AWD and 4WD are essentially the same, I’ll address that first. All-Wheel-Drive generally applies to full-time, one front and one rear driven wheel under slippery conditions. They either use a transaxle for the front axle-shafts and a rear driveshaft powering the rear axle, but there are systems that use a regular longitudinally mounted transmission coupled to a transfer case which has transfers engine power to a front driveshaft to the front axle and a rear driveshaft to the rear axle. And yes, I said ONE front and ONE rear wheel. Generally this is what happens on slick surfaces if the front and rear differentials in the front and rear axles are the standard “open” type. This means the front and/or rear axle differential have not been equipped with locking or limited slip devices. True, it is possible for all four wheels to be driven, but generally this causes some very hairy handling conditions when attempting to turn the vehicle on ice because all four tires will usually be turning at the same rate and therfore will tend to keep the vehicle going in a strait line when the driver attempts to turn the wheel. For this reason most “All Wheel Drive cars” do not truly drive all four wheels in the worst, slippery conditions. But locking and limited slip differentials can be added on many AWD cars as an option for extreme driving conditions. I’m sure there are also computer controlled AWD systems that drive all four wheels but avoid poor handling on ice by altering power output to each wheel as needed according to wheel speed sensors coupled with lateral acceleration meters, denoting the vehicle is turning a corner. Generally, the typical AWD system is designed with “slip” in the systems so they can be driven on dry pavement without causing adverse handling or excessive drivetrain stresses. Four-Wheel-Drive is essentially the same as AWD, but 4WD was a term used on pickup trucks. These trucks had a heavy transfer case behind the transmission which the driver could manipulate, usually by a shift lever, that would allow the power to be applied to the front driveshaft — to the front axle (which, if out of 4WD, was generally free-wheeling or unpowered, under normal driving conditions.) These transfer cases usually had heavy gears inside the heavy, cast-iron transfer case. If the vehicle had no locking or limited slip differential in the axles, once both the front and rear axle were driven, they still only had two wheels, (one front and one rear wheel) driving on ice, but they could still handle slipperier conditions than just a rear wheel drive truck which also had no locker or limited slip differential in the axle. Also, locking axle differentials and limited slip differentials could be installed in the axles of the 4WD truck, thus making these vehicles true “Four wheel drive” vehicles. But handling on ice with all four tires driving was dicey here, too. These 4wd systems are not to be driven on dry pavement when locked into 4WD mode since there is not “slip” built into the system, and the front and rear driveshafts locked together will cause poor turning radius and excessive front wheel hop when turning tight corners. Finally there was a revolutionary fulltime 4WD system, installed exclusively in full-size Jeep vehicles from 1973-1979 which was called “Quadratrac.” It was a lightweight, aluminum transfercase which did not use heavy gears but a sturdy drive chain and a limited slip differential inside the transfer case. This limited slip differential in the transfer case also allowed one front and one rear wheel to drive in slippery conditions like the locked-in, earlier gear-driven 4WD systems. But unlike the gear driven systems, this system could also be driven readily and easily on dry pavement due to the limited slip differential. This attribute was excellent for driving on patchy roads where you could encounter dry pavement, snow, and ice all within a few yards or miles. The cool part about these systems was the limited slip differential in the case “sensed” slip from the front or rear driveshaft and sort of locked the front and rerar driveshaft together so if the front was slipping and the rear had traction, it would drive and vice-versa. This system was simple and soon obsolete. Now these limited slip, fulltime 4wd systems are every where, and generally this type of system could be called an AWD system though they were never offered on Jeeps with locking or limited slip devices in the axles. There was also a locking feature in the Quadratrac system that would mechanically lock the front and rear driveshafts together for use in very slippery conditions, and this feature was also not to be used on dry pavement or it would cause the same adverse handling effects and drivetrain wear. Front Wheel Drive — This is a good idea from one point of view only, the weight of the engine and transmission is over the driven wheels and so the tractive force is better, however, aside from “pushing” or understeering, which any vehicle can be made to do under the wrong conditions, it also adds complications to the front end due to the need for constant-velocity U-joints included in the front axleshafts to allow steering of the front, driven, wheels. Also these cars can get light in the rear end since, under extreme braking, even more of the vehicle’s weight is transfered to the vehicle’s front wheels which can lighten the rear end if driven on ice, and down hill, this can allow you to see the rear end it come around on you. These systems also suffer from “torque-steer” when the throttle is punched, which means the engine’s torque effects the steering in ways you won’t like. Rear Wheel Drive. — this is a great idea if you have alot of power to put down to the ground in a heavy vehicle and you wish to go in a relatively strait line very quickly. Yes, the rear-end can easily break loose under hard acceleration in a corner, but then that can happen in any vehicle under the wrong conditions, even AWD and 4WD, trust me. Usually RWD in an unloaded two seater pickup truck is the worst performer in poor traction conditions, allowing more undesired wheels spin on only slightly slippery condidtions. However, doing a few simple things can help this problem — adding weight over the rear end, and if you drive in icy conditions often, get snow and ice tires like Blizzaks or have your mud and snow tires studded. You can also have a locking or limited slip differential installed to allow both rear wheels to be driven as opposed to only one on ice, if the truck is equipped with a standard “open” differential. Trucks aside, most older cars were built with RWD systems, and got around fine in most conditions, even ice. The usual axiom applies though, drive smart — think ahead. doing 90mph through a patch of ice and turning in almost any car will cause an accident due to loss of driver control because of loss of traction. Be that as it may, horrendous amounts of power can still be effectively delivered to the ground through a front-engined, RWD car, better suspension and transferring weight to the rear of the car is beneficial for this. These days, with computer controlled traction-control systems in many of the cars available, there is no “One drive system is better than the others” in all categories, everything is a compromise — generally you compromise money for traction and stability. Also, alot of this depends on the vehicle’s weight distribution, center of gravity, and how it shifts its weight in certain handling situations. It also can depend on the conditions under which the driving will happen and the intended uses of the vehicle. ANY car, truck, or SUV can be made to handle at various speeds on ice, or in mud, it just may take more customization and/or money to accomplish this depending on what you wish to do, where you wish to go, how you wish to get there, and what you are driving. Category:Home › Autos • Will the trend toward economy-size car models lead Americans back to buying smaller-sized cars? — part 2 • You should never use after-market auto parts when maintaining or fixing your car • Should a new car be rust-proofed? • Will the trend toward economy-size car models lead Americans back to buying smaller-sized cars? — part 1 • Five safest cars in North America • How to ensure your teen drives safely after getting a license • Pros and cons of using an hydrogen generator for your car • DIY automotive maintenance: How to change the oil
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