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XT was 4wd
XT6, SVX, Legacy, Impreza, Outback are all AWD
-mike
I bet it's a part-time system. The Auto 4WD probably has automatic engagement of the front axle when needed, but part-time. I doubt it has a center differential, so it ought to be for slippery surfaces only.
The 4WD High probably forces power to both axles, but should be used on slippery surfaces only. 4WD Low is the same but with much shorter gears.
-juice
-mike
-mike
-mike
Personally, I like to work out the transfer case every once and a while by using Full & Part Time as well as Low. I've never heard it being necessary or that it makes any difference at all. Many of the people looking for parts for older (or broken
Full-time 4WD... Traditionally, or historically, this has meant the ability to drive all four wheels with no threat of driveline damage. And that almost always meant three open diff'l, or some other type of "free-wheeling" arrangement wherein each of the four driven wheels could have different, independent, rotational rates.
In other words the only time you really had 4WD was when all four wheels' traction was fairly close to equal, otherwise ALL of the engine torque would be routed to the wheel or wheels with the lowerest traction level.
Part-time 4WD has some form of locking center diff'l/transfer case that does NOT ALLOW different turning rates bewteen the front driveline and the rear driveline. Ordinarily when turning a corner the front driveline will turn at a different rate than the rear, since in this case it absolutely cannot then something else must "give". That's where the LTS, Low Traction Surface comes in, it allows slippage between the tires and the surface so the driveline will not be damaged.
You might want to think of that LTS as your center "open" diff'l, because that is exactly the function it provides.
AWD should really be called AAWD, Automatic All Wheel Drive. Some mechanical arrangement is made such that the majority of the time the front driveline and the rear driveline can have fully independent rotational rates. With the older 4WD systems it was the operator who determined the roadbed conditions and took the appropreate action. In one way or another today's AWD system actually "detect" roadbed traction conditions and "adjust" automatically.
The Ford AWD Aerostar, bless it's poor departed soul, had a "normal" torque distribution of 30/70 F/R and if the ECU detected rear wheel slippage it was electronically switched to 50/50.
-mike
The vehicle operator was in the LOOP. Not an automatic system by any means.
Most of today,s AWD systems do not require any operator intervention, they are self-determing and self-activating, AUTOMATIC.
Maybe it should be "A4WD".
You're trying to distinghish between AWD, as in the HL, RX, MDX, and a vehicle that slips into 4WD FROM 2WD automatically.
Is there even one of those in the market?
If one does exist I would think it would go immediately into locked 4WD mode on slippage detection, but then how would it know when to safely return to 2WD?
That's the advantage of the Ford Aerostar system, if no slippage occurs in 50/50 it will "test" for LTS after a time by returning to 30/70.
Vehicule that are always in 4WD/AWD mode (no 2WD only mode) most Volvo AWD vehicule, another one is the Acura MDX. These vehicule are Full Time AWD , they put most of the torque on the front wheels exept when slippage occurs, they then transfert some power to the back wheels or they can also send torque to the back wheels in some condition even if there is no slippage ie.: at a stop sign/red light.
There is alot of way to implement Auto4WD/AWD on a vehicule, so each model has its own subtilities, but at the end - it is more or less the same, as the principal system that will keep you on the road whatever are the condition, is yourself.
Hope that I'm answering your question.
Explorer
Envoy
Tahoe
Trailblazer
CRV
Escape
Tribute
QX4/Pathfinder (with auto-4wd option)
MDX (100% fwd til slippage)
Ones that put *some* power to both axles at all times when in AWD Mode:
ML series and 4-matic MBs
BMW 3series and X5
Trooper
Subarus
Axiom
Sequioa
Montero, Montero Sport
Toyota Land Cruiser
Audi Quattros
-mike
mike's list looks just about right. Add Mercedes-Benz 4Matic to the bottom list.
-juice
-mike
They also have a limited slip rear differential and a limited slip viscous center differential with an open front differential. There are no buttons to push and there is nothing for the driver to decide upon. The effect of the setup is a minimum 38% power distribution to the front tires all the time.
The limited slip rear ensures the rear tire with better traction gets the power and the limited slip center diff distributes power automatically between the front and rear tires, favoring the end with better traction.
While there is no low 4wd setup, this drivetrain setup assures the vehicle will always have power to at least three tires even under the worst conditions and there is no time needed for any power transfer since its present all the time.
-mike
Now, the CR-V's system is a part-time 4WD system. While this system does work for most road conditions, it is not as effective as full-time 4WD because the system needs to slip before activating.
Finally, LSD serves a similar purpose as traction control by transfering torque from the spining wheel to the other. The main difference is that traction control uses brakes to slow the spining wheel. The main drawback is that you loose energy through braking heat. A LSD, on the other hand, uses a locking mechanism to transfer power.
Here's a very good summary on 4WD:
http://4x4abc.com/4WD101/need.html
Note that the CR-V falls under the category of "True AWD" or TOD.
Ken
An AWDer would send just 50 ft-lbs to each wheel, and that makes each wheel less likely to break traction. Add to that the fact that the job to get your vehicle moving is divided into four contact patches instead of two.
But the CR-V starts out as FWD, and then gets traction assistance from the rear wheels. I still think this is better than FWD+traction control, though not as good as systems sending power to both axles proactively.
It's hard to compare an Accord to a CR-V because they carry different tires. I bet the one with better tires (for snow) would win.
-juice
The RT4WD is a reactive system which benefits a vehicle 99% of the time. Why? Why do I need 4WD if my front tires do not slip at all?
Does the FWD traction control work by dethrottling the engine, as the Cadilac does, or does it apply the front brakes, individually(?), or simulatneously?
-mike
That statement is true only when looking at 4WD/AWD as a means to get your vehicle moving from slippery surfaces.
The other major benefit to full-time 4WD/AWD is during cornering. By having both the rear and front tires powered, you can acheive more neutral handling characteristics than with a FWD vehicle.
Check out the right column on the same website:
http://4x4abc.com/4WD101/need.html
That's why many performance vehicles from Porsche and Audi employ AWD transmissions.
RT4WD is okay for getting you moving in snow, for example, but it doesn't help during cornering. In fact, the system would bind during turns.
Ken
On the LSD vs. T/C, it depends. Traction control is only as good as the software. The best systems are very good and manage the axle very effectively. The systems weigh very little and can use existing ABS hardware, but it's all in the programming. They can also heat up the brakes, and could potentially create brake fade in prolonged use.
LSDs are split into two main categories: torque sensing (torsen) and viscous. Torsens are wonderful for the track because they distribute power quickly with little loss. Their achilles' heel is that in zero friction they are useless, i.e. no good for icy conditions. They also tend to be heavier and more enpensive.
A viscous LSD takes longer to act. They are fluid filled and the fluid heats up and shears very quickly, so send power to the other side. They are effective in snow and ice. They are also light and inexpensive.
So, T/C is good for ice, VLSD is good for ice, and torsen is best for the track. Each has its stregths and weaknesses.
-juice
Another thought, how could your porsche have plowed through as deep a snow as the all-road? You would have most likely gotten stuck earlier on due to the low ride height. You say you tried the exact same manuver with the porsche as the audi, I take it you did the same manuver on the same day at the same time, etc, etc. Or else your arguement just can't wash, conditions are always different.
-mike
No fair comparing one car with snow tires to another without them. Snow tires are a major factor, of course. Plus, in climbing hills, most of the weight transfers to the rear anyway.
A 944 is close enough to the ground that it becomes a snow plow, though. :-)
-juice