Question about AWD, FWD, RWD

[mbeige]

Is it true that on rolling starts, mashirap yung 4wd? And that the advantage of 4wd is when you're on a standing start?

[KoKoNAT]

Maybe its true especially for a 4WD truck like a Hummer, a Land Rover or a Land Cruiser. Even with a V8 engine, it'll be hard-pressed to move a 3-tonne hunk of metal from a standing start.

But for an AWD car like an Audi or a Subaru, it shouldn't be a problem.

[basti08]

original question:

i think its gonna be a win for the RWD.

RWD propels rather than pulls (FWD). it (RWD) will push the weight forward easily unlike the FWD, which will collect all the weight first then will accelerate. as for the AWD, as OTEP said is tricky to launch.

i remember one time Vicky, when she was still on Top Gear (UK) made a test regarding the differences of FWD, RWD and AWD but she focused on the handling. 3 cars were up as a subject car: Alfa Romeo 156 2.0 AT (FWD), BMW 323i AT (RWD) and Audi A4 1.8T AT back in 2000. they set a point in a curve na biglang may maghahagis ng stufftoy and she had to avoid it.
sa RWD, it oversteered but she was able to avoid the animal toy thrown at her. sa AWD, it's almost neutral verging on the understeer (a bit) but she manage to not hit the target. dun sa FWD, it understeered to the point na din nya naiwasan yung stufftoy.

[niky]

RE: Top Gear test... That's an Alfa Romeo 156... compared to the BMW and Audi, it handles like a dog. It would've been fairer to compare a sportier FWD to the other two. Understeer in emergency handling depends much more upon chassis design and overall balance than mere drivetrain layout.

First off:

If you use the exact same car, and make it available in FWD, RWD and AWD, it will not weigh the same for all variants. The FWD will be lightest, the RWD next, and the AWD will be heaviest.

Let's say our theoretical car is a Mitsubishi Evolution. This is a good car to start with, because you can make it FWD by removing the rear differential (God knows why you'd want to) or RWD by welding the front diff and removing the front drive axles. People have actually done these things with the Evo engine, so it's a good start for comparison.

1. Acceleration

Depending on conditions, either the RWD or AWD vehicle will come in first. The Evo engine is too powerful for the front wheels, and it comes in last... by a long margin. If the surface is slippery enough, the AWD will win. If the surface is sticky enough, the RWD may win, because it has a more efficient drivetrain. The FWD Evo has the most efficient drivetrain, but the least traction, so it's no match for the other two.

At high speeds, the RWD will have the acceleration advantage, but the AWD will be somewhat more stable.

2. Handling

Corner Entry

Here's the tricky part. When diving into the corner on the brakes, the weight balance of the RWD car will make it understeer less. The AWD car will understeer a bit more, and the FWD car will likely understeer about as much as the AWD car. Given that the Evo is a front-engined car, and that its suspension is designed with lots of "snap" in it... all three cars will possibly break out and oversteer under trail-braking in the same manner... it's just that the RWD version will have the option of maintaining a tighter line if you're trying to drive smoothly.

FWD understeer? Not true in this case. Until you get back on the gas, it'll act just like the AWD and RWD car.

Mid Corner

Here's where the FWD car starts to falter. You have to stay off the gas to keep it from understeering (same with the AWD), or, you need to make it snap-oversteer to get it aligned with the corner exit (same with the AWD, again)... and get it back in line by pressing on the gas again, although the AWD is more effective at this, and understeers less under power.

The RWD car, with nearly identical weight balance to the other two (engine in the same place), will act nearly the same without throttle, but has the distinct advantage of being able to balance on the throttle between understeer and oversteer.

Corner Exit

This is what kills the FWD car. On the exit, it can't put down power as early as in the other cars, otherwise it'll understeer to hell... but with Mitsubishi's fancy differentials, it shouldn't be as bad as in the nasty Alfa Romeo Top Gear tested.

The RWD car can take a tighter line around the corner, and can get on the gas earlier, but some conditions might force you to feather the gas. Still, it's usually first to accelerate from the corner.

This is where AWD reigns supreme. Once you're lined up for the Corner exit, boot it, and you're gone... leaving the other two cars in the dust.

Overall, in dry handling, RWD is better, but AWD is easier. FWD is very easy to drive fast, but it's difficult to make it competitive (although there are some FWDs that can kick RWD ass on track, they're the exception, not the norm).

3. Handling in the Dirt

In the dirt, things are a little different. In the dirt, you're often driving sideways on loose surfacing. RWD cars can drive on the dirt, but they're not optimum for it, as you need to be a little more conservative with slip angles. FWD cars are great for neophyte rally drivers, the engine is right over the drive wheels, giving good traction, and you can control your slip angles by giving it a little more throttle... in fact, many levels of rallying below the Group N use FWD cars (like the Junior WRC) to good effect.

AWD is king of dirt. Period.

[the_wildthing]

I was thinking more of using the 911 as the theoretical car (though walang FWD), more of AWD vs. RWD.

If the original poster's intent was to keep the weight the same, the AWD will have to be stripped of some stuff, simply because the additional drive shaft and front drive components add more weight.

Now, given the electronics of their "Intelligent All Wheel Drive", the novice driver might make better time through corners than in a 2WD, but as the driver skill increases, the difference will probably become negligible.

But having driven neither 911 variants, this is just a theory.

[niky]

It would be nice if we could find someone to lend us an Evo, so we can find out for sure...

That's the biggest downfall of any FWD-AWD-RWD comparison... you cannot find cars of identical weight, power and suspension tuning (this is the most important) spread out amongst the three types of drivetrains.

Like the aforementioned Top Gear test. They used a BMW 3-series, an Audi A4 and an Alfa Romeo 156. The 2000 Audi A4 isn't the best available example of dynamic balance in AWD, and the Alfa Romeo isn't exactly the best FWD. For this test, I would have specified a 3-series, a Honda Accord Euro R, and maybe a Subaru Legacy.

Heck, if the RWD in question were an old Mercedes C-Class... would it have performed as well as the BMW? The BMW always skews the comparison, because it's built with near 50:50 balance, whereas most other mass-market RWDs are designed with a 55:45 weight balance. The Alfa and Audi would likely both be 65:35 or worse.

With an Evo or a WRX STi (or even a stock Impreza 1.6 or 2.5 non-turbo), it should be relatively easy to compare the three drivetrains on a platform with the same suspension and chassis tuning.

[AG4]

Corner Entry

Here's the tricky part. When diving into the corner on the brakes, the weight balance of the RWD car will make it understeer less. The AWD car will understeer a bit more, and the FWD car will likely understeer about as much as the AWD car. Given that the Evo is a front-engined car, and that its suspension is designed with lots of "snap" in it...

Here's something else, high-performance AWD cars(like Impreza STI, Audi RS4, Skyline GT-R etc.) are RWD-biased, meaning more power is sent to the rear wheels than the front, this lessens its FWD-like tendencies of understeering when entering a corner.
With less power on the front wheels, its easier to point(steer) the car where you want it to go.

[basti08]

esp. the Skyline with its advanced AWD (ATTESA + SUPER HICAS 4WS). it can even send 100% of power to rear wheels if needed.

[niky]

The Skyline is actually at 100% RWD most of the time. It's only when it detects slip that it starts juggling drive to the front wheels. Gives it a killer corner entry, and makes it handle better than the Evos and STis of its time...

But the new STi, with its rearward bias, and the active-yaw control Evo have pushed the game on further since the Skyline left.