How caravan and tow vehicles interact

by | Sep 20, 2020 | Articles, Caravan/Tow Vehicle Behaviour & Suspension, Roll

by Collyn Rivers

How caravan and tow vehicles interact

How caravan and tow vehicles interact is basically this. A trailer towed via an overhung hitch is fundamentally unstable. Minimising the causes ensures stability within limits.

By the mid-1920s vehicle-drawn caravans were common. From their beginning, they had handling problems. Now, (2020) reports of rigs jack-knifing and overturning still increase. Most now relate to long end-heavy twin-axle trailers towed by lighter vehicles.

Identifying the cause

In the early 1900s, central axled, heavy transport trailers towed via overhung hitches, were unstable. This worsened as towing speeds increased. Fruehauf (USA) realised hitch overhang imposed lateral forces on tow vehicles. As trailers yawed clockwise, that overhang caused tow vehicles to yaw anti-clockwise. And vice versa. The longer that hitch overhang, the greater the effect.

Caravan and tow vehicle interactions

Figure 1. This shows the inherent problem with a conventional caravan. If one part of the rig yaws it causes the other to yaw in the opposite manner. Pic: copyright RV Books, Mitchell Island, NSW, Australia.

Locating the hitch over the tow vehicle’s rear axle/s eliminated side forces (Figure 2) solved the problem. It led to the semi-trailer concept. The transport industry adopted it world-wide. It has used it ever since.

Caravan and tow vehicle dynamics

Figure 2. The fifth wheel concept. Yawing of one or other part of the rig barely affects the other. Pic:

Early vehicles used for towing rarely exceeded 30-40 mph (approx. 50-65 km/h). Nevertheless, their overhung hitches caused rollovers. Curiously, early caravan makers and owners seemed unaware of this – let alone the known cause. Many still are!

Caravan and tow vehicle dynamics – early understanding

Caravan and tow-vehicle dynamics began to be understood in the 1970s. Studies, plus practical testing, revealed the causes of instability. These included trailer yaw inertia, inadequate nose weight, poor weight distribution and incorrect axle positioning. Tow vehicle tire pressure and side-wall stiffness affect stability. Furthermore, that all such causes interact.

It was initially believed that excess trailer weight relative to the towing vehicle was a major concern. It is only recently realised that excess trailer length is an even greater issue. Furthermore, poor loading and excess speed are always involved.

Caravan and tow-vehicle dynamics – terms used

Mass and weight: these are different concepts.

Mass: is the amount of matter within a body.

Weight: is a measure of the force caused by the downward pull of the Earth’s mass (gravity) on mass. It is that which keeps your feet (and an RVs tires) on the ground. For, the purposes of the article, unless stated otherwise, mass and weight can be seen as identical.

Laws of Motion: In 1668, Newton defined the laws of motion. They usefully describe caravan and tow vehicle behaviour.

Law 1. Unless influenced by an external force, mass remains at rest. If a force causes a mass to move it continues to do so at a constant speed. Unless deflected by an external side force it moves in a straight line.

An otherwise stable vehicle towing an equally stable caravan will normally stay in a straight line. A side wind gust, however, may deflect it.

Law 2. The rate of change of a masses’ momentum is proportional to any applied force. It acts in the direction the force is acting. For example, a powerful vehicle can accelerate a rig quicker than a less powerful vehicle.

Law 3. To every action, there is an equal and opposite reaction. Jump backwards off a skateboard, and that board is propelled strongly in the opposite direction.

Force: is any influence that causes a mass to accelerate. The greater the force applied, the greater the rate of change of acceleration. That rate of change is directly proportional to the force acting upon it. It is inversely proportional to the mass of that body. Force has both magnitude and direction. Describing it requires both terms.

Moment arm: A moment arm is a lever. A simple example is a wheelbarrow’s handles. Others include tow-hitch overhang and weight along a caravan‘s length.

Torque: is the effect of a force causing something to roll or rotate. It enables revolving wheels to cause a car to move. Or the action of using a spanner to tighten nuts.

The terms ‘torque’ and ‘moment arm’ means much the same.  The term ‘torque’ is used where there’s some form of powered turning. An example is closing a heavy door. Pushing near its hinge requires more force but less movement. Pushing further from the hinge requires less force but more movement. The work that is done and energy exerted, however, is the same.

