Who has 4x4?

Hello dear readers. Passing more than one thousand miles in their native land in search of coveted trophies of the underwater world, I once watched a very amusing picture when "cool jeep" digs into the planet, often for no reason. And when quite firmly bury, leaves a solid citizen who wears his fashionable jacket and began to swear and walk around the excavated them as well. As you may have guessed, it goes on the booty, and that at least they should go. Therefore, this article I will cover the topic in detail as all-wheel drive car, therefore this article will be very interesting to those readers who enjoy hunting, fishing, or those who are going to get all-wheel iron friend, because he lives where no all-wheel drive does well!

Let's start with the fact that the very essence of the system was invented around 100 4x4 years ago. At the dawn of the automobile industry, as you might guess, there was no road but need to get from point A to point B has always been. And the biggest need for this was in the military, so they became the first major customer. But back to our days.

Many of you may say that I have a four-wheel drive, but let's face it. Sticker, label or inscription 4x4, Off Road, All road, 4matic, 4 wd, Full time 4 wd, AWD, Xdrive, controller of the drive system 2wd; 4wd; lock lever and even transfer gearbox can not guarantee that your car is truly 4x4, but why? The system itself is 4x4 drive involves the transmission of torque to each wheel independently. That is, if 3 out of 4 wheels to slip frustrated, we still do not get stuck, but thanks to a wheel can get out and continue driving.

I propose to consider all possible variant of drive systems.

1. 4x1, ie four-wheel torque is transmitted only by 1. Incidentally, in the automotive industry, these cars are called 2 wd.

For clarity of the experiment, imagine that we are in a car Toyota Corolla are starting to rise, with any of the front wheels, we are on the ice (a very low coefficient of friction). At the time of launch there is slippage of the wheels, which is on the ice, the second front wheel spins no (or almost no spinning). The reason is very simple - by the presence of all the differential torque goes to the wheel, which is easier to twist, that is something that has a lower coefficient of traction. Torque (as electricity) always follows the path of least resistance.

This is the most common category. It get almost all cars. Starting from all the known Camry (front drive) to BMW 525 (rear-drive). Although the latter model can not only be rear-wheel drive, depending on version.

2. 4x2. From 4-wheel torque is transmitted only by 2. This class and all the subsequent four automakers are called wd.

If we apply the same experimental conditions as described in option 1, then in this case also there will be a slip of the front wheels and ... car would not go again! But as you say, now that torque is not just for the front, and rear-wheel drive! That's right, only now the torque goes to one of the front wheels, not only from the front, but with the rear axle. In order for this not happening, there is a locking center differential. Here dwell on this in more detail.

Locking center differential are mechanical (usually with a switch on the transfer gearbox), such as Mitsubishi Pajero Sport and electronic (with a convenient switch on the dashboard or near the checkpoint), this includes almost all SUVs Toyota Rav 4, Nissan X-trail, and many , many others. Each of these systems, their advantages and disadvantages. 

The advantages of mechanical systems: hard redistributing torque between the front and rear axle (usually 50/50) thus provide a more confident driving on the road. The disadvantages of mechanical interlocking is the restriction on your connection speed (on some models up to 100 km / h, and the other only at a standstill), and that the movement is locked in the center differential is only possible on slippery surfaces (ice, snow, mud , deep sand), otherwise distributing CAT may be destroyed due to high friction. This blocking option is also called "hard block", it is set mainly on the planet.

Electronic locks provide less interaxial solid progress on the road, and they tend to shut down when overheated (mode of self-defense), but they can easily turn on or off at any speed, and go at it on any surface. This option is called a "soft lock" and set it in the main on SUVs or sports cars.

There is a third option interaxle Block - limited-slip differential. Such differentials are hydro-mechanical and electronic. The principle of operation of such differentials is based on the difference in speed front and rear wheels. If the speed of rotation of the front wheels relative to the rear wheels exceeds a predetermined threshold, it triggers the lock and torque begins to act not only on the front but the rear axle. On a machine with such a lock may not be a mode switch, there simply is blocking it. This is also a variant of "soft lock" for the crossovers.

