by Collyn Rivers
Caravan and tow vehicle dynamics
In the early 1900s, trailers with central axles, towed by trucks with overhung hitches, were unstable. This escalated as towing speeds increased. Around 1920, Fruehauf (USA) realised hitch overhang caused (not just allowed) trailers to yaw anti-clockwise. And vice versa. Furthermore, the longer the hitch overhangs the greater extent of and the lower the road of its onset.
To this day, this is an inherent problem with conventional caravans. In the 1970s studies plus practical testing revealed the causes. These include excess trailer length, inadequate nose weight, poor weight distribution and incorrect axle positioning. Tow vehicle tyre pressure and side-wall stiffness too affect stability. Furthermore, such causes interact.
It was initially believed that excess trailer weight relative to the towing vehicle was the major concern. It is, however, increasingly realised that excess caravan length (and excess speed) are more the cause.
An otherwise stable vehicle towing an equally stable caravan will normally stay in a straight line. A side wind gust, however, may deflect it.
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. The unit is often shown as ‘G’ (but correctly as ‘g’). A driver cornering at an advised road sign speed will experience about 2 g.
Caravan and tow vehicle dynamics – tyre 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.
Tyres 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 tyres.
A caravan’s tow vehicle acts physically much as those horses. The caravan depends on the stability of whatever pulls it – as did horse-drawn carriages. This is often overlooked.
Early pneumatic tyres
The pneumatic tyres used on early cars were like oversized-bicycle tyres (and solid tyres). They rolled more or less where pointed. When forces exceeded their grip, such tyres slid progressively and predictably.
Then cars became heavier and faster. Tyres became balloon-like. Owners sought a softer ride. Doing so, however, caused them to handle poorly. And often unpredictably.
By the mid-1930s it was understood how suspension and tyre interaction dictates handling. This particularly applies to caravans and tow vehicles. Their ultimate behaviour is dictated by their suspension and tyres. Not all caravan makers and caravaneers know this. Let alone how.
An inflated tyre 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 tyre construction and air pressure.
A typical tow vehicle tyre (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.
Steering a tyre is like twisting a rolling balloon. Torque is applied, via the wheels’ rims, to the tyres’ 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.
The steered 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 tyre width, sidewall and tread stability and tyre pressure, the lesser the slip angle.
A typical tow vehicle tyre (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.
The term slip angle can mislead. In normal driving, the footprint does not slip. That footprint is caused (by torque applied to the tyre’s sidewalls), to stretch and distort. It is only when side forces overcome footprint grip that tyre slides.
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 tyres increases their cornering power. It does so, however, by only 0.8 or so of that increase in grip.
Interaction of tyre slip angles
Interacting front/rear tyre slip angles dictate vehicle handling. Passenger vehicles have front slip angles that normally exceed their rear 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 some opposite steering lock, the rear slip angles increase until their footprints lose control. The vehicle then jack-knifes or spins.
Understeer and oversteer in extreme. 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. The above is from www.driversdomainuk.com/img/oversteer.jpg (original source unknown).
Rear tyre 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 tyre pressure, loading, or road camber will cause understeer or oversteer.
Caravan and tow vehicle dynamics – maintaining footprint balance
A rig’s dynamic behaviour depends ultimately on tow vehicle tyre behaviour. This necessitates its tyres firmly gripping the road. Despite this, some trailer makers maintain that leaf-sprung 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 prejudice electronic stability systems. These rely totally on caravan braking. Brakes, however, are only effective when tyres are firmly on the ground. Without adequate spring damping, they are not.
Caravan and tow vehicle dynamics – slip angles and load/tyre pressure etc
A tyre’s cornering power decreases with load and increases with tyre pressure. Adding tow ball mass necessitates increasing (tow vehicle) rear tyre pressures to retain the required slip angles. Those rear tyres need to be 50-70 kPa (7-10 psi) higher when towing. Never increase tow vehicle front tyre pressure.
If a vehicle’s front/rear weight balance is unchanged, it’s front and rear slip angles increase proportionally while cornering. The vehicle’s balance is maintained. But if its rear tyres loading only increases (as when a caravan yaws), front/rear slip angles change accordingly. If that induces oversteer, the rear tyre footprint may lose all grip. If that happens the rig is instantly triggered into a rig jack-knifing sequence.
Adverse effects of tow vehicle suspension changes
The relative tyre 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 tyre 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 buying airbags on eBay.
Caravan and tow vehicle dynamics – 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.