‘Moment arm’ relates to levers. An example is an adult and a child on a see-saw. Balance is only possible by the adult sitting closer to the pivot. Or the child sitting further away. A similar effect is a weight on a caravan‘s rear. Its effective is far greater than if close to its axle/s.

Inertia: Inertia is virtually any resistance to change. It’s a tow vehicle’s ability to keep moving at the same speed and in a straight line unless steered otherwise. Or, if jack-knifing – to be straightened.

Momentum: is a measure of the quantity of motion. A moving trailer and its tow vehicle’s momentum is its combined weight times its speed.

Acceleration: relates to change in a mass’s rate of movement. It may be positive (e.g. increasing speed). Or negative (e.g. when braking). It is measured by dividing velocity (metres per second) by seconds. In Imperial units, it is 32.2 ft/s. The unit is often shown as ‘G’ (correctly it is ‘g’). A driver cornering at advised road sign speed experiences about 2 g.

Moment of inertia: is a measure of an object’s resistance to changes in rotation. In imperial (US) units it is shown in pound-foot-second squared (lbf.ft.s2). In metric units, it is shown in kg/m².

A trailer’s such resistance to rotational change can be calculated. It is done by theoretically ‘cutting the trailer into thin slices’. Each slice has a mathematically describable shape.

The moment of inertia can also be measured. It can be done by locating the trailer on a friction-free turntable. That turntable is then rotated by about 30 degrees against the force of springs. It is then suddenly released. The time taken for the trailer to re-centre is a measure of its moment of inertia.

Radius of Gyration: this where a trailer’s centre of mass would be, were all its weight in one place. That centre of mass should ideally be just ahead of its axle/s. A caravan must never be rear-end heavy.

Work has a specific meaning. It refers to transferring energy or applying force over a distance. One example is lifting a heavy object.

Energy is the ability to perform work. It can be expressed as force times displacement in a given time. An example is stacking goods on a high shelf.

Potential Energy is the capacity of something to do work by virtue of its position or configuration. A compressed spring contains potential energy. So does the water in an elevated tank.

Kinetic Energy is associated with motion. Moving objects perform work as a result of moving. Kinetic energy is proportional to the square of a mass’s velocity. A tow vehicle and trailer at 60 mph (just under 100 km/h) have four times the kinetic energy than at 30 mph (just under 50 km/h). This is why it is dangerous to tow at excess speed. Never tow above 60 mph.

Power: is the amount of work done in a unit of time. When you tow your trailer up a hill the work done is always the same. Doing so at 60 mph, however, needs more power, but for a shorter time, than at 30 mph.

Yaw: is a rotational or rocking movement. An example is a trailer rocking around its axle/s. Many caravan owners refer to this as ‘sway’. This confuses. When a trailer sways (rolls) its centre of gravity moves sideways.

Yaw Force: is the effect of (say) a side wind gust that causes a trailer’s front or rear to be pushed sideways. The greater that force, the greater the rate of change of that movement.

Yaw Inertia: can be seen as the resistance of a caravan to yaw when subject to a side force (‘yaw’). It can also be seen as the reluctance to cease yawing once started. (It’s like staying in bed on a cold morning).

Tire behaviour

Horse-drawn carriages had pivoted front axles. This ensures their wheels aligned with the pulling force. But if cornered too fast, the carriage’s inertia overwhelmed the horse’s grip. They would lose control. The carriage’s momentum, however, would cause it to keep moving. Then often overturn.

Tires back then had to revolve, but not sink nor fail under load. Their marginal grip only partly resisted sliding. Braking was by ordering the horses to slow down. Also levering against a tyre to prevent it rolling. The main forces: for traction, steering and slowing, were external, via animal power.

A powered vehicle has similar limitations, but with a major difference. Forces for moving, braking and steering are applied and reacted only by its tires.

Caravan and tow vehicle history

A caravan’s tow vehicle acts physically much as those horses. The trailer depends on the stability of whatever pulls it – as did horse-drawn carriages. This is often overlooked. Pic: courtesy of

Early pneumatic tires

The pneumatic tires used on early cars were like oversized-bicycle tires (and solid tires). They rolled more or less where pointed. When forces exceeded their grip, such tires slid progressively and predictably.

Then cars became heavier and faster. Tires became balloon-like. Owners, particularly in the USA, sought a softer ride. Doing so, however, caused cars to handle poorly. And often unpredictably.

