4WD vs RWD

[djones]

How much of an advantage is 4WD over RWD in a road car (lets say a 300 BHP car)?

I've thought about it a lot and as far as I see it, on a dry road there is little to no advantage from a standing start, no advantage around a corner but maybe an advantage out of the corner. As a downside you have higher drive train loss and extra weight.

I only have very basic knowledge of this kind of thing so I'm probably very wrong but this is how I see it.......

Standing start - A large amount of the weight goes to the back of the car so the front wheels hardly have any weight over them anyway. (take a drag racing car for an extreme example)

Around a corner - The fact that it's 4WD cannot increase lateral grip.

Out of the corner - maybe it will have better traction due to the fact its not a standing start and there will be less weight transfer.

Like I said my theories are probably wrong so any correct information would be great.

The only thing so far that's making me think I'm wrong is the fact Audi always used to win in touring cars with their 4WD cars.

[NickT]

Ok simply:

Advantages:

Better traction in almost every situation.
Less tyre wear, driving 4 rather than 2 wheels.
More stability under breaking and in transient conditions.

Disadvantages.

Weight
Complexity
Cost

Audi set up their latest systems with a rear wheel bias to give the car a more neutral to over steer feel, allowing more of the grip of the front wheels to do the steering. But this can be altered automaticly if the rear starts to break away allowing the front wheels to puul the car back onto the straight and true.

The best comparison I saw recently was Tiff Needel in an Audi S4 and Jason Plato in a BMW M3 CSI on Channel 5's Fifth Gear - Awsome and cool when they were trying to power slid them both together

[DaveKillens]

On cars with approximately same weight and power, the 4WD usually beats the RWD on a dry race track. But not every day is perfect, and when things turn sour and you have sand, or rain, or snow, the 4WD suddenly stands heads and shoulders better than it's RWD counterpart.

[m3_lover]

Say your tires had about 25000 km on them, and if one of your tires pops a hole and you changed it, does that mean you have to change the other tires as well on a 4WD because of the computers reading an unusal tires as in one tire has all of its tread and the others have been used?

[NickT]

No thats what the differentials (3 of them) are for, plus the software is already designed to deal with this type of thing as they have to deal with differing tyre pressures and punctures.

[flynfrog]

its not realy that simple 4wd cars tend to oversteer horibly in the dry because like a fwd car you are asking the front tires to both steer and drive the car. (im sure someone will chime in about computer controled diffs and what not) in reality id have to say that 4wd is the best only when it weights the same at a rwd and is coupled with 4ws so that you can split the steering and drivng between all 4 wheels



ps what about 6 wheelers

[NickT]

I think you mean understeer, and yes the early systems did horribly with a 50:50 torque split.

The latest systems have a desirable rearward bias, that can be controlled electronically or can be purely mechanical. This lessens the work being done by the front wheels and gives a much more balanced handling setup.

[mini696]

I race 4wd radio controlled RC cars, and during a race I snapped the dogbone effectively making it a RWD... MAJOR OVERSTEER.

However I didnt have another spare so I adjusted the setup (MAJOR MAJOR differences between my 4wd and RWD setups (like a much softer rear and much harder front when in RWD mode).

My lap times were "similar".

If you change a RWD car into a 4wd without any other setup changes then the 4wd car will understeer very badly.

4wd gives more grip in corners because you still have 4 wheels driving, even though lateral loading doesent change.

[NickT]

What is the torque split and can it be varried

[RH1300S]

4WD seems to win in most situations (rpobably any situation where the car has enough power to make it traction limited). AFIK - 4WD can increase corner speed because the tyres are sharing the corning force and power distribution better. With RWD the rear tyres are trying to corner and accept power - at some point they won't be able to deal with the conflict and the back gets a bit waaahayyyyyyyy. With 4WD you delay that point as you can send some power to the fronts, which would have been doing far less cornering work at the point the back gets loose.

I still like the idea of rear wheel drive 'though

[NickT]

I still like the idea of rear wheel drive 'though

Definately on a track and the road

But would leave the Ferrari at home in more extreme weather and take the Audi or perhaps the Lamborghini Gallardo