its wheels and tires to be interchangeable with the vehicle’s, they need to have interchangeable wheel rims with identical tire diameters. However, trailers with wheels that have a larger diameter than the vehicle’s will pull better through sand. If you can carry a spare specifically for your trailer, fit over-sized wheels and tires on the trailer. These tires can then be deflated to pressures below that of the vehicle and the trailer will cause far less drag.
Most trailers are stored outdoors and therefore are prone to rapid decay by corrosion and a trailer cover is a good idea. A stainless steel body is only necessary if you intend to use your trailer extensively on the beach. Galvanized mild steel is an excellent alternative and is almost as good, easier to repair and cheaper. For use inland, sealer-protected mild steel is fine if the trailer is stored under cover. Trailer components that seem to deteriorate first are attachments such as hinges and clamps. These should be stainless steel and attached with stainless bolts. When looking at the many South African off-road trailers, most manufacturers have placed rust prevention high on their priority list.
Stability at speed
Stability depends more on weight distribution in the trailer and in the towing vehicle than on trailer design. There is no simple way of testing a trailer’s stability at high speed other than to tow it yourself. Weight distribution is critical to stability. If you have deflated trailer tires to assist progress through sand, this will cause instability when you get back on the road. Remember to re-inflate your trailer tires.
The distance from the tow ball to the trailer axle will determine ease of use. The shorter the distance the better it will handle off-road but the penalty comes with reversing and on-road stability. The longer the tow ball to trailer axle distance the easier it will tow in all conditions other than severe off road.
Many off-road trailer manufacturers omit over-run brakes or offer them as an option. This is because in very heavy dust conditions brake drums tend to fill up with sand which wears out the shoes. This only occurs in excessive dust conditions. If the wheels are as large as the vehicle’s, which they should be, the same dust problems occur to the vehicle brakes as well. If dust does become a problem simply remove the shoes from the drums and lock the brakes open at the tow arm, an hour’s job for both wheels. As for me, I would rather take the safe route and the unlikely risk of troublesome brakes than drive at 120kph with a ton of unbraked load behind me.
Another argument against the fitting of brakes is that when driving over uneven terrain the brakes engage and disengage as the trailer bumps around. All over-run brake systems are fitted with a locking device on the tow arm. This is a hinged piece of steel that wraps around the arm to prevent the brakes from activating when reversing. This must be engaged when driving over uneven ground where speeds are low and over-run brakes are no longer required.
Trailer manufacturers may place a weight restriction plate stating 750-kgs on a trailer capable of carrying a ton or more, because with a stated payload of over 750 kilograms the trailer must, by law, be equipped with a braking system. The choice is yours; are you prepared to take a risk and tow such a large mass without a braking system? Under normal driving conditions you may not realize the risk, but do an emergency stop and it could mean the difference between stopping clean or rolling your vehicle.
This wheel supports the nose of the trailer when standing alone. Because off-road trailers are often left parked on uneven ground, the longer the jockey wheel the better. It must be able to be removed completely and stored on the trailer when driving off-road. If it is simply raised, it is vulnerable to damage off-road.
Trailer-top racks and tents
A tent on a trailer is a excellent, practical idea because the tent need not be taken down when going on a game drive. The trouble is that unless the trailer has side access doors you are unable to get to the trailer’s contents once the tent is erected. A tent does raise the trailer’s center of gravity and those with raised tent platforms tend to be a bit top-heavy.
A vast range of equipment can be specified when ordering a trailer
as most manufacturers build to order. Articles fitted range from
built-in water tanks with camp showers, dual battery systems linked to the vehicle, fridge/freezer units and simpler items like Jerry can and gas tank racks.
In Chapter-8, I discuss in detail why it is a bad idea to locate a freezer in a trailer and why it’s even a worse idea to fit a battery in a trailer. Briefly, the long cables and trailer connector cannot take the current and the system works at very low efficiency. Freezers and batteries must remain in the vehicle. However, a series of power points and a fluorescent light fitted inside the lid and connected to the vehicle electrics is very handy.
Consider that the more you fit into your trailer, the heavier it will become and the more difficult it will be to handle in the bush.
Weight distribution in a trailer is very important. Some trailer designs have all the Jerry can brackets and water tanks fitted behind the axle which can cause low trailer hitch to total weight ratio. This in turn can cause instability, especially when climbing steep hills, where the trailer lifts the rear of the towing vehicle. I have seen a Pajero battling to get up Sani Pass, which is normally effortless for a 4×4, because its traction was seriously compromised by the badly packed trailer behind it. Consider carefully where the optional extras you choose for your trailer are fitted. Nose-cones storage boxes and boxes on the mudguards are very handy and must be dust-proof and lockable. Jerry can mounts on a trailer must be low down to keep center-of-gravity low. Trailers are ideal for carrying spare fuel.
If your vehicle carries its spare wheel on the tailgate make sure that it can be swung free with the trailer hitched on. Spare wheels carried here can make hitching awkward and any boxes, clamps or other attachments above the tow arm can make matters worse. Another reason for limiting attachments on the towing arm is that anything that could obstruct the wheel carrier from being opened can also hit the spare wheel when driving through a dip, when the trailer lifts and the vehicle drops. The option of removing the spare wheel carrier and placing it on the trailer is an alternative, but remember that the vehicle needs to carry a spare when the trailer is left at ‘base camp’.
When judging if a trailer is suitable, open and close all of the boxes and the lid. Pretend you are at your camping site and you need to find something in the trailer. Many trailers are fitted with a tailgate – a very useful feature as you can pack traveling items such as the day’s lunch or a tool box at the back which can be easily accessed when traveling. Other features include cubby holes, exterior boxes or interior compartments, some of which are cleverly designed and some impractical for reasons such as a narrow aperture which become frustrating when packing and unpacking. Some trailers fitted with tailgates require that the roof be opened in order to open the tailgate. That is bad enough but some go even further in poor design – to open the tailgate the lid must first be opened. But in order to support the open lid the tailgate must be closed. Draw your own conclusions!