More specifically, what are the pros/cons of each? Why do some cars use one system over the others and which is “better”?
EDIT: Thanks everyone for the explanations. It’s been very useful.
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Front wheel drive: better traction and control over RWD because the tires are weighted by the motor and are pulling the car rather than pushing. FWD is much less likely to cut the rear end loose in a hard turn or acceleration versus rear wheel drive.
4WD. Switchable from 2WD (rear) to 4WD either electronically through a switch inside the vehicle, or through a transfer case and hubs. Great traction as all four wheels are pushing/pulling. In modern four-wheel drive, the computer will deliver power to the wheels with the most traction. Worst gas mileage as there is an element of “fight” between wheels.
AWD. All four tires are engaged all of the time. In modern vehicles, to improve gas mileage, the computer manages the power going to each wheel, and generally leaves the vehicle in what is essentially 2WD until it detects a loss of traction. It then engages or disengages wheels as needed. This leaves you with a combination of great traction when you need it, but also great gas mileage, plus the convenience of not having to switch in and out of four-wheel drive manually.
Front wheel drive: better traction and control over RWD because the tires are weighted by the motor and are pulling the car rather than pushing. FWD is much less likely to cut the rear end loose in a hard turn or acceleration versus rear wheel drive.
4WD. Switchable from 2WD (rear) to 4WD either electronically through a switch inside the vehicle, or through a transfer case and hubs. Great traction as all four wheels are pushing/pulling. In modern four-wheel drive, the computer will deliver power to the wheels with the most traction. Worst gas mileage as there is an element of “fight” between wheels.
AWD. All four tires are engaged all of the time. In modern vehicles, to improve gas mileage, the computer manages the power going to each wheel, and generally leaves the vehicle in what is essentially 2WD until it detects a loss of traction. It then engages or disengages wheels as needed. This leaves you with a combination of great traction when you need it, but also great gas mileage, plus the convenience of not having to switch in and out of four-wheel drive manually.
It’s not very easy to explain without going in depth but I’ll try my best.
So, a car has four wheels, right? The engine power must be transmitted to the wheels for the car to move, hence why that part is called “transmission”. That part may transmit the engine power to the two front wheels, the two rear wheels or to all wheels at the same time.
Front wheel drive means that the front wheels of the car are being powered by the engine. This setup may provide cost savings and lower weight, they are also good when it comes to stability.
Rear wheel drive means that the rear wheels only are being powered by the engine. This setup may provide cost savings and simplified mechanical components, as well as some sporty feeling, although it might be unstable without stability and throttle control.
4WD and AWD mean basically the same thing: The engine is powering all the four wheels of the vehicle. The difference between the two is AWD is always engaged, and 4WD may be turned on and off at the driver’s desire. This setup usually performs better when offroading and in rough terrain, as well as better acceleration overall, at the expense of adding on weight and complexity.
Overall, it doesn’t really matter which setup you choose if you’re only using your car for day-to-day stuff like normal driving. If you’re going offroading or track racing thought, each choice has more definitive pros and cons.
These are indications of which wheels the engine actually turns. Front and rear wheel drive means that only 2 of the 4 wheels are connected to the engine.
Front wheel drive is usually more efficient in designs where the engine is at the front of the car, keeping all the drive components under the hood and no long shafts connecting the front engine to the back wheels. Brakes also like it because the heavier front end means when you brake and your weight is thrown forward, that’s good for the front wheels’ grip while braking.
Rear wheel drive is typically seen in performance cars where the fact that acceleration causes vehicle weight to shift backwards which helps the drive wheels grip better, but you also see it in trucks which have their heavy cargo or trailers at the rear for the same reason – grip while accelerating.
For AWD and 4WD, you need to understand that not all wheels spin at the same speed. When turning especially, each wheel does need the ability to turn at different speeds. In 2-wheel drive configurations, a device called a differential splits the power between the drive wheels while also allows that speed difference. With that said…
In AWD all wheels get engine power all the time. This is generally good for handling since no wheel(s) getting stuck should make it so you can’t move. However it requires 3 differentials – one to split the power between front and rear wheels, and then one for each set of wheels – or another very creative design. That’s a lot of gears to waste a bit of engine power, and a lot of parts to build the car, so these cars are usually most expensive.
4WD is a selectable mode, but it doesn’t provide that differential that splits power to the front and rear wheels and allowing their speeds to vary. You mostly see it on vehicles designed to off-road a bit, like pickup trucks, which are normally RWD. It’s meant for when you’re in a sticky situation, and spinning tires is expected. It will deliver power to the front wheels, but they must turn at the same rate as the rear wheels and the lack of that differential may force the tires to grind against the ground a bit. That’s fine for driving in the dirt, but not for real roads.
