Differential in automobiles

What is a differential?

Differential is a mechanical device used in rear axles and it's primary function is to vary the power delivered to the rear wheels when the car is negotiating a turn.

Differentials also acts as a final gear reduction, to slow down the rotational speed of the transmission before it is delivered to the wheels. Another function of a differential is to transfer power at right angle from the propeller shaft to the wheels.

Why is differential used?

When a car is negotiating a curve, the inner wheel has to cover a smaller distance compared to the outer wheel. Therefore, the inner wheel must rotate at a lower speed compared to the outer wheel.

Without a differential, both the driven wheels would rotate at the same speed. If a solid shaft is used without differential, then wheels will have to slip to accomplish the turn. Differential is not required on non-driven wheels i.e. if a car is a front wheel drive, then differential would be installed at the front axle and will not be required at the rear axle.

History of differential:

There is no clear fact about the invention of differential. Some researchers day that it was first used in China way back in 30 B.C. However, Onésiphore Pecqueur, A French mechanical engineer patented the modern day differential gear in the year 1827.

In 1874, Aveling and Porter, a British agricultural engine and steam roller manufacturer listed a 2 ton crane engine with a rear differential to permit tight cornering without disconnecting both the rear wheels.

In 1876, James Starley who was known as the father of bicycle industry invented a chain drive differential for bicycles. Interestingly, Karl Benz used this invention on his automobiles, as we can see in Benz patented Motorwagen of 1885. Power was transmitted by means of two roller chains to the rear axle.

Working of a deferential:

Differential is an integral part of a driven axle. Wheels receive power from the engine via driveshaft. The main function of a differential is to allow the wheels to rotate at different rpm while receiving power from the transmission.



The ingenious mechanism of the differential allows wheels to rotate at different rpm, while transferring power to both wheels. Lets learn a differential's construction in a step by step manner:

• Power from the transmission is transferred to a ring gear via a pinion gear.


• The crown wheel is attached to a differential cage, which contains the 'sun' and 'planet' gears.


• There are 2 sun gears which rotate in the same axis of the ring gear. There are 2 planet gears which rotate in a axis perpendicular to the ring gear.


• The planet gears are attached to the ring gear and meshed with the sun gears.


• Each wheel is connected to the sun gears independently with the help of half axle shafts. The drive from the sun gear is transferred to the wheels

Case 1 (Vehicle moving in a straight direction):

Consider a vehicle moving in a straight direction. At this point, the planet gears rotate along with the ring gear but does not rotate along it's own axis. Therefore, both the sun gears rotate at the same speed, so does the wheels. In other words, the entire differential cage unit will rotate as a single solid unit.

Case 2 (Vehicle taking a right turn):

Consider a case where vehicle is taking a right turn. In this case, the planet gears play a pivotal role. The planet gears not only rotates along with the ring gear, but also rotates about it's axis. The left wheels has to rotate faster than the right wheel.

The effective combined rotation of the 2 sun gears should be equal to the planet gears i.e. peripheral velocity of the sun gears should be equal to the planet gears.

Another simple way to understand the mechanism is that the speed of the left wheel is equal to the sum of the rotational speed and the spinning speed of the planet gears. Whereas, the speed of the right wheel is the difference between the rotational speed and the spinning speed of the planet gears. This helps in making the left wheel rotate faster than the right wheel.

It is the vice-versa when a vehicle is taking a left turn. The planet gears spin in the opposite direction.

Speed reduction at the differential:

This is also one of the functions of a differential, where the speed from the transmission is reduced by having a larger ring gear compared to the pinion gear. This set-up increases the gear ratio, thus reducing the speed. This is also known as final gear ratio. If the final gear ratio is 1:5, that means the ring gear has 5 times more teeth than that of pinion gear.

Disadvantage of a standard differential:

The main disadvantage of a standard or an open differential is that it is not effective on surfaces offering different traction. Consider a situation where one wheel is moving on a slippery track (water) and the other wheel is on a rough  surface. In this case, the standard differential will send the majority of the power to the slippery wheel and hence the vehicle is stuck.  To overcome this problem, Limited Slip Differentials (LSD) are used.

