Diff or differential lock, limited-slip differential, locked diff or sport diff. If you read such terms and can't do anything with them, you don't need to despair. We explain what is meant, what the limited-slip differential does in the car and what you can use it for on some trips. Before you look for the differential lock, you should know: Not every vehicle has this part. And if you do, you usually can't see it. But you will feel the difference as soon as it is active.
What is a differential lock?
Let's start with the normal differential in the car, also known as the differential gear: Differential wheels in the transmission compensate for the speed differences between the wheels on an axle that occur in every corner. That's why every car has a differential, whether it's a small car or a super sports car. Depending on the front-wheel drive or rear-wheel drive, the lock is located in a different place in the car. In vehicles with front-wheel drive, the differential is usually integrated into the transmission. In vehicles with rear-wheel drive, it sits as a rear-axle transmission in a kind of bell.
A differential lock does exactly the opposite. Instead of compensating, it locks so that the full power is transferred to the wheels. This prevents a wheel from spinning. The differential locks either partially or completely the force that goes to the wheel. A rigid connection between the wheels of an axle or, in the case of all-wheel drive vehicles, is created between the front and rear axles.
In all-wheel drive vehicles, the differential is used to compensate for the speed differences between the front and rear axles. This is not necessary when driving in a straight line, but it is when cornering. The wheel on the inside of the curve rotates more slowly than the wheel on the outside of the curve due to the shorter distance and the different speeds. With a rigid connection, the inner wheel would spin or start hopping. A differential compensates for these differences, allowing for a smooth ride.
There are different types of limited-slip differentials that work differently. They are called multi-disc locks (classic differential locks for sports cars) or Torsen differentials (widely used in sporty front-wheel drive cars). Speed-sensing differentials react to speed differences between the drives, either by means of a viscous clutch or a hydraulic pump. Mechanical differential locks are usually used in hard off-road vehicles, electronic differential locks are more likely to be used in SUVs with all-wheel drive.
Some modern limited-slip differentials compensate for the friction loss with an electronic differential lock (EDS). The function: If a wheel spins when starting off, it is braked down to reduce the speed difference to the other wheel. Many vehicles with electronic differential locks regulate this slip with the intervention of the ESP. In this way, the power is not redirected, but the strongly spinning wheel is simply braked. The energy then evaporates as heat in the brake. Thanks to modern control technology, these electronic differential locks work quickly and well – but they are not a real limited-slip differential.
What are the benefits of the differential lock?
A limited-slip differential helps in situations where there is often a risk of friction loss. In other words, where full power is required on both or all wheels. In snow, ice, slippery conditions and mud, it is an advantage that both wheels of one axle are driven. Depending on the road conditions, such as black ice on one side, it can happen that one wheel spins while the other wheel is stationary. The differential lock then blocks the slip, creating a rigid connection and driving both wheels again. Vehicles that are often on the road in such problematic road conditions therefore rely on diff barriers.
Do differential locks also have disadvantages?
Disadvantages of the limited-slip differential relate only to the additional weight and price. With most vehicles, you can't order a limited-slip differential separately anyway. This is either only found in certain vehicles such as off-road vehicles or can only be combined with certain engines or all-wheel drive transmissions.
The price of a vehicle with a limited-slip differential is usually higher than that of a car without. Another disadvantage is mechanical wear – the locking effect can decrease with increasing mileage. In addition, locks react sensitively to the circumference of the tyre, i.e. also to pressure differences. When the lock is activated, driving on normal roads becomes an ordeal for the driver, car and tires. In curves, the inner wheel starts to spin and hop. This massively increases wear and tear and the vehicle tends to understeer severely.
What else should you pay attention to?
It depends entirely on the use of the vehicle what you need for a differential lock. If you often drive in snowy regions and mountains, a car with all-wheel drive and lockable differentials has its advantages.
Many modern SUVs with all-wheel drive, on the other hand, rely on ESP. This regulates the force on the wheels, but works quite differently compared to a differential lock. While the diff lock allows the power to pass through to the non-rotating wheel so that it turns again, the ESP brakes the power on the spinning wheel so that the speed difference is reduced.
This may lead to no wheel turning. Then you as a driver have to deactivate the ESP and rock yourself out of snow or mud, for example. You will hardly experience this situation in a city. But if you often drive in mud, snow or sand, choose a vehicle with all-wheel drive and differential lock.
Which cars have a differential lock?
As a rule, only hard off-road vehicles have a limited-slip differential. SUVs, on the other hand, do not have it, and normal passenger cars such as small cars, compacts or sedans also have no locks on board. Sports cars rely on a partially locked differential with a lock of 30 to 60 percent. This is supposed to improve cornering speed, because then the driven wheels deliver full power.
To avoid tension in the drivetrain, some off-road vehicles rely on several limited-slip differentials on the front and rear axles as well as between the two drive axles. In this way, the same power can be transmitted to all four wheels in hard terrain and the car can burrow through mud or sand. The Audi quattro (1980) was the first vehicle to have two standard differential locks. At that time, the driver still had to operate it by cable pull.