matt d

"It's one of the great motoring myths that four-wheel drive increases grip. It doesn't."

- this month's "Autocar"

Right or wrong? Btw they were discussing cornering grip, in their lateral g test article.

I reckon they are wrong. 4wd does not increase total grip, but it does increase cornering grip under power vs a similar car with 2wd.

Andy W

I had a quick look at one of the magazines today (can't remember which one) and it had a comparison of the grippiest cars, the UK300 was only slightly behind lightweights like the elise, the noble M12 won.
Judge for yourself.

Andy W

[This message has been edited by Andy W (edited 04 August 2001).]

matt d

Sadly their results are almost meaningless with regard to the effectiveness of 4wd, since other variables account for much more of the difference in cornering grip (e.g. tyre type and size, suspension setup, chassis, weight etc)

SDB

They are dead right in the 4WD does not increase cornering grip as such.

The way to get your head round it is this..

4WD increases traction, but it only does this by using the traction available from all of the tyres rather than just two.

In cornering, the demand for longitudinal grip (traction) from the tyres does not increase the amount of lateral grip they can produce (in fact, demanding longitudinal grip from the tyres actually reduces their effective lateral ability - but this is a side issue).

The fact is, 4WD only increases the *cars* overall traction, not it's cornering grip.

Cheers

Simon

matt d

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR>Originally posted by Simon de Banke:
<B>They are dead right in the 4WD does not increase cornering grip as such.

The way to get your head round it is this..

4WD increases traction, but it only does this by using the traction available from all of the tyres rather than just two.

In cornering, the demand for longitudinal grip (traction) from the tyres does not increase the amount of lateral grip they can produce (in fact, demanding longitudinal grip from the tyres actually reduces their effective lateral ability - but this is a side issue).

Simon[/quote]

Ok, the traction bit is obvious. BUT - doesn't the use of grip for traction reduce the lateral grip available to the tyres? This "side issue" seems like it would reduce the cornering ability of the car.

john banks

But only if you are accelerating through the corner?

SDB

matt & john - dead right..
although an addition, decelerating also reduces cornering traction.

This is one of te main reasons why *typically* 4WD has reduced turn-in bite than RWD.

But... don't let that fool you into thinking that just because one car 4WD and another is RWD that the RWD can go round corners quicker than 4WD...

The speed a *car* cam go round a corner is not JUST about the overall cornering grip of individual tyres.. (although it is probably safe to say this is the MAJOR factor)....

but it is the car as a whole which creates the corner speed, and this includes using the available grip at each contact patch, so it's never as simple as a blanket statement.

But it is definitely the case that 4WD does not (on it's own) increase cornering grip.. in fact it is more likely to reduce it (if you had to generalise).

cheers

Simon

matt d

Ok my "hypothetical example" would be 2 identical cars, one with 2wd and one 4wd, let's assume that 4wd weighs nothing. Under no power or braking, the cornering grip is identical. Under power, well below the limit of grip, both corner at the same speed. But let's look at the limits of the 2wd car - it only has so much lateral grip. Now under power it also requires some grip to the rear (or front) wheels for traction to maintain or increase speed mid-bend. This takes away from lateral grip (cornering grip). Given identical power, shared between two wheels, and the same power, shared between 4 wheels, surely the 4wd car will retain more grip for cornering than the 2wd car?

I'm not quite sure how the 4wd car will have the same, let alone less, cornering grip available under power than the 2wd car. If this is true, why do WRC cars have 4wd despite the weight disadvantage???

SDB

matt.. great questions..

WRC = traction. like I said, this does *not* mean that the *car* will go round corners slowers than RWD, just that you are using up more of the tyre's grip in traction when under power.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR><B>This takes away from lateral grip (cornering grip). Given identical power, shared between two wheels, and the same power, shared between 4 wheels, surely the 4wd car will retain more grip for cornering than the 2wd car?[/quote]

Yes this is true, but, not necessarily.. RWD cars (particulrly performance cars) will be more likely to be set-up to utilise the benefits of the extra available lateral grip at the front, so the entire chassis set-up will be tuned differently. If you put that set-up into 4WD you would probably end up with serious power on understeer (although it is impossible to make such a generalisation).

You can't either say "the two cars are exactly the same, what then?" as some set-ups will favour the 4WD and others favour the RWD.

The facts are... there are some AWD cars that go round corners quicker than RWD cars, and there are some RWD cars that corner quicker than AWD cars.

actual mid-corner, neutral throttle cornering is a TINY part of the actual bend, let alone and entire road, so it (again) does *NOT* mean that RWD cars will go round corners quicker.

cheers

Simon