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4WD and AWD systems explained

Every manufacturer devises its own scheme to supply power to each wheel yet they are all called 4WD and AWD. Sometime they qualify 4WD with marketspeak like "Active 4WD", "Quadra-Drive", and AWD as "Real Time 4WD". Here are our definitions:

  • Four-wheel drive (4WD) - Generally when manufacturers call a vehicle four-wheel drive, it is a part-time 4WD. These systems are meant only for use in low-traction conditions, such as off-road or on snow or ice
  • All-wheel drive - These area also referred as full-time four-wheel drive. All-wheel-drive systems are designed to function on all types of surfaces, both on- and off-road, and most of them cannot be switched off

So both 4WD and AWD are 4x4s, the only difference is that AWD is 4WD all the time while most 4WD are part-time 4WD where the vehicle runs on 2WD until you switch on 4WD functionality in off-road conditions. The confusion results when a manufacturer sometimes calls an AWD vehicle a 4WD or uses some other term. Why should we bother about figuring out the exact meaning of 4x4. Well first and foremost it will help you if you are in the market for a 4x4. If you are a committed off-roader you would want to invest in an AWD vehicle rather than a part-time 4WD but if you are generally driving in the city and you encounter bad roads only occasionally part-time 4x4 is just great because you don't have to spend extra on fuel and purchase price since an AWD consumes more fuel and is expensive to purchase. If you find this confusing and you need help in purchasing your vehicle please don't hesitate to contact Jim Autos Thailand at jim12cars@gmail.com and we will be glad to give you honest and objective advise even if it means that you won't buy from us.

The best Part-time and full-time four-wheel-drive systems send exactly the right amount of torque to each wheel that won't cause that tire to slip. All 4wd vehicles consume more fuel but with 4WD you have peace of mind because you won't get stuck in snow or mud if ever find yourself in such driving conditions.

Part-time 4WD

Part time 4WD can - as the name suggests - only be used part of the time in four wheel drive, its normal operating mode is two-wheel drive mode. A driver activates 4WD functionality by either moving a lever or pressing a button. When the vehicle is running in a 2WD mode, power is sent to the two rear wheels and the two front wheels just come along. When you shift to four wheel drive mode the front and rear axles are locked together and the front and rear wheels rotate at the same speed thus improving straight-line traction.

A word of caution: part-time systems lack a center differential so the two axles cannot rotate at different speeds in a corner which means that they work great on very slippery road condition such as deep snow, sleet, rain and mud in a Part-time 4WD you must switch to 4WD only in adverse terrain conditions. If you use 4WD functionality  on dry pavement or other hard surfaces, you will feel that you are bound as you turn a corner because both axles are locked together,  high strain is placed on the drive shafts and transmission, eventually causing one of two things to happen: either one of the wheels slips or spins to overcome the stress or the drive-shaft/transmission breaks. In addition to damaging drive system components this also wear down the tire prematurely. This is why part time 4WD’s should never select 4WD on tarmac or a surface with good grip. A central differential within the transmission allows Permanent 4WDs to overcome this problem.

The settings range from 2WD, 4WD High, and 4WD Low. When you turn on 4wd High or 4WD Lo in a part-time 4WD vehicle, the system provides 25% of the torque to each of the four wheels constantly to prevent wheel spin. In newer models, the driver can shift from 2WD to 4WD High while driving, but must come to a complete stop to shift into the 4WD Low setting. 4WD Low should only be engaged in extreme off-road terrain, in low gear system provides improved torque. This type of system is ideal for part-time off-roader who wants the extra security in case he encounters snow or adverse terrain. It requires a transfer case, two driveshafts, two axle differentials and two powered axles but no center differential. 

Some examples: 1942 Willys, Jeep Wrangler, Nissan XTerra, Toyota Tacoma

Pros

  • Cheaper to buy since part-time 4wd system is cheaper to incorporate
  • Higher Fuel Economy than a full time system
  • Less wear and tear since 4WD is only being used sparingly so it will last longer
  • Quieter than 4WD since 2WD creates less noise

Cons

  • Some older systems require driver to come to a full stop before you can switch from 2WD to 4WD. In emergencies you may not always have the time
  • User must decide when the 4WD is warranted, he may switch when it is not needed and damage the transfer case or switch too late and cause the vehicle to slip

Full time or Permanent 4WD

center differentials are a feature of permanent 4wd - <a href=Jim Autos Thailand is home to great 4x4 pickups and SUVs" src="images/auto-articles-images/center-differential.png" width="250" height="153" />As the name suggests in full-time or permanent four-wheel drive you can use 4WD in all terrains from rough terrain to dry pavement. You won't damage the transfer case by using 4WD on dry pavement because the transfer case is fortified with a center differential. As a matter of fact is does not even offer a two-wheel-drive mode all four wheels are powered all the time so no bothering about pressing a button or pressing a lever. In extremely rigorous off-road terrain, a driver can engage low range for improved torque. Permanent 4WD requires a center differential, transfer case, two driveshafts, two axle differentials and two powered axles.

