The four-wheel drives and when to use them.
Maybe you have always owned four-wheel drive trucks? Maybe this is your first four-wheel drive vehicle? Four-wheel drive gives the driver options. Knowing how to best use those options is the key to success. Does four-wheel drive allow you to get through anything? Not even remotely close! Sometimes it just allows you to get more stuck and broken. To understand how to best use your four-wheel drive options we need to go over a few basic principles. First, the drive axle differential. All vehicles use a differential on the drive axle to allow for smooth turns. When a vehicle goes around a corner the outside tire needs to cover more ground than the inside tire. Power from the engine/transmission comes into the center differential and is distributed to the drive tires via the drive axles. The differential allows for the difference in wheel speeds while providing power to both drive tires. During most driving situations this happens very smoothly. If one drive tire is on a high traction surface and the other is on a low traction surface. Like ice, for example. The tire on ice will spin and the vehicle will not move. In this situation, four-wheel drive can help.
When the vehicle is shifted into 4H or 4WD or Part Time 4 (all the same thing) it splits the outgoing power 50/50. Half to the front axle and half to the rear axle. In the given example above, the vehicle will move if the front tires both have traction. 4H should only be used is situations where one or more of the tires are likely to lose traction. If 4H is used in a high traction situation, like in a paved parking lot, it can cause the driveline to bind. This can cause hard steering and may lead to broken driveline components.
Moving on to AUTO-4WD or Part Time 4 (again, both the same thing). AUTO-4WD is like 4H except it also incorporates differential function in the transfer case. Most of the time this is accomplished using a clutch pack or viscus coupler. Allowing the power distributed to each drive axle to vary as needed. For instance, when making a tight turn in higher traction situations, like our parking lot from above. Each of the four tires will need to turn at a slightly different speed to complete the turn. The tires nearest the center of the radius of the turn will turn slower than the tires farther away from the radius. While the drive axle differentials allow for the side-to-side difference in tire speed, the transfer case clutch pack allows for front to rear differences in paired drive tire speed. This slip must be allowed and controlled at the same time. If the transfer case had an open differential, like the drive axles, then you could still get stuck in a
parking lot if one tire was on ice. All the power would be sent to the tire with the least traction. The transfer case clutch pack limits the slip so power can be directed to the other axle with tires that have traction. Some vehicles just use a clutch pack with springs to control the slip while others use a computer-controlled viscus coupler. The after effect of controlling the slip is heat. Which breaks down the fluid in the transfer case. AUTO-4WD should not be used when it is not needed or the ambient
temperature outside is high. Regular fluid services of the transfer case or viscus coupler can prolong the life of each component. The best situation to use AUTO-4WD is when you are likely to lose traction with some tires but, overall the roads are fairly clear of snow and ice.
Next, we have 4L (Four Low). 4L is like 4H in the sense that it splits the outgoing power to each drive axle 50/50. The difference is 4L also provides additional gear reduction. 4L is ideal when slow speed control is needed. Fox example, moving a heavy trailer around your yard or pulling on a stump. I use it almost exclusively when driving on off-road trails for superior vehicle control. Top speed is limited in 4L. Ideally to 30 MPH or less.
Finally, we have AWD (All Wheel Drive) vehicles. AWD vehicles do not have a 2HI or 4LO range. Like most Subaru vehicles for example. They operate like an AUTO-4WD vehicles but are designed to be used in all temperatures and conditions. Most AWD vehicles use computer controls to vary the amount of
power sent to each drive axle. They may be front wheel drive biased where 60% of the power is sent to the front drive axle and the remaining 40% is sent to the rear. The driver has limited control over this. Sometimes those vehicles will have drive modes. Like the Subaru XMODE. The vehicle can be switched to Snow/Dirt or Deep Snow/Mud modes. The vehicle computer can adjust the amount of power sent to each drive axle and also control how the engine/transmission function to provide the ideal application of force for the given driving conditions. AWD is very user friendly and functions without much driver input. There are limits to how far these systems can be pushed before problems occur. They are ideally suited for on road driving.
Driving four-wheel drive vehicles can be very helpful and fun at times. They can also get you in some challenging situations if not used properly. Take some time and try out each of your four wheel drive options in different operating conditions. Then you will best know what option to use when for your vehicle. While you are out and about stop in and see the experts at Sanderson Auto Repair. We can help you properly maintain your four wheel drive systems and make sure they are ready to go when you need them most.