Most commonly, AWD vehicles send more power to the front wheels often called front-wheel biased , while 4WD systems send more to the rear ones rear-wheel biased. Many AWD systems run almost entirely in front-wheel drive until they react to situations, such as slipping or acceleration, whereupon they send some power to the back. Older systems could take time to react, and you sometimes got stuck before they could help you out, but most today are far more sophisticated and almost instantaneous. Some even measure speed, braking, steering angle and ambient temperature to predict slippage, and distribute power proactively as necessary.
All of this happens automatically and with no action from the driver. Some systems also have torque vectoring, which can further distribute that power between the left and right wheels. This is more often found on performance vehicles, where providing extra torque to an outside rear wheel can help nudge the front end tighter into a curve for better handling. Most only work at low speeds, and are meant for such situations as getting out of a snowy driveway. So how do you know what type of system your AWD vehicle has?
Most AWD systems are meant for slippery conditions or mild cottage trails, not for heavier-duty off-roading. A true 4WD system, such as that found on pickup trucks or sport-utilities like the Jeep Wrangler, runs in rear-wheel drive until you engage 4WD, usually with a dial or a lever.
This is where you have to be careful, because using it improperly could potentially damage it. Under almost all conditions, you keep it in rear-wheel drive only, which is the 2WD setting on the dial. All-wheel drive systems differentiate themselves by providing power to all four wheels either all the time or automatically.
A differential allows for varying torque distribution and speeds. In the front and rear axles, the differential allows for different wheel speeds, comparing left to right, when turning.
In an AWD vehicle, the center differential is critical, because it allows for different wheel speeds, front to rear, when turning. In addition to the always-on 4WD system, some AWD vehicles may also include options for locking up either the front, rear, or center differentials, depending on traction requirements.
These may be automatic, based on wheel slip, or manual, based on driver demand. However, RWD provides less traction on slippery roads. These days, most high-end cars offer all-wheel drive either standard or as an option. All body-on-frame pickups and SUVs are offered with optional four-wheel drive. As the name implies, all-wheel drive AWD can feed power to each corner. Depending on the system designs vary , AWD can provide maximum forward traction during acceleration.
It is especially helpful in sloppy road conditions and when driving over moderate off-road terrain. It can help get you going and keep you moving through mud, sand, and other loose surfaces.
Most AWD systems deliver power primarily to one set of wheels, front or rear. When slippage is detected at one axle, power is diverted to the other axle, in hopes of finding more traction there. Sophisticated clutch systems and, more recently, the use of electric motors allow for precise control over the power sent to each wheel. High-performance vehicles with AWD often send power to each corner at all times but can alter the balance among the wheels depending on conditions or drive mode to change the performance characteristics of the car.
Not all AWD systems are equal. Many other systems fitted to front-wheel-drive vehicles operate with percent of the power normally going to the front wheels; the rear wheels then receive power only when the front wheels start slipping, or proactively based on the vehicle conditions or drive mode selection.
This transfer of power is traditionally achieved with a mechanical driveshaft running the length of the car, but electric vehicles and some hybrids utilize individual motors at each axle with no physical connection between them. AWD systems are especially helpful in rapidly changing conditions or when driving on a road with intermittent snow and ice.
It is commonly used for car-based SUVs, as well as certain cars and minivans. Although four-wheel drive 4WD and AWD are designations that are often used interchangeably in advertising and sales literature, there is a difference.
Generally, 4WD is optimized for severe off-road driving situations such as climbing over boulders, fording deep water, and tackling steep hills with loose, low-traction surfaces. Most 4WD systems use a heavy-duty transfer case with a high and a low gear range, the latter used to increase torque at the wheels for low-speed climbing. Some have differentials which allow left and right wheels and front and rear axles to turn at different speeds that can be locked for maximum traction.
Modern 4WD systems are full-time, which means they stay engaged; automatic, where the vehicle automatically switches between two- and four-wheel-drive mode; or part-time, which requires the driver to manually shift between two- and four-wheel drive.
Aside from serious off-road enthusiasts, most drivers never come close to needing the capability that 4WD systems provide over and above AWD systems. Photo: Ford Photo: Ford.
0コメント