CAN YOU TOW AND DODGE?
What’s to be done if that kudu remains defiant, regardless of how hard you are braking and hooting? Either you are going to hit it or you will have to swerve.
Surely all of us know that the slower you drive, the more control you have over your towing combination... but Francois and Marèt demonstrates empirically with a simple calculation how a slower towing speed can make an observable difference when a driver suddenly has to reduce the speed of his towing combination in order to swerve.
For the purpose of this exercise, let’s assume that you’re driving a double-cab bakkie with a tare weight of 1 900 kg, luggage that weighs 300 kg, two occupants weighing 100 kg each, and a halffilled fuel tank of 40 kg (this comes to a total of 2 340 kg): In the first test, we hitch a trailer without run-in brakes. The trailer has a tare of 360 kg and it’s carrying a load of 390 kg (giving a total of 750 kg).
If this towing combination is driving along a level road at 120 km/h and it has to brake suddenly, it will take 5,3 seconds (including your reaction time) to come to a complete stop over a distance of 101 m. If towed at 90 km/h, it would’ve taken 4,2 seconds to come to a stop over a considerably shorter distance.
Now let’s suppose we hitch a caravan to the same double-cab bakkie with a tare of 1 120 kg and 100 kg of luggage inside (so a total of 1 220 kg). In this case, you do need overrun brakes for the caravan.
Even though this combination is considerably heavier, the overrun brakes should do their job. If towed at 120 km/h, the bakkie and the caravan can be brought to a halt within 4,2 seconds over 83 m. At 90 km/h, the combination can be brought to a halt after 3,3 seconds and 52 m.
A QUICK DUMMY?
You may believe that you’ll be able to brake in an emergency. But what happens if you also have to swerve to avoid hitting something in the road ahead of you? How easy is it for your towing combination to start skidding over the road surface – or even worse – to start toppling over?
“The stability of a vehicle depends on a number of factors. The most important of these is the ratio between the wheel alignment of the towing vehicle and its centre of gravity above the road surface,” says Francois. “The centre of gravity is that point where you can assume all of the weight of the vehicle to be centred. The position of this point of course depends on the design of the vehicle and the way in which you load the luggage into it. For example; the centre of gravity of a car with a roof carrier loaded with heavy luggage moves upwards.
“The Static Stability Factor (SSF) of the vehicle is an indication of its resistance to roll. The static stability is calculated by dividing the width of the wheel alignment by double the height of the centre of gravity. The bigger this value, the more stable the vehicle will be while moving.
“A typical sedan has a SSF of about 1,4, while that of an SUV is about 1,17 and >
that of a bakkie is 1,18,” explains Francois.
“The SSF serves as an indication of the tendency of a vehicle to fall over when involved in an accident. A vehicle with an SSF of 1,03 or less stands a 40% chance of falling over, while the chance of crashing with an SSF with 1,45 or more is only 10%. The SSF of most towing vehicles ranges from 1,13 to 1,24, so they consequently stand a 20-29% chance of rolling over.”
THIS IS HOW THINGS PLAY OUT
Francois made a few more calculations to illustrate how towing speed, in combination with the stability of your towing vehicle and caravan, affect your ability to avoid a collision.
Suppose the headlights of a towing combination with a high average SSF of 1,25 suddenly reveal a stationary truck 45 m ahead in the road. Will there be enough time for the driver towing at 120 km/h to switch lanes successfully (that is, to swerve within 3 m), provided he is able to suddenly turn at a sharp enough angle to allow the towing combination to only just avoid skidding or rolling over? But at 90 km/h, things look better (see illustration).
As you continue to load luggage and other items higher and higher, both in your bakkie and the caravan, the height of the centre of gravity increases and the SSF value decreases. The moment that the SSF value has dropped to below 0,6, it will be impossible to avoid hitting anything that you notice ahead of you in the road – not even at a speed of 90 km/h.
When a towing combination is moving ahead at 120 km/h, the combination will already have moved ahead 26,7 m within the driver’s 0,8 second reaction time. This means you are already 18,3 m from the truck, while you need 25,9 m to swerve out in time. So it takes you 52,6 m in total to react.
Suppose the towing combination is moving ahead at 90 km/h, then it will have moved 20 m in the reaction time of 8 seconds and the driver will be able to perform the maneuvre in 19,3 m – with a leeway of 5,7 m before it will hit the stationary truck.