4WD is the abbreviation for 4-Wheel Drive and drives all four tires using transfer, etc. There are two types of 4WD, full-time 4WD and part-time 4WD. 4WD vehicles can also be classified into those based on FF (Front-engine, Front-wheel-drive) and those based on FR (Front-engine, Rear-wheel-drive). Furthermore, 4WD vehicles can also be classified into those for driving off-road and those for driving on-road.

1. Operation of differential

The vehicle uses the differential to absorb the rotational difference during cornering. With full-time 4WD, there is a center differential in addition to the front differential and the rear differential in order to absorb the rotational difference between the front and rear wheels. Providing three different differentials makes smooth driving possible by ensuring the equal transmission of power to all four wheels, even during cornering – this is the major advantage of full-time 4WD.

When part-time 4WD vehicles, which do not have a differential to absorb the rotational difference between the front and rear wheels, turn sharply in the 4WD mode, the tight corner braking phenomenon occurs and smooth cornering becomes impossible. For this reason, with part-time 4WD, switching between 2WD and 4WD in response to driving conditions becomes necessary.

2. Tires and installing tire chains

All four tires mounted on a 4WD vehicle must be of the same size and type. If the front and rear tires are of different diameters, the center differential will have to operate at all times, even on a straight, level road. And if the diameters of the left and right tires are different, the front or rear differential will have to operate at all times (depending on whether the tires that are of different sizes are in the front or rear). Full-time 4WD offers outstanding drive-through performance on rough and snowy roads, but in some cases, it is necessary to install tire chains. At such times, the following point should be considered. When a 4WD vehicle is based on an FF model, tire chains should be mounted on the front tires. When the 4WD vehicle is based on an FR model, the tire chains should be mounted on the rear tires.  If a mistake is made, the tire chains could hit the body and damage it, so exercise caution at all times.

Types of 4WD

4WD can be classified into the following types according to differences in the drive type and control.

1. Part-time 4WD

With this type of 4WD system, the driver switches between 2WD and 4WD in accordance with road conditions by the transfer. Ordinarily, the vehicle is driven in 2WD, and 4WD is used for rough roads and snowy roads, etc. However, because the part-time 4WD system has the front and rear drive trains connected directly together, the tight corner braking phenomenon occurs during cornering, and the vehicle cannot be turned smoothly. For this reason, it is necessary to switch from 4WD to 2WD when driving on ordinary roads.

2. Full-time 4WD

The vehicle which can be used at all times under all road conditions and driving conditions, from ordinary roads to rough roads to low coefficient-of-friction roads, is full-time 4WD. Also, the full-time 4WD vehicles are equipped with the center differential.

3. V-Flex full-time

4WD V-Flex full-time 4WD system enables the vehicle to operate practically in the 2WD mode during normal travel when there is almost no rotational difference between the front and rear wheels. When a rotational difference is created between the front and rear wheels, such as during cornering or driving on a snowy road, this system transmits the drive force to the rear wheels. Thus, it ensures stable maneuverability while driving in various roads and driving conditions.

Speedometer Test or Other Tests

A speedometer tester or chassis dynamometer is used.

NOTICE:

Confirm that the vehicle is securely immobilized. Never operate the clutch or brakes suddenly, suddenly drive the wheels, or suddenly decelerate. Be sure to observe the maximum testing speed and maximum testing time.

On-vehicle Wheel Balancing

1. When doing on-the-car wheel balancing on a full-time 4WD vehicle, to prevent the wheels from rotating at different speeds or in different directions from each other (which could lead to damage to the center differential or transaxle gears), always be sure to observe the following precautions:

(1) All four wheels should be jacked up, clearing the ground completely.

(2) The mechanical locking type center differential should be in the LOCK position (with the transfer gear in the “H” position). The hydraulic multi-plate clutch type center differential with center differential control switch and mode select lever should in the LOCK mode. Center differential control switch: OFF Mode select lever: Lock. The viscous coupling type center differential should be in normal driving condition.

(3) The parking brake lever should be fully released.

(4) None of the brakes should be allowed to drag.

(5) The wheels should be driven with both the engine and the wheel balancer.

NOTICE:

When doing (5), be careful of the other wheels, as they will rotate at the same time.

2. Avoid sudden acceleration, deceleration and braking.

3. Carry out the wheel balancing with the transmission in 3rd or 4th gear (A/T “D” or “3″ range). Sub Category

1. Free wheel hub

These are generally used on the front wheels of FR based part-time 4WD vehicles. This system uses clutch mechanisms to connect and disconnect the wheel and axle shaft. In 2WD mode, the front wheels transmit rotation to the front axle, front differential, and propeller shaft. This causes vibration and noise in the vehicle and lowers fuel efficiency. Thus, on unpaved roads, bad roads, and snowy roads, the free wheel hub is locked with 4WD mode, but on normal roads and high-speed roads, the free wheel hub is freed with 2WD mode. The types for switching the hub between “LOCK” and “FREE” are the manual type, which must be operated from outside the vehicle, and the automatic type, which can be switched automatically. There are also dual types that combine the manual type and the automatic type.