By the mid-1930s it was understood how suspension and tire interaction dictates handling. This particularly applies to caravans and tow vehicles. Their ultimate behaviour is dictated by their suspension and tires. Not all caravan makers and even fewer caravan owners know this. Let alone why and how.

Tire basics

An inflated tire does not roll over a surface. It has a caterpillar-like action. It lays down and picks up an elongated oval of tread (called its footprint). That footprint’s stability is determined by tire construction and air pressure.

Steering a tire is like twisting a rolling balloon. Torque is applied, via the wheels’ rims, to the tires’ sidewalls. The sidewalls flex, and via their stiffness and air pressure, cause the footprint to distort as directionally required. That footprint’s grip is partly molecular and partly frictional.

Tyre slip angles

The steered tires’ footprint’s distortion creates an angular difference between where wheels point and the vehicle travels. That angular difference is called ‘slip angle’. The greater the tire width, sidewall and tread stability and tyre pressure, the lesser the slip angle.

The term slip angle, however, can mislead. In normal driving, the footprint does not slip. That footprint is caused (by torque applied to the tire’s sidewalls), to stretch and distort. It is only when side forces totally overcome footprint grip that tires actually slide out of control.

How caravan and tow vehicles interact - caravan and tow vehicle, weight distributing hitch, trailer independent suspension, tow trailer for car, how much overhang on trailer, tire slip angle

A typical tow vehicle tire (green) increases ‘cornering power’ as its slip angle increases. It then levels off and starts falling away sharply. The latter introduces major and possibly terminal oversteer. It can result in jack-knifing.

A tire’s footprint grip is not linear with imposed weight. When cornering, weight, (or any weightless downforce such as that from the so-called ‘wind spoiler’ used at the rear of racing cars) imposed on tires increases their cornering power. It does so, however, by only 0.8 or so of that increase in grip.

 Interaction of tire slip angles

Interacting front/rear tire slip angles dictate vehicle handling. Passenger vehicle front tires have slip angles that normally exceed their rear tire slip angles. This effect, called understeer, causes vehicles to veer away from side-disturbing forces. (So, likewise, do correctly-trimmed yachts and aircraft).

If cornered too fast, an understeering vehicle automatically increases its turning radius. This reduces side forces, and hence slip angles. If, however, rear slip angles exceed front slip angles, the vehicle adopts an ever-tightening spiral. This causes its rear slip angles constantly to increase. Unless the driver applies opposite steering lock, the rear tire slip angles increase until their footprints lose control. The vehicle then jack-knifes or spins.

How caravan and tow vehicles interact - caravan and tow vehicle, weight distributing hitch, trailer independent suspension, tow trailer for car, how much overhang on trailer, tire slip angle

Understeer and oversteer. In mild form, understeer adds stability. If the vehicle is corned too fast, it automatically adopts a wider radius turn, thus reducing undesired forces. Too much understeer, however, can result in the (upper) example above.Oversteer (in all except rally cars) is undesirable. Once oversteer sets in, unless instantly corrected – by applying opposite steering lock – it rapidly escalates and usually results in the vehicle spinning out of control. Pic: (original source unknown).

Rear tire distortion can cause oversteer. Such distortion can result from a yawing caravan. It imposes side forces on the tow vehicle’s rear. Other oversteer causes are excess tow ball weight or too low tow vehicle rear tyre pressures.

Neutral steer may seem desirable. It is not. Neutral steer requires constant steering correction to overcome road camber. It causes a vehicle to be demanding and tiring to drive. Neutral steering is also impossible to maintain. Even minor changes in tire pressure, loading, or road camber will then cause understeer or oversteer.

Maintaining footprint balance

A rig’s dynamic behaviour depends ultimately on tow vehicle tire behaviour. This necessitates its tires firmly gripping the road. Despite this, some trailer makers maintain that their products do not need shock absorbers. They argue that inter-leaf friction provides adequate damping. Such damping, however, only acts as the spring’s compresses. On the rebound, however, the spring leaves are no longer held in firm sliding contact. As a result, release their rebound energy instantly. That energy jack-hammers the wheel back down. As the wheel impacts the ground it imposes shearing forces on wheel studs and stub axles. This causes those studs to snap. Stub axles break. Wheel bearings needing ongoing replacing. See Wheels Falling off Trailers.

Inadequate or non-existent spring damping also prejudices electronic stability systems. These rely totally on trailer braking. Brakes, however, are only effective when tires are firmly on the ground. Without adequate spring damping, they are not.

