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.
In: 10
Fwd/rwd have been well covered. Important to note that 4wd/AWD both have a variety of implementations and there’s enough corner cases and abuse by the marketing team to make you bang your head on a wall.
The other important thing is that all of the wheels of a car turn at different speeds because of their different paths as the vehicle turns. Hence why axles have differentials and for the simplest AWD system there’s also a centre differential to allow the front and rear wheels to turn at different combined speeds. This can be improved by limited slip differentials that only allow a certain difference in speeds between them or various other computer controlled cunningness.
A 4wd on the other hand is designed to function in a low traction environments and so can take advantage of that to have the wheels slip instead.
So they have the front and rear propshafts locked together to give more traction. Additionally a 4wd normally has a selectable low range to give more torque at low speeds (plus allow slow speeds without burning a clutch out or overheating an automatic gearbox). For on road usage of a 4wd they’re either selectable 4wd and disconnect the front axle (making them rwd) or are constant 4wd’s and have a centre diff that’s lockable.
Some obnoxious corner cases are disco 2’s which are 4wd’s complete with solid axles, high/low range but didn’t come factory with a linkage to lock the centre diff, various Subaru’s which had hi/lo in a car, Mitsubishi’s super select system for their 4wd’s that gives you rwd, AWD, locked centre diff/4wd and then hi/low.
There’s also some clever systems that normally only drive one axle but when loss of traction is detected engage the other axle.
**RWD:** The engine powers the rear wheels, and the front wheels are free-spinning, like the wheels on a matchbox car. RWD is popular on sports cars because in the right hands, it offers better control than FWD in spirited driving, since the duties of steering and power are completely split between the front and real wheels.
**FWD:** The engine powers the front wheels, and the rear wheels are free-spinning. FWD is popular on ordinary passenger cars because it’s easy to keep under control under normal driving for an unskilled driver – if you encounter conditions where the back end of the car might slide out a little, the front end has a chance to just pull it back in line. Whereas an RWD in novice hands can be prone to spinning around 180° if you give it too much throttle.
**4WD:** The engine powers all 4 wheels equally. This is popular for off-roading because it means you can still make the car move even if 1 or 2 wheels are lifted off of the ground entirely. It’s bad for driving on a clean road though, because the wheels aren’t allowed to turn at slightly different speeds from one another as they would, say, when you’re taking a turn, so it puts strain on the tires and drivetrain.
**AWD:** Power is given to all 4 wheels with allowances for slightly different wheel speeds, and nowadays, sometimes some intelligent control over how much power is sent where and when. It’s kinda a compromise between the other 3 types: Safe for everyday driving, decent for spirited driving, and competent in poor driving conditions like rain, mud and snow/slush.
RWD, this is the OG way that we made cars. The rear wheels push the vehicle, the front wheels steer. Iw a modern context RWD is actually fairly rare, used mostly in very specific kinds of vehicles. These days some sports cars are RWD (it’s considered more fun, and more techinicall to drive). Vehicles that are designed to tow are often RWD as well because of traction related reasons.
FWD, Front wheel drive. Most modern cars are FWD. This is where the front wheels “pull” the car behind them. It’s MUCH easier for a novis to control a FWD car, much harder to get into a bad situation and so on.
It’s worth pointing out. A very skilled driver will have more control in a RWD car, but it’s easier for a less skilled driver to get themselves into trouble. A FWD car is less good for the skilled driver, but much better for the unskilled (read, average) driver.
A FWD car is more technically complicated to build, but also tends to leave more space in the passenger area for passengers.
4WD. Four Wheel Drive. This is a car that can “sometimes” use all 4 wheels to drive the car at the same time. Normally these are RWD vehicles and there’s a switch of button that changes it into 4WD. It’s commonly seen in trucks of off road vehicles. Generally a 4WD vehicle should not be driven at highway speed in 4WD mode, the special 4WD mode is normally engaged after the truck gets stuck. 90% of the time these vehicles operate in normal, 2 wheel drive mode.