HINT: When the transfer is in 4WD, the hub must not be “FREE”.

NOTICE: The automatic type can only be switched when moving straight ahead. Check the Owner’s Manual carefully before switching.

2. ADD (Automatic Disconnecting Differential)

This is the front differential and contains a mechanism that connects/disconnects the axle shaft to which power is not transmitted when a part-time 4WD vehicle is in 2WD according to whether the transfer is switched to 2WD or 4WD. An ADD connects/disconnects the left or right axle shaft with an actuator linked to the switching of the transfer, so there is no need to switch with a free wheel hub. Further, in order to make it possible to switch from 2WD to 4WD even while running, the transfer has a synchronization mechanism for starting up the rotation of the drive system that is stopped during 2WD. Also, there is a one-touch 2-4 selector (push-button switch) or 4WD control switch inside the vehicle.

A-TRAC (Active Traction Control) System

During rugged off-road driving, this function controls the engine output and the brake fluid pressure that is applied to the slipping wheel, and distributes the drive force that would have been lost through the slippage to the remaining wheels in order to achieve a LSD effect. As a result, the vehicle’s off-road drivability and ability to free itself from the mogul have been increased.

Braking Force Test

1. Vehicle speed: below 0.5 km/h (0.3 mph)

When doing low-speed type brake tester measurements, observe the following instructions.

(1) Position the wheels to be tested (front or rear) on the tester.

(2) Put the center differential in Free position. Mechanical locking type:

• ·Shift the transfer select lever to “H” position (HF2A only)
• ·Turn the center differential lock switch to OFF and check that the center differential indicator light goes off. Hydraulic multi-plate clutch type:
• ·Turn the center differential control switch to OFF and check that the center differential indicator light goes off.

(3) Shift the transaxle shift lever to Neutral or “N” range.

(4) Idle the engine, operate the brake booster and do the test.

A vehicle must be capable of driving on various types of roads, not only rough roads or snowy roads, but also on roads that surface characteristics change due to changing weather conditions. The best way to drive on rough roads is with a vehicle where all four wheels transmit power. In this respect, 4WD vehicles have several outstanding advantages over FF, FR or MR (Mid ship-engine, Rear-wheel-drive) 2WD vehicles.

1. Cornering stability

Since power is transmitted equally by all four tires instead of by only two, the load on each tire is reduced and the tires’ cornering force can be effectively utilized, providing excellent stable cornering.

2. Straight-line stability

With 4WD, since the amount of surplus grip provided by each tire is increased, external turbulence has no effect on the vehicle. Thus stable straight-line stability is achieved.

3. Starting and acceleration performance

The amount of tire grip with 4WD is approximately double that of 2WD vehicles, so even if the vehicle is equipped with a high-output engine, the tires do not spin when the vehicle starts off or during acceleration. This greatly improves starting and acceleration performance.

4. Hill-climbing performance

Since the available power is approximately double that of a 2WD vehicle, a 4WD vehicle can climb hills that a 2WD vehicle is incapable of climbing.

5. Driving on snowy/rough roads

In 4WD, since all four wheels are transmitting power, the power applied to the road can be double that provided by 2WD vehicles on snowy roads, and drivethrough performance on roads with low coefficients of friction () is outstanding. Greater power is needed when driving on sandy, muddy, or extremely rough roads. Since all four wheels are transmitting power with 4WD, the front and rear wheels help each other, and high drive-through performance is realized.

1. Tight corner braking phenomenon

When the propeller shaft between the front and rear axles is connected directly, the differences between the rotations of the front and rear axles cannot be absorbed. This causes undue force to be brought to bear on the drive train. On roads with low coefficients of friction, if any of the tires slips, the difference between the rotations of the front and rear axles can be absorbed, but on roads with high coefficients of friction, such as dry paved roads, it is difficult for slipping to occur, creating a condition very similar to braking. This is called the “tight corner braking phenomenon”.

2. Weight is increased

The number of parts increases, so the weight increases.

3. Cost is increased

The more complex construction and the increased number of parts result in an increases in the cost of the vehicle.

4. Construction is complex

A transfer and propeller shaft, differential, etc., are required in order to distribute power to the front and rear wheels, making the construction complex.

5. Sources of vibration and noise increase

The increase in the number of rotating parts (transfer, propeller shaft, etc.) causes an increase in the number of possible sources of vibration and noise.