One of the most important elements in the preparation of a race car is to correctly choose the type of differential that best suits your needs. If you already have some basic knowledge of auto mechanics, you may be already familiar with what differentials are and how they work – the power comes from the engine, goes through the transmission, then the differential to be shared between the drive wheels. That’s a good start. But do you know all the different types of differentials available and the advantages and disadvantages of each?
There are several types of differentials and not all have the same influence on a car’s behavior. Let’s take a Nissan 240SX built for drifting. The ideal differential choice would be something like a Kaaz 2.0 way, a differential not suitable for everyday use. But since the car is built with track-use in mind, a differential like this one becomes a necessity.
But why such a sacrifice? How does a differential work? What are the differences? And how can you choose the appropriate differential to suit your needs? Here are some of the questions we will try to answer in this article.
Working principle of open differentials
Open-type differentials are the most commonly used on mass-market vehicles. If you have no idea what type of differential your vehicle is equipped with, here’s a quick test to find out: lift the car so that the drive wheels don’t touch the ground and rotate one of the drive wheels by hand. If the opposite wheel turns in the opposite direction, your car is equipped with an open differential. If the direction is the same for both wheels, you have a limited-slip differential (Torsen type differentials are an exception).
Open differentials are used to allow the drive wheels to rotate at a different speed from each other. When the vehicle moves in a straight line, the rotation speed is the same for both wheels. However, at the beginning of a turn, the outer wheel must then turn faster than the inside wheel. The principle is quite simple: if one calculates the circumference of a circle as follows: (radius X 2) X π, the radius of the inner wheel will inevitably be smaller than that of the outer wheel.
Now, if the drive wheels are directly connected, the rotation speed will the same for both, at all times. In a cornering situation, the outer wheel would be unable to rotate at a higher speed than the inner wheel. In practice, this would translate into difficulty in initiating the direction change and a tendency to understeer.
An open type differential is therefore installed to allow for a different rotation speed of the wheels of the same axle. To achieve this, a differential uses spider gears. These gears are connected directly to the outside housing which rotates with the ring gear. In a straight line, the spider gears follow the movement of the housing, but remain motionless on their vertical axis of rotation, making the wheels turn at the same speed. In a curve, the spider gears turn on a vertical axis around the side gears, thus allowing the outer wheel to rotate independently of the inner wheel.
The disadvantage of open differentials
An open differential will always try to balance the torque between the left wheel and the right wheel. The best way to imagine this principle is to think of what happens when driving on a snowy road. When only one tire is on the pavement while the other wheel is driving on a snowy surface, one of the side gears will encounter a lot of resistance on the side of the wheel on the asphalt and almost no resistance from the other wheel. In this case, the differential will try to balance it out and all the torque will be sent to the side having the least traction. In short, the wheel slipping on snow will start to rotate very quickly and the other one will remain almost still. If little to no torque is needed to rotate the tire on a slippery surface, almost no torque will be transmitted to the other side.
Different types of limited-slip differentials
To any problem, there is a solution and the same applies to the problems related to having and open-type differential. The solution here is called a limited-slip differential. Although there are several different types of LSDs, all have the same goal: to allocate more torque to the wheel that has a better grip.
Viscous limited-slip differential
Viscous type differentials are mainly used to manage the difference in traction between the front and the rear axles of four-wheel-drive vehicles. On this type of car, the differential is composed of two shafts, an input shaft, and an output shaft, instead of three. The input shaft is made of a series of propeller-shaped plates, while the output shaft is designed as a cylindrical box surrounding the input shaft’s plates with a series of perforated plates. Both types of plates are only linked using a silicone-based viscous fluid that has the property of thickening when submitted to heat.
When both axles have the same traction, the two shafts rotate together with the fluid. If one of the two axles loses traction and turns faster than the other, it creates friction. This increases the temperature and therefore the viscosity of the fluid, which then locks the plates together, and thus transfers a portion of the torque to the axle having the most traction.
Clutch-type limited-slip differential
On this type of LSD, the side gears, the pinion as well as the pinion gears are located behind two pressure rings. This characteristic is specific to Kaaz LSD models though. Behind each of the rings is a series of clutch plates. Half of the plates are connected to the cage and the second half, to the left and right shafts.
When the gas pedal is pressed, the differential cage rotates on a horizontal axis. When it happens, the pinion sinks between the two pressure rings, pushing them outward and crushing the different clutch plates against each other. The plates will lock together, allowing the driveshafts to turn at the same speed as the cage and thus sharing the torque evenly between the two sides.
It is important to note that this is a principle specific to Kaaz LSD units, a big name in the LSD market. The majority of traditional clutch-type differentials use a spring pack between the side gears. As the spider gears move, the spring applies pressure that crushes the side gears against the plates to lock them instead to transmit torque equitably between the two wheels.
It is important to note that clutch-type differentials are divided into three subcategories: 1 way, 2 way, and 1.5 way. A clutch LSDs are then categorized according to their ability to lock – and to send power to both wheels – during acceleration or deceleration. Concretely, a 1-way unit only works under acceleration. With a 2-way, the system goes into action during acceleration and deceleration. Finally, a 1.5-way LSD means that the differential locks only partially when decelerating but fully during accelerations. On a Kaaz unit, this characteristic is determined according to the design of the pinion and the pressure it exerts on the rings.
The use of a 2-way differential is strongly discouraged if you make use of your vehicle on a daily basis. It is mainly used in drag racing and drifting. By locking both rear wheels during braking, it is easier to make the car slide. For most car owners, a 1.5-way system is a good compromise, especially if you want to use your car daily.
A Torsen type differential is a completely mechanical application. Its ability to limit tire slippage comes from its pairs of helical wheels and gears. In a normal cornering situation, when no slippage occurs, the Torsen differential acts as if it was a standard open-type differential.
When one of the wheels, the right one, for example, loses its traction because of the condition of the road it’s driving on, this is where the pairs of helical wheels and gears come into play. With an open differential, no torque would be directed to the left side. In the case of a Torsen differential, the torque would be rerouted thanks to the right helical wheel which would transmit its energy to its gear pairs, resulting in the left synchronization gears transmitting the torque to the left wheel.
As explained above, this has the effect of locking the right side to the left and sending some of the torque to the wheel with the most traction. The amount of torque transmitted depends on the design of the gears since it directly determines the ratio of shared torque. For example, a Torsen differential with a ratio of 5:1 will allocate 5 times more power to the wheel with the best traction.
Torsen differentials have the advantage of operating smoothly. It’s a truly effective mechanism with only one notorious disadvantage. Since it’s based on a multiplier ratio to transfer the torque on the other side, it transmits no torque at all if one of the wheels leaves the ground. Multiplying a factor by 0, the result will always remain 0. Simple as that. Another major problem is its price. Frequently priced over $1,000, it’s not really a budget-friendly option.
A limited-slip differential brings real benefits to your car’s performance. Examples are numerous among car manufacturers. For example, after offering its Sentra SE-R Spec V without an LSD, Nissan soon realized that with a 2.5-liter producing 180 lb-ft of torque, a LSD was more than necessary to efficiently transmit the engine’s power to the front wheels. As a matter of fact, the SE-R was later offered with a Torsen-type LSD. Without a proper LSD, an FWD car can’t be pushed to its limits. When it comes to finding a limited-slip differential that fits your needs, it is always better to choose according to the use you want to make of your car and your budget. On the other hand, one thing is certain: without a LSD, even the most powerful car in the world becomes as useless as a car without tires.