Towing with an Automatic Transmission
One of our members recently looked at my towcar and, noticing it had an automatic gearbox, said “Are automatics any good for towing? I thought they sapped power, drank fuel and over-heated. Oh, and you’ve no engine braking when going downhill!”
Whilst some older automatic gearboxes may indeed have exhibited one or more of those characteristics modern ones are very good indeed. In this article we’ll have a look at the differences between automatic and manual transmissions and the potential benefits and drawbacks of each for towing a caravan.
Why do cars need gearboxes anyway?
The reason cars have gearboxes is to match the torque (turning force) and speed of the engine to that required by the driving wheels. A gearbox allows the relatively narrow power band of the engine to be matched to the wide speed and load variations of the car. When moving off from rest you need lots of torque but when cruising on the motorway much less is required. Torque and speed are inter-related. If you want more of one it is at the expense of the other. A gearbox performs this balancing act to enable a car to get the most out of its engine.
Manual Transmission
A manual gearbox consists of discrete sets of cogs of different sizes. These are engaged and disengaged as each gear is selected. If the output shaft of the gearbox turns once for each two revolutions of the engine then the effective torque of the engine is doubled, and so on. In a manual gearbox the drive has to be interrupted in order to change gear. This interruption is provided via the clutch which allows the gears to be changed without crunching. One potential problem with manual gearboxes is that the gear ratios chosen by the car manufacturer for solo driving may not be so good when towing a heavy caravan.
Conventional Automatic Transmission
With automatic transmission the cogs are arranged sets that look a bit like planets going round a sun. For the technically minded this type of gearing is known as epicyclic gearing and is similar to that used in the old Sturmey Archer hub gears found on bicycles many years ago. Unlike a manual gearbox the cogs are in constant engagement with each other and the gear ratio is changed by stopping individual sets from rotating using special brake bands (sometimes known as clutches). As one set is stopped from rotating others will spin faster or slower depending on the gear selected. The advantage of this arrangement is that power can continue to be transmitted during gear changes
To enable drive to be engaged when the vehicle is stationary and to cushion the effect of changing gear under power a device known as a torque converter is used. This effectively replaces the conventional dry plate clutch used with manual gearboxes.
A torque converter is basically a turbine comprising consists of two sets of curved vanes. These face each other very closely and sit in oil. One set is connected to the engine, the other to the gearbox. With the engine just ticking over very little torque is transmitted to the gearbox. When drive is engaged the car will creep forward, but the force is small and easily countered by applying the hand or foot brakes.
As the engine speed increases, centrifugal force causes the oil to be forced under pressure into the outer parts of the vanes thereby dramatically increasing the torque transmitted. This enables the car to surge forward, even with a caravan on the back! Modern torque converters can actually increase the torque of an engine by up to 3 times (albeit with a corresponding reduction in rotation speed).
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Here is a conventional automatic gearbox. The torque converter vanes can clearly be seen towards the left of the picture. To the right are the epicyclic gears and underneath them are the clutch operating mechanisms that determine which sets of cogs can rotate and which sets are locked . |
The ability of a torque converter to multiply torque on a continuously variable basis means that automatic gearboxes do not need as many gears as manual ones. Indeed for many years most automatic gearboxes had just three forward gears. In recent years however the strive for performance and economy has seen this increase to four or even five. Of course during the same period manual gearboxes have gone from having four forward gears as standard, to five or even six.
The downside of a torque converter is that because it does not provide a direct connection between the engine and gearbox some power is lost in transmitting drive through the oil from one set of vanes to the to the other. This loss of power causes increased fuel consumption and results in the transmission oil getting hot as the vanes churn through the oil. To counter this effect, most modern torque converters have a lockup clutch. When the two halves of the torque converter get up to speed, the clutch locks them together, eliminating slippage and improving efficiency. The clutch usually operates just on the higher gears at speeds above 30-35 mph, and is normally undetectable by the driver. It means that transmission losses can be greatly reduced, especially in motorway or open road driving, where average speeds are relatively high. With this arrangement the fuel consumption of a modern 4 or 5 speed automatic gearbox can be comparable to that of a manual gearbox.
Some automatic gearboxes have ‘sport’ and ‘winter’ modes. These basically determine when the gearchanges occur. In ‘sport’ mode changes will be delayed until the revs of the engine are higher. This gives better acceleration and can be useful when towing, especially in hilly terrain. The downside is an increase in fuel consumption. Selecting ‘winter’ normally results in first gear being locked out thereby giving much gentler acceleration from rest to reduce the chance of losing traction on snow or ice. Additionally some automatic gearboxes come with adaptive programming, meaning that over time, the shifting pattern adjusts to suit the driver.