Slip angles and load/tire pressure etc

A tire’s cornering power decreases with load and increases with tyre pressure. Adding tow ball mass necessitates increasing (tow vehicle) rear tire pressures to retain the required slip angles. Those rear tires need to be 7-10 psi ( 50-70 kPa) higher when towing. Never increase tow vehicle front tire pressure beyond that in normal driving.

If a vehicle’s front/rear weight balance is unchanged, its tires front and rear slip angles increase proportionally while cornering. The vehicle’s balance is maintained. But if its rear tires loading only increases (as when a trailer yaws), front/rear slip angles change accordingly. If that induces oversteer, the rear tire footprint may lose all grip. If that happens the rig is instantly triggered into a jack-knifing sequence.

Adverse effects of tow vehicle suspension changes

The relative tire loading front/rear (and hence slip angles) is not just a function of weight distribution. It depends on how the suspension resists roll.

Never stiffen rear suspension without stiffening the front proportionally. Stiffening the rear alone causes more of the vehicle’s resistance to roll to be borne by its outer rear tire whilst cornering. That increases its slip angle. If that footprint collapses or slides, jack-knifing is likely. This is not just theory. It happens.

Minor spring rate changes are rarely detectable in normal driving. This is why many claim it’s safe. But by stiffening rear springing alone a strong yaw force can trigger that vehicle into sudden and terminal oversteer.

If a vehicle’s suspension needs upgrading it’s being overloaded. Suspension changes require serious expertise. Not adding airbags sourced from eBay.

Tow ball weight

To keep a caravan straight, front end weight is essential. That in Australia has long since been taken as 10% of gross trailer weight. Now (2020) many have as little as 4%. The British, whose caravans are 40% lighter (per metre) opt for 6-7%. Americans may use as high as 14%.

Basing a trailer’s tow ball weight on a percentage of caravan weight has long been routine. What matters far more, however, is a caravan’s length. Furthermore, where mass is distributed along that length. Because of this, even 10% may be too low for a long end-heavy caravan. This is an ever-increasing problem. Vehicle makers continue to reduce tow ball weight limits. And tow vehicle weight decreases.

To keep sway from building-up, Australian and US-made caravans typically need 550-770 lbs (250-350 kg (550-775 lb). nose weight. Such weight, however, thrusts the tow vehicle’s rear end downward. As with pushing down on the handles of a wheel-barrow, that nose weight causes the vehicle’s front to lift. This undesirably shifts weight from the tow vehicle’s front (steering) tires.

The Hensley hitch

In 1971, the USA’s N. Gallatin obtained a patent (US 3790191 A) for a trapezoidal trailer hitch. This hitch  comprised first and second, spaced apart hitch members pivotally connected at their rearward ends to the forward end of the trailer and pivotally connected at their forward ends to the rearward end of a truck or the like. Shortly after (in 1971), the US Hensley company patented a not-dissimilar version, but not integral to the tow vehicle.

That effect of both patents was to geometrically extend the virtual tow ball further toward the tow vehicle’s rear axle. The Hensley unit became widely used. The hitch weighs about 42 pounds (approx. 20 kg [44 lb]). Most US caravans over about 20 feet (about 6 metres) use one.

Weight distributing hitches – and drawbacks

Developed initially in Australia (in 1950), but adopted almost immediately in the USA, a weight distributing hitch (WDH) forms a semi-flexible springy beam between tow vehicle and trailer. This reduces the weight otherwise imposed on the tow vehicle’s rear tires. It also restores some of the otherwise reduced weight on its front tires.

A WDH, however, can only counteract downforces on the tow vehicle’s rear tires. Although the downforces on those rear tires are reduced by the WDH, those tires are still carrying much of the tow ball mass. They must still resist caravan yaw forces, but a WDH cannot reduce those yaw forces.

That not realised by almost all caravan owners, and even some makers, is that a WDH inherently reduces a rig’s ultimate cornering ability. It does so typically by about 25%. This issue is recognised and addressed by the US Society of Automobile Engineers in its current SAE J2807 recommendations. These recommendations are now followed by all US (and the top three) Japanese vehicle makers.

That (SAE J2807) recommendation includes advising to adjusting a WDH to correct no more than 50% of the tow vehicle’s rear end droop. Never the full amount. It suggests correcting 25% of that rear end droop is better. Such advice has long been given by Cequent in the USA. (Cequent owns Hayman Reese). Hayman Reese locally used historically to advise levelling the rig. It now follows the Cequent (USA) advice.

