5. GEARING
Gearing will also affect towing performance. All likely towcars now have at least
five gears in manual gearboxes and probably at least four, if not five or more, in
automatics. More gears give improved ability to match the engine to the road speed
in an optimum way, giving better fuel economy, better emissions performance and
quieter high speed cruising.
A partial guide to a car’s towability is the road speed at which maximum torque is
produced for every 1000 rpm in top gear. If the figure quoted is 21 mph (in top gear)
and maximum torque is quoted at 3000 rpm, then multiply 3 (thousand) by 21 = 63
mph. This suggests that at about the maximum legal towing speed on a UK
motorway, such a car would be ‘pulling’ at its strongest - this should mean it will be
able to maintain a steady speed in spite of moderate hills, for instance. Too high a
figure for this calculation and the car will ‘run out of breath’ very easily in top year,
forcing you to change down at the slightest up slope. It is also said that a vehicle
showing good midrange acceleration times (normally 50-70 mph) should be a good
towcar. Modern cars will regularly achieve times of under 7s for this measure, but
those which do so in 4th or higher gears are likely to be best for towing – watch out
for figures which only show good performance when in 3rd, since this could be an
indication of a ‘peaky’, inflexible engine.
6. AUTOMATICS
One way to bypass problems created by less than ideal torque is to choose an
automatic gearbox rather than a manual. An automatic is ideally suited for caravan
towing, and with its torque converter allows crawling pace where a manual's clutch
would slip and wear. Also when starting off from rest the action of the torque
converter gives a torque magnification factor approaching 2:1, ie when the converter
output is at rest or turning slowly the torque is maximum and the output torque
exceeds the input torque - ideal when towing.
Other benefits are less wear and tear on the transmission, easy re-starts on hills and
reversing to hitch up (not having to cope with three pedals and only two feet!), and
perfectly adequate gear control with the selector. As the mood takes you, the gearbox
will do all the work if left in Drive, or you take control of changing up and down.
With all these advantages there must also be disadvantages! One penalty with older
three speed automatics, but less for those with four-speeds or more was higher fuel
consumption. Another was slight loss of performance over the manual equivalent and
increased heat production. Later generations of five, six, seven and soon eight(!)
speed automatics have virtually eliminated any significant fuel consumption penalty
in most cases and generally deliver performance effectively equal to their manual
counterparts. As there are occasional exceptions to this, it is still worth checking the
manufacturer’s fuel consumption data, though. Occasionally, the manufacturer’s
towing limit is lower with automatic transmissions – be sure to check (see information
leaflet ‘Automatic Transmission for Towing’). Over 20% of Club members choose
an automatic.
One annoyance when choosing a new model is that diesel automatics remain
comparatively rare. This seems strange, since the ability of a diesel to perform well at
low revs should make it ideally suited to use with an auto ‘box. The combination of a
(20 Seiten)
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