4 WD 911-997= Driver has limited abilities??????

[crbox]

A non motoring enthusiast friend asked me why Porsche make 4WD and 2WD versions of the same model.

I replied,

In absolute, 4wd may ofer higher cornering speeds but in reality, it translates to more grip in
all conditions.

So more grip equals higher cornering speeds?

Yep - usually

So whats the downside?

More weight, cost, complexity and a reduction in responsivness.

But it's faster right?

Nor necessarily,but definitely yes for inexperienced drivers

So what's the point of them producing the 2wd like you run?

Because some of us enjoy controling the progress near the limit, directly - not have a computer do it for us. To me the 911's draw is that, it could go out of control if you're weak - but can offer amazing cornering if you stay committed.

So who'd be quicker - you in the 2WD 911, or an average driver in a 4WD 911?

I'd like to think I'd be quicker - everything equal.

So really only drivers that are scared of the car go for a 4WD?

Um.............

[sidicks]

I think that although 4WD requires a different driving technique than 2WD, you can drift a 4WD just as well as 2WD - In real world conditions, for most people, I reckon the 4WD would be as much fun and just as fast.

Of course the 4WD also looks nicer and will be much faster in inclement weather...............

Sidicks

[Steve_M]

I believe the 4wd carrera is basically for the mainland european market, they are great in winter condition because of the extra grip and 4 wheel traction, they chew up snow and ice so much safer and easier than the 2wd car.

[housemaster]

crbox said:

In absolute, 4wd may ofer higher cornering speeds but in reality, it translates to more grip in all conditions. So more grip equals higher cornering speeds?

I see this statement often when 4WD is discussed, usually on certain Japanese fan sites.

4WD does not offer higher cornering speeds if the road is dry and often if the road is wet. There is a distinct difference between mechanical 'grip' and 'traction', and the two are very different but often confused when the 4WD debate pops up its head. No question that more traction is delivered by 4WD in most conditions, but not necessarily more lateral grip or cornering ability. I would also agree that for most drivers, 4WD will offer a more 'secure' feeling in poor conditions but when it comes to absolute grip 4WD is not typically any better. In some case it may be worse due to the disruption of physics driving the wheeels rather than just steering them.

I am sure someone with better understanding of physics than I can afirm this, but I would use traction as an advantage of 4WD (or if you like forward/backward grip) rather than lateral 'grip' or absolute cornering ability.

[DanH]

Are you not confusing traction with grip?

[sidicks]

housemaster said:

I would also agree that for most drivers, 4WD will offer a more 'secure' feeling in poor conditions but when it comes to absolute grip 4WD is not typically any better.

In principle, I agree, but the 4S has wider rear tyres than the 2S, hence does generate more grip as well as traction......

Discuss!!

Sidicks

[crbox]

The wider the tyre the more compromised the steering geometry is. More grip yes, but due to an increase in friction - not load. The load supported by a wider tyre is less a than a narrower one, per lateral unit. They also consume more power to turn, from standstill and they also wear out quicker.
That's why dragsters have wide tyres and mpg record cars run on circular razor blades.
The wider the tyre the more the scrubbing - due to differential in turning radius across the footprint. That's why there's little steering feel on wide front tyred cars like Cobras.
Not sure I agree 4wd 911's look better but it's clearly subjective. I would expect anyone looking for maximum real world performance would want the narrower car.
That way you get a wider road as there's more room for error.
In fairness I s'pose you 4wd mob would say - We don't need a wider road - we won't be spinning it anyhow.......

[ponkyporky]

Good comment, well described.

My view was always traction on a tyre would be a function of:

load - which has no bearing on tyre width, the wider the tyre the more dispersed the same load
Surface area/ footprint - everything to do with tyre width and includes tread pattern too
Road surface - smooth, dry, wet
Compound adhesiveness - temperature and softness/stickness/wear/age of compound.

I consider this all to be in the traction category.

Correct me if wrong but I have always considered Grip would come in to the 4wd equation, when four points controling and driving the weight with the C of G being contained within the 4 wheels impliesg greater grip than two wheels behind the C of G confined to a single axle having less grip and general control.

Is that seem right to the technitions?

[damianc4s]

As I've said on many occasions, no foot on the throttle no power going to any axle, so in this situation is a 2wd Porsche slower or has less grip than a 4wd Porsche?

As Sidicks said it does require a change of driving style to make use of this possible extra traction, but just now many of us actually do that?

[persuader]

damianc4s said:

As I've said on many occasions, no foot on the throttle no power going to any axle, so in this situation is a 2wd Porsche slower or has less grip than a 4wd Porsche?

With 4wd though surely the fact that engine braking is spread over 4 wheels must give some benefit?