A WDH is only required when the download on the tow vehicle’s rear tires is not acceptable. If it is acceptable you can readily compensate for that weight shift. To do so, increase tow vehicle rear tire pressures by 7-10 psi (50-70 kPa).

Trailer independent suspension can have downsides

Passenger car independent (front) suspension stems from the USA in the 1930s. It resulted from a buyer demand for softer suspension. Softening and increasing spring travel, however, resulted in beam front axle wheel ‘tramping’. The wheels would alternately jump up and down and swing violently from lock to lock. This particularly happened with poorly damped and/or soft suspension long-travel suspension.

Around 1934, General Motor’s Maurice Olley established this was a ‘gyroscopic precession’. You can experience this by holding a bicycle’s front wheel off the ground, spinning it and then swinging it in an arc. It imposes an unexpected swaying effect. This can also be shown via a gyroscope.

How caravan and tow vehicles interact - caravan and tow vehicle, weight distributing hitch, trailer independent suspension, tow trailer for car, how much overhang on trailer, tire slip angle

Here, (US) teacher Gary Rustwick demonstrates the effects of gyroscopic precession. He swings the spinning wheel in an arc whilst standing on a free-moving turntable. As he does so precession forces cause the turntable to rotate.

Wheel precession is dangerous. If it builds up, the vehicle becomes unsteerable. Worse, reducing speed (as one must) decreases the tramping frequency but increases its amplitude.

Need for steered wheel stability

In the early 1930s, General Motors’ Maurice Olley realised precession was only totally preventable by ensuring steerable wheels rose and fell vertically. Not forced to move in an arc created by a tilting beam axle. Achieving this required steered wheels to be suspended independently.

This concept was not new. It was used on a road-going steam locomotive in the late 1800s. Lanchester used it in 1901, Morgan in 1911, Lancia and Dubonnet in the 1920s. But all did so to reduce unsprung mass and improve the ride. Olley knew that too, but also that independent (and vertical) front wheel travel was necessary for soft suspension.

Non-steerable wheels are subject to the same precession forces. As they cannot swivel, however, such forces do not matter. This is why many cars and almost all trucks and many 4WDs retain beam-axle rear suspension.

Caravan wheels do not steer

As caravan‘s wheels do not steer there is no inherent need or benefit for independent suspension. Nor is there any need for suspension travel greater than that of their tow vehicles. For much of the time, a caravan rocks on an axis around its tow hitch. Many, for seemingly marketing reasons, have ultra-soft suspension like some American cars of the mid-1930s. Almost all currently-made cars are much firmer. They also have less suspension travel. And do not wallow.

The US-made Airstream is an exception. Right from its beginning in the early 1930s, its pre-tensioned rubber suspension provides a firm but adequately-soft ride. The suspension is independent.

Suspension issues

Human physiology dictates passenger vehicle suspension. The result is compromised by the brain’s response. Nausea is created if the suspension is too soft, and physical discomfort if too hard. Such constraints do not apply to non-human carrying trailers. It is thus absurd for their makers (particularly in Australia) to base the suspension on huge US cars (such as Chevrolets) of the mid-1930s. For optimum road holding, caravan suspension needs to be firm. This can readily be done and with no risk to any contents.

Fifth-wheel trailers more stable

A fifth-wheel caravan pivots from a hitch above the tow vehicle’s rear axle/s. Side-wind gusts may cause the trailer to swing slightly, but the forces are low and quickly self-damp. They do not affect the tow vehicle. Drivers are rarely aware of them. As long as a fifth wheeler’s rear wheels are well back, the weight on the tow vehicle is within that vehicles’ limits. A well-balanced fifth wheeler is stable at any speed.

Action and reaction

As described earlier in this article a hitch distanced behind a tow vehicle’s causes a trailer to yaw if that tow vehicle yaws – and vice versa. This would not overly matter if the trailer yawed in the same direction. That overhung hitch, however, causes the opposite. If the tow vehicle yaws clockwise, its overhung tow ball yaws anticlockwise. As it does, it takes the nose of the trailer with it.

Likewise, if the caravan yaws clockwise, that overhung tow ball swings the rear of the tow vehicle anticlockwise. This is the root cause of conventional trailer instability. The longer that overhang, the greater the (undesirable) effect.

