FWD vs. RWD

I am just wondering why people are so against FWD cars. I mean you can get alot of horsepower outta them and they are fast. Also in many car specs they say whether an engine block is cast-iron or alluminum. Which is better and why? Thanks.

The problem with FWD is basicly two things, traction for the tires and understeering.

During acceleration the cars center of gravity is moved back in the car, this means less weight on the frontwheels and vice versa for the rear wheels. This makes it more difficult to get traction for the front wheels.

When a front wheel driven car is accelerating it generally understeer, but if the throttle is released the car can have tendences to oversteer, or at least it will oversteer more than it did when the car was accelerating. A rear wheel driven car is just the opposite, and that can make the car easier in turns but more difficult to handle on straight parts of the road, but there the FWD will have more problem with traction for the driving wheels when it accelerate. But in higher speeds the aerodynamics can affect the traction, which can give it good traction.

And about the engine block material, there is no one that is better, it just depends of what you're using it for and who you ask the question to.

Well, aluminum is lighter.

Originally posted by SaabJohan
And about the engine block material, there is no one that is better, it just depends of what you're using it for and who you ask the question to.

Why would you want aluminum though? for racing? Yes, it is lighter but i think iron is stronger. :D

Not ot mention that FWD cars can't do cool donuts. And yes...donuts in reverse are lame.

Iron blocks are stronger.

Aluminum is lighter

This doesn't mean that aluminum isn't good. Unless your looking at running HIGH horsepower...go with aluminum if you can (especially if you have aluminum heads since you have thermal expansion).

Sebastian "Seabass"

During acceleration the cars center of gravity is moved back in the car
It might make more sense if you say that the car's center of gravity is shifted backwards.

I think there is another thing too. Torque steer. I've noticed it in quite a few in FWD cars because the car will pull to one side or the other when u slam on the gas. Ive never known a RWD car to do that. Plus I also like how the steering wheel feels while driving a RWD car compared to a FWD, as the FWD tries to pull it self staight if you are on the gas and cornering.

The side to side thing that happends with FWD is because of two basic things, wrong torque distribution and loss in traction on the front wheels.

Wrong torque distribution can be a result of a chassi that is too weak or a ineffective diffirential.

A 4WD car also have tendenses to go from side to side, while a RWD instead throw it's rear end out like when doing a donut.


Usually aluminium blocks have more problems with thermal expansion, cause since most liners are made of iron.

The question you pose is pretty non-specific, how about we analyze it in specific venues?

For example, if you are looking for small, lightweight and affordable performance cars in the US right now, there really aren't any rwd alternatives. So fwd is currently the king of this market niche, and the Hondas are quite a fad with most of the younger generation too. There's nothing wrong with the cars and many are quite capable performance wise in their classes, but if you are talking about no holds barred racing with no specific rules, it's doubtful any fwd car will compete favorably with a well chosen and built rwd car. The driving dynamics of rwd are superior in two equally well designed and built cars, it's just that simple. But the gap is not as large as it used to be, thanks to better fwd drivetrain and suspension setups.

The problem that's impossible to get around is that the front wheels have to do nearly all the work in terms of braking, accelerating and turning. With only so many rubber molecules available to do this work, and with most performance driving demanding two of these three modes of traction at once, it's just a better system when you split acceleration duties to the rear wheels and leave the fronts to specialize in turning and braking.

Finally, the line "you can get alot of horsepower outta them and they are fast" is all relative. If you were to say that to a Pro 5.0 car builder and competitor, they'd laugh til they cried. These cars make in excess of 1200hp and run very low 7's to very high 6's in the quarter mile, and they do it with quite a few challenging class rules. Tube chassis fwd cars with no limitations on engine power or configuration aren't even close to that mark of speed or power output, and they have a good bit more money thrown at the engines. But of course if you got your average 17 year old kid to jump in a turbocharged Integra GSR they'd think it was wickedly fast, even though it's still making less horsepower than a bone stock Camaro Z28 (which would cost you about 1-2g more out the door on new models). Just remember all these arguments are relative to the money one has to spend, the car's intended use, and the owner's specific tastes. Some love the feel of fwd, some hate it. I'm somewhere in between, but chances are the next car I buy will be rwd.

Originally posted by SaabJohan
The side to side thing that happends with FWD is because of two basic things, wrong torque distribution and loss in traction on the front wheels.

Wrong torque distribution can be a result of a chassi that is too weak or a ineffective diffirential.

A 4WD car also have tendenses to go from side to side, while a RWD instead throw it's rear end out like when doing a donut.


What about a AWD car like the evo or the skyline.. Do these cars have tendencies to go from side to side? is AWD better than RWD?


