Leading accident investigation expert, Mark George, explains why towing a caravan is much safer if the driver stays below the ‘system critical speed’
Every car-trailer combination has what is referred to as a system critical speed, which has a close relationship with system damping ratios for both yaw and roll.
The system critical speed is rig specific and relates to the speed at which the yaw damping ratio becomes zero, thereby defining the boundary of the system’s dynamic stability of a particular vehicle-caravan combination.
It is influenced by many factors, including car and caravan design, car/caravan mass ratio, tyre construction, tyre pressures, suspension properties, hitch set-up and load distribution, but it is important to know that it is speed specific.
Some researchers have reported system critical speeds in the order of 113-115km/h for common car-caravan rigs. Others have found system critical speeds in the order of 150km/h for a correctly loaded conventional trailer, but reducing to 105km/h when the trailer was loaded incorrectly.
For practical purposes, three important factors that significantly influence yaw damping ratios are suspension, cornering stiffness of the tyres, and load distribution … all of which are the responsibility of the driver to maintain.
If you happen to be travelling at or above the system critical speed when caravan sway occurs (for whatever reason), you will quickly lose control, and electronic stability control (if you have it fitted) may not be enough to regain control.
The scary part is, because it is such a complex issue, you will never know what your rig’s system critical speed is; it will never be the same for your rig for any trip, it may reduce during your trip due to load shift, fuel consumption, slow puncture, other mechanical defect, and/or transient road/ weather conditions.
From a physics aspect, if you are travelling below the system critical speed when caravan sway occurs, your vehicle and caravan will exhibit positive oscillatory response, that is, the oscillations will slowly decrease with time.
However, if you are travelling at or above the system critical speed, the oscillations will increase exponentially leading to control loss. The key takeaway is that one must take robust precautions in relation to making sure your suspension is adequate and serviceable for towing, you have appropriate tyres for towing, your tyres are correctly inflated for the applicable loads, your load is distributed correctly, and your travel speed is safe.
Rigs with a GCM of over 4.5 tonnes are limited to a maximum legal speed of 100km/h. But is this a safe speed for your loaded car-caravan combination? My advice (and it’s a legal requirement) is to strictly follow the Manufacturer’s User Manual for rig set-up, load limits/ distribution and maximum travel speed for your particular tow vehicle and caravan.
If your rig is prepared and Every road has its challenges loaded strictly in accordance with manufacturers’ specifications, your maximum safe speed will be below the system’s critical speed, and will likely range between 80-100 km/h.
Some manufacturers (e.g. Ford) may stipulate variable maximum speeds for the trailer mass you are towing. Whatever maximum speed your manufacturer stipulates, I recommend further erring on the side of caution and discount another 10% for good measure. Do not use cruise control whilst towing as it is likely to accentuate a sway event if one occurs.
* Mark George is the Director and Principal Investigator of Accident Investigation Services Pty Ltd. He has been involved in accident investigation for 43 years, and is a current ACTAR Accredited Traffic Accident Reconstructionist