At low levels, yaw interaction is mainly annoying. It is reducible (at low speed) by friction and other forms of damping. It typically dies out after two or three cycles. If it does not, it indicates instability. That needs resolving at its source. Friction damping is almost useless at speed. This is because the friction stays constant. Yaw forces, however, increase with the square of the rig’s speed.

Severe yaw is serious

If severe yawing occurs above a critical speed (specific to each rig and its loading) the yaw may self-trigger into jack-knifing. It is fuelled by the rig’s kinetic energy. Once triggered, if travelling at speed, this sequence is almost impossible for a driver to correct.

Musicians and public speakers experience a similar effect. If their microphone picks up the sound from the loudspeakers, that sound suddenly develops a full-on yowl. This is only stopped by drastically reducing the volume (akin to braking a caravan). Or by moving back from the loudspeakers (akin to reducing tow hitch overhang).

Critical speed

Depending also on loading, every combination of a tow vehicle and caravan has a so-called critical speed. Once above that speed, yawing can irreversibly escalate out of the driver’s control.

That critical speed, and the degree of yaw, is directly associated with the tow vehicle’s mass relative to the trailer’s mass (and particularly mass distribution). It is also associated with trailer length, hitch overhang, tyre type and size, sidewall stiffness and pressure etc.

All of the above (and more) is involved. The longer and the lighter the tow vehicle (and its tow ball mass) the lower that critical speed. The onset of critical behaviour is sudden. Because of this, the still-common suggestion ‘accelerate to dampen yawing’ is risky except at very low speed.

The critical speed effect does not imply that the rig jack-knifes if that speed is exceeded. If, however, a rig is travelling at or above its critical speed, a strong side wind gust, or a strong swerve makes jack-knifing more likely. Few owners encounter this, so many dismiss its possibility.

Avoiding jack-knifing

When a caravan yaws, it transfers the yaw force via the overhung hitch to the tow vehicle. The transmitted forces are resisted by the tow vehicle’s weight and the grip of its tires. Minor trailer braking assists straightening the rig. Heavy trailer braking, however, may overwhelm the trailer’s tires as they are already stressed by yaw forces.

If a caravan yaws never apply tow vehicle braking. Doing so may trigger that tow vehicle’s already stressed rear tires into terminal oversteer – such that it spins.

Beware of cruise control

Cruise control detects the minor drop in speed when yawing occurs. It nevertheless attempts to restore the set speed and the tires slip angles increase. While convenient, it is thus better not to use cruise control when towing a heavy trailer at speed.

Wind effects

A further cause of major caravan instability is wind forces from fast-moving trucks. This is particularly so of trucks towing trailers; and even more so if the truck has a flat front (rather than a bonnet). That bluff front creates an ongoing strong bow wave plus a vortex (i.e. a rotating wind gust) along its side.

If overtaking (or being overtaken) a tow vehicle and caravan will experiences wind buffeting. As the trailer’s tow vehicle approaches the rear of the truck cab, a side wind vortex initially causes the tow vehicle to be drawn toward the truck. As the tow vehicle draws closer to the front of the truck cab it is hit by the truck’s strong side-going bow wave. This causes the trailer to swing slightly away from the truck. The overhung hitch causes the front of the trailer to sway toward the truck. A vortex pulls it in further. This initiates a rapidly developing yaw cycle. Jack-knifing can result.

A generally similar but less common effect occurs when a truck and a caravan rig are approaching each other at speed on narrow roads.

Electronic stability systems

Electronic stability systems monitor trailer yaw. AL-KO’s applies caravan braking when it detects ongoing yaw forces exceeding about 0.2 g. The maker warns the system is an emergency aid. It is intended to prevent accidents. It does not enhance stability.

The Dexter system applies the caravan’s brakes asymmetrically (i.e. out of phase with the yaw). It does so at lower yaw acceleration levels. As testing is done at 60 mph (just under 100 km/h) the ability (except as a yaw reducer) to prevent a catastrophic incident at speeds above the critical speed is unknown. Both Dexter and AL-KO (now one company) emphasise their products cannot override the laws of physics.

Enhancing rig stability

The major factors include everything that affects front/rear tyre slip angles. Those within owner control include:

Loading and load distribution of the trailer and tow vehicle.

Excess tow ball overhang caused by unnecessary hitch bar extension.

The speed at which the rig is driven.

Fitting and use of yaw control devices, WDHs etc.

