Why are there 4 wheels on a car?

Q: Why do cars have four wheels instead of three?

Because cars are generally rectangular in shape, its crucial that its weight is equally distributed at its four nodes (corners). Plus, since cars operate on axles that support the mass, each axle would get two tires at the ends. Thats why more axles are added to longer cars.

Q: Why dont cars use 3 wheels?

A three-wheeled car doesnt do anything a four-wheeled car cant do, and it comes at a cost of grip and stability. More tires equal more grip, almost always. And despite their advantages, three-wheelers have many of the same drawbacks of motorcycles and cars. That puts them somewhere in the middle, a novelty.

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What company manufactured the first Jeep for U.S. military use?

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The Jeep got its name from a cartoon character named _____________.

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In a four-wheel drive vehicle, how many wheels receive power from the engine?

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If all four of the car's wheels can be driven and if the differentials can be locked, the car can keep going even if only one of its wheels is

gripping the surface. This feature is of obvious value to cross-country vehicles, but it also means that a four-wheel-drive road car will be able to keep going on icy or muddy roads.

Torque splits

Although all the wheels have to revolve at the same speed, the amount of torque —turning force reaching each end of the car can be varied. Ford's Sierra 4x4, which was based on a rear-wheel-drive car, has a torque split of 27:63 (27 per cent reaches the front wheels, 63 the rear) to preserve its handling balance. The Audi Quattro, which was originally front-wheel drive and is nose-heavy, has a torque split of 50:50, all four wheels being equally powered.

Permanent 4WD

Some road cars are basically two-wheel drive with a four-wheel-drive option which you engage when the road surface demands it. But others are permanent four-wheel drive.

With permanent four-wheel drive, power is fed through the gearbox to a centre differential, usually mounted just behind the gearbox itself. This differential divides the power between the front and rear

wheels in such a way that it allows the front and rear wheels to rotate at slightly different speeds when necessary, for instance when the car is being manoeuvred at low speeds.

If this didn't happen the slight difference in rotational speed between the front and rear wheels would impose severe strains on the transmission - a condition known as wind-up. The wind-up would only be released if a tyre lost its grip.

From the centre differential, the power goes to the front and rear wheels via a prop shaft and drive shafts - the exact layout depends on whether the car was originally front- or rear-wheel drive.

Two additional differentials are required to make four-wheel drive work properly - one between the front wheels and one between the rear ones. As on an ordinary twowheel-drive car, these differentials allow the wheels on one side to turn

faster than the others, as happens when the car turns a corner.

4WD on the road

Four-wheel drive for road use came into prominence in 1966, when Jensen launched their FF model, which was based on the Interceptor bodyshell.

The FF (standing for Ferguson Formula after the company who designed and developed the system) had permanent four-wheel drive and anti-lock brakes, but the idea did not catch on.

The effectiveness of four-wheel drive for road cars was proved by the Audi Quattro, which was subsequently developed into a successful rally car. Since then, four-wheel drive has appeared as standard equipment or as an optional extra on cars from Ford, Alfa Romeo, Toyota and Fiat, among others.

Ford four-wheel drive

To add drive to the front wheels of the rear-wheel-drive Sierra and Granada Scorpio, power is taken from the side of the gearbox by a wide transfer chain after running through an epicyclic centre differential and viscous coupling. A short prop shaft runs forward to the front differential, which is mounted on the right-hand side of the sump. The only way to get power to the left-hand front wheel is to run a drive shaft through the sump itself inside a sealed tube.

Epicyclic differential

The epicyclic differential splits the torque unequally between the front and rear axles - tow thirds to the rear and one third to the front - yet it still drives each axle at the same speed.To add drive to the front wheels of the rear-wheel-drive Sierra and Granada Scorpio, power is taken from the side of the gearbox by a wide transfer chain after running through an epicyclic centre differential and viscous coupling . A short prop shaft runs forward to the front differential, which is mounted on the right-hand side of the sump . The only way to get power to the left-hand front wheel is to run a drive shaft through the sump itself inside a sealed tube.

Diff locks

With permanent four-wheel-drive systems, there needs to be some means of locking the differentials, especially the centre one, or all drive could be lost if one wheel met a patch of ice.

Some systems have a self-energizing locking differential - the Ford Sierra 4x4 has a centre differential which incorporates a viscous fluid. When the difference in speed between the front and rear wheels increases, the fluid resists this tendency and locks up the differential so that power is supplied to both the front and rear wheels.

Audi's forthcoming quattro models will use a Torsen central differential. This device (Torsen is short for torque sensing) uses worm gears to limit the amount by which one of the car's axles can over-run the other.

Most other permanent fourwheel-drive vehicles, including the Range Rover, have a manual differential lock. When the driver engages this, power is directed to both the front and rear wheels, whether or not any wheel is spinning.

Some four-wheel-drive cars have a rear differential lock as well. Engaging both the central lock and the rear lock forces both rear wheels to turn even if one front wheel is slipping.

Part-time 4WD

Some four-wheel-drive cars have 'occasional' four-wheel drive for off-road use and for coping with ice, snow and other slippery surfaces.

These cars have no centre differential, just a transfer box or dog clutch to transmit drive to the other pair of wheels.

