{"id":59190,"date":"2016-09-06T16:24:07","date_gmt":"2016-09-06T21:24:07","guid":{"rendered":"http:\/\/www.artofmanliness.com\/?p=59190"},"modified":"2026-01-08T14:05:54","modified_gmt":"2026-01-08T20:05:54","slug":"gearhead-101-how-part-time-four-wheel-drive-works","status":"publish","type":"post","link":"https:\/\/beta.artofmanliness.com\/skills\/cars\/gearhead-101-how-part-time-four-wheel-drive-works\/","title":{"rendered":"Gearhead 101: How Part-Time Four-Wheel Drive Works"},"content":{"rendered":"


\n<\/a> \"Gearhead<\/a><\/p>\n

Welcome back to Gearhead 101<\/a> — a series on the basics of how cars work for the automotive neophytes out there.<\/p>\n

In our last primer, we discussed the basics of the drivetrain.<\/a> The drivetrain consists of a series of parts that transfer the rotational power produced by the car\u2019s engine to the car\u2019s wheels to make the vehicle move. On most cars, known as two-wheel drive (2WD) cars, the drivetrain transfers power to just two wheels — either the two rear wheels (aka, rear-wheel drive) or the two front wheels (aka front-wheel drive). The movement of these two wheels makes the car move, and the other wheels in turn.<\/p>\n

For most vehicles and in most driving conditions, having just two wheels to move your car is all you need. But when your driving surface is covered in snow or consists of loose sand and gravel, it helps to have all the wheels working together to give your car extra traction and move it over the terrain.<\/p>\n

Enter four-wheel drive. 4WD for short.<\/p>\n

4WD is a type of drivetrain system that, as the name implies, transfers engine power to all four wheels. You see 4WD primarily on trucks and SUVs.<\/p>\n

While you\u2019ve probably heard about 4WD plenty of times, and have a hazy idea of what it does for your car, there\u2019s a good chance you don\u2019t really<\/em> know what it means or how it functions. So in today\u2019s Gearhead 101, we\u2019ll discuss the basics of the 4WD drivetrain system including its benefits and how it works. Our focus for this article will be part-time 4WD because it\u2019s the most common type of 4WD you see out there. We\u2019ll take a look at full-time 4WD next time.<\/p>\n

This can be pretty tricky to understand. So shift your brain to 4WD and let\u2019s get started.<\/p>\n

Why 4WD Comes in Handy: Traction and Power<\/u><\/h3>\n

Before we get into in the specifics of how 4WD works, we need to understand why you\u2019d want the engine powering all four wheels on your car in the first place.<\/p>\n

Traction<\/h3>\n

As we discussed in our Gearhead 101 installment on how a car engine works<\/a>, your engine produces rotational power called torque. The drivetrain (consisting of the transmission, drive shaft, and differential) transfers the engine\u2019s torque to the wheels. Applying torque to the wheels is what makes your car go.<\/p>\n

But to get the torque that\u2019s been transferred to the wheels to actually move the vehicle, your tires need to have traction on the road. Without tire traction, you can apply as much power as you want to your wheels and still go nowhere. Your tires will just spin really fast while your car stays in the same place. This ineffectual tire rotation is called wheel spin<\/em>. You\u2019ve probably experienced wheel spin when trying to get your car unstuck from the snow or mud.<\/a><\/p>\n

So traction is what transforms engine torque into vehicle movement. <\/strong>Yes, this is extremely simplified (we could go into the details of friction\u2019s role in traction, but won\u2019t), but this is a good working definition.<\/p>\n

What 4WD does is increases the chance you can get traction when you\u2019re driving on dicey surfaces. Instead of relying on only two powered wheels for traction (2WD), you\u2019ve got four moving wheels that could possibly hit good traction to keep the car moving.<\/p>\n

For example, let\u2019s say you\u2019re in a rear-driven 2WD vehicle and the rear wheels are in some mud while the front wheels are on dry ground. Because there\u2019s less traction in mud, your rear wheels are likely to just spin and spin while your car stays stationary. It\u2019d be nice to have those front wheels turning, coefficient, because that\u2019s where the traction is.<\/p>\n

That\u2019s what 4WD drive does.<\/p>\n

If you\u2019re in a 4WD vehicle in the same situation, the front wheels that are on the high traction surface will be getting power from the engine and thus be able to propel your car forward.<\/p>\n

4WD traction is a bit more complicated than what I just described above, and we\u2019ll get into some of those nuances below, but the big take-away is that overall increased traction is a major benefit of 4WD. <\/strong><\/p>\n

Power<\/h3>\n

Increased power is another benefit.<\/p>\n

If you\u2019re off-roading up a steep hill or trying to drive over obstacles (like you see in this video<\/a>), you\u2019ll not only want more traction, but also more power sent to your wheels. 4WD can provide it.<\/p>\n

