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Changing gears – Gearboxes explained and how it actually works

2017-09-27 06:40

Image: Supplied by AutoTrader SA

Partner Content - AutoTrader SA


Johannesburg - Aside from parallel parking, the gearbox is probably the most feared aspect of learning to drive.

What gear to use when, how to balance the clutch and how to prevent the car from stalling are all challenges that we face when learning how to drive.

In the simplest terms, a gearbox (or a transmission if you’re American) is a system of toothed gears that either reduce or increase the speed of rotation.

A 2:1 (pronounced as “two to one”) ratio will see the output shaft make two revolutions for every one revolution of the input shaft.

This allows the wheels to turn slower, or faster than the engine speed as needed.

Pull away from the lights and while your engine is doing 2000rpm, your wheels will only be doing a few hundred RPM.

All images: Supplied by AutoTrader SA

Not all gearboxes are created equal though, so let’s look at the most common types available so that you can decide which one is best for you.


Manual

This is the oldest and simplest gearbox that is still in use today. Outside of America and Australia, this was the most common gearbox for many decades. With the manual gearbox, the driver is responsible for changing the gears through a selecting lever, most commonly mounted alongside them or in older examples, on the steering column or dashboard. 

In order for the driver to be able to change the gears, the engine has to be disconnected from the gearbox. The driver does this by disengaging the clutch by pressing the clutch pedal. The required gear is selected with the clutch pedal depressed and when the selection is completed, the clutch pedal is released and the clutch reengages. 

Manual gearboxes are the simplest and most cost effective to produce and as such are regarded as being the most durable, requiring the least maintenance. 


While requiring more effort and adding additional input from the driver, the manual gearbox is still regarded as the purist approach to motoring.

For the longest time, high-performance and racecars were all fitted with manual gearboxes.

Advances in technology have done away with this norm, but enthusiasts will still gravitate towards the manual gearbox for ultimate driver engagement.


Automatic

The automatic gearbox is, as the name suggests, a gearbox that changes gears automatically. The driver selects the desired range of gears (most commonly indicated by a D for Drive) and allows the car to do the rest.

At set RPM intervals, the gearbox will select the following gear to best correspond with the throttle input, engine and road speed.

When slowing down, the gearbox will change to a lower gear until the vehicle comes to a stop, where it disengages automatically to prevent the engine from stalling.

A torque converter is employed to transmit the rotational forces from the engine to the gearbox and still allow for disengagement when bringing the vehicle to a standstill.

Torque converters are known to be parasitic in that they are prone to rob the engine of power, only transmitting a percentage of the engine power to the gearbox and thus the wheels.

The elimination of the clutch system means that there is no need for a clutch pedal and the driver only has to contend with a throttle / accelerator and a brake pedal. As such, the automatic is regarded as the easiest to drive and the most relaxing or comfortable.

While the automatic has its advantages in high volume traffic scenarios, many enthusiasts will still scoff the idea, despite the modern advances in power handling capabilities and shift times.



Continuously Variable Transmission

The Continuously Variable Transmission (CVT) is very similar in operation to the automatic in that it offers an automated driving experience. Its principle is significantly different though. Where an automatic gearbox has a system of planetary gears, the CVT uses a system of conical cylinders and belts so that a near-infinite range of ratios is available. This creates a smooth drive with seamless “changes”.

The easy of use and linear gearing means that a CVT system should, in theory, be the most fuel efficient of all the gearboxes as it allows the engine to remain at a single, effective and efficient RPM while altering the road speed.

This fluidity unfortunately dulls down the driving experience and steals some of the character that driving enthusiasts long for when driving. As a result, manufacturers may program the gearbox to simulate a small shudder to replicate the sensation of changing gears.



Semi-Automatic

Semi-Automatic gearboxes include a range of shifting and selecting options but we’ll concentrate on the two most common examples found on our roads today, traditional torque converter and dual-clutch systems.

Both systems will function like a full automatic where the driver selects Drive and no clutch operation is needed, even when coming to a stop. Many semi-automatic offerings, regardless of type, will offer steering wheel or column mounted shift paddles to allows the driver to select the desired gear, either changing up or down as the need arises.

Traditional torque converter-style

These gearboxes function in the same manner as a regular automatic with the additional option of driver input without needing a clutch. These still use a torque converter to transmit the power and as such, the shifts between gears, even when reacting on driver input, are often slower. Sometimes also referred to as Tiptronic / Steptronic / SelectShift / S-Tronic.

                                                              All images: Supplied by AutoTrader SA

Dual-Clutch

Dual-Clutch gearboxes are based on the manual gearbox system with two computer-controlled clutches; one for the odd gears and one for the even gears. This allows for a near instantaneous switch between gears with many manufacturers claiming shifts faster than a manual gearbox.

The most common Dual-Clutch Transmission (DCT) is the Direct-Shift Gearbox (DSG) as found in VW offerings with a similar system found in Audi models. Drivers can select between full automatic mode, which leaves the changes in the computers hands or opt to change gears themselves by either toggling the shift paddles behind the steering wheel or tapping the gear lever to either +/-.

DCT gearboxes have progressed extensively over the past few years and despite their cost to produce, are being found in smaller vehicles with many high-performance cars opting to fit DCT gearboxes as standard over manuals.

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