If the transfer box were engaged so that four-wheel drive were used for ordinary driving, the absence of a centre differential would soon cause wind-up. This doesn't occur if the system is used correctly because the strain is released every time a wheel loses grip.

Subaru 4WD

The Subaru system can supply power to the rear wheels to get out of sticky situations, but there is no centre differential, so four-wheel drive cannot be used all the time. Instead of a centre differential behind the front-mounted transaxle , there is a transfer box which incorporates a dog clutch and allows no slippage.

The Audi quattro system

Audi Quattro layout

To carry power to the rear wheels of the original front-wheel drive design, an extra drive flange emerges from the back of the transaxle and connects to a prop shaft running to the rear differential. Ordinary independent suspension-type drive shafts carry the drive out to the rear wheels. The Torsen centre differential automatically locks for maximum traction on very slippery going, yet still allows use of the anti-lock braking system. The rear differential can be manually locked for slippery starts.

TORSEN DIFFERENTIAL

Torsen differential

The Torsen differential consists of two sets of sliding worm gears - one for each drive shaft - the two sets being linked via spur gears. When one drive shaft over-runs the other, the worms turn and transfer torque to the other drive shaft.The Torsen differential consists of two sets of sliding worm gears — one for each drive shaft — the two sets being linked via spur gears . When one drive shaft overruns the other, the worms turn and transfer torque to the other drive shaft.

Advantages of 4WD

For the driver who wants to cross muddy fields , there is no argument - two-wheel drive just will not do. But even for the ordinary road driver, there are some benefits to be had from four-wheel drive.

When conditions are bad, four-

wheel drive gives more traction Audi have calculated that the quattro system gives more than one and a half times the traction on wet roads and more than twice the traction on ice, compared with two-wheeldrive cars.

Even when the roads are dry, four-wheel drive enables the car to put the power down better. A tyre has only so much grip on the road, so the more grip that is spent in transmitting power, the less it has left to resist sideways cornering forces. Therefore, if the power is shared among the wheels, each one can contribute to both driving and cornering.

Disadvantages

The disadvantages of four-wheel-drive systems spring largely from their complexity. The extra machinery that is needed for a four-wheel-drive system is expensive and heavy. The extra friction in the more complex transmission means a small increase in fuel consumption although tyre wear will probably be more even compared with a twowheel-drive car because all four wheels are sharing the work.

The other problem with four-wheel drive is that, although the system will keep you going in slippery conditions, it cannot stop the car much more quickly. For this reason, it should be used in conjunction with anti-lock braking.

The need for four-wheel drive on the road has only arisen since the development of powerful frontwheel-drive cars, when manufacturers found that the steering imposed a limit on the amount of power that could be put through the front wheels. From that point of view, four-wheel drive is perhaps no more than a fashionable trend for upmarket cars. Whether the trend continues, so that it becomes the norm on ordinary family cars, will be seen in the next few years.

Mercedes 4-Matic

Mercedes-Benz are developing a very sophisticated electronic microprocessor-controlled system called 4-Matic that will provide on-demand drive to the front wheels of an otherwise rear-drive car. The system uses wheel sensors similar to those found on anti-lock braking to detect when one wheel is over-running so that it can lock the differentials as necessary. If traction loss still persists, 4-Matic can reduce this by cutting torque to the slipping wheels.

On-demand 4WD with synchro

VW Golf synchro

An alternative form of part-time four-wheel drive, as fitted to the VW Golf synchro, gives all-wheel drive automatically when it is needed. The car's driveline to the front wheels is identical to the normal front-wheel-

drive model, but there is a prop shaft emerging from the rear of the transaxle to drive the rear wheels when required.

In normal use the car has only its front wheels driven, but if the front wheels start losing traction and overrun the rear ones, the viscous coupling at the end of the prop shaft locks up to transmit drive to the rear wheels.

Viscous coupling

An automatic alternative to the locking differential is the viscous coupling.

The coupling consists of a drum which contains a number of slotted discs and some silicone fluid that is resistant to shear — that is, slicing between the discs. Half the discs are connected to one drive shaft and half to the other.

Why do cars have four wheels instead of three?

Because cars are generally rectangular in shape, it's crucial that its weight is equally distributed at its four nodes (corners). Plus, since cars operate on axles that support the mass, each axle would get two tires at the ends. That's why more axles are added to longer cars.

What is the function of four wheels?

4WD improves traction in dangerous driving conditions, such as snow, ice, rocks, and other scenarios that can make control difficult. By engaging both sets of wheels, traction and control improves. Additional weight contributes to better grip on the road.

Why don't cars use 3 wheels?

A three-wheeled car doesn't do anything a four-wheeled car can't do, and it comes at a cost of grip and stability. More tires equal more grip, almost always. And despite their advantages, three-wheelers have many of the same drawbacks of motorcycles and cars. That puts them somewhere in the middle, a novelty.

Do all 4 tires move in 4 wheel drive?

A 4WD vehicle that has two locking differentials provides true 4WD — all four wheels turn with the same amount of power no matter the situation. Even if the wheels on one side of your vehicle are completely off the ground, the wheels that are still on the ground will still continue to get a steady amount of torque.