All cars have differentials have one 4x2 and 4x4 have three. Sports performance or outside carreteran improve with self-locking differential or limited slip. The objective of these systems is to control the differences in rotation inside an axis (between wheels) or between two shafts, to improve grip and traction.
Limited slip differential Ford F-150 on the rear axle
Spreads also distribute engine power
Straight, we believe that all wheels rotate at the same speed, but not so in the curves. The outer wheels then make a longer journey inland, and produces a Unlike revolutions. This, in a shaft in which the wheels are not connected to each other, is not a problem.
When there is a mechanical connection between the wheels, it is necessary differential, mechanical element that allows the wheels to turn at different speeds while receiving power from the engine. There are several types and do not intend to bore you with technical details, but we can distinguish three kinds:
- free differential: Allows rotation differences of 100%
- Limited slip differential or LSD: Allows rotation differences to a certain limit
- Differential blocked or "soldier": Forcing both wheels to turn equally, or two axes have a deal 50:50
In most road cars, free differential used in the drive shafts. That is, in the front-wheel drive, no differential front and rear axle no mechanical connection between wheels and no differential. In contrast, high-performance models or SUVs, the issue is more complex.
The Subaru WRX STI allows three differential lock to suit the driver, depending on whether you want maximum efficiency with good adhesion or instead have better traction when very scarce adherence
Imagine a situation where one wheel loses traction. If we speak of a free differential, it may be the case that a wheel turn crazy (100% torque) and the other stay still. It is normal when an area is depressed with very low grip such as ice or gravel.
AWD also have a center differential
The simplest solution to avoid this problem is to force the wheel with more grip to turn, and that is slowing the other. With calls electronic differential, It is selectively braking one of the wheels until the other actually receives force and traction capability is recovered.
This solution is the most economical, because it depends on the braking system and the ABS sensors, which are already pending what is the speed of rotation of each wheel. The problem is that it makes more intensive use of the brakes and worsening performance to the limit. However, it may be useful off-road use, and is used in many SUVs as well.
Ford Focus RS (II) with 300 hp to the front wheels. Neither the front differential Quaife could do miracles, once the available tire grip
Unless we want to dedicate ourselves to Drifting (We need a differential "soldier"), What interests us is a limited slip differential. In this way, we allow a speed difference between the wheels, and when you get to a particular point, acts to rotate more severally.
In all-wheel drive models, want to control which axis rotates, depending on the effect you want. It is the function of the center differential. We have three types of locking differentials, which can go both ahead and behind and in central position:
- mechanics or viscous coupling
- Torsen or sensitive pair
- Slip-controlled multidisc clutches
Haldex systems belong to the latter type
Depending on the type of car, we may want more than one or two. For example, mechanical, viscous and are suitable multiembragues central differential that normally circulate models with only one drive shaft, and react to the loss of traction force sending the other axis. This saves fuel by having lower friction.
In this situation we are not interested to turn a wheel in the air, and force must pass the rear right wheel that has more grip. The Juke 2012 onwards allows such dynamic cast in its 4x4 version
However, if what you want is maximum efficiency, the Torsen solutions are often the best. Not only provide all-wheel drive when accelerating or traction is lost, braking and retention. In addition, they can send most of the force (torque) to the wheel that turns least, unlike the others.
The most effective systems are those that allow also the wheel passing through the outside of the curve get tighter, reducing understeer (the car turns less than expected) and improving cornering by increasing the yawing effect. However, this efficiency is always limited by the available tire grip and how much force can be transmitted to the ground.
Such systems are often referred to as torque Vectoring or Vector torque control. That as regards sports models, SUVs we care that can block the differential to force a deal 50:50, especially useful in difficult situations.
The Mercedes-Benz G-Class allows three differential lock and forcing all four wheels to rotate at the same speed. While a single wheel to pull the SUV advance
Best SUVs allow block the three differential, front, center and rear. Instead, on the road, that does not interest us anything: torture tires and subejes (or driveshafts), unless the situation is very difficult, like driving on icy snow.
The negative side of spreads that are not "electronic" They are comprised of more parts, weigh more, and consume more fuel. The ideal is to drive with only two-wheel drive in the easiest situations (highway with dry roads), all locked in the worst situations, and have control in the intermediate steps.
We saw in a previous article how all-wheel drive Audi quattro works. They are not the same needs of a A3 (transverse front engine) that an R8 (central rear engine) or A6 (longitudinal front engine). There is no better than another system, but more appropriate or less what you want of each car systems.