Choosing a vehicle for towing can be a daunting proposition. Jake Venter explains what to keep in mind when shopping around for something that can tow a heavy caravan.
A heavy and powerful towing vehicle is ideal, but for many buyers this will be too expensive in terms of purchase price and fuel usage.
The choice of vehicle therefore depends to a large extent on how often the towing will be required, and the distances involved. An everyday towing vehicle usually needs to be more towing-friendly than one that will be needed to tow a caravan to the coast once a year.
There is such a wide range of vehicles that choosing the correct one for your motoring needs could be a frustrating and lengthy business.
Any evaluation should start with a close look at the following factors:
The law specifies mass limits for the vehicle, as well as for the wheels and axles, and the car/trailer combination. These should be adhered to for legal, safety and warranty considerations.
Commercial vehicles usually show these details on a plate, but private cars seldom display the complete mass breakdown. The manufacturer or local importer should be able to supply the information.
South African towing regulations are complicated and have sometimes been promulgated without any information being given to motoring associations. The result is that many people are towing illegally.
Power to mass ratio
It is obvious that a high power to mass ratio is desirable, but this will inevitably lead to higher fuel consumption. Divide the engine’s maximum power in kilowatts by the car’s kerb mass in metric tons (1000kg) to calculate the kW/ton ratio.
Torque to mass ratio
Torque is the rotational equivalent of a force and is the correct unit to measure the force developed by a rotating engine. Therefore the torque to mass ratio is more important and is a better guide for towing performance than the power to mass ratio.
In addition, maximum torque is not as important as the way the torque delivery is spread over the range of engine speeds. An engine that develops more than 90% of its maximum torque from 2000 to 4000 r/min will be happier while towing than an engine that develops the same percentage of maximum only between 2700 and 3000 r/min.
This information can be obtained from the manufacturer or from magazines that publish power and torque curves with every road test.
Engine torque can be expressed as a force acting between the tyres and the road by taking the overall gearing and the wheel radius into account. This means that the overall gearing, as well as the steps between the gears, are important. The overall effect can be calculated but an experienced driver can get a good idea of how the numbers work together to create a good or indifferent towing vehicle.
Choosing a suitable engine and transmission
Petrol or diesel? Petrol engined vehicles are less expensive to buy and maintain, and they cope better with infrequent servicing, poor quality fuel and even neglect, but tend to use about 20% more fuel than diesel engines.
Old-fashioned diesel engines without turbo-charging are even more robust than petrol engines, but the modern turbo-charged diesel engine cannot tolerate any dirt in the oil, air and fuel. If they are not serviced promptly and correctly, the consequences are often dire.
Many turbos rotate at more than 200 000 r/min and the impeller inside the exhaust manifold runs red-hot.
On the plus side is the wonderful feeling of driving a vehicle that develops a high level of torque over a range of engine speeds from as low as 1200 r/min to 2500 r/min.
Front, rear or all-wheel drive?
Front-wheel drive normally gives a vehicle more interior space because the engine and gearbox can be mounted sideways and there is no driveshaft tunnel to encroach on the rear passenger footwells. It does, however, increase the steering effort so that power steering becomes mandatory.
Many front-wheel driven cars are fitted with electronic wheel-spin controls because they tend to lose traction and spin their wheels at the slightest provocation.
Most light commercial vehicles are rear-wheel driven. This is normally a better option for carrying a heavy load, or towing, especially when going uphill. It also tends to give a sporty car better handling.
Four-wheel drive is useful when going off road or on a slippery tar road, but has no advantage when towing on a good surface. Such a layout also increases fuel consumption and maintenance costs. Part-time four-wheel drive that either engages automatically or manually is a better alternative.
Many people prefer a manual gearbox, and there is a certain pleasure in changing a gear when you feel like it. This normally ensures the best fuel consumption under most conditions, and may even be the preferred choice for petrol engine vehicles.
However, it is better to select an automatic transmission for towing with a turbodiesel engine because it will ensure that the vehicle is always in the correct gear. This is an engine-saver because diesels do not take kindly to “lugging” – employing a large throttle opening at low speed in a high gear. Such treatment tends to set up torsional vibrations in the crankshaft that may, at the very least, cause transmission damage.