>> Edited by persuader on Tuesday 21st March 22:11

[tertius]

ponkyporky said:

Good comment, well described. My view was always traction on a tyre would be a function of: load - which has no bearing on tyre width, the wider the tyre the more dispersed the same load Surface area/ footprint - everything to do with tyre width and includes tread pattern too Road surface - smooth, dry, wet Compound adhesiveness - temperature and softness/stickness/wear/age of compound. I consider this all to be in the traction category. Correct me if wrong but I have always considered Grip would come in to the 4wd equation, when four points controling and driving the weight with the C of G being contained within the 4 wheels impliesg greater grip than two wheels behind the C of G confined to a single axle having less grip and general control. Is that seem right to the technitions?

I agree completely ... except the other way round ...

Grip is about holding on to the road; traction is about transferring power to the road.

4wd cars will have (potentially) more traction but (all other things being equal) no more grip.

[WILL_T]

4WD got me back from Scotland through a snow storm when loads of Mercs & BMWs had to park up

Will

[aceparts_com]

VW/Audi discovered that driving all 4 wheels actually increases top speed slightly due to the front tyres contact patch moving slightly ahead compared to unpowered front wheels.

Don Palmer demonstrated to me how 4wd can improve overall cornering ability by basically keeping the front end pointing where he wanted to go and having all 4 wheels burning rubber. Not entirely practical in all conditions though.

[nel]

I went straight for the 4WD option because the turbo was my dream car, but have had occasional regret pangs when reading about the "purer" 2WD 911 experience. Somehow that regret fades when the going gets wet and slippery, and you can boot the gas coming out of a well-sighted bend, get a lairy twitch of 4WD scrabbling then howl up the road like an exocet.



But hey, if you guys are right that 4WD is for pussies, I better start saving for a GT2.

[tony.t]

Disadvantages of 4wd is that, like for like, it will increase the weight of the car thus reducing acceleration, increasing braking distance and reducing cornering speed. There are also increased transmission power losses and hence less power at the wheels. Then there is the problem of power to the front wheels inducing understeer.

The obvious advantage of 4wd is that each wheel has to put down a smaller portion of the engine power to the road and hence the tyres are less likely to exceed their maximium available grip thus improving traction. The other advantage of most 4wd systems is that they act as a de facto traction/stability control shifting power from wheels that are losing grip to those that aren't and it's this that generally allows higher conering speeds to be reached for most drivers since it negates the need for the driver to worry about controlling under/oversteer.

[foryorent]

any one know the differences between the "old" awd system on current and older cars and the suposed "new" one one the new turbo?

why cant they just come out with a system like Infiniti has on the GSx? It's like 85/15 rear bias for launching, rear drive above 15 unless there is wheel spin, then it modulates power accordingly to the various wheels. Up to 40% in the front I think.

I would take the additional weight this adds in exchange for the ability to go awd when you need it.

one thing - YOU MUST BE ABLE TO TURN IT OFF

go to strait rear wheel drive with zero intervention.

better yet, have control over the amount of rear drift that causes the awd to kick in. even get control over how it kicks in. that would be perfect.

[crbox]

WILL_T said:

4WD got me back from Scotland through a snow storm when loads of Mercs & BMWs had to park up Will

No offence Will, but Monte carlo Rally Winners look like this

'68 - 911T
'69 - 911S
'70 - 911S
'78 - 911 Carrera

All 2WD
All over snow
All snow studded tyres (I think)
From a company that had little interest in rallying - what a prospect if they were eligible today!

[GravelBen]

crbox said:

[quote=WILL_T] No offence Will, but Monte carlo Rally Winners look like this '68 - 911T '69 - 911S '70 - 911S '78 - 911 Carrera All 2WD All over snow All snow studded tyres (I think)

...All against other 2wd cars.


4wd does give huge traction in slippy conditions, but 911's have good traction anyway so may not be such an issue.

[Raven Flyer]

A couple of points...

Have a read up on slip angles and how power distribution affects overall grip.

Secondly, once the limit of grip has been exceeded, whether that be due to conditions or simply due to the cornering speed being used in the dry, then 4WD will be much easier to pilot.

Porsche's system is 95% rear 5% front, in normal use. Because the central diff is a viscous coupling, this ratio will only change when a wheel spins, so in all full grip situations, the car will be have like a rear wheel drive car.

[nbetts]

It is a subjective thing really... unless you are a racing driver who is able to extract the nth degree out of your car.

I think the Wide Body 911's as seen on the latest 4s look great.

I prefer the feel of the 2WD cars to drive. The steering feels 'nicer'...

I do not dispute the advantages of 4WD over 2WD in something like snow but I do not think that 4WD cars are faster round bends in dry conditions.

Without having a Physics degree - I can't back it up... but if a tyre is turning and propelling how can it do both tasks equally as well. In a 2WD car the front wheels are just for steering... they do not have to propel.