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
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.
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