The Clutch and Gears.
Torque is the power in an engine and horsepower is the speed in the engine. The gears are the means of changing torque into horsepower or power into speed. For any given engine speed it is your choice of gear that determines the speed and power of your vehicle.
The clutch is the means of connecting the gears to the engine and allowing the turning of the engine to turn the wheels of your vehicle. Torque is the twisting power or ability of an engine to turn the vehicle wheels. The bigger the engine the greater its ability to turn the wheels.
The engine at low speed (tick over) does not generate enough power (torque) to move the vehicle from stationary. In order to do this it needs gears. Once the vehicle is moving you do not need so much power but you do need more speed.
Imagine you are pushing a vehicle from a stationary position. At first you need a lot of power but once it starts moving you then need speed.
The Clutch.
The clutch serves a number of uses:
- Starting and stopping the car
- Low speed control when manoeuvring
- The selection of neutral while temporarily stationary allowing you to be stationary with the engine running
- The selection of a gear when the vehicle is moving
- The selection of reverse gear
Use of the Clutch.
When using the clutch while the car is moving, it should be fully depressed and then brought up in a controlled manner. The clutch should be balanced with the accelerator for a smooth change.
When moving from stationary and for low speed manoeuvring you will need to find the bite. This is the point when the engine is starting to connect with the gears and trying to turn the wheels of your vehicle. This is known as slipping the clutch and it is where the clutch plates are starting to touch.
If the hand brake was not on the vehicle would start to move. As the clutch is brought through to full contact and transmission of the engine power, if the handbrake was still on the engine would stall. Either the engine turns and the wheels move or the wheels stay stopped and the engine joins them.
Finding the Bite.
There are 3 ways of knowing you have found the bite.
By the Sound of the Engine:
- As the clutch is raised by your foot the clutch plates in the engine start to touch.
- At this point the engine note will change as it comes under load.
By Observation:
- With the hand brake on as you raise the clutch the vehicle tries to move.
- It will be prevented from doing so by the hand brake which has secured the rear wheels.
- In first the front will rise up.
- In reverse it will dip down as the vehicle pivots around the fixed rear axle.
By Feel:
- By raising the clutch to the bite you will feel the vehicle move as it tries to pivot around the rear axle.
- There will be a vibration in the car
Controlling the Clutch:
- If your heel is kept on the floor and you find the bite by pivoting with your ankle.
- Let your heel slide back on the floor if necessary.
- This will result in a lot more control and less fatigue than would get with your foot in the air.
- If you have your foot in the air, you are supporting the weight of your leg.
- But you also have to overcome the pressure of the clutch pedal.
With your foot on the floor, once you have the bite and the vehicle is trying to move. Release the handbrake and let the spring on the clutch pedal push your foot away. Please note you must coordinate with the accelerator and make the correct observations. Wearing the right footwear will help.
The Gears.
Gears give mechanical advantage. Low gears are most powerful and the high gears are the fast ones. As you move up through the gears you trade power for speed.
A low gear (1st) might have a ratio of 4:1. This is 4 turns of the engine for 1 turn of the wheels. This applies a lot of power to the vehicle wheels and is used for moving the vehicle from a stand still.
A high (4th) gear may have a ratio of 1:1. This is 1 turn of the engine for 1 turn of the wheels. This gives a lot of speed.
The intermediate gears (2nd & 3rd) and ratios are known as the working gears. The lower gears being more powerful are used for moving from a stand still and moving up hill where more power is required.
Lower gear may also be used to slow a vehicle down. This is a form of braking called engine compression. This would be used going down hill or on a slippery surface where braking the wheels may cause skidding.
The higher gears will be used for situations where more speed is required such as the open road. Because higher gear means less torque (power) to the drive wheels they should be used on slippery surfaces.
Overdrive known as 5th Gear may have a ratio of 1:0.8 which means that the vehicle wheels are turning faster than the engine. This is used for fuel economy.
Use of Gears.
The correct gear is the one that gives the best response from the controls. Every time you change gear because you disconnect the engine from the driving wheels. You are in more control if your engine is connected to the driving wheels.
When you are disconnected there is no engine compression if you are braking. And there is no response from the gas pedal if you are accelerating.
You can stop in any gear you like. You should always be in gear when moving apart from when changing gear.
Block Changing.
