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 learnt
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.