In addition to the 4WD High and 4WD Low settings, you may also have automatic 4WD setting. Full-time 4WD is best for serious off-roaders and for people who regularly drive in slippery or snowy conditions where constant shifting between 2WD and 4WD is inconvenient. Off-roading capability is enhanced when it is combined with axle differential locks.

Some examples are: pre 2006 Mercedes M-Class, Mercedes G500, the whole LandRover line including RangeRover, Toyota Prado, Lexus GX470.

Pros of Permanent 4WD

  • Since 4WD is always on, you don't have to be on lookout for driving conditions that may require you to turn 4WD on
  • You don't have to wait for 4 Wheel Drive to come on as you must do in Automatic system which won't come on until they sense that road conditions warrant its start

Cons

  • More expensive to buy than part-time or automatic 4WD system due to higher costs of heavier duty drive trains
  • More expensive to maintain
  • Higher fuel budget than that of part-time 4WD or automatic 4WD systems
  • It is difficult to turn the vehicle when the center differential is locked

Automatic Four Wheel Drive

This system as system suggest is designed to automatically apply torque when added traction is needed. A good example of Auto 4WD was Control Trac offered by Ford which has three modes, "Auto", "4H(igh)", and "4L(ow)". The "Auto" mode worked transparently to provide four wheel drive when needed and one could push four wheel drive high or low buttons to get more traction. These were offered on Ford SUVs and a viscous coupling was used to control rear wheels in a front wheel driven vehicle.

When "Auto" mode is on, engine power is routed to the rear differential until  electronic sensors detect wheel slippage and activate an electromagnetic clutch in the transfer case and transfer power to the front differential in progression of seconds. The drive will generally keep the vehicle in Auto mode until he encounter extreme terrain as deep snow or sand where he will first turn "4H" mode on. The "4H" mode will lock the center clutch, thus forcing the front and rear drive shafts to turn at the same speed. This is great to navigate sand or snow but if you are on a treacherous road turning again and again, you might end up damaging the drivetrain as center clutch's locking may cause it to bind. In even more extreme circumstances "4L" utilizes additional low gearing to maximize torque, such as towing a boat out of water. Your vehicle will crawl at low gears.

When the vehicle engages 4WD, it manages and transmits the power, as necessary, to both front and rear wheels or just to the wheels with the most traction.  System requires a center differential or some kind of viscous coupling or multi-plate clutch, a transfer case, two driveshafts, two axle differentials and two powered axles.

Pros

  • Less Expensive than permanent 4WD
  • Fuel Efficiency is better than a permanent 4WD system
  • The driver can concentrate on driving, rather than having to decide whether to shift into four-wheel drive. The appeal of these automatic four-wheel-drive vehicles is they monitor and sense their own traction needs as they travel, and automatically adjust how the power is delivered to the wheels.

Cons

  • Some four-wheel-drive experts say that in severe off-road conditions the automatic system can hamper a driver's efforts. For example, because the system is quick and automatic, a driver may find the power shifting from one axle to another abrupt and unsettling while the vehicle attempts to climb over rocks or traverse a gulch. In this case, a driver should set the vehicle in 4WD high or 4WD low, rather than automatic 4WD
  • The automatic system is more complex, typically with more components, than a part-time four-wheel-drive system—and it can be more costly. Trend Automatic four-wheel drive has been a growing offering on vehicles and is available on such models as the Ford Escape and Lexus GX 470

Full-time symmetric AWD

Like full-time 4WD vehicles, the All-wheel drive (AWD) is always sending power to the wheels but unlike 4WD it has no 2WD switch. When the road gets slippery, the AWD system locks the axles and automatically distributes power to all four of the tires. In AWD, 4WD is always on with no need for an on/off switch. Not all AWD are suitable for severe off-roading since it lacks the slow speed torque enhancing low range feature. Only those AWD vehicles that allows the driver to switch to a "low" range gearing setting, like in Toyota 4Runner, can be used for severe off-road use. AWD systems work well in cars and crossover vehicles because they don't add much weight and make them true all-weather vehicles. These cars and crossover vehicles will not be suitable for rough terrains but have added stability and performance in bad weather.

Some examples: 2006 Mercedes M-Class, Audi Quattro, most Subaru, pre 2006 RAV4

Another example of Asymmetric AWD is Automatic asymmetric AWD. Auto AWD is essentially a 2WD car with 2WD handling characteristics but if it encounters bad driving conditions the 4WD kicks in automatically. Don't use it in adverse terrain beyond graded dirt roads. Some examples are 2006 RAV4 and Honda CRV.

Importance of tires

Good quality tires are very important to get the performance from a 4WD vehicle. If you saddle the most advanced 4WD vehicle with regular performance or touring tires, it will be spinning in snow, sleet or rain.

 

 

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