4WD and AWD systems explained

燕子山

4WD and AWD systems explained

/ Y3 t+ V0 r' t. F2 BA word of caution first. Manufacturers tend to obscure the true nature of their 4WD/AWD systems behind phantasy names like "Active 4WD", "Quadra-Drive", etc. - others might call their automatic AWD "Real Time 4WD".
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5 P3 g9 a9 N# P- dThere is plenty of confusion about what is what. Does it matter? Can't you just call "four wheel drive" "all wheel drive"? Yes you could. If all eight wheels of your big truck are driven, isn't it all wheel drive? Yes it is.
  \6 Q5 x8 N5 s/ GHowever, there are so many different 4WD systems on the market now that it is important to be precise and specific. It is important to call AWD when it is AWD and full time 4WD when it is full time 4WD. Just to say 4x4 is not sufficient any longer.
: i2 e% q* j3 |4 @Wrong terms lead to misinformation - intentional or not. Wrong terms might make you buy something you neither want nor need! Do your homework before buying a 4WD - to get what you really need. If necessary, ask me.

6 V) m# U  K! L9 w( gThere is no sanctioning body who ever established the definition of terms regarding 4x4. The terms I use below are the ones used internationally by engineers and competent magazines.
. p  B* C- Q: ], M( f4WD has been invented a very long time ago and various concepts have been tried. Here is a simplified synopsis of what is what. Links within this page provide detailed insight.
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3 }# H2 J, ~9 ^  q/ {#1 Part time 4WD is a system that can only be used part of the time in four wheel drive.
This 4WD system was created to provide a vehicle with more traction to either carry higher loads and/or to travel in adverse terrain conditions. Clearly purpose built to do hard work. It can only be used for adverse terrain conditions - not for dry pavement.
Typical lever settings are 2WD, 4WD Hi, 4WD Lo.
6 r* a1 c. ]7 Q, f* e1 W# H. u# JVery good off-road. Most competent when combined with axle differential locks.
" m2 x  l* h( n• 2WD setting must be used on dry pavement.
• If 4WD is selected, all 4 wheels are permanently powered.
Prominent examples: 1942 Willys, Jeep Wrangler.


# y/ {8 K* _3 O2 x7 @  H) F2 X#2 Full time 4WD - also called permanent 4WD, can be used full time on all surfaces including pavement.
Full time 4WD was created to provide a vehicle with more traction and to make 4WD more useful for everyday life. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time.
2WD is no longer available. Can still be a strong workhorse. Some rough terrain competence is retained - the priority is added stability as a safety gain for everyday driving.
3 A7 [! W4 \& K3 n7 B8 R5 d2 XTypical lever or switch settings are 4WD Hi, 4WD Lo. Very good off-road when center diff is lockable. Even better when combined with axle differential locks.
; [; F& W, }/ {4 W8 K: `4 Q8 J• All 4 wheels are permanently powered.
Prominent examples: pre 2006 Mercedes M-Class, Mercedes G500, LandRover, RangeRover, Toyota Prado, Lexus GX470.

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) d) T, c' c  U8 Q* d( c$ d7 k#3 Full time symmetric AWD is similar to full time 4WD - only it lacks the slow speed torque enhancing low range feature. Can be used full time on all surfaces including pavement.
/ w! w( L5 s& pSymmetric AWD was created as a safety feature for modern day vehicles. Not designed for hard work. Clearly a safer, more comfortable vehicle. Rough terrain competence has almost vanished - focus is on added stability and performance.
7 z3 I6 i" }5 mLimited use for off-road.
• All 4 wheels are permanently powered.
; e) ^* O0 F2 k" DProminent examples: 2006 Mercedes M-Class, Audi Quattro, most Subaru, pre 2006 RAV4
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1 S' ~$ W  Q5 C#4 Automatic asymmetric AWD was created solely as a stability enhancing system. Auto AWD can be used full time on all surfaces including pavement. AWD only briefly activates automatically when stability threatening conditions arise. Essentially a 2WD car with 2WD handling characteristics. Absolutely no adverse terrain capabilities. Clearly built for added road stability and safety.
Not recommended for off-road beyond graded dirt roads.
4 I) a" n0 J2 }: z8 B• 2 wheels are powered most of the time
% v% P/ v7 z  J4 Y% H6 p. A0 n• 4 wheels are only temporarily powered.
1 x' G7 U% Q3 VProminent examples: Volvo AWD, 2006 RAV4, Honda CRV.

& D4 S6 T! }, @2 gA more detailed explanation of the differences between 4WD and AWD.

! b. |  N' @  gSo, is "real time 4WD" really 4WD? No, it is a sophisticated 2WD car with automatic asymmetric AWD. There is no commonly accepted standard how to name the different 4WD systems - companies and especially their PR departments use terms for 4WD systems very loosely - transparent consumer information is not their priority.

Many vehicles are offered with a combination of 4WD systems outlined above. Very confusing for consumers.

燕子山

What is the difference between Full time Four Wheel Drive (4WD) All Wheel Drive (AWD) and automatic All Wheel Drive (auto AWD)?

