The making of a good tow car

Press, 19 October 1978, Page 9

The making of a good tow car

The very first essential for anyone to consider when deciding on a towcar is weight — of both car and caravan.

The car obviously needs enough reserve power to cope with the extra load of the caravan but almost all modern cars can cope with a suitably chosen caravan. Much more important is the weight ratio between car and caravan to ensure stability under all conditions.

Weight Guidelines: There are some general rules of thumb for establishing an equitable towing ratio between car and caravan. If you stick to these you will not go very far wrong. But it is important. to remember that your car and caravan may not perform entirely to the formulas expectations; the car may be happy towing a greater weight or it may not even manage safely the formulated weight. Each rig and each circumstance has to be judged individually.

However, here are the recognised formulae: The fully laden weight of the caravan should not exceed the unladen weight of the car; and for each lOOcu.cm engine capacity , the car should be able to tow 63.5 kg of caravan. (This rule falls down somewhat when big six-cylinder or VB-engined cars are used. Often these cars have neough power to pull any caravan made here). Obviously, the caravan weight, given by the formulae for any particular car must be interpreted with sense. It would be ridiculous to say that a certain car can tow 700 kg but not 712 kg under any circumstances. The difference, after all, represents no more than that 13 litres of water in the water tank.

Kerbside weight: This is the weight of the car, including towing bracket, withou driver or passengers but with a full supply of fuel, oil, and water and with loose tools and equipment with which the vehicle is normally supplied.

Manufacturer’s gross towing limit: This is Ise car manufacturer’s suggested limit for the all-up weight their car should tow. This means the caravan complete with al! food, bedding clothes and equipment. It you exceed this, it is possible you may invalidate the car warranty. It pays to check with the manufacturer).

Manufacturer’s recommended caravan noseweight: Car manuiacturer’s tend to be ultra conservative tn selling tins figure. In many cases, caravanners exceed these limits without undue ill effects.

Rear suspension: The rear springs of the car can greatly eliect the stability of the whole outfit. There is nothing wrong in principle with independent rear suspension systems for towing, but tse tendency is for independent coil systems to be softer than conventional leaf springing. Soft systems should be avoided, as they poroduce a poor towing performance.

A simple test is to put ail your weight on one rear wing of the car and see how much the car sinks down. If a man. of average weight has no effect on the rear of the car, it is likely that the rear suspension is good for towing. A car that sags badly under the weight may need the rear suspension beefing up. If the noseweight of the caravan is less than about 50kg it is unlikely that it will overload the rear springs of the towing car. If it is considerably in excess of this figure check with the service department of the car manufacturer for the maximum ratings for the rear springs.

Tail overhang: The distance from the centre of the rear wheels to the coupling point should be as short as possible. A long tail acts as a lever,

allowing the caravan noseweight to compress the cars springs. A long tail leads to pitching especially if the springs are soft. Engine capacity: Engine size is usually of no great significance, except as an indication of where to start looking.

Maximum BHP at RPM: Maximum brake horse power at stated engine revolutions governs the cars top speed. High bhp is not, by itself, important to the towing man. Of greater interest is' the cars torque characteristics.. Maximum torque at RPM: Acceleration and hill climbing, the muscle of an engine, is governed by torque. A car with plenty of torque at the right engine speeds has plenty of slogging power. Hill starts with a caravan demand lots of torque at the low engine speeds. If maximum torque is produced about 1800 to 2000 rpm. engine characteristics are good for towing. But you must still respect the ac-

tual amount of torque produced, the more the better.

Number of forward gears: Five is better than tour is better than three for manual gearboxes. Overdrove, often available as an extra, is useful especially as some manufacturers provide a lower ratio back axle so that everj' gea’r is lower and better for towing. km/h per 1000 rpm in top gear: The higher the road speed the higher geared is the car. Most family cars run between 24km/h and 32km/h per lOOOrmp. High gears lead to fuel economy and easy engine performance at high road speeds, but the caravanner will find such a car calls for much gear changing when towing. Wagon version sometimes have lower overall gearing. or a lower ratio rear axle.

Automatic transmission: Most automatic boxes use a converter which transmits engine torque to the

rear wheels. Unlike a manual gearbox-clutch setup, the automatic allows the maximum engine torque to be fed to the driving wheels when the vehicle is still stationary. With a normal clutch, only part of the engine’s turning effort actually reaches the rear wheels as the friction surfaces begin to engage. So, with an auto delivering the right sort of effort at the right time, hill restarts become much easier. Most automatics feature three-speed ranges which, because of the flexiblity provided by the torque converter, in practice provide a better coverage of ratios than a four-speed manual.

A note of caution for drivers of automatics is to ensure that the gearbox does not overheat. A safety precaution would be to fit an oil cooler before any extensive towing was undertaken, that is, if the car does not already have one fitted standard.