Thank you all for your posts! :ylsuper

I'm a big FWD fan. Also, I'm the first to admit that it all goes down to your personal preference. For brute power RWD is somewhat better. But it is also much more unstable (at the same time it's easier to bring it under control, but it's easier to lose that control in the first place). As for competition (and not just drag racing), in DTM Audi are currently whooping Mercedes's ass and in BTCC I can't even recall when an RWD car has won anything. Surely there's a reason for that! In the hands of a good driver FWD can be at least as fast as RWD. I remember a few years ago in Super Touring Wagen championship in Germany there were huge BMW vs. Peugeot battles. The BMW won, but narrowly. It was a close and even fight till the last ace of the season.

And there's nothing wrong with alluminium blocks :D

Alex

Since a car got 4 wheels, 4WD = AWD.

I think the fastest FWD does the 1/4 mile in around 8 second, don't know how much power but I can guess of around 800 hp. If we compared that car against a HKS tuned Supra with around 800 hp we will found that it is almost 2 secons slower... so RWD must be really bad, 2 seconds... No, we can't compare these cars that way, and the same between 1200 hp rwd dragsters and fwd:s with tube chassis.

Comparing done with the cost of the car can also be bad... if the only thing that matters were speed/dollar, why don't buy a rocket and a skateboard instead, one can die trying but at least it was inexpensive...

Originally posted by TrvlynAlec


What about a AWD car like the evo or the skyline.. Do these cars have tendencies to go from side to side? is AWD better than RWD?



it depends on the car as there are smany different versions of AWD. depending on the setup it will behave partly like a FWD and partly like a RWD. the skyline will behave like a RWD most of the time (oversteer) whereas the evo would tend more towards understeer (FWD), as a generalised example. with a big launch and decent power an AWD car can spin its tyres so it will move around alright.

AWD gives better grip in slippery conditions but RWD is the most entertaining setup in my opinion. it depends what you want in a car.

4WD isnt AWD, get that straight. Two very differnt setups. I'f i were to drive my pathfinder around with 4WD conencted all the time, not only would i waste gas, and not get 20 miles before i hear the transfer box start grinding off gear teeth, but steering at speed (as if i could get very fast in the first place) would be nearly impossible. :) just so you know.

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?

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.

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.

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.

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.

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.

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.

This page describes awd and 4wd:

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

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.

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.

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 :)

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)

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.

i think he meant variable torque splitting.

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 :( :(

If you look at just the differentials, then there are about 3 main type that you'll find, with a few sub-types crammed into those:

* Open differential (commonly used on front axles of 4wd vehicles): there is no slip-limiting mechanism. Torque to each wheel is equal, and speed of each wheel is completely independent of the others. The downside is that when one wheel starts to slip, the non-slipping wheel gets the same amount of torque as the slipping wheel, which means that the torque to the non-slipping wheel is severely restricted.

* Limited slip differential (several varieties): there is a slip limiting mechanism that works counter to a speed difference between the wheels, effectively transfering torque from a slipping wheel to a non-slipping wheel.
- viscous limited slip uses a fluid coupling between the axle shafts. The torque difference between the shafts is roughly proportional to the square of the speed difference between them
- frictional limited slip uses a clutch mechanism between the axle shafts. When one starts to slip, the torque difference between the shafts is sometimes (there are also schemes that alter this behavior) a fixed quantity independent of slip speed.

* Locked differential (there are many varieties, usually they are equipped with some sort of a mechanism to unlock the differential for driving situations where traction is available): These differentials force the tires to spin at exactly the same speed. For straight-line driving, if one tire is not slipping, then the other one is prevented from slipping. Torque is split based on available traction at each wheel


If you look at the whole 4wd/AWD system, then the biggest differences besides the above are:
- whether or not the transfer case includes a differential, and what type, if any, is included (open, limited slip, lockable, etc)
- how the differential is controlled, if there is one (manually mechanically shifted, manual air or electronic control, automatic electronic control, etc)


more info:
http://www.howstuffworks.com/four-wheel-drive.htm
http://www.howstuffworks.com/differential.htm

blah.

If you look at the whole 4wd/AWD system, then the biggest differences besides the above are:
- whether or not the transfer case includes a differential, and what type, if any, is included (open, limited slip, lockable, etc)
- how the differential is controlled, if there is one (manually mechanically shifted, manual air or electronic control, automatic electronic control, etc)


I would also add the qualifier of whether or not the system is intended to be used full time or part time. That's one of the biggest functional differences present in this symantic argument.

Whether or not they're going to be used full time is the primary factor driving the choice for a "center" differential, right? (the one in the transfer case) Part time systems can get away with not having a center differential, while full-time systems absolutely have to allow slip between the front and rear driveshafts?