4 Wheel Drive Systems: Their differences...

[wolfpack]

ALL WHEEL DRIVE

All wheel drive is exactly what it says, it drives all 4 tires of a vehicle when running all time when the vehicle is in use, the difference is that it does not "LOCK" the front and rear differentials of the vehicle mechanically due to what is called the "Tight Turn Braking Phenomena" this happens when both front and rear differentials are Syncronized in Distribution of torque to both front and rear Diffrentials on a vehicle equipped with a Central Diff Lock which is engaged. When making a u turn on pavement a vehicle with the Central Diff lock engaged, the vehicle will experience a braking effect due to the difference (Hence the term differential lock) in the distance travelled by the front and rear tires, This is similar to why a 2 wheel drive vehicle is not equipped with a locking device to syncronize both left and right tire, continous usage of your diff lock on pavement will eventually lead to severe drive train damage and extreme tire wear.

It is for this reason why All Wheel drive came into being, an all wheel drive system does not "Syncronize" the spin of the front and the rear diffs to avoid the tight turn braking phenomena, and provides the superior traction and handling afforded by "4wd" minus the thight turn braking effect. This is made possible by a British development called "Vicous Coupling" It is a front and rear diffrential "syncronizing" system using Hydraulic fluids to propel the Front and/or rear diffs of the vehicle. Since the propeling medium is fluids it does not "LOCK" the front and rear Diffs and drive torque can be manipulated to distribute more torque to the front or rear diffs of the vehicle which is offered in some Rally cars, on production vehicles the manufacturer usually sets the torque split.

Locally manufactured vehicles equipped with all wheel drive only (no Central Diff Locks) are:
Rav 4 2nd Gen, CRV 1st and 2nd Gen,

COMING SOON! PART TIME 4WD AND HYBRID ALL-WHELL DRIVE/PART TIME 4WD

i hope this helps in you guys apprecieating your vehicles capabilities more. :D

[Ungas]

Cool infos! :D

[roydok]

Nice one there. :D

[marwen]

galing... bilib na talaga ako sa mga 4x4 nyo.... punta na kayo sa cebu.. para makasama ako kahit passenger lang...

[kiper]

so meron ang escape ng central differential lock and viscous coupling..okay..

[OTEP]

kiper,

The RBC viscous coupling is the center differential but it is not of the fully locking type. Mahirap kasi i-classify ang sa Escape because it breaks new grounds.

[Karding]

ayun naman pala....great info 8)

[mazdamazda]

kiper,

The RBC viscous coupling is the center differential but it is not of the fully locking type. Mahirap kasi i-classify ang sa Escape because it breaks new grounds.

You can classify the Escape & CRV's as Automatic Asymmetric AWD.

While the Rav4 & X-Trail would be classified as Full-time Symmetric AWD.

Here's another good write-up about them:

AWD System one (true all wheel drive - or full time symmetric AWD) has a conventional differential inside the transfer case - each of the wheels gets about 25% of the torque as long as traction is equal. However, the center diff cannot be mechanically locked.

To prevent a complete loss of traction when one wheel or one axle would spin, a viscous coupling or a similar device like a Haldex coupling (see note) will try to "glue" both driveshafts together to keep enough torque flowing to the axle with traction. Works kinda OK on slippery pavement when the vehicle has already sufficient momentum and the connecting device has to kick in very infrequently. Off-road or in other situations with slow speed and high demand for torque the glue box (viscous coupling or Haldex etc.) is overstressed and fails to deliver the needed torque. High torque transfers and continous use make especially viscous couplings fail. Haldex units are much more reliable but cannot satisfy the constant high demand for torque at all wheels either.

AWD System two (automatic asymmetric AWD - and in a way actually only a sophisticated 2WD system) might not have a differential in the transfer case (Volvo, Honda, etc.) but some do (Jeep Grand Cherokee). Primary power goes only to one axle (makes spinning tires much more likely due to inefficient use of traction - as likely as in any other 2WD car). However, both drive shafts are joined by a viscous coupling or a similar device (see note) and as long as all 4 wheels turn at the same speeds the control unit remains inactive. Once the powered axle or one of the powered tires loses traction, the powered drive shaft rotates faster than the one that is just rotating along. The control unit reacts to the speed difference and kinda glues both drive shafts together. This way the previously unpowered shaft will get some of the torque and rescue the failing tires. Same story as in system one: Works kinda OK on slippery pavement when the vehicle has already sufficient momentum and the control unit has to kick in very infrequently. Fails miserably when need of high torque arises or when activated frequently. Cannot satisfy the constant high demand for torque at all wheels when off-road.

[Field Master]

Nice one Wolfpack. Looking forward for the next episode.

[nugundam93]

um. super bump lang.

anong klaseng full-time 4wd ang sa outlander?

[mazdamazda]

um. super bump lang.

anong klaseng full-time 4wd ang sa outlander?

Full-time Symmetric AWD, same as the Rav4 and X-Trail.

[jaeger]

.
What do you call the 4WD system that's full-time AWD (with torque-split system) but has a 2-speed transfer case (HI & LO) just like in the Fortuner's Torsen drivetrain and the Sorento's Borg-Warner Torque-on-demand drivetrain?

Most of the full-time AWD of the compact SUV (like CRV & Rav4) only has a torque-split system (i.e. locking center diff.) in the transfer case without the Hi & LO 2-speed gearing.

Isn't it that when you say AWD, you normally refer to a torque split system without necessarily incorporating a 2-speed transfer case, so what do you call an AWD with a 2-speed transfer case? Is there a particular term for this kind of 4WD system?
.

[OTEP]

You call the Fortuner a dual-range full time system. Variants of Land Cruisers also use a similar system.

The dual-range part is what separates the men from the boys.

[nugundam93]

so would the evo then have symmetric awd? i know the impreza wrx has assymetric awd...

[OTEP]

If the default torque split between the front and rear is 50/50 then its symmetric. Otherwise its not.

GEN II Pajeros were symmetric in 4H (unlocked) mode. The GEN III is asymmetric (rear drive bias to improve handling daw).

[jaeger]

.
So that's what it's called - Dual Range Full Time system. Thanks. Expert talaga si boss OTEP pagdating sa 4WD. Saludo ako.

Would you know if the series of discs which make up the viscous coupling in a full-time 4WD system - are they replaceable specially if you're a regular off-roader?

Like if I have a Fortuner and go hard-core trailing every weekend, diba there will come a time when that viscous coupling starts to slip due to wear-n-tear. Can you just change the discs or replace your entire center diff.?
(I guess if you're really into regular hard-core 4WD trailing, it's wiser to get a part-time 4WD with real 2-speed gearing, right? instead of getting a rig with dual range full-time system - is this a safe conclusion? or not neccessarily?).
.

[nugundam93]

ahhh okay. thanks!

[OTEP]

Hindi naman in contact ang clutch packs kapag viscous coupling. The 'wear' occurs when the fluid is overheated and looses its viscosity. This results in reduced transmission of drive force from one blade to the other.

Solution: replace fluid. :D

Wala bang center difflock ang Fortuner so you can just go into direct-drive and bypass the center differential's effects?

[rj_tim]

e sa tucson anong klaseng AWD?

[Hayward]

Very informative and educational even for those that don't have 4X4 rigs especially when experts join in the discussion. Let us have more of this please.