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All wheel drive explained

Why is it important to know how your all wheel drive works? First, it may appear that your all wheeldrive system is not meant to be used on-road. For example, part-time all wheel drive cannot be used innon-slippery conditions - you'll have to drive this car in rear-wheel drive mode, even when it is raining

or snowing - in the weather conditions where all wheel drive might be needed. Second, depending onthe type o all wheel drive, your car behaves di erently when driving and cornering in slipperyconditions. !ou might want to know what to expect.

"on't get con used by the abbreviations the manu acturers use# $%W"$ is not necessary a ull-time allwheel drive, $&W"$ is not ust or o -road vehicles. (here is a do)en o brands the car manu acturersare using to distinguish their our-wheel drive vehicles - $*uattro$, $&motion$, and so on. +one o theseactually represent the type o all wheel drive system used on the particular vehicle.

n act, ust our types o all wheel drive systems exist#

art-time all wheel drive Full-time all wheel drive

%utomatic all wheel drive

Selectable all wheel drive

+ote# n this web site, when we describe details o the all wheel drive system used on a particularvehicle, we use the de initions that are listed here.

Part-time all wheel drive

(his is a $temporary$ all wheel drive system. n normal driving conditions, ust one axle /the rear axlenormally0 is driven. n slippery conditions, another axle is engaged by the driver, whether by a lever or a

button. (his type o all wheel drive does not have a center di erential - when all wheel drive is engaged,the ront and rear drivesha ts are mechanically connected and rotate at the same speed.

When a vehicle is turning, the ront wheels travel greater distance than the rear wheels. 1ecause the part-time all wheel drive system does not have a center di erential, the ront wheels cannot go asterthan the rear wheels. (his type o all wheel drive cannot be used on pavement. (urning on pavement/even on a wet pavement0 with all wheel drive engaged causes transmission windup and increases thechances o the transmission breakdown. When all wheel drive is engaged, the vehicle heavilyundersteers and this can lead to an accident.

(he all wheel drive mode shoud only be used on sur aces with low traction /mud, snow, ice, sand0, orshort periods, and at low speeds. n these conditions the transmission windup is eliminated by slippingo the wheels.

+ote# $ art-time &wd$ mode o the 2eep 3herokee's Selec(rac transmission means $locking o centerdi erential$. 2eep's Selec(rac is a selectable all wheel drive system.

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Full-time all wheel drive

(his is a permanent all wheel drive or permanently engaged all wheel drive system. %ll wheels are powered at all times. (he vehicles with ull-time all wheel drive are e*uipped with a center di erentialthat lets all wheels travel di erent distances while turning. (his type o all wheel drive can be used both

on and o road. n slippery conditions, the center di erential can be locked, whether manually orautomatically, depending on the vehicle.

When a manual center di erential lock /available on o -road vehicles and some S45s0 is engaged, thetransmission's behavior is similar to part-time all wheel drive , i.e. the ront and rear drivesha ts rotate atthe same speed. (he use o ull-time all wheel drive with locked center di erential is limited to sur aceswith low traction.

n case o an automatic lock, a (orsen di erential, viscous coupling , multi-plate hydraulic clutch, orsimilar traction device is used in con unction with the center di erential. When a wheel slip occurs /onedrivesha t rotates aster than the other0 the device locks the center di erential and the tor*ue is

trans erred rom the axle that slips to the other axle that has traction. %s soon as the wheel slip iseliminated, the device unlocks.

Some vehicles /6and 7over "iscovery , pre-x"rive 18W 9:0 do not have a locking centerdi erential, but are e*uipped with an electronic traction control system /known as ;lectronic"i erential 6ock - ;"60 on all our wheels. (his electronic system detects slipping wheels by reading%1S sensors, then it applies brakes to the slipping wheels and the tor*ue gets trans erred to the wheelsthat have traction. While it per orms well on slippery roads, the system cannot compete with a realmechanically locking di erential when driving o -road.

Automatic all wheel drive

(his is an $on-demand$ all wheel drive system. 4nder normal driving conditions, only one axle is powered. When wheel slipping occurs /the driving drivesha t rotates aster than the driven drivesha t0, amultiplate hydraulic clutch, viscous coupling, or other similar traction device locks and engages anotheraxle. (he tor*ue gets trans erred to another axle. %s soon as the di erence in the ront and rear axlespeeds is eliminated, the device unlocks and the vehicle goes back to the two-wheel drive mode.

(he di erence between the traction devices that are used in ull-time all wheel drive and automatic allwheel drive systems is that the device used in automatic all wheel drive system replaces the centerdi erential.

%dvanced electronically controlled all wheel drive systems can be pro-active and lock the tractiondevice even be ore wheels start to slip - the need o all wheel drive is determined in real-time, based onthe in ormation that is collected rom di erent sensors /i.e. g- orce sensor, accelerator pedal position,etc.0.

Some vehicles let the driver to lock the multiplate hydraulic clutch manually when the driver eels thathe needs all wheel drive engaged permanently and be ore wheels start to slip. For example, in +issan 9-

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(rail, this is accomplished by pressing a button on the dashboard console. n Subaru 6egacy, the clutchis locked when the automatic transmission gear shi t lever is at the position $wd mode, &wd mode with automatic distribution o tor*ue via viscouscoupling /acts like the ull-time all wheel drive 0, &wd with locked di erential /acts like the part-time allwheel drive0 and &wd with low gearing /low range part-time all wheel drive0.

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BMW's xDrive all wheel drive system

ro-active automatic all wheel drive system. (he rear wheels are powered at all times. (or*ue can betrans erred to the ront wheels via electronically controlled multi plate clutch that is located in thetrans er case. 9"rive does not have a center di erential. 8ost o the time, the clutch is partially lockedand power is trans erred to both axles in proportion &@AB@ ront to rear. (he power distribution can bealtered to continuously variable levels / rom :@A:@ to @A

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