$ J4 D7 k1 L. W& W) GFull time 4WD, also called permanent 4WD, (not to be confused with: part time 4WD ) is a system that powers all four wheels at all times and can be used full time on all surfaces including pavement. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time.
' y0 I% ^  q/ o2WD is not available (only part time 4WD offers that option). Each tire creates about 25% of the available torque when the ground is level with a consistant surface. Driver has a choice of a "4-high" (that's your every day setting) and "4-low".
0 G$ `9 M& O8 T/ J  k0 w4 A' tFull time 4WD vehicles work very well on-road and are very capable off-road.
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When "4-low" is selected the wheels create substantially more torque (on a Grand Cherokee its 2.72 times more) than in "4-high" - at the same time the vehicle moves at substantially slower speeds (2.72 times slower on a Jeep Grand Cherokee).
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Important: "4-low" does not create more traction - it creates more torque and that can be detrimental when traction is marginal. Slipping tires are more likely in "low" than in "high"!

The low setting is an advantage for drivers who need to tow and maneuver a heavy trailer etc. and for drivers who at one point or another may want to negotiate difficult off-road terrain, when more torque and/or slower speed is needed.

% V9 i- x  f$ M6 J- W4 ZAll wheel drive (AWD) is almost the same thing as full time 4WD - it is a system that powers all four wheels of a vehicle at all times as well. Full time symmetric AWD would be the best term to be used. Difference to full time 4WD is that a "4-low" setting is not available in AWD cars. Due to the lack of "low range" AWD vehicles are much less capable in off-road settings than full time 4WD vehicles, but work perfectly well on-road.

Automatic AWD system is the newest kid on the block. PR agency generated names like "Real Time 4WD", "intelligent AWD" or "active AWD" are hiding the fact that automatic AWD is essentially a sophisticated 2WD system. Automatic asymmetric AWD would be the best term for them.

* p7 T! e8 x+ _: G8 X. VHere is how they work: Under normal conditions one axle gets 100% of the torque - meaning you are driving in 2WD. During traction loss at the driven axle (could be front or rear) a fully automatic system (hydraulic, mechanical or electronic) makes up to 50% of the torque to the axle with traction available. This means you have to lose traction in 2WD on your driven axle first and then the other axle will be added and try to keep the car moving and stable. Once the primary driven axle regains traction and both axles rotate at the same speed again, the system reverts back to 2WD. So, for a moment you had AWD.

Automatic asymmetric AWD is much less capable in off-road settings than full time AWD systems and inferior to full time 4WD. However, automatic asymmetrical AWD is becoming more and more sophisticated and offers pretty much everything consumers expect for everyday (pavement) driving.
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" P, B4 R. W" T6 B) |- B9 [Examples: Honda CRV, (newer) Toyota RAV4, LandRover Freelander, Isuzu Trooper (TOD), Volvo V70, 1999 and later Jeep Grand Cherokee (in high range).

Recently some magazines have called the automatic AWD system "part time 4WD", since it offers AWD only part of the time. They have a point - however, the term "part time 4WD" has been used since WW II for cars like the Willys and Jeep Wrangler and their part time 4WD . A manual system where the driver had to select 2WD or 4WD. The name coming from the fact that 4WD was designed to be used only part of the time (when off-road), most of the time it had to operated in 2WD (on-road).
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A frequent consumer complaint about some older automatic AWD is, that all 4 tires need to be replaced even if only 1 or 2 are bad.

Confusing but true: some vehicles have a combination of part time and full time 4WD systems, or even a combination of 2WD, automatic asymmetric AWD and part time 4WD (low range). Impossible to give those custom mixes a name..
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; O+ Q. P, V- M4 ~Learn from professionals to get the most out of your 4WD :

HRU

燕子山的解释应该是最全面的之一。
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2 [* g; d0 e- ~+ C. I3 U如果你能用中文解释，更多的人会受益的。

* X1 h/ f# R. l7 y“Confusing but true: some vehicles have a combination of part time and full time 4WD systems, or even a combination of 2WD, automatic asymmetric AWD and part time 4WD (low range). Impossible to give those custom mixes a name..“

这是非常有用的一段。ｓｕｚｕｋｉ　ＳＸ４　ＡＷＤ　就是这种情况。　它有三种模式可选：　２ＷＤ（前轮驱动）Ａｕｔｏ　Ａｓｙｍｅｔｒｉｃ　ＡＷＤ（自动全轮驱动）和　Ｌｏｃｋ　（５０－５０锁定），这就是一种组合了分时四轮驱动　ｐａｒｔ－ｔｉｍｅ　４ＷＤ，ＡＷＤ，Ｆｕｌｌｔｉｍｅ　４ＷＤ　的三合一模式
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8 ?, T- T$ e2 d5 y+ n6 i5 fＭｉｔｓｕｂｉｓｈｉ　Ｏｕｔｌａｎｄｅｒ（０７－ｎｅｗｅｒ）也是这种情况。
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谢谢燕子山的详尽解释。