Most 4WD vehicles offer the ability to switch between two ranges of 4WD: Hi or Lo. 4WD-Hi allows your vehicle to power all four wheels while going fast over dicey terrain — like a gravel or a snow-covered road. There\u2019s just less power sent to the wheels.<\/p>\n

If you need more oomph to get over an obstacle, you\u2019ll shift to 4WD-Lo. It gives your wheels more power, but moves the wheels at a slower speed allowing you to conquer that roadblock<\/a> just by driving over it.<\/p>\n

Now that we know why 4WD would come in handy, let\u2019s get into the nitty-gritty of how it works.<\/p>\n

How Part-time 4WD Works<\/u><\/h3>\n

Part-time 4WD is a vehicular system that allows the driver to turn on the 4WD only when needed. On regular driving surfaces without the 4WD engaged, it works just like a rear-driven 2WD vehicle. One of the big benefits of part-time 4WD vehicles is increased fuel economy. Powering all four wheels requires more fuel than just powering two. So you can save some money on gas by only using 4WD when you need it.<\/p>\n

Parts in Part-time 4WD<\/h3>\n

\"How<\/a><\/p>\n

To get the front and rear wheels moving at the same time when 4WD is engaged, part-time 4WD vehicles utilize a transfer case, a separate front drive shaft (in addition to the rear drive shaft), a front and rear differential, and locking hubs. Let\u2019s take a look at these different parts one by one.<\/p>\n

Transfer Case<\/strong><\/p>\n

Without a transfer case, your part-time 4WD vehicle would be a 2WD vehicle.<\/p>\n

The transfer case (also called the T-case) is what splits power from the engine 50\/50 to both the rear and front axles by way of the front and rear drive shafts. The transfer usually sits right behind the transmission in your drivetrain.<\/p>\n

\"How

Red indicates power flow from engine<\/p><\/div>\n

In part-time 4WD vehicles, when 4WD is not engaged, the rear wheels get 100% of the torque from the engine, just like a 2WD vehicle. The power flow in this scenario looks something like this: Power produced by engine goes to the transmission. From there it goes to an output shaft and then to the transfer case. Inside the transfer case, the output shaft is connected to the rear drive shaft. The rear-drive shaft then transmits the torque to the rear differential. The rear differential then spins the wheels causing the car to move.<\/p>\n

Alright. So how does the transfer case transfer power to the front wheels when 4WD is<\/em> engaged?<\/p>\n

Inside the transfer case there\u2019s a series of gears and chains. When 4WD is engaged, the gears mesh together causing a chain to move a gear that\u2019s connected to the front drive shaft. The front drive shaft starts spinning at the same speed as the rear drive shaft and delivers torque to the front differential, which then transmits torque to the front wheels. Boom. 4WD.<\/p>\n

If you\u2019re still a bit confused, this diagram will give you a big picture understanding of the transfer case\u2019s role in delivering engine torque to both the front and rear axles.<\/p>\n

\"How<\/p>\n

In addition to the gears that sync the front and rear drive shafts, transfer cases on most part-time 4WD systems have gear sets that allow the vehicle to shift to a low range when in 4WD. As mentioned earlier, this allows the vehicle to deliver extra torque (power) to both the front and the rear wheels. You get that extra power at the cost of speed, though. When in 4WD Lo, your vehicle can only go up to 15 mph.  <\/strong><\/p>\n

Front Drive Shaft<\/strong><\/p>\n

Because 4WD vehicles also send engine power to the front wheels, it needs a front drive shaft to do so. The front drive shaft connects the transfer case to the front differential. When 4WD is engaged, the transfer case splits the torque 50\/50 between the front and rear drive shafts. The front drive shaft spins at the same speed as the rear drive shaft, transferring torque to the front differential. The front differential then transfers that power to the front wheels by way of the axle shafts.<\/p>\n

Differentials<\/strong><\/p>\n

We talked about differentials in our article on the basics of the drivetrain. On 2WD vehicles, a single<\/em> differential sits in the middle of the front or rear axles (depending on if the car is front- or rear-wheel drive). Power from the drive shaft is transferred through the differential to each wheel, causing them to turn. On a 4WD vehicle, because all four wheels are getting power, it needs two<\/em> differentials — one for the front axle and one for the rear axle.<\/p>\n

But the differential isn\u2019t just a power transmitter. The reason it\u2019s called a \u201cdifferential\u201d is that the gears inside of it allow the wheels on a single axle to move at different <\/em>speeds. You\u2019re probably thinking, \u201cWhen would my wheels move at different speeds?\u201d Well, a common instance is whenever you go around a corner. When you make a right turn, your inside wheel (the right wheel) travels less of a distance than your outside wheel (left wheel). To keep up with the inside wheel, the outside wheel must spin slightly faster. The differential makes this possible. If there was a solid connection between both wheels, the inside tire would need to skid or skip in order for the axle to keep moving. For a video demonstration of how a differential works, watch this:<\/p>\n