Front wheel drive allows you to package the engine and gearbox tightly together in the front of the car giving better room. Rear wheel drives give better weight distribution and allow you to package the gearbox and other driveline components in the center console so you can have a bigger engine. It also gives better grip during acceleration and gives better handling. 4 wheel drive allow you to connect the front and rear axle together so all wheels turn at the same rate. This is heavier and takes up more room but gives you grip on all four tires. So this is used when there is less grip available such as in mud or on sand. However when going around corners on a hard surface the wheels end up spinning so the handling is terrible. Therefore almost all 4WD systems allow you to disengage this so the vehicle becomes front or rear wheel drive so you can drive it on normal roads. All wheel drive have a looser coupling between the front and rear axle using a center differential or some sort of slip coupling. This gives better handling on road while still allowing power to go to any of the wheels for off road use.
So in general front wheel drives are good for compact cars and vans that need the room and weight. Rear wheel drives are good for road going vehicles with more power and better handling. 4WD is for off road vehicles that might only occasionally go on the road. AWD is best in bad road conditions such as lose gravel or snow.
You should also explain the stuff about how 4WD is engaging 2 ties at the same time and it can be any of the 4 tires. All wheel drive is engaging 1 tier at a time but it can be any of the 4.
In front wheel drive, the power is sent to the front wheels. The wheels pull the car behind them and control the the direction, which means the direction of the power delivered and the intended direction of motion are the same. This means easier handling, which is why many passenger cars use this.
Rear wheel drive delivers power to the rear wheels. It’s better for delivering more power from the wheels to the road. If you loom at a car from the side, when the wheels rotate one way, the rest of the car attempts to rotate the other way (action and reaction). This means the fron of the car tries to lift and the rear to slump. In a rear wheel drive, there’s a lot of weight in front to try to rotate, counteracting this effect. This means more force onto the road, which means better grip. Better grip means more torque and more power can be delivered to the wheels before they slip. But it also means the driven wheels are not the steering wheels, so the direction of motion snd direction of power delivery are not the same. This makes handling more difficult, and why people keep crashing sports cars.
All wheel drive drives both front and rear wheels, so it’s sort of best of both worlds, but adds weight to the car, so it’s mostly used in cars which are already bigger and heavier, so they need the help with power delivery AND handling.
4 wheel drive is like all wheel drive, but with one difference: all previous drives use differentials, which allow wheels to spin at different speeds. But it also means that in rough terrain, you get a situation where one wheel goes off the ground and is free to spin, meaning the other wheel gets no power. That’s how people get stuck in mud on the side of the road. In 4WD, the differentials are locked. All wheels spin exactly the same, and get equal power. This means that no matter the terrain, the wheel touching the ground always delivers power. But this is also really bad for driving on paved roads, as you want wheels to spin differently when turning around corners, and preventing that is actually detrimental to handling and causes excessive tire wear.
So AWD – roads and mild off-roading. 4WD – heavy duty off-roading and heavy lifting.
Front wheel drive means the front two wheels only are connected to the engine to provide traction that moves the vehicle. Rear wheel drive means just the rear wheels. In both 4WD and AWD systems both front and rear axles are connected to the engine at least some of the time. In general the term 4WD is used with pickup trucks, jeeps, and other vehicles made for off road use, and AWD is used for vehicles that are meant for on on road usage. The key difference between 4WD and AWD is the center differential.
Every drive axle has a differential, this is a mechanical device that takes a single input shaft and splits the rotation to two output shafts, which can spin at different speeds (hence the name differntial). Differentials are needed because when turning the inner and outer wheels need to spin at different speeds. The inner wheel travels along a shorter path than the outer so it must spin slower. Not only do the inner and outer wheels travel at different speeds when turning but so do the front and rear axles as a whole. This is where the center differential comes into play, it allows the front and rear drive shafts that run to the front and rear axles to spin at different speeds.
In a 4WD system without a center differential engaging the system on pavement or other high traction surface can result in damage to the driveline as the front and rear drive shafts want to spin at different speeds but cannot. Using 4WD on a low traction surface like snow, dirt, mud, etc means tires can slip rather than spin at different speed. In an AWD system the center differential allows the system to be used in all conditions. Many AWD systems use a slightly more complicated center differential which allows unequal power split between the front and rear axles, which can have benefits for performance or handling.