Now, we understand options interaxle locks, let's go back to our experiment. So now we start the car with a locked center differential. Terms of experiment 1 (ice only under one of the front wheels). The driver starts, and observers dressed in winter jackets, again surrounded the car. Due to the fact that now the torque split between front and rear axle, the car begins to move through the rear wheels. Let's complicate the task. Now, create the conditions where the ice is not only under one of the front, but also one of the rear wheels. Start as well as on a small rise. In this case, we will experience not just slip the front but the rear wheel, which is on the ice, and those wheels that are on the asphalt, spin will not (or hardly any), because due to cross-axle differentials torque will be spin those wheels, where there is less resistance. That is, again, came - come out and pull this all-wheel drive.

3. 4x3. From 4-wheel torque is transferred to the 3.

From the experimental conditions, Table 2 shows that for the beginning of the movement requires not only interaxle lock and cross-axle.

Interwheel differential lock on the principle of operation can be divided into mechanical and electronic, but of the connection are forced and self-locking. But the goal of all these systems a - when a large difference in speed between the right and left wheels of one axle to brake, the wheel, which has great speed and transfer torque to the one that has a lower speed.

Now, in experiment 2 we start a car with a blocked cross-axle differentials. For concreteness let it be the rear differential. In this case, our SUV, a little scrolling wheel being on the ice begins to move forward confidently. This category includes virtually all SUVs. Mitsubishi Pajero Sport, Mitsubishi L-200, Nissan Navara, Toyota Land Cruiser Prado, Lend Rover Discovery 3.

As you may have guessed, we once again complicates the problem, and now 3 out of 4 wheels will be on the ice at the start of the hill (two rear and one front). Under these conditions, even our SUV, overcame the conditions of experiment 2, there is only sand and ice fiercely growling engine, moving slowly with the back ?. Why back, but because the friction of wheels on ice, the latter begins to heat up and turn into water. The level of grip will drop even more (agree that wet ice more slippery than dry), and the machine under its own gravity will begin to move back from the hill, despite the fact that the wheels are spinning forward. Therefore, when driving on ice or snow polished try to never let failure of the drive wheels to slip, ie operate the gas pedal very gently.

4. 4x4. At last!

We continue to fight the slip three of the four wheels. To overcome the three experimental conditions, it is necessary, the presence of all 3 locks: interaxle, front and rear cross-axle cross-axle. Thus, incorporating all three locks and starting conditions of the experiment 3, we get a full slip rear axle, but at the expense of the front cross-axle lock, the car will still move forward by just one wheel, which is not on the ice.

And now let's the last time complicate the task. In this variant of the experiment, we will start with all four wheels on the ice! It is unrealistic to say to you, but let's see what we get?? Now we need to remember what I wrote earlier, but rather the condition of inclusion and cross-axle lock interaxal. Let me remind you, they start to work when there is a significant difference in speed between the front and rear axle (hole spacing), or between right and left wheel (cross-axle). A little correction: start is made on the usual factory tires, no studs, velcro, etc. Well, let's go ...

With all the locks included (front, back and interaxle), there are two versions of events, which were made me. Option 1 - Start with a sharp click on the gas pedal. The miracle did not occur, and due to the formation of water ice at the start, the car lift does not take. Option 2. With a continuous pressing on the gas pedal the car still starts uncertain moves forward. Now let's see, what's the secret here. At the start we have a slip all four wheels, but weak, and when they begin to work the lock, each wheel has at least very similar, but different coefficients of traction (due to the heterogeneity of the surface). Due to this torque all the time "jumps" between all the wheels. That is, if you are currently the most traction is the front right wheel, then a second later it starts to get bogged down in the slip, and then torque "being transferred" to the wheel with more grip, for example, on the right rear wheel, and so constantly among all wheels. Thus, it appears that the car is moving forward little thrashing each of the 4 wheels. The experimental conditions 4 passed!

Let's look at what well-known off-road vehicles can be overcome by the experimental conditions 4? So, the shoes winter men's shoes , dress, and sit in one of these mass-produced cars 4x4: Land Rover Defender, Mitsubishi Pajero 4, Nissan Patrol (in the previous body, the new did not go - do not know) or the majority of military equipment on 4 wheels! If anyone else knows, please write. Well, summing up what has been said, I want to say that the level of off-road the following factors: experience and professionalism of the driver – 70, tires - 10%, the system full of drive - 10%, geometric traction, with a weight - 10%.

Good luck on the road.