Traction Control System (TCS)

What is TCS?

TCS stands for Traction Control System and is a safety feature used in cars to maintain the traction between the road and the driven wheels. In the past, driver had to remove his feet odd the accelerator pedal on slippery roads to avoid slipping of the wheels. Today with the help of electronic traction control system, the power from the engine delivered to the wheels can be limited.

When was TCS introduced?

Before TCS, rear wheel drive cars were installed with limited slip differential, a mechanical system that delivers more torque to the non slipping wheel, while still allowing some wheel spin to occur.

In the year 1971, Buick introduced MaxTrac system to detect the rear wheel spin with the help of a computer system. MaxTrac system modulated the engine power to those slipping wheels to provide more traction. Cadillac came up with Traction Monitoring System (TMS) in 1979 on its Eldorado model.

How TCS works?

Electronic Traction Control system makes use of the same wheel speed sensors used for Anti-lock Braking System (ABS). When the TCS system detects that one wheel is rotating slower than the other wheels, it modulates the brake valves to reduce the brake pressure on that particular wheel and avoid slipping. In addition to this, TCS also regulates the engine power delivered to the slipping wheels.

Traction Control System does not increase traction, but only prevents the wheels from slipping by maintaining traction. A vehicle might still get stuck or lose control even with TCS on it. For better protection, a car should feature both TCS and ABS.

The powertrain can reduce the torque supplied to the slipping wheels by cutting off the fuel supply, limiting throttle action, retarding spark ignition, shutting down some cylinders, and many other methods.

Traction Control can also help in cornering. If too much throttle is supplied, then the wheels might lose traction and slip sideways. TCS limits the power supplied to those wheels and thus help in increasing the grip of the tires.

Electronic Brakeforce Distribution (EBD)

What is EBD?

EBD stands for Electronic Brakeforce Distribution, an automotive technology used as a safety feature.

Why is EBD used?

EBD is a automotive brake technology used to vary the brake force on each wheel of a vehicle automatically based on the road conditions, speed, load, etc. It always works with the help of ABS (Anti lock Braking System).

The weight of a vehicle may not be evenly distributed among all the wheels, so in that case, the brake force required on each wheel would also vary. EBD automatically senses the variation in loads and distributes the brake pressure to wheels based on the requirements. This helps in stopping the vehicle without losing control over it.

How EBD works?

Friction is the most important aspect in making a car move in the right direction. Whether we accelerate or apply brakes, friction between the tires and road helps in making the car move forward and also stop it without losing control. But in some conditions (for e.g. icy roads), there is not enough friction to stop the vehicle and also to steer the vehicle.

It is important to drive carefully to avoid losing control of a vehicle when driving on roads providing less friction. When one suddenly applies brake, there is always a chance of wheel lock, which can lead to skidding. ABS helps in preventing wheel lock by sensing the slip ratio ( slip ratio is the difference of the speed at which a vehicle is moving to the rotating speed of a tire ). ABS can calculate the slip ratio of individual tires and vary the brake force applied to each tire, therefore avoid skidding.

EBD is also useful when a car is negotiating a turn. While turning, the outer wheels of a vehicle rotate faster than the inner wheels. There might be a scenario where too much brake force is applied to the inner wheels, therefore leading to over steering. EBD can sense the slipping of inner wheels and reduce the brake force applied to the inner wheels and help the vehicle negotiating the curve safely.

EBD components:

• Speed sensors: To sense the speed at which each wheel rotates and also to sense the speed at which a vehicle is moving. There is no specific sensor used to sense the speed of a vehicle. Average speed of all wheels rotating is considered as the vehicle speed.


• Brake force modulator: It is used to vary the brake force supplied to each wheel hydraulically. The ECU actuates the valves to vary the brake force.


• Electronic Control Unit (ECU): It receives input from the speed sensors, calculates the slip ratio of each wheel and actuates the brake force modulator to vary the brake pressure on each wheel to avoid skidding.