What Is The Double Clutch Shift?
Very little skill is required in changing gear on a modern car fitted with a synchromesh gearbox, for anyone who is capable of depressing a foot pedal and pushing a lever around can successfully change gear. To use a gearbox skilfully is another matter entirely. A gearbox of some sort is necessary on a car because no internal-combustion engine has sufficient power or torque to cope with all the demands of road speed, gradient etc, without having some means of keeping the engine working at its most efficient speed. The driver also requires some means of disconnecting the drive when it is necessary to stop, and some form of gearing is required in order to reverse the car.
The History of Double-Clutching
In the early days of motoring and, indeed, until the 1930s, car gearboxes were generally provided with no means of synchronising the gears as they meshed, so that great skill was needed to select a gear noiselessly, especially when changing down through the gears. In fact, so demanding were some gearboxes that they would refuse to engage a gear unless the gear speeds were perfectly synchronised; the driver would be reminded of his clumsiness by a fearsome grating noise from the gearbox and very often with smarting knuckles as the gear lever whipped out of engagement.
By the 1930s, gearboxes were often fitted with
synchromesh on some of the ratios but first and reverse were seldom supplied with
synchromesh, and indeed it was only in the mid 1960s that the vast majority of cars were fitted with
synchromesh on first gear. Reverse gear was not usually fitted with
synchromesh because it was only engaged when the car was at rest. There are various types of
synchromesh, but they all work on the principle of equalising the speeds of two gears which are required to mesh with each other before allowing the teeth to engage. This method of gearchanging is virtually foolproof except on the few cars which have exceptionally poor gearbox designs, or when the
synchromesh has worn so badly that it is ineffective.
Drivers soon learn how and when to change gear when they are under instruction, and it soon becomes almost second nature to the vast majority of drivers. The sound of the
engine is the most appropriate signal to the driver as, if the
engine reaches the upper end of its revolution range, acceleration begins to tail off and it is time to change into the next higher gear. Likewise, if the
engine begins to labour when climbing a hill, then it is time to change down to prevent the
engine stalling. The learner quickly discovers how to do this, but few bother to progress beyond the stage of depressing the clutch pedal and moving the gear lever in the required direction.
Mastering The Use Of The Clutch
Despite the use of
synchromesh, there is still scope for the good driver to demonstrate his or her skill in the use of the clutch and gearbox. The clutch is quite a delicate mechanism, and a clumsy driver can wear one out in 20,000 kilometres, so a feather-light touch on the clutch pedal will be more than repaid in extra clutch life. The clutch should always be allowed to engage smoothly and progressively so that the car moves away without jerking; this requires the driver to use just the right amount of engine revolutions as the clutch engages - too many and the car will jerk or the clutch will slip; too few and the car may well stall or hop away from rest in an untidy way. A driver who allows the clutch to judder frequently will find that the clutch plates become ridged, causing severe vibrations through the
transmission.
Keen drivers tend to change gear as quickly as possible, depressing the clutch pedal only far enough to free the plates, then whipping the gear lever rapidly from gear to gear. With a good
synchromesh gearbox, this causes no problems, although it will tend to accelerate wear on the
synchromesh cones, but on cars with a poor design of
synchromesh or a worn gearbox, this will cause a good deal of noise and, again, it will accelerate wear. Some older cars have particularly obtrusive
synchromesh which refuses to be 'rushed' - in other words, if the driver attempts to change gear rapidly, the
synchromesh prevents a rapid change, and may even prevent the gear being engaged at all. Sometimes, this tendency disappears after the gearbox has covered a few thousand miles. Other times, it it is inherent in the design - and not a sign to a potential buyer that the
synchromesh is worn.
Using The Engine To Assist Braking
Many drivers use the gearbox as a means of braking the car. Instead of using the brakes to slow from say 70 kph to 50 kph to take a corner, they may change down from top to third gear and allow the
engine compression to slow the car with the throttle pedal released. When the speed is low enough, they then accelerate through the bend. This method of braking is frowned on in official circles, mainly because of the inherent danger of the gear not engaging properly or the driver missing the gear and finding neutral; it may then be too late to apply the
brakes. This problem can be overcome by using the heel-and-toe method of braking and gear changing. This method is used almost all the time by
racing and rally drivers who need to brake at the latest possible time in order to negotiate a corner as rapidly as possible. Obviously, if a driver has to brake to their correct cornering speed and then change gear, they are losing time on the driver who is braking and changing gear at the same time.
Heel-And-Toe Gear Changing
The techniques used in heel-and-toe gear changing vary from driver to driver and from car to car. Some cars have the brake and accelerator pedals well placed for heel-and-toe changes, while others have very badly positioned pedals. Depending on the position of the pedals and the driver's preferences, there are two different methods of heel-and-toe braking and gear changing. One is to brake with the toe part of the foot, angling the heel outwards to work the accelerator pedal, while the other is to brake with the left side of your foot, and use the right side to operate the accelerator. The technique is usually combined with double de-clutching which was an essential before synchromesh became available, but is now rarely used except by racing drivers whose cars are not fitted with
synchromesh gearboxes. However, it can be useful on cars which do not have
synchromesh on first gear as double de-clutching does allow the gear to be engaged while the car is moving and also prevents grating of the gears as they are engaged.
The Art Of Full Double-De-Clutching
The full double-de-clutching routine for a downward change combined with heel-and-toe braking goes as follows: place the toe of the right foot on the brake pedal and begin braking, at the same time depressing the clutch with the left foot and moving the gear lever into neutral. Now release the clutch pedal and flick the accelerator pedal with the heel of the right foot so that engine revs rise briefly, then fall again. Now press the clutch pedal to the floor again and move the gear lever into the appropriate gear, at the same time removing the right toe from the brake and transferring the right foot back to the accelerator pedal. This method sounds rather clumsy and time consuming, but the skilled driver can double declutch, brake and change down in little more than one second.
It is necessary to de-clutch twice and blip the accelerator pedal in order to equalise the speed of the two gears which are to engage. By blipping the throttle, the driver speeds up the gear on the gearbox primary shaft to the same speed as that of the lower gear. Some practice will be required to perfect this technique as only several attempts will show the driver exactly how hard he must blip the accelerator pedal to achieve the correct gear speeds.
It is obviously important not to change down to a lower gear at a speed which is too high for that gear. A tachometer or rev counter is an important aid in preventing this because the driver can see the exact rev drop between each gear on the dial. If, for instance, the driver changes from third gear at his maximum permitted revs of 6000 rpm into top gear and the revs drop to 4000 rpm, the rev drop between third and top is 2000 rpm. The driver then knows that they must not change down into third gear when travelling at more than 4000 rpm in top gear, otherwise the engine will be over-revved when he changes down. This task is assisted on some cars which have gear speeds marked on the speedometer, although these are usually somewhat conservative figures well below the maximum permitted engine revolutions. mt
