Photo: pixabay/edited by autoevolution
As some discussions with different persons from a few days ago revealed to me, there are still people out there driving and having absolute no idea why does their econobox comes with a front-wheel drive setup and not a rear one, or giving me a strange look telling them all-wheel-drive is not the same with four-wheel-drive (or 4x4).
So, as a result I decided to write this short guide on different powertrain layouts. Those already initiated in the automotive techniques will most probably find it boring and redundant, but keep in mind I wrote this for the absolute novice who just ended up with a driving license having no idea about the world he/she is stepping into.
As a side note before we begin, I’d like to tell you all of the following statements are not necessarily always true. Modern technology, hybrid powertrains and advanced materials can affect these layouts enough to put some of them on par regarding the benefits you get from each type.
This combo is also good during winter, because all the weight of the engine pushes right down on the power wheels, which get more grip on snowy roads this way.
Ultimately, since the drivetrain is short, power loss is smaller and you get better efficiency, which translates into a smaller fuel consumption. Maintenance is somewhat cheaper too.
Turning radius tends to get bigger because the wheels’ steering angle is smaller “thanks” to all the mechanical parts crowded in one place. And acceleration will be poorer too because the car’s center of mass will move towards the rear where no power is transmitted, thus the front wheels will lose grip.
Along with poor acceleration comes torque steering, which is the car’s tendency to pull left or right when accelerating hard because FWD cars with transversally mounted engines have different sized prop-shafts to turn each wheel.
The last nail in the coffin comes from the understeering characteristic of this type of cars. Understeering is a car’s tendency not to go right or left properly when you turn the wheel. This happens because at high speed, all that concentrated mass’ inertia at the front is too big to be handled by the front wheels. They can’t simply “grab” the weight and point it towards the desired direction of travel, lose grip and take the corner very wide.
Since the wheels at the front have less weight to deal with and the engine bay isn’t that cluttered, they can turn more and can put the car on the desired line faster.
It basically outcomes the FWD’s problems: smaller turning radius, better cornering, acceleration and no torque steering since the differential is right in the middle of the two wheels and both drive shafts are the same.
Then you get less passenger and luggage space due to the aforementioned tunnel and the differential at the back on which the trunk is located.
Also, since there’s less weight pushing on the driving wheels, they tend to lose grip faster on snowy roads, thus making it harder to accelerate. Finally, the same weight imbalance will make the rear end stick out and make the car oversteer.
While oversteering could be dangerous, with the car tending to spin out of control and crash, it could also be fun, since applying the right amount of countersteer and acceleration turns your slide into a controllable drift.
Some vehicles can be all-time 4x4s meaning all wheels are being powered at all times with the same amount of power being sent to each axle.
Acceleration also gets a big plus for the same reason.
It may be able to snap your neck during acceleration, but it won’t go too high with the numbers on the speedometer. These things usually make the most out of torque, not horsepower, so top speed isn’t that great and taking them out of the “comfort zone” will rocket the fuel consumption sky high.
When you suddenly need power, in situations like hard acceleration or having to go up a steep climb, the system automatically switches the distribution up to 50-50 on the two axles, turning your FWD or RWD into a 4WD for a short period of time.
Of course, trying to be best of two word means you have to make compromises. Although not needed all the time, your car gets to carry the extra weight all the time. The AWD system is also more complex which in result turns it more expensive to own and repair.
As a side note before we begin, I’d like to tell you all of the following statements are not necessarily always true. Modern technology, hybrid powertrains and advanced materials can affect these layouts enough to put some of them on par regarding the benefits you get from each type.
FRONT-WHEEL DRIVE (FWD)
This is the most common layout you can find today. The engine/gearbox combo is located at the front, usually transversally to the car, and all the power is sent to the front wheels, as the name suggests.Why it’s good
First of all, it is an economic solution, reason why econoboxes are called that way. There is no need to send the power to the rear axle so you don’t get the extra driveshaft running along the center of the car. Reason why you also don’t get the big transmission tunnel and a rear differential which usually eats up passenger and baggage space.This combo is also good during winter, because all the weight of the engine pushes right down on the power wheels, which get more grip on snowy roads this way.
Ultimately, since the drivetrain is short, power loss is smaller and you get better efficiency, which translates into a smaller fuel consumption. Maintenance is somewhat cheaper too.