Being in gear, means being in more control of your vehicle. So for this reason you can block change gears where appropriate.
The money change is 4 down to 2 so called because it can be expensive if you have not dropped enough speed. When block changing up be careful not to spend too long in the lower gear as this puts a strain on the engine. Also excessive power to the driving wheels can cause wheel spin.
When changing down, losing speed by braking is better than using engine compression. This is because it is better to match your gear to a speed than your speed to a gear. Also the following traffic gets the benefit of your brake lights.
Brake to slow and gears to go.
Coasting.
Moving your vehicle when in neutral (out of gear) is called coasting. This could be dangerous for the following reasons.
- You will have no response from the accelerator if you have to accelerate out of danger
- This might occur if you looked in your mirrors and were about to be hit from behind
- When braking there would be no engine compression to help you slow
- By disconnecting the power from the driving wheels you may also affect the handling of your vehicle
Engine Compression.
This occurs when you are in gear with the clutch up and foot off the accelerator. You may also be braking as well.
Some of the energy of your vehicle that would have been forward motion is now being used to turn the engine over. You have also reduced the flow of fuel to your engine. By dropping down a gear, speed is now being turned into power.
Slowing down using engine compression is not recommended for normal driving as there is no signal given by the brake light. Engine compression can be a useful supplement to your braking on slippery road surfaces.
Remember to touch the brake pedal to signal with the brake light if using engine compression only.
Automatics.
Automatics are considered to use more fuel than vehicles with a manual gearbox. The reason for this is that automatics use a fluid flywheel. This is less efficient at converting the power of the engine to the turning of the driving wheels.
A manual gearbox has a direct mechanical connection to the driving wheels which is more efficient. Because a manual has a direct connection between the engine and the wheels you can either push or tow start a manual vehicle.
The advantages of an automatic are ease of use particularly in heavily congested areas. This ease of use lends them to use by drivers with physical disabilities.
In normal driving only the right foot is to be used. Drivers who are used to manuals will find it takes a while to become used to using only the right foot. It is important to use just the right foot as when you are accelerating you are not braking and vice versa.
Automatic Gears.
These will vary by make and model. In addition vehicle manufactures sometimes allow the selection of different modes of gear. For example you may have a sports or an economy mode.
Most gears selectors will have the following:
- P- Park
- R- Reverse
- N- Neutral
- D- Drive
- 1- First gear
- 2- Second gear
Park will lock up the transmission so should only be used when stationary.
Reverse and neutral are self explanatory.
When in Drive the speed and load on the engine are sensed by the transmission. This then enables the correct gear to be selected by your vehicle. The numbered gear selections allow the vehicle to be kept in lower gear. This is useful for maneuvering and going down hill.
Kick down.
This provides extra acceleration when needed. Most commonly used for acceleration to overtake. It does this by dropping down a gear within the automatic transmission. By releasing the accelerator and returning to normal pressure the transmission will select the appropriate gear.
Creep.
When you are in Drive but foot off the accelerator your vehicle will move forward. The higher the tick-over of your engine is set the greater the tendency to creep forward. A manual vehicle in 1st gear with the clutch up and foot off the accelerator will also move forward.
When stationary use the hand brake as appropriate. Do not rely on creep to hold you on a slope as the engine could cut out.
Manoeuvring.
In a manual vehicle low speed control is achieved by clutch control.
With an automatic this ability to use the clutch is absent. When manoeuvring an automatic at low speed your vehicle speed will be determined by a number of factors:
- Your engines tick-over speed which gives rise to creep.
- The amount of acceleration offered via the foot pedal.
- The rise and fall of the camber and gradient.
- The gear you have selected; use the first gear facility if available.
When manoeuvring at low speed use both feet. The right foot for the accelerator and the left foot for the brake. This 2 footed method should increase your control and help prevent any power surges.
Types of Automatic.
In general there are 3 types of Automatic:
- Full Auto where all gear changes are facilitated by the vehicle.
- Semi-auto where a paddle change on the wheel or steering column will allow a change up or down.
- And lastly what is sometimes known as Constant Variable Transmission (CVT) which is a cone with a belt that moves up and down it.
Consult the manufacturer’s handbook as these systems are different from manual boxes and are different from each other.
Syllabus For Learning To Drive – Car Controls & Instruments
© Liam Greaney