Those outside direct owner control (but subject to the choice of rig) include:

Length of the trailer, the unladen weight of the trailer.

Weight and stability of the tow vehicle.

Those determined by the trailer builder include:

Length of the trailer.

Weight of the trailer.

Distance from trailer tow hitch to axle centre/s.

Distribution of weight along the length of the trailer (particularly at its rear).

Centre of mass (i.e.weight) in both planes.

Height of the roll centre and roll axis (as imposed by the geometry of the trailer’s suspension).

Moment Arms about the roll axis, particularly at the far rear.

The magnitude of yaw inertia.

The radius of gyration.

Damping of yaw and roll.

Tires with good sidewall stability (such as light truck tires).

Optimising towing stability (summary)

Tow vehicle behaviour is now well understood and proven. That required is a long-wheelbase vehicle with a short rear overhang that weighs at least as much as the trailer. Towing three or more tonne behind a 2.5-tonne dual-cab ute is an accident awaiting the circumstances to trigger it.

A major undesirable factor with caravans is excess length. Excess weight matters, but excess length is now known to be a far greater issue.

Reducing trailer perimeter weight, and particularly rear-end weight, is vital. If feasible house a trailer spare wheel below the chassis and in front of or just behind the axle. Batteries are best located centrally between the axles. Water tanks should be wide but not long and located as centrally as possible.

Friction devices smooth low speed snaking, but have a negligible effect at high speed. One that works well at low/medium speeds is likely to be less than 1% effective at 100 km/h (62 mph). Elastic energy held within sprung-cam devices may suddenly be released when such devices are overwhelmed –  and ‘fed into the system’.

Lateral sidewall stiffness of all tires assists.

The major factor, however, is excess speed.

Driver reaction

Most big rigs feel stable in normal driving. There is also usually sufficient stability to enable an experienced driver to cope with scary but not accident-resulting situations.

A major issue is that (particularly) with heavy rigs, unless grossly unbalanced, it is not possible for a driver to know (by feel or ‘experience’) how that rig will behave in an emergency. Most big rigs feel ultra-stable. Short vans are more stable but may feel twitchy (particularly if twin axle). The concern is how the rig behaves in situations that cause major yaw. These include sudden strong side wind gusts on a motorway, braking hard on a steep winding hill at speed, and swerving at speed.

‘My rig always seemed so stable’

Police say the most after-accident reaction is: ‘my rig always seemed so stable until it suddenly jack-knifed’. Such apparent stability is typical of container ships and car ferries, until a rogue wave or turning too sharply proves otherwise.

There is increasing evidence that the safe maximum speed for big rigs is under 60 mph (about 100 km/h). This is related to tow ball weight. Furthermore, the lower that weight, the lower the safe speed.

Trailer and tow vehicle dynamics – Summary

The above is a precis of some of the most relevant parts of RV Books Why Caravans Roll Over – and how to prevent it. The book is written in plain English but has a fully referenced final technical section.


My articles in this area primarily summarize current thinking. They stem from my interest and involvement while employed by Vauxhall/Bedford’s Research Dept in the 1950s, and particularly by the influence of Maurice Olley.

Maurice Olley was born in Yorkshire in the late-1800s. Following time as Rolls-Royce’s Chief Engineer, he worked with General Motors Research Division. He later returned to Vauxhall Motors (UK). I was privileged to attend his lectures during my years at Vauxhall Motors Chaul End Research Centre

Collyn Rivers is a semi-retired automobile research engineer.  He is the author of seven books, five of which are about making RVs and solar work optimally.

Why Caravans Roll Over
and how to prevent it

Explains in detail every aspect of tow vehicle and caravan stability. This is what you need to know to be safe on or off the road.

eBook versions

Paperback version

The book retailers set their own prices which can vary substantially. We'll aim to keep a selection of the better prices above.

Any bookshop, whether online or bricks and mortar, can order copies of Why Caravans Roll Over.  Just ask.
ISBN: 978-0-6483190-6-1

Why Caravans Roll Over
and how to prevent it

Explains in detail every aspect of tow vehicle and caravan stability. This is what you need to know to be safe on or off the road.

eBook versions

Paperback version

The book retailers set their own prices which can vary substantially. We'll aim to keep a selection of the better prices above.

Any bookshop, whether online or bricks and mortar, can order copies of Why Caravans Roll Over.  Just ask.
ISBN: 978-0-6483190-6-1