|Last changed: 2013/05/16 05:52 / History||Edit|
Understanding all wheel drive systems(Edit)
Why is it important to know how your all wheel drive works? First, it may appear that your all wheel drive system is not meant to be used on-road. For example, part-time all wheel drive cannot be used in non-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 on the type of all wheel drive, your car behaves differently when driving and cornering in slippery conditions. You might want to know what to expect.
Don't get confused by the abbreviations the manufacturers use: "AWD" is not necessary a full-time all wheel drive, "4WD" is not just for off-road vehicles. There is a dozen of brands the car manufacturers are using to distinguish their four-wheel drive vehicles - "quattro", "4motion", and so on. None of these actually represent the type of all wheel drive system used on the particular vehicle.
In fact, just four types of all wheel drive systems exist:
Note: On this web site, when we describe details of the all wheel drive system used on a particular vehicle, we use the definitions that are listed here.
part-time all wheel drive(Edit)
This is a "temporary" all wheel drive system. In normal driving conditions, just one axle (the rear axle normally) is driven. In slippery conditions, another axle is engaged by the driver, whether by a lever or a button. This type of all wheel drive does not have a center differential - when all wheel drive is engaged, the front and rear driveshafts are mechanically connected and rotate at the same speed.
When a vehicle is turning, the front wheels travel greater distance than the rear wheels.
Because the part-time all wheel drive system does not have a center differential, the front wheels cannot go faster than the rear wheels. This type of all wheel drive cannot be used on pavement. Turning on pavement (even on a wet pavement) with all wheel drive engaged causes transmission windup and increases the chances of the transmission breakdown. When all wheel drive is engaged, the vehicle heavily understeers and this can lead to an accident.
The all wheel drive mode should only be used on surfaces with low traction (mud, snow, ice, sand), for short periods, and at low speeds. In these conditions the transmission windup is eliminated by slipping of the wheels.
full-time all wheel drive(Edit)
This is a permanent all wheel drive or permanently engaged all wheel drive system. All wheels are powered at all times. The vehicles with full-time all wheel drive are equipped with a center differential that lets all wheels travel different distances while turning. This type of all wheel drive can be used both on and off road. In slippery conditions, the center differential can be locked, whether manually or automatically, depending on the vehicle.
When a manual center differential lock (available on off-road vehicles and some SUVs) is engaged, the transmission's behavior is similar to part-time all wheel drive, i.e. the front and rear driveshafts rotate at the same speed. The use of full-time all wheel drive with locked center differential is limited to surfaces with low traction.
In case of an automatic lock, a Torsen differential, viscous coupling, multi-plate hydraulic clutch, or similar traction device is used in conjunction with the center differential. When a wheel slip occurs (one driveshaft rotates faster than the other) the device locks the center differential and the torque is transferred from the axle that slips to the other axle that has traction. As soon as the wheel slip is eliminated, the device unlocks.
Some vehicles (Land Raver Discovery II, pre-xDrive BMW X5) do not have a locking center differential, but are equipped with an electronic traction control system (known as Electronic Differential Lock - EDL) on all four wheels. This electronic system detects slipping wheels by reading ABS sensors, then it applies brakes to the slipping wheels and the torque gets transferred to the wheels that have traction. While it performs well on slippery roads, the system cannot compete with a real mechanically locking differential when driving off-road.
automatic all wheel drive(Edit)
This is an "on-demand" all wheel drive system. Under normal driving conditions, only one axle is powered. When wheel slipping occurs (the driving driveshaft rotates faster than the driven driveshaft), a multiplate hydraulic clutch, viscous coupling, or other similar traction device locks and engages another axle. The torque gets transferred to another axle. As soon as the difference in the front and rear axle speeds is eliminated, the device unlocks and the vehicle goes back to the two-wheel drive mode.
The difference between the traction devices that are used in full-time all wheel drive and automatic all wheel drive systems is that the device used in automatic all wheel drive system replaces the center differential.