AWD. All Wheel Drive. This is a kind of evaluation of 4WD that’s intended to be “on” all of the time. Different manufacturers have different AWD systems and they all act a little differently. Sometimes they are FWD systems that only sometimes can send power to the back wheels, sometimes it’s the opposite, sometimes it’s power to all the wheels all of the time. The important distinction between an AWD car and a 4WD car is that in an AWD vehicle it’s the car that chooses when to engage or disengage it. And the system might sometimes just be turned on all of the time without harming the vehicle or the drivetrain.
AWD is less good for things like offroading, but much better dubious traction situations that the average driver might encounter (such as a little snow in the driveway). AWD is getting more and more common in modern cars and in the past 15 years basically every manufacturer offers some kind of AWD vehicle.
Front wheel vs. rear wheel drive are easy to understand. With front wheel drive (FWD), the motor is connected to the front wheels through the transmission (the gears that control how much power goes from the engine to the wheels). In rear wheel drive (RWD), the motor is connected to the rear wheels through the transmission and then a long drive shaft to the rear axle.
There are pros and cons to each of these. Generally, it is considered that FWD is better for traction and RWD is better for power basically because FWD is pulling you forward and RWD is pushing you forward.
4WD vs. AWD is a little more fuzzy. 4WD came first. Both, as their names imply, allow any or all of the four wheels to be powered by the motor. It’s the implementation of how this is done that differs. To go back to the original versions of these systems that were purely mechanical is the best way to understand them.
In a vehicle, the axles that are powered must have something called a differential. This is a set of gears between the two wheels on that axle that is connected to the drive shaft (the shaft that the motor spins). It is called a differential because the wheels on a vehicle must be able to spin at different speeds. This is because when you go around a curve, the inner tires travel a shorter distance than the outer wheels. A solid axle between each side wouldn’t allow for this and the wheels would wear unevenly or could even break at the hub from the forced use of power on the inner wheel.
Now, with a modern 4WD vehicle, there is also a center differential between the front and rear axles. As you might guess, this is because the front and rear axles need to be able to spin at different speeds for similar reasons that there are differentials between the wheels. Additionally, since both the front and rear axles can receive power from the motor, both of them have differentials. Early 4WD systems lacked a center differential and the front and rear differentials were manually locked. This meant they could only be used off-road and in some cases you had to exit your vehicle to switch modes.
Now, you have a vehicle that can receive power from the motor at any wheel. But you need to add some other stuff to make it practical. It’s no good having a 4WD vehicle if one wheel that loses traction starts spinning and hogging all the power since it will follow the path of least resistance through the differential gears. So you need something called a limited slip differential that can detect a spinning wheel that has lost traction and send power to the ones that haven’t. Also, 4WD uses more energy. It’s just not very efficient to power all four wheels all the time. So, you add in something called a transfer case that lets you control whether the center differential should be engaged with both the front and rear wheels or just one axle. This will let you pick between 2WD (typically the rear wheels) or 4WD. So your car has two shifters – one for the transmission gear and one for the transfer case.
Then we come to all wheel drive (AWD). AWD is a form of 4WD. But it is generally simplified for the operator. Typically, AWD is automated and using an automatic transfer case to select which wheels need power. Sometimes you may have some choice over high or low gears, but usually not. Think of this like the difference between manual transmissions and automatic transmissions. Modern AWD systems can offset the balance of power to the front and rear axles which can allow for better fuel economy when running in 4WD such that there is minimal loss of efficiency and the vehicle can run in 4WD at all times.
Nowadays, fans of either system generally consider manually-operated 4WD systems to be superior for off-road driving and automatic AWD systems to be optimized for on-road driving. High-end 4WD systems feature things like locking differentials which does force the wheels to spin at the same rate which is better in low-traction conditions like snow, sand, or mud. Meanwhile, high-end AWD systems optimize traction for slippery road conditions like rain, snow, or ice. They use forms of traction control to limit the speed of a slipping wheel through the brakes or the differentials to prevent it from losing traction. They can also allow for more output from the motor to the wheels since there are more paths for that power to travel from the motor to the wheels which is why some performance vehicles have AWD systems.
Not many people are talking about RWD. RWD is good for acceleration because the car’s weight will shift to the back when moving forward, applying more force to the back tires. More weight in the back = more friction with the ground = better traction when accelerating, which is why most sports cars will have RWD.
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