Australian-made caravans typically need 250-350 kg nose weight. Such weight, however, thrusts the tow vehicle’s rear 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) tyres.
Weight distributing hitches
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 tyres. It also restores some of the otherwise reduced weight on its front tyres.
A WDH, however, only counteracts downforces on the tow vehicle’s rear tyres. Although the downforces on those rear tyres are reduced by the WDH, those tyres are still carrying much of the tow ball mass. They must still resist caravan yaw forces, but a WDH cannot reduce those yaw forces.
Weight distributing hitch drawbacks
That not realised by almost all caravan owners and makers is that a WDH inherently reduces a rig’s ultimate cornering ability, 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 tyres is not acceptable. If it is acceptable you can readily compensate for that weight shift. To do so, increase tow vehicle rear tyre pressures by 50-70 kPa (7-10 psi).
Caravan independent suspension has next to no benefit
Passenger car independent (front) suspension stems from 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.
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 the 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. He particularly knew that independent (and vertical) front wheel travel was vital for soft suspension.
Non-steerable wheels are subject to the same forces. As they cannot swivel, however, such forces do not matter. This is why many cars and most trucks and 4WDs retain beam-axle rear suspension.
Caravan wheels do not steer
As caravan‘s wheels do not steer there is no 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 pointlessly have suspension like the wallowing US cars of the mid-1930s. Almost all currently-made cars are much firmer. They also have less suspension travel. And do not wallow.
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 discomfort if too hard. Such constraints do not apply to non-human carrying trailers.
Caravans do not carry passengers. It is absurd for their makers to base the suspension on huge wallowing Chevrolets of the mid-1930s. For optimum road holding, suspension needs to be firmer. This can readily be done and with no risk to any contents.
Caravan and tow vehicle dynamics – fifth wheel caravans 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 caravan 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 caravan 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).
A conventional caravan and tow vehicle are inherently unstable. A sanely designed, laden and driven rig is nevertheless safe as long as the speed is not excessive for that rig.
Depending also on loading, every combination of tow vehicle and caravan has a so-called critical speed. Once above that speed, yawing can irreversibly escalate out of driver control.
That critical speed, and the degree of yaw, is directly associated with the tow vehicle’s mass relative to the caravan’s mass (and particularly mass distribution). It is also associated with caravan 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 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 puts it at risk. Few owners encounter this, so many dismiss its possibility.
A demonstration of the effect of excess rear end mass can be seen at: www.towingstabilitystudies.co.uk/stability-studies-simulator.php
When a caravan yaws, it transfers the yaw force via an overhung hitch to the tow vehicle. The transmitted forces are resisted by the tow vehicle’s weight and the grip of its tyres. Minor caravan braking assists straightening the rig. Heavy caravan braking, however, may overwhelm the caravan’s tyres as they are already stressed by yaw forces.
If the caravan yaws never apply tow vehicle braking. Doing so may trigger that tow vehicle’s already stressed rear tyres into terminal oversteer. It may cause it to spin.
Beware of cruise control
Cruise control detects the minor drop in speed when yawing occurs. It attempts to restore the set speed. Meanwhile, the tow vehicles tyres heat up and slip angles increase. While convenient, it is better not to use cruise control when towing a heavy rig at speed.
A further cause of major caravan instability is wind forces from fast-moving trucks. This is particularly so of those 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 caravan‘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 caravan to swing slightly away from the truck. The overhung hitch causes the front of the caravan 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 the truck and the caravan rig are approaching each other at speed on narrow roads.
Electronic stability systems
Electronic stability systems monitor caravan 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 caravan 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 caravan, the unladen weight of the caravan.
Weight and stability of the tow vehicle.
Those determined by the caravan builder include:
Length of the caravan.
Weight of the caravan.
Distance from caravan tow hitch to axle centre/s.
Distribution of weight along the length of the caravan (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 caravan’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.
Tyres with good sidewall stability (such as light truck tyres).
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 caravan perimeter weight, and particularly rear-end weight, is vital. If feasible house a caravan 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 tyres assists.
The major factor, however, is excess speed.
Caravan and tow vehicle dynamics – 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 100 km/h. This is related to tow ball weight. Furthermore, the lower that weight, the lower the safe speed.
The above is a precis of some of the most relevant parts of RV Books’ Why Caravans Rollover – 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 summarise 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 Research Centre
His work lives on in the 620-page Chassis Design: Principles and Analysis. The book was prepared from Olley’s notes, some 27 years after his death by Milliken and Milliken.