Front wheel drive is cheaper and lighter, but asking the front wheels to do the acceleration and steering lowered the high end performance. Rear wheel drive is simpler because the front steering wheels do not also need drive shafts and in situations where high torque is needed (towing heavy trailers) a RWD system will be cheaper and more effective than a comprable FWD system. 4WD is cheaper than AWD and often more robust for the same cost which is why its choosen for off road application where speed will be less important than durability. 4WD vehicles are almost always primarily RWD with the option to engage the front wheels when conditions are right. AWD is the most expensive and complicated of all but offers the best on road traction and performance. Modern AWD system can be used to vector torque to certain wheels enabling better traction and stability control system to help people in bad weather or in high performance driving. AWD systems typically have a nominal power that is bias in favor of sending more power to either axle and computer controls in the vehicle will adjust the power split based on driving conditions.
FWD = only the front wheels are driving the car. A driveshaft comes from the engine and connects to the front wheels. This is the lightest & least complex system available, and therefore the cheapest. There is no long driveshaft or axles that run to the back of the car. If driving in the snow, this system will have significantly better traction than a RWD system because the entire engine bay is over the driven wheels, unlike a RWD system which usually has only an empty trunk as “weight”. Handling isn’t as good as a RWD system.
RWD = the front wheels steer, and the rear wheels drive the car. Superior handling at high speeds and through turns is the only advantage. This system is slightly more expensive and complex since there’s a driveshaft and multiple clutches connecting the engine at the front of the car, to the driven wheels at the back. Plus there is a usually a “hump” in between the seats to account for the driveshaft. This system usually has poor snow traction since there is no weight on the driven wheels. It also weighs more and has more pieces.
AWD = This implies that all four wheels, or both axles are being powered simultaneously. Full-time = this happens constantly. Part-time = one axle is powered only until “slip” is detected or the system requires the other axle to engage, in which case it engages. The part-time system saves gas by only driving one set of wheels until the full system is required (for example if the rear wheels spin in the mud). This system generally has poor fuel economy compared to a FWD system (which has the best), or a RWD system (which has 2nd best). This is because there are two axles, at least 2 drive shafts, and multiple clutches that power this system. Unlike a FWD system which has a driveshaft usually less than 3 feet, this system might have 30 feet of connections. As a result, energy is lost at each connection point. This is why a FWD Toyota Corolla will always have better fuel economy than an AWD Subaru Impreza. The advantage of this system is better handling in poor weather. So if you constantly drive in the snow or mud, this system is superior for safety. All four wheels can receive power (either full-time or part-time), meaning it’s less likely for the car to get “stuck”. An AWD uses open or limited slip differentials so that each wheel & each axle can turn at different rates. In an AWD system, the engine is connected to a transmission, and there are often multiple drive shafts to connect to each axle. There are at least 3 differentials on all AWD systems (a front, a rear & a center).
AWD & 4WD are sometimes interchangeable terms in 2023. However 4WD may sometimes refer to a 4×4 system on trucks and jeeps. This is a very rudimentary system that DOESN’T employ a center differential. This means that unlike a Subaru, both axles spin at the same rate. This type of system was created years ago, and gives 50/50 torque to each axle so that the truck or jeep can power out of the mud of the snow. The main limitation of this system is that it’s not designed to be driven on roads with traction. Since each axle turns at the same rate, the rear right wheel, the front left wheel, and the rear left wheel try to turn at the same rate. If you try to make a turn, you’ll notice that the truck or jeep starts to “lurch” or “squeal” since when a normal car makes a turn, all four wheels are moving at separate speeds. Most trucks in 2023 still have this system, and even in the owner’s manual, it’s recommended to only turn it on when you’re in deep snow or mud, and NOT drive it on regular road. In this type of system, you would drive the truck or jeep into deep mud or snow, and then you’d flip a switch and “engage” the 4×4 system. The system would be “locked” with 50% of the torque going to the front axle and 50% going to the rear axle. Each axle would spin at the same rate to power the car out of the mud or snow. Once you get back onto smoother roads, you would disengage the system and it would go back to a two-wheel drive system.
I’ve driven FWD cars, I’ve had 2 pickup trucks with a 4×4 locking system, and I’ve owned 4 Subarus which have an AWD system. The Subaru’s are the safest and require zero input in order for their system to work. The trucks require constantly turning the 4×4 system on and off.
In 2023, the biggest success of traction depends on the type of tires you are using. Even a sophisticated AWD system won’t help you much if you are using (for example) worn summer tires in deep snow. If the are you live in routinely gets snow or gets below 5 degrees celsius, you should use winter tires for months with snow and low temperature.
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