Advanced electronically controlled all wheel drive systems can be proactive and lock the traction device even before wheels start to slip - the need of all wheel drive is determined in real-time, based on the information that is collected from different sensors (i.e. g-force sensor, accelerator pedal position, etc.).
Some vehicles let the driver to lock the multiplate hydraulic clutch manually when the driver feels that he needs all wheel drive engaged permanently and before wheels start to slip. For example, in Nissan X-Trail, this is accomplished by pressing a button on the dashboard console. In Subaru Legacy, the clutch is locked when the automatic transmission gear shift lever is at the position "1".
selectable all wheel drive(Edit)
In this category fall Mitsubishi Pajero(Montero) with its Super Select transmission, Jeep Grand Cherokee with SelecTrac transmission, and a few other off-road vehicles. Mitsubishi, for example, has in fact a full-time all wheel drive transmission with two wheel drive possibility. In Mitsubishi, the driver can choose between the 2wd mode, 4wd mode with automatic distribution of torque via viscous coupling (acts like the full-time all wheel drive ), 4wd with locked differential (acts like the part-time all wheel drive) and 4wd with low gearing (low range part-time all wheel drive).
|Last changed: 2013/05/16 05:52 / History||Edit|
I am looking for an all wheel drive or a 4 wheel drive that can be towed behind an RV. Is it a reality or should I stick to a FWD.
Haldex or similar electronically-controlled automatic all wheel drive can be towed with one axle off the ground. Any manually-engaging part-time all wheel drive can be towed this way too.
I presume an automatic all wheel drive that is actuated using mechanical parts, such as oil pumps in Honda CR-V (...-2012), or any full-time all wheel drive should not be towed that way.
However, regardless of what I said here, please make sure you refer to the user's manual or ask a dealer before buying a vehicle.
Kia Sorento 2007 4WD (with 4X4 auto and low gear )
Car runs perfect but when turning 90 degree at very low speed eg doing parking or turning into garage, can feel the car trans jerking.
please advise what are the problems?
so if I'm not mistaken, xDrive uses a Multi-plate clutch coupling. On the other hand the 4Matic uses a non-locking center differential. Does that mean that the Merc falls under the category of Full-time all wheel drive the BMW under Automatic all wheel drive? Up to how much rear/front torque they can transfer(65/35 or 55/45, etc). Pros and cons, please.
I am looking for a parts page(s) for the transfer box of a 2000 Suzuki Grand Vitara 2,0l with part numbers. Can anyone assist ??
i have a 98 jeep cherokee quad track and the system shows only front two tires illuminated and it sounds funny and drives funny, what do i need to do to fix problem
Reply to George, depend on the region, the US models SI4 have permenant all wheel drive
'permanent all wheel drive' is meaningless nonsense.
The Evoque does not have a center differential
It continues in the path of the Freelander, which started with front wheel drive + viscous coupling clutch powered rear axle.
The computer decides how much/how little clamp load is applied to the clutch coupling.
No clamp/all slip = front wheel drive
Full clamp/no slip = off-road drive [rigid]
variable clamp/variable slip = normal operating mode.
George is right, Evoque has Haldex, similar to Freelander.
Would you please explain us the new Range Rover Evoque system ? which category it falls under ?thanks
It is front wheel drive with a PTO [power take off] system (Haldex 4, I think) for powering the rear differential.
2010 RAV4 falls into this category:
"Advanced electronically controlled all wheel drive systems can be pro-active and lock the traction device even before wheels start to slip - the need of all wheel drive is determined in real-time, based on the information that is collected from different sensors (i.e. g-force sensor, accelerator pedal position, etc.)."
I have Lite ace Noah full time 4WD,how to use in mud road,the gear shifted in L or 2?
tengo un problema con mi carro, al conectar la doble traccion, al momento de que camina el carro se escucha un tromido en el tranfer. mi pregunta es se puede reparar el tranfer o lo tengo que cambiar por otro ?