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How 4WD Works

Date: 10/16/2017

Every car is limited by the surface it travels on. The wheels need to find traction, if they want to deliver the power of the engine to the ground in any usable form. Off the road however, there is no guarantee that the driven wheels will find traction, let alone a surface to drive on. 

4 wheel drive systems were invented for just this scenario. They allow power to be sent to either two or all four wheels, thereby allowing the car to grip in any condition. The driver can switch between different modes depending on the driving conditions. These vary from 2HI (all power to rear wheels) which is best suited for everyday driving on paved roads, 4HI (power is split 50-50 between the front and back wheels) for slippery or uneven conditions and 4LO (same as 4HI but geared towards providing more torque) when the going gets really tough and the car needs to literally crawl over the terrain. How does a 4WD system benefit in different conditions? For that we need to understand 4WD first. 

A 4WD system can send power to all wheels. The power from the engine goes through the transmission, like any other car. This could then go to a reduction gear setup (also known as a high/low gear setup) or directly to a transfer case. This reduction gear setup allows the driver to get a lower output speed for the same torque. The transfer case allows the car to switch between 2WD and 4WD and can also choose whether it runs on low gear mode or high gear mode. Power is then sent to the rear differential by an output shaft. The output shaft connecting the front differential, is powered by a chain drive (more commonly) or by gears. The differentials on the axles can themselves be locking or open. An open differential allows both wheels on the axle to move at separate speeds. A locking differential can lock the wheels on the axle and force them to move at the same speed. These differentials themselves, can be electronic or mechanical. Limited slip is another term that gets thrown around a lot in these circles. Using the brakes on the wheels or a differential, excess power from the engine or slip on either wheel is controlled by braking the wheel and sending the torque from that wheel to the other wheel on the axle which is not slipping. If there is already a lock on the differential, then obviously there is no need for the limited slip. 

From this brief itself, we can gauge that the whole system is designed so that in any given condition, there is at least one wheel that keeps getting power from the engine. If there is only one tire without traction (maybe it’s slipping or completely off the ground) then the wheel would normally just spin uselessly. In a 4WD system, the center differential can be locked using the transfer case along with the rear differential. This means that for the free wheel to spin, the rear wheels also needs to spin as they are connected by the drive shafts. Since the other tires, indeed do have traction, the free wheel does not spin and the car can move on. 

If instead, beyond all probability, three wheels end up in the air or start slipping, a much different and dangerous situation arises. All these three wheels can end up spinning even if they are in lock as demonstrated in the last situation. However the wheel with traction can be braked using the limited slip differential. This stops the other slipping wheels from spinning and diverts all the torque to the usable wheel. Therefore even in such a dire situation, 4WD comes through nicely.

The effect of the three layouts can also be shown through an acceleration test as demonstrated by a recent YouTube video. The test involved measuring the 0-30 mph acceleration time of a Nissan Frontier Pro-4X, in 2WD HI with no traction control, 2WD HI with traction control, 4WD HI and 4WD LO. This revealed that the fastest time of 3.2 seconds was achieved for 4LO, which was a quarter of a second faster than the other timings. Although there was a gearing advantage involved with the 4LO setup, the role of increased traction cannot be denied. 

In this way, 4WD allows everyday people to brave the outdoors (or cross a driveway covered by particularly slippery leaves) in the same cars that take us to our offices. All thanks to some clever engineering.

If your 4WD vehicle needs to be repaired contact Spencer Auto Repair in Mesa Az.

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