CVT Automatic Transmissions
Continuously variable transmission (CVT) gearboxes dispense with traditional cogs in favour of a special belt running between two sets of interlocking cones. The gear ratio is varied by adjusting the degree of interlock of the cones and hence the effective diameter as seen by the belt. This gives an infinite variability between the highest and lowest gears with no discrete steps or shifts. Some CVT gearboxes are connected to the engine via a torque converter as with a conventional automatic, others (e.g. Honda’s Multimatic system) use a special multi-layered clutch.
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| CVT Theory | CVT Practice |
CVT gearboxes have been around for some time - remember the little DAF 66 Variomatic of the 1970’s? The problem with these cars was the belt itself, which was essentially made of rubber. This made frequent changes necessary and if you tried to use the system on a car with any significant power the belts quickly failed.
In recent years design improvements and the use of steel belts has greatly improved both the reliability and the power handling capabilities of CVT gearboxes. As a result they can now be found in more powerful cars such as the Audi A6 (Multitronic), Land Rover Freelander, MG Rover 75 (Steptronic), and the superb Lexus RX 400H – a petrol electric hybrid car with an amazing driving experience.
So how do CVT automatic compare to conventional ones? Well, in theory at least, they are more efficient, especially those that dispense with the torque converter. However from a driving point of view they can seem little strange as there is often little correlation between the sound of the engine and the speed of the car. At times it can seem a bit like driving a car with a slipping clutch. To counter this some manufacturers provide a manual mode allowing the driver to select preset ratios and thereby producing a more conventional driving experience.
Transmission oil cooling
Even where torque converters are fitted with a lock up clutch, the transmission oil can still get quite hot. To prevent overheating most modern cars now come with an oil cooler as standard. The cooler is often situated in the bottom of the radiator where it can take advantage of the relatively cool water to be found there. This means that it is often unnecessary to have an (additional) oil cooler fitted before towing with an automatic - but do check first!
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Above are a couple of |
Here is an oil cooler incorporated |
The problem is that the hotter oil gets the thinner it becomes and the less effective it is as a lubricant. In extreme cases mechanical damage can occur. See what the manufacturer recommends with regard to towing and do make sure not to exceed the towing limit or train weight (i.e. car & caravan) rating of the vehicle. Beware that the towing limit for a car equipped with an automatic gearbox may be less than for the same car equipped with a manual gearbox.
Taking to the road
For most journeys in an automatic it is sufficient simply to put the gearbox into drive and let it make all the decisions on when to change gear. These will depend on the load on the engine, the speed of the vehicle and the position of the throttle pedal. For overtaking, flooring the throttle normally produces a downchange (or even two) resulting in good acceleration. If a Sport mode is available this can be quite useful when towing by making the outfit more responsive to the throttle pedal.
Of course gears can be selected manually and so lower gears can be engaged to give engine braking when negotiating long descents. Selecting ‘3’ for example will prevent the gearbox from going to a higher gear, but will not prevent it changing down should conditions require it. Some gearboxes (e.g. Volvo Geartronic, Audi Tiptronic) improve on this by allowing the driver to do fast manual shifting for twisty mountain roads and similar situations when the demands on the gearbox are constantly changing.
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This is Audi’s Tiptronic gearbox. In addition to the normal P(ark), R(everse), D(rive), 4,3 & 2 selections the gear lever can me moved over to the gate on the right where gears can be changed quickly up and down by flicking the lever between the ‘+’ and ‘-‘ positions |
Some times, on hills, it is not even necessary to apply the handbrake as the natural tendency for the car to creep forward is countered by the pull of the slope. To eliminate the danger of rolling back on steeper slopes most modern automatics are fitted with a device to prevent it.
One of the best features of towing with an automatic is the ability to move off quickly and smoothly from rest. This is particularly noticeable on hills or at roundabouts where it can be difficult to nip into any short breaks in the traffic flow.
Another is the elimination of the manual clutch. Who hasn’t suffered from an aching left leg whilst trying to keep in the optimum gear in slow moving traffic? In these situations it’s so easy to let the clutch in too fast thereby imposing shock loads on the engine, transmission, towbar and caravan. An automatic gearbox eliminates this.
Conclusions
The significant benefits of towing with an automatic cannot be ignored. If you don’t already have one, and can live with the (normally) higher purchase cost and slight increase in fuel consumption, then do consider buying an automatic when you next change the car. But do choose wisely and make sure the vehicle is well up to the job of towing your caravan.
Advantages and Disadvantages
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Advantages |
Disadvantages |
Manual |
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Conventional Automatic |
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CVT Automatic |
All the above for conventional automatics plus:
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