The so-called transfer box is a device that distributes the power of the engine, which can output power to the rear axle or both front and rear axles. From this perspective, it can be seen that the transfer box is actually an accessory on four-wheel drive vehicles. With the development of four-wheel drive technology, the transfer box has been constantly changing and gradually formed a completely different style of transfer box, which is matched with different demands of four-wheel drive vehicles, and their basic principles and functions are also different.

A time-sharing four-wheel drive car is a car that can be a two wheel drive vehicle on off-road conditions and converted to a four-wheel drive vehicle. The time-sharing four-wheel drive transfer box is a purely mechanical device. When the transfer box with this structure is equipped with 4-wheel drive mode, the front and rear axles are rigidly connected, which can achieve a 50:50 distribution of front and rear power, which is very beneficial for improving the vehicle's passability. In addition, due to its pure mechanical structure and high reliability, it is crucial for vehicles that often drive in wilderness areas lacking rescue. Even now, there are still a large number of hardcore off-road vehicles using this transfer box. On the transfer gear lever of this type of vehicle, we will see the shifting gears of 2H, 4H, N, and 4L. When equipped with a 2H, this type of vehicle is a rear wheel drive vehicle, where the power of the engine passes through the gearbox and is directly connected to the rear axle through a transmission shaft. The function of the transfer box is to introduce an output end onto the gearbox and transmit power to the output shaft of the front axle through a silent chain. Of course, this is not a direct connection, otherwise it would be impossible to switch between 4-wheel drive and 2-wheel drive. In fact, it is separated and connected through two sets of gears, and its structure and principle are similar to the first and second shafts of a gearbox. When switching, pull the gear lever of the transfer box to connect and disconnect the power from the front transmission shaft through the fork. Unlike mainstream gearboxes with synchronizers nowadays, there is no synchronizer for switching in this area, as it requires a complete match between speed and wheel speed. This is the basic principle of this transfer box.

But the actual situation is not so simple. In order to improve performance, this type of transfer box will also have an afterdrive gear, which is the 4L mode on the gear lever. On the gearbox, there is a gear with a larger gear ratio that, when engaged, can provide a much higher main transmission ratio than daily driving. We found that when we need to shift to 4L, we must go through an N gear. At this time, the gearbox will separate the power from each transmission shaft, and when shifting to 4L, the gear with a larger gear ratio will be connected. This switching process also lacks a synchronizer.

Knowing this principle, let's take a look at the operating characteristics of each mode of this type of transfer box. As car enthusiasts familiar with traditional off-road vehicles know, this type of transfer box does not require parking when switching between 2H and 4H, and can generally switch freely at a speed of 80 kilometers per hour. When switching to 2L, it is necessary to stop and switch, otherwise it cannot be hooked up at all. Why is this?

Whether in 2H mode or 4H mode, the power is always connected to the transfer box structure diagram of the rear axle, and the wheel speed of the rear wheels is completely matched with the engine speed. At this point, as long as the wheels do not slip, the wheel speeds of the front and rear wheels are the same. Therefore, when switching between 2H and 4H, the engine speed matches the speed of the front output shaft, and even without a synchronizer, it is completely possible to switch. Therefore, switching between 2H mode and 4H mode is completely possible while driving, without the need to stop and switch. But when it comes to the 4L mode transition, the situation is completely different.

To switch from 4H to 4L mode, it is necessary to first shift the transfer box to N gear. At this time, the engine power is disconnected from each wheel and the engine is put into idle mode. At this point, if 4L is engaged, it will be difficult to match the wheel speed with the engine speed, which is equivalent to a car without a synchronizer wanting to shift into first gear while driving, which is obviously difficult.

There is no differential between the front and rear axles of this transfer box, so driving on high adhesion roads can only be done with 2H. The four-wheel drive mode is only designed to improve passability on sandy and unpaved roads. Therefore, four-wheel drive vehicles using this type of transfer box are generally hardcore off-road vehicles, which perform well on unpaved roads but perform mediocrely on highways.

In the early days, time-sharing four-wheel drive was completely manually switched. Later on, there was the emergence of electric switching of time-sharing four-wheel drive. Its basic principle is the same as manual switching of time-sharing four-wheel drive, except that all switching is completed through an electric motor.
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