Why it’s bad
Well, first of all, those front wheels are put through a lot of stress as they need to put the power down, steer the car and dampen the road irregularities. Combine this with the unbalanced weight of the car and you end up with poor maneuverability.Turning radius tends to get bigger because the wheels’ steering angle is smaller “thanks” to all the mechanical parts crowded in one place. And acceleration will be poorer too because the car’s center of mass will move towards the rear where no power is transmitted, thus the front wheels will lose grip.
Along with poor acceleration comes torque steering, which is the car’s tendency to pull left or right when accelerating hard because FWD cars with transversally mounted engines have different sized prop-shafts to turn each wheel.
The last nail in the coffin comes from the understeering characteristic of this type of cars. Understeering is a car’s tendency not to go right or left properly when you turn the wheel. This happens because at high speed, all that concentrated mass’ inertia at the front is too big to be handled by the front wheels. They can’t simply “grab” the weight and point it towards the desired direction of travel, lose grip and take the corner very wide.
REAR-WHEEL DRIVE (RWD)
As the name implies, the engine sits at the front and the power is sent to the rear wheels via a driveshaft and a differential right in the middle of the axle. It is the classic layout and is usually used on sportscars as well as luxury ones.Why it’s good
First of all, it does let engineers play with the weight distribution, since some cars also have the transmission/differential combo placed at the back, a combination known as a “transaxle”.Since the wheels at the front have less weight to deal with and the engine bay isn’t that cluttered, they can turn more and can put the car on the desired line faster.
It basically outcomes the FWD’s problems: smaller turning radius, better cornering, acceleration and no torque steering since the differential is right in the middle of the two wheels and both drive shafts are the same.
Why it’s bad
More weight is added, since you get a driveshaft and the transmission tunnel along the car’s length. More weight and more linkages means power loss, lower efficiency and higher fuel consumption. If that’s not enough, the extra stuff adds more to the final cost of the car.Then you get less passenger and luggage space due to the aforementioned tunnel and the differential at the back on which the trunk is located.
Also, since there’s less weight pushing on the driving wheels, they tend to lose grip faster on snowy roads, thus making it harder to accelerate. Finally, the same weight imbalance will make the rear end stick out and make the car oversteer.
While oversteering could be dangerous, with the car tending to spin out of control and crash, it could also be fun, since applying the right amount of countersteer and acceleration turns your slide into a controllable drift.
FOUR-WHEEL DRIVE (4WD or 4x4)
This system is usually used by hardcore off-roading machines needing maximum traction all the time. Most of them will work as RWDs and thanks to a transfer case power is then split to the front wheels as well.Some vehicles can be all-time 4x4s meaning all wheels are being powered at all times with the same amount of power being sent to each axle.
Why it’s good
To put it straight, with 4x4 cars, slippery roads and off-roading won’t be a problem anymore as you get to use all the available power without the wheels getting overwhelmed by it and slip.Acceleration also gets a big plus for the same reason.
Why it’s bad
Weight plays a big role here since you need two sets of axles with two differentials and the extra transfer case. The system’s complexity also results in power losses, decreased efficiency and increased fuel consumption.It may be able to snap your neck during acceleration, but it won’t go too high with the numbers on the speedometer. These things usually make the most out of torque, not horsepower, so top speed isn’t that great and taking them out of the “comfort zone” will rocket the fuel consumption sky high.
ALL-WHEEL DRIVE (AWD)
Generally, the AWD system powers all the wheels, but NOT always. All four wheels get to be powered at the same time only when certain situations require that; like a slippery road or rough terrain. It’s found both on off-roaders and certain performance-oriented machines.Why it’s good and bad
The AWD is trying to be the best out of the two worlds. When cruising or going at low speeds, the power ill take the shortest route from the engine to the wheels to minimize losses and increase efficiency.When you suddenly need power, in situations like hard acceleration or having to go up a steep climb, the system automatically switches the distribution up to 50-50 on the two axles, turning your FWD or RWD into a 4WD for a short period of time.
Of course, trying to be best of two word means you have to make compromises. Although not needed all the time, your car gets to carry the extra weight all the time. The AWD system is also more complex which in result turns it more expensive to own and repair.
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