XF Forum banner

1 - 20 of 20 Posts

·
Registered
Joined
·
9,965 Posts
Discussion Starter #1
http://www.tirerack.com/winter/tech/tec ... echid=168&

recommended higher pressures for winter use.


http://www.tirerack.com/winter/tech/tec ... echid=163&

recommended deeper tread depth remaining at replacement time. For the UK this means you could use your cold weather tires for longer if it was just cold and not snowy. Ice performance is not adversely affected by lower tread depth.

http://www.tirerack.com/winter/tech/tec ... echid=122&

Computerized driver aids and awd not necessarily advantageous.

http://www.tirerack.com/winter/tech/tec ... echid=126&

Narrower tires may be better, especially in deep snow. Ice traction is not affected much by tire profile except for sidewall height effects.

http://www.tirerack.com/winter/tech/tec ... echid=168&

Static "cold" tire pressures need to be set in anticipation of actual ambient temperatures expected when driving.

Winter is still a factor over here, until May. Tire Rack's main facilities are in Colorado which has quite similar conditions to ours. Your current winter conditions are worse than what we are experiencing over here, although the Trans Canad highway has been closed due to avalanche risk for the last three or four days. People living or vacationing in the closed zone are actually running out of food.......and we have the usual rash of avalanche fatalities after every major winter storm in the Mountains, three this weekend, locally. This added to three who died when their softroader mini SUV slid off a mountain road, rolled over and broke through the ice. Rescuers were unable to release the seat belts of three of the victims as they were of course hanging from their belts, they drowned. Don't know if they were using winter tires but I suspect not. The rescuers were up to their chests in freezing water and are lucky to be alive themselves, needed seat belt cutting tools which everyone should have in their car, actually.

Winter tires are more important than good brakes, in winter. They may seem like an unnecessary expense until your life depends on them.
 

·
Registered
Joined
·
6,985 Posts
jagular said:
Computerized driver aids and awd not necessarily advantageous.
I don't know why you try so hard to make this point, when the Tire Rack article, every serious publication, and driver experience tells exactly the opposite - i.e. that AWD provides a real advantage in traction for acceleration and getting unstuck.

Here is the section on the TireRack article you linked:

"AWD (All-Wheel Drive Systems)/4WD (Four-Wheel Drive Systems)

All-wheel/four-wheel drive cars, SUVs and light trucks have become very popular among drivers living in the snowbelt. While their year-round versatility certainly plays a role, they are often selected primarily because their all-wheel/four-wheel drive systems make winter driving easier.

The ability of these systems to divide the vehicle's power among all four tires provides a real advantage when accelerating on slippery roads.
A 200 horsepower rear-wheel drive vehicle with a limited slip differential requires enough traction from each tire to accept about 100 horsepower. The best four-wheel drive systems divide that same 200 horsepower among all four tires. Each tire then requires only enough traction to transmit about 50 horsepower. Doubling the driving wheels virtually doubles the acceleration on slippery surfaces."

The article goes on to say that they don't provide any/much advantage in stopping and cornering, which, no one ever claimed it did. But braking and turning don't even come into play if you can't first get going.

I have never seen ANY serious writer say anything other that that, of two similar modern passenger vehicles, equipped with similar tires, the AWD version is better in wintry conditions than the RWD or FWD version. It is one of those settled issues - settle a long time ago. Well, I take that back, if you are interested in doing some drifting, or donuts in the parking lot, the RWD version is better - but, you know what I mean :) :)
 

·
Registered
Joined
·
9,965 Posts
Discussion Starter #3
Because awd is not a drive system for inexperienced or unskilled drivers and it is sold as suitable for same.

Unless actually mudplugging off road at low speeds, fwd is far superior to awd for the ordinary driver.

In poor traction conditions it is the braking limitations that are important, not the traction limitations.

If you are skilled you do not need awd for road driving and if you are unskilled awd can be downright dangerous.
 

·
Registered
Joined
·
6,985 Posts
So lets see. For similar vehicles, equipped with similar tires, in winter slippery conditions:

AWD has better acceleration traction and better get going traction - no question (though you think that is not important).

AWD is no worse under braking (note that weight is not a factor for braking so long as your brakes have the power to lock the wheels given the available traction). It is likely marginally better than a FWD car due to slightly better weight distribution.

AWD is no worse under cornering (again, weight and tipping moment are not a factor in slippery conditions). In general it is marginally better in cornering than most RWD and FWD cars because of better weight distribution. e.g. A Carrera 4 is much, much easier for the average driver to make turns in snow than a Carrera 2. For FWD, cars the "plowing" effect - going head on straight is reduced slightly by the typically enhanced ballance when AWD gear is added.

So exactly, what areas is an AWD version of a vehicle worse in wintry conditions than a RWD or FWD version of the same vehicle?

I have yet to find out.
 
G

·
Guest
Joined
·
0 Posts
baron95 said:
So exactly, what areas is an AWD version of a vehicle worse in wintry conditions than a RWD or FWD version of the same vehicle?
The answer to this question is probably in the UK where most awd vehicles are fitted with summer tyres. The acceleration traction far exceeds the cornering ability & the ability to stop in a straight line. Remember most UK inhabitants are inexperienced at winter driving & are primarily concerned with getting going until the inevitable happens. I had expected the Scots to be far superior drivers in these conditions but having seen numerous news reports recently I think we are all equally as bad.
 

·
Registered
Joined
·
1,412 Posts
baron95 said:
So lets see. For similar vehicles, equipped with similar tires, in winter slippery conditions:

So exactly, what areas is an AWD version of a vehicle worse in wintry conditions than a RWD or FWD version of the same vehicle?

I have yet to find out.
I cannot verify the results for the same vehicle but I recently had some experience on a skid pan with using both AWD and my RWD XF. Now, I am perhaps just outside the 'top ten best racing drivers in the U.K.' ;) but I did notice that when the AWD lost traction completely it was very unpredictable. When my XF lost traction it was easier to correct. If that is what jagular is getting at then he has a valid point :)
 

·
Registered
Joined
·
231 Posts
raregrin said:
baron95 said:
So lets see. For similar vehicles, equipped with similar tires, in winter slippery conditions:

So exactly, what areas is an AWD version of a vehicle worse in wintry conditions than a RWD or FWD version of the same vehicle?

I have yet to find out.
I cannot verify the results for the same vehicle but I recently had some experience on a skid pan with using both AWD and my RWD XF. Now, I am perhaps just outside the 'top ten best racing drivers in the U.K.' ;) but I did notice that when the AWD lost traction completely it was very unpredictable. When my XF lost traction it was easier to correct. If that is what jagular is getting at then he has a valid point :)
But surely your rwd loses traction long before the awd? My experience with awd on slippery surfaces is that you get better steering because the wheels are not pushed ( ie the fwd advantage) while retaining an awd braking effect going downhill where the fwd would get slippage on the front wheels. Personally, I prefer rwd to fwd because you get steering and push on separate axles, but I'm certainly aware of the advantages inherent in awd. The stuff about abs and dsc/tc/ not being of value on snow is in my opinion somewhat misguided. We did have voices voicing this in Norway when these aids were still uncommon, but I believe consensus now is that they are indispensable. But you do need winter tyres... It is also a well-known (in other words experience based) fact that narrower tyres go deeper in snow and may increase grip by compressing the snow or reaching through it to better surface underneath, just as a heavier vehicle will have better grip (for the same reason). Main thing is still to remember that less friction means longer stopping distance and less ability to handle acceleration (and cornering).
 

·
Registered
Joined
·
6,985 Posts
Gemset said:
baron95 said:
the acceleration traction far exceeds the cornering ability & the ability to stop in a straight line
OK, I was talking about the car capabilities.

If you are talking about the car/driver capabilities and the thinking is, "it is safer because without AWD the car won't move anyway, and a stationary vehicle is safer", fine. I can't argue with that.

I thought a car was a machine designed to take you from one location to another. If it doesn't even move, it is not even starting to do its job.

If it moves but gives you ammunition to kill yourself (or your neighbors), well that is natural selection.

If we follow this logic, we can say that the BMW M3 is a worse car than the BMW 320 because it has more speed potential, which in the hands of most drivers will make them get in trouble. I.e. the acceleration will exceed they ability to brake, turn, etc.

Good luck with making that argument at a car enthusiasts forum.

However, your local politician may pick up on that and start taxing cars based on performance potential.

Oh wait. They already do it in the UK. Bummer. FWD Prius for all :)

At least the ricers know how to make their cars "look fast", right? :)
 
G

·
Guest
Joined
·
0 Posts
I think you are just about getting there Baron :lol:

One last go & I'll keep it simple........

If it's ability to go is matched by it's ability to corner & stop it should be reasonably safe.
 

·
Registered
Joined
·
6,985 Posts
Gemset said:
If it's ability to go is matched by it's ability to corner & stop it should be reasonably safe.
Over here we only care about the GO part, even in the snow - stopping is for wimps :)


But, for all those who have anxieties about driving those unsafe AWD vehicles in the snow

Here is a cheap and safe way to practice :)

[youtube:10md2nek]hr3958ZPj3k[/youtube:10md2nek]
 

·
Registered
Joined
·
1,412 Posts
dyrlege said:
raregrin said:
I cannot verify the results for the same vehicle but I recently had some experience on a skid pan with using both AWD and my RWD XF. Now, I am perhaps just outside the 'top ten best racing drivers in the U.K.' ;) but I did notice that when the AWD lost traction completely it was very unpredictable. When my XF lost traction it was easier to correct. If that is what jagular is getting at then he has a valid point :)
But surely your rwd loses traction long before the awd? My experience with awd on slippery surfaces is that you get better steering because the wheels are not pushed ( ie the fwd advantage) while retaining an awd braking effect going downhill where the fwd would get slippage on the front wheels. Personally, I prefer rwd to fwd because you get steering and push on separate axles, but I'm certainly aware of the advantages inherent in awd.
You are absolutely correct Dyrlege :)
I sometimes forget that this is a worldwide forum and it is important to be unequivocal with one's explanations. I am guilty of not stating what I believed to be obvious. As you say RWD does lose traction before AWD and therefore AWD is inherently better equipped to deal with the treachery of winter roads. However, my postulation was that when the limits of safety have been exceeded, an inexperienced driver who lost grip in an AWD vehicle would perhaps fare worse than the same driver skidding in a RWD vehicle :)
 

·
Registered
Joined
·
231 Posts
raregrin said:
You are absolutely correct Dyrlege
I sometimes forget that this is a worldwide forum and it is important to be unequivocal with one's explanations. I am guilty of not stating what I believed to be obvious. As you say RWD does lose traction before AWD and therefore AWD is inherently better equipped to deal with the treachery of winter roads. However, my postulation was that when the limits of safety have been exceeded, an inexperienced driver who lost grip in an AWD vehicle would perhaps fare worse than the same driver skidding in a RWD vehicle
I absolutely, totally and wholeheartedly agree with you on this one! When you get stuck or when you exceed safety limits with an awd, then you are in trouble, a lot more trouble than with a rwd or fwd, and yes, an inexperienced driver would get there quickly. :lol:
 

·
Registered
Joined
·
6,985 Posts
I'm not sure I agree entirely (it depends on the situation), but I'll agree to this extent...

A driver of a RWD car in slippery conditions (assuming he/she is not stuck), is MADE aware of the available traction during the acceleration phase. The rear end will get twitchy and snaky (even with DSC) if you push too hard. So you are put on notice that there is not enough traction before you get to the need to turn or break.

FWD and to a greater extent AWD will mask the slippery conditions from the driver on the acceleration phase, and that driver is more apt to be surprised during the turn or break phase.

That is true.

However, in many other circumstances, the RWD driver can get a hand full - accelerating out of the first turn of the day - is just one. I've seen many a RWD car do a full 360 when pulling out of a stop to make a turn.

The FWD driver also often gets caught going wide on a turn by plowing straight out due to the perverse front biased weight distribution that is MANDATORY on all FWD cars.

But let me also say, at least for the American driver in the cold climates, these distinctions are totally lost - AWD is the only thing that those worried about winter driving care about.

*Interesting Discussion Though*
 

·
Registered
Joined
·
6,985 Posts
Not sure what Jagular would recommend for this New York City car owner.

AWD - it has it - it is a US-spec Audi after all.

Snow tires - hard to tell.

Me? I recommend a blow torch :)


P.S. temperatures were about 1F (about -16C) in the New York Area this weekend. Only side effect to the XF was that the trunk (boot) became hard to open/close - I think the oleo/struts gelled and didn't flow right. Sottozeros got a bit hard and flat spotted for first 3 miles or so, but gripped well after a short workout.
 

·
Registered
Joined
·
6,985 Posts
Of course, when all the snow we got start to melt in the spring - I'm taking the bus.

Thinking of hiring the driver of this bus:
[youtube:36yp99oh]iAs8scrGTzk[/youtube:36yp99oh]
 
S

·
Guest
Joined
·
0 Posts
baron95 said:
Not sure what Jagular would recommend for this New York City car owner.

AWD - it has it - it is a US-spec Audi after all.

Snow tires - hard to tell.

Me? I recommend a blow torch :)


P.S. temperatures were about 1F (about -16C) in the New York Area this weekend. Only side effect to the XF was that the trunk (boot) became hard to open/close - I think the oleo/struts gelled and didn't flow right. Sottozeros got a bit hard and flat spotted for first 3 miles or so, but gripped well after a short workout.
Crikey it must be Bleeding Means new Ice Wash treatment.Alfonso what have you done :shock:
 

·
Administrator
Joined
·
22,463 Posts
baron95 said:
Of course, when all the snow we got start to melt in the spring - I'm taking the bus.

Thinking of hiring the driver of this bus:

[center:xef10e34][youtube:xef10e34]iAs8scrGTzk[/youtube:xef10e34]
[/center:xef10e34]

Fair play Baron, I say hats off to the drivers :shock: :shock: :shock:
Quite amazing. My other though was that it is a good job that the bus was not the lead vehicle in an operation described Here :lol:
 

·
Registered
Joined
·
6,985 Posts
No kidding, Pekem.

True story - I once was driving to a vacation spot in the mountains and there was a lot of fog on the curvy road - it is about a 45 min ascent with the fog. After a while, I was leading a pack out of about half a dozen cars. Nearing my destination, I pulled out of the main road into the side road - carefully not to brake as I didn't want to collect anyone in the rear.

Proceeded up the road and up the winding driveway to the house....only to realize that all the lights following me were continuing in tow.

Result - got to the house and had to tell 5 or six motorists that that was our house and they had to drive back to the main mountain road.

Some of them had this puzzled look on their faces - "You mean this is not the road up the mountain?"

Back to winter driving....

My car will be at the airport lot - soaking cold for 5 nights - it will be between -5 and 15 most days - lets hope it starts :0
 

·
Registered
Joined
·
9,965 Posts
