AWD - it really is useful in slippery conditions

Just had a read of this, and can't let it go without comment:

http://www.wheels.ca/article/800946

To begin with, tire grip is only a function of the interface of your tire tread and the road surface.

Plus weight on the tyre, really important because a large part of car control is managing weight shift.

And tyre grip is split into adhesive and mechancial grip.

What determines the amount of grip is the rubber compound of the tread and how well it reacts to cold temperatures, the construction and age of the tire, the depth of the tread, air pressure and the size and shape of the actual contact patch. A rolling tire will give the driver only as much traction as its above-stated characteristics dictate. Putting engine power through that tire will not make it deliver more traction.

I think I see the point of this statement now.  It's when people say "AWD gives you more traction".  That's not strictly true - what it does is allow you to use more the tyre's available traction more efficiently.

In other words, a given tire on a skid pad will only develop “x” amount of grip. If you try to power that tire by putting engine torque to it, that tire will not make any more grip than if it was freewheeling.

That's true, but misleading.  Comparing 2WD to AWD, of that "x" per tyre less of the "x" now needs to be devoted to moving the car as opposed to providing a cornering force.  The total traction available per tyre is the same, 2WD or AWD, just tha AWD uses more of it hence it appears more is available.

Precisely the same claim is made about cross-axle locking differentials - that they 'increase' traction.  They don't, they simply permit more of the tyre's traction to be used.  The same is true of electronic traction control.  The principle is best illustrated by a skilled race driver taking a car around a corner vs a novice driver.  Both drivers may be on their limit, with tyres squealing, but the race driver will go faster simply because he has made better use of the traction available, not because he has more traction available.

Having said that, AWD or 4WD will not help a vehicle turn (steer) with more traction.

That's wrong.  It certainly can help in many situations, as described below.

For example, if we had a vehicle in which AWD could be turned off so the vehicle also drove with only 2WD, that vehicle will generate the same amount of lateral grip on the skid pad whether it was in AWD mode or 2WD mode.

This has been done and the results provide otherwise.  The reason is simple.  When a vehicle is turning the tyres must provide lateral grip, to keep the turn going, and some measure of longitudinal grip to overcome drag and resistance, otherwise the car will slow down. 

For the purposes of this example, the lateral (cornering) grip can simplistically be thought of as being distributed equally between the two outside wheels.  The writer is correct about there being "x" amount of grip - but that grip needs to be divided between lateral and longitudinal grip.  In the case of a 2WD, one set of tyres has to do all the longitudinal gripping.  This, simply, leaves less grip available for lateral grip.

An AWD car by contrast splits the longitudinal grip requirement between both wheels, therefore leaving more of the "x" for lateral grip and therefore permitting higher speeds, or the same speed on a lower-traction surface.

This is the reason why AWD rally cars, which work on slippery surfaces, are much quicker than rear wheel drive (RWD) or front wheel drive (FWD) cars.  Once you get into racecars with ultragrippy tyres and where weight and other factors are critical the equation changes, but the article is about winter driving in normal roadcars.

Bottom line, AWD or 4WD will not enhance the active safety of steering, such as collision avoidance or cornering grip.

Wrong, for the reasons explained above.  And in addition driving the front wheels will pull the car around, similar to what happens with FWD.  The problem with FWD is that the front tyres are doing the work of pulling and turning, and that's not good in slippery conditions.  With a RWD the front wheels aren't being actively pulled, they're pushed and that won't help them bite in.

Now if we look at another active safety feature of vehicles — braking — AWD does not help shorten braking distances.

Mostly wrong.  Winter drivers are taught to use the engine to slow down to avoid lockup.  This is engine braking, and it works by the engine attempting to turn the wheels slower than the roadspeed, and the enforced rotation mitigates lockup.  Now what's better, slowing down with four wheels or two?  Four, so yes indeed AWD can help when you're braking if you use the engine as is good practice in very low traction conditions.  The difference may not be huge, but it is there.

It actually can make braking distances longer due to the added mass of the AWD system.

Technically correct, but the extra mass isn't that much in roadcar terms.  It's a passenger or so. 

So what benefit is there to having AWD on a vehicle? It allows the vehicle to accelerate better in slippery conditions.

And turn and brake.  So if the car can accelerate better, it has more traction...which has to be a good thing, right?  What if you're turning and driving uphill, or turning and accelerating, for example at a road junction?  Surely AWD would be a benefit there?  Physics says it is!

It is basically a performance feature, not a safety feature. AWD will distribute the torque to the tires with the most traction and to all four tires if required.

Not if required, it does this by definition, except that differentials distribute torque to the tyres with least traction, but that's not a AWD problem.  It is however an AWD advantage. 

An on-demand AWD driving just one axle and the other as required will distribute torque as required but that's not true AWD, and traction control will do much the same job as far as it can.

This translates into less tire slip and better acceleration. This is also a double-edged sword. Many readers have told me because there is less chance of the tires slipping (only when power is applied) that this in itself is a safety feature. Not necessarily so. That slip of a tire under power can be a good thing. It tells the driver there is very little traction. If the tires do not slip as can happen with AWD, it can mask how slippery the road surface really is and contribute to an overconfident driver.

That's a good point about confidence.  But I think the tyres not slipping is a safety feature. 

What should also be covered is skid recovery.  Recovering a RWD car from oversteer takes some skill, FWD less so, AWD even less.

There is no place better to show this concept than at our winter driving schools. When we discuss tire grip and AWD in the class, I’ll get my share of strange looks from the students as this is contrary to what they have been told. It’s not until they are driving on the ice and realize they cannot negotiate the skid pad, slalom, collision avoidance or emergency stopping any better than front wheel or rear wheel drive vehicles.

This is interesting...my tests and training definitely show improved control with AWDs in slippery conditions in acceleration, steering and to a small extent in braking too.  However, if all the student exercises are under brakes then I wouldn't expect to see much difference.  If they involve any non-braking there would be.

One point worth making is that of winter tyres, or tyres designed to operate at low temperatures, something the UK has just discovered!  These make a huge difference, so much so that a 2WD can outperform an AWD in some conditions.  But put the winters on an AWD and the AWD is back in front again.

Anyway, don't take my word for it.  Fifth Gear have come to the same conclusion:

http://www.youtube.com/watch?v=jV18ZBXwwYM

The presenter did say she'd take the RWD, but she's an ace driver capable of snapping an oversteering car back under control, and stability control is available on AWD and FWD cars too.

Here's another one:

http://www.youtube.com/watch?v=UE9TRnjidXw

And this is not well made, but the same point is brought home. Whether it's gravel or snow, it's still low-traction.

http://www.youtube.com/watch?v=DDOtoaBEAPs

This one's not bad, not specific to snow or ice but does demonstrate AWD traction and the advantage of full-time AWD as opposed to 'on-demand'.  It should be said that not all on-demand systems are that bad, but none are as good as proper full-time AWD.

http://www.youtube.com/watch?v=ooQRxlChvMw

The author of the original article, Ian Law, has read this page and kindly added his views below.   All comments welcome!