Four Wheel
Four Wheel
Drive The general layout of a four- wheel drive system is shown here. A representation of how torque is distributed
is also shown.
Torque distribution in a 4WD system
The variation in torque is achieved by differential action. This is examined in some detail later in this programme.
Summary To produce enough torque at the road wheels, a fixed gear reduction is required. This is known as the final drive. It consists of just two gears. On rear wheel drive systems, the gears are bevelled to turn the drive through ninety degrees. On front wheel drive systems, this is not necessary. The drive ratio is similar for front or rear wheel drive cars.
Describe the purpose of final drive gears.
Look back over the previous section and write out a list of the key bullet points here:
DIFFERENTIAL OPERATION
Introduction The differential is a set of gears that divides the torque evenly between the two drive wheels. The differential allows one wheel to The outer rotate faster than the other. As a car goes around wheels travel a
greater
a corner, the outside driven wheel travels further distance than the inside one. The outside wheel must therefore rotate faster than the inside one to cover the greater distance in the same time. Tyre scrub and poor handling would be the result if a fixed axle were used.
Main Components The differential consists of sets of bevel gears, and pinions within a cage, attached to the large final
drive gear. The bevel gears can be described as sun and planet gears. The sun gears provide the drive to the wheels via halfshafts or driveshafts. The planet gears either rotate with the sun gears or rotate around them, depending on whether the car is cornering or not.
Final Drive Gears The small pinion brings the drive from the gearbox to the larger final drive gear. A fixed gear reduction is produced by the crown wheel and pinion. On rear wheel drive cars, bevel gears are used to turn the drive through ninety degrees.
Differential Casing and Bearings The bearings support the differential casing, which is in turn bolted to the final drive gear. The casing transmits the drive from the final drive gear, to the planet gear pinion shaft.
Sun and Planet Pinions The planet gears are pushed round by their shaft. The sun gear pinions, which are splined to the drive shafts, take their drive from the planet gears. The sun gears always rotate at the same speed as the road wheels.
Planet Shaft The planet shaft is secured in the differential casing so that it pushes the planet gears. If the sun gears, which are attached to the road wheels via the driveshafts, are moving at the same speed, the planet gears do not spin on their shaft. However, when the vehicle is cornering, the sun gears need to move at different speeds. In this case, the planet gears spin on the shaft to make up for the different wheel speeds.
Travelling In a Straight Line When the vehicle is travelling in a straight line, the bevel pinions (planet gears) turn with the sun gears, but do not rotate on their shaft. This occurs because the two sun gears attached to the driveshafts are revolving at the same speed.
Cornering When the vehicle is cornering, the bevel pinions (planet gears) roll round the sun gears, and rotate on their shaft. This rotation is what allows the outer wheel to turn faster than the inner.
Torque Equaliser A standard differential can be described as a torque equaliser. This is because the same torque is provided to each wheel, even if they Differential are revolving at different speeds. At greater speeds, more power is applied to the wheel, so the torque remains the same.
Extreme Example One further way to understand the differential action is to consider the extreme situation. This is when the corner is so sharp, the inner wheel does not move at all! Now of
All the drive is transferred to
course this is impossible, but it can be the free wheel simulated by jacking up one wheel of the car. All the drive is transferred to the free wheel. The planets roll around the stationary sun wheel but drive the free wheel because they are rotating on their shaft.
Stuck In The Mud! The example, given on the last screen, highlights the one problem with a differential. If one of the driven wheels is stuck in the mud, all the drive is transferred to that wheel and it normally spins. Of course, in this case, drive to the wheel on the hard ground would be more useful. The solution to this problem is the limited slip differential.
Summary As a car goes around a bend, the outside driven wheel travels further than the inside one. The outside wheel must therefore rotate faster to cover the greater distance in the same time. The differential allows this difference in speed.
Describe what happens to the planet gears when a vehicle is driven with one wheel spinning in mud.
Look back over the previous section and write out a list of the key bullet points here:
TOOLS AND EQUIPMENT
Introduction Components will usually be removed, inspected and repaired or replaced when a defect has been diagnosed.
Other components are replaced, or stripped and cleaned, at scheduled mileage or time intervals. Refer to the Routine Maintenance Section for details on these items.
Recommended Procedures The descriptions provided in this section deal with the components for individual replacement, rather than as a part of other work. Always refer to a workshop manual before starting work. You will also need to look for the recommended procedure, special tools, materials, tightening sequences and torque settings. Some general and specific tools and pieces of equipment are described on the following screens.
General Toolkit General tools and equipment will be required for most tasks. As your career develops you will build a collection of tools and equipment. Look after your tools and they will look after you!
Soft Hammers These tools allow a hard blow without causing damage. They are ideal for working on driveshafts, gearboxes and final drive
Some hammers
components. Some types are made of special hard contain metal