FWD vs. RWD

[cadet]

the way i see it a car with four wheels and four wheel drive is all wheel drive although some manufacturers choose to use one or the other name for marketing reasons.
obviously the setup on your pathfinder is not designed for use on sealed surfaces, does it overdrive the front wheels or what?

[SaabJohan]

4WD is AWD if the car got 4 wheels, which most cars have. The name AWD/4WD has nothing to do with the setup used, it's just what the maufacturer prefers to call it. But if the car got more than 4 wheels, like light and heavy trucks can have, then 4WD is not the same as AWD.

[enginerd]

Marketers call it whatever the hell they wish. 4WD is a low-geared system with a transfer case, front and read differentials. AWD is used all the time with (normally) a viscous coupling, center, front and rear differentials.

[Steel]

the 4wd isnt front differentiated either, cause i can her rubber grinding if i turn the wheel and drive. Trust me, 4WD like what trucks have, is different than AWD like what subaru's have. Sure all four wheels are turning, but its still differnent, otherwise it would be ok, albeit a bit gasoline intensive, to drive my pathy around in 4wd. Its just like theres different engines. Sure, eventually all of them make a driveshaft turn in the end, but they can do it in very different ways.

By the way, if you have a 2 axled 6 wheeled truck, doesnt make it much differnt, its just that 2 wheels are connected to each other.

[SaabJohan]

I say it one more time, the names AWD and 4WD have nothing to do with the setup used. Low geard, permanent drive, hydralic couplings, it doesn't matter, it only tells you how many wheels that can pull the car forward.

If a truck have 2 axles it just got 4 wheels, even if they are 4 on the rear axle it counts as 2, at least when we talk about these things.

[Gonthrax]

Now I can't answer for the Evo, but in the GTR's case it will go into power oversteer or brake biased oversteer (a FWD trait) before it starts to understeer.

Now as for 4wd and awd, the difference is larger then you think Saab. Lets take the USDM WRX and the Skyline GTR. The WRX is 4wd, if you launch hard all 4 wheels will start to spin. If you give a bit to much gas in a curve, you will get the rear wheels moving in a slightly higher arc then the fronts. After that the fronts will start devoting more % traction to acceleration then turning and you will understeer.

Now with the GTR we have AWD (ATTESSA ETS to be exact). If you launch hard the rear wheels will get about two revolutions of wheel spin while the fronts don't move. Then torque is slit to the front wheels, depending on power they may spin a few turns or they may not. If you slip the clutch right they won't spin at all. Now while this is happening your ATTESSA computer is figuring out which wheels are spinning, which arn't and taking torque from the ones that are and sending it to those that arn't (10 times a second). Now lets take the curve in the GTR. You aproch 90% capacity on the tires (they start to squeal) you start to accelerate, you give it a bit to much gas. If you only give it a little to much gas ATTESSA will divert the torque to a wheel that won't spin so you continue on around the corner. But if you give alot of gas, the back end will break loose and start traveling in a much higher arc then the fronts. If you keep your foot planted the back tires will keep spinning and a steady speed while the remaining torque is split to the front and you will drift right through the turn. I haven't heard of any model GTR that will understeer as a matter or course accept perhaps for some of the 1st gen R34 Vspecs because of some little bug in the setup.

[SaabJohan]

Originally posted by Gonthrax
Now as for 4wd and awd, the difference is larger then you think Saab. Lets take the USDM WRX and the Skyline GTR. The WRX is 4wd, if you launch hard all 4 wheels will start to spin. If you give a bit to much gas in a curve, you will get the rear wheels moving in a slightly higher arc then the fronts. After that the fronts will start devoting more % traction to acceleration then turning and you will understeer.

Now with the GTR we have AWD (ATTESSA ETS to be exact). If you launch hard the rear wheels will get about two revolutions of wheel spin while the fronts don't move. Then torque is slit to the front wheels, depending on power they may spin a few turns or they may not. If you slip the clutch right they won't spin at all. Now while this is happening your ATTESSA computer is figuring out which wheels are spinning, which arn't and taking torque from the ones that are and sending it to those that arn't (10 times a second). Now lets take the curve in the GTR. You aproch 90% capacity on the tires (they start to squeal) you start to accelerate, you give it a bit to much gas. If you only give it a little to much gas ATTESSA will divert the torque to a wheel that won't spin so you continue on around the corner. But if you give alot of gas, the back end will break loose and start traveling in a much higher arc then the fronts. If you keep your foot planted the back tires will keep spinning and a steady speed while the remaining torque is split to the front and you will drift right through the turn. I haven't heard of any model GTR that will understeer as a matter or course accept perhaps for some of the 1st gen R34 Vspecs because of some little bug in the setup.

And once again two examples on diffrent setups and the names that the manufacturer has chosen to call it... starting to get gnagy now.

4WD and AWD is just like biturbo and twinturbo... diffrent names, same thing but exist in many setups.

[SaabJohan]

This page describes awd and 4wd:

http://4wd.sofcom.com/A.hints/AllWheelDrive.html

[Steel]

well duh, i know that 4wd and awd are the same, in definiton, but we all know that they carry different meanings, similar to how we use Formica, Band Aid, Coke, and the like.

[SaabJohan]

Actually some manufacturers, like Subaru, are trying to get people to think that AWD is better than 4WD, it isn't. One AWD system can be better that one 4WD system, but AWD can never be better that 4WD just like 4WD can't be better than AWD, at least when we talk four wheels, otherwise it's a little different.

What is more interesting to know is how it works, is it permanent och not. Is it electronicly controlled, diff locks and so on.

[ales]

Well, technically, AWD is a broader term than 4WD (6X6 & 8&8 cars)

BUt I agree that one a car with four wheels AWD is just the same as 4WD. Come on, they both mean the same - that engine delivers power to all four wheels. Manufacturers may call it either of the two and use whatever drivetrain management system they can.

Edit: and that's exactly what the article linked to above says

[Gonthrax]

I may be mistaken, but from my experience with 4wd (in the WRX and other vehicles) it is locked 4wd and there is no torque splitting. AWD on the other hand distributes torque to non spinning wheels, so if you sat one wheel on some ice, one in a puddle, one in mud, and one on pavement it would break down into somthing like this. 0% Torque to wheel in ice, 25% to Puddle wheel, 15% to mud wheel, and 60% to wheel on pavement (if you could split it that way depending on the diff)

[ivymike1031]

it is locked 4wd and there is no torque splitting.

Maybe I don't understand what you're trying to say, but if you meant that the differentials were "locked," then you'd see better torque splitting than with either a limited slip or "open" configuration. Since the wheels would be forced to spin at the same speed, torque to each would be proportional to the load and traction at each.

[Steel]

i think he meant variable torque splitting.

[Gonthrax]

Originally posted by Steel
i think he meant variable torque splitting.

Correct


Am I right about this? I'd hate to think I've gone for several years thinking I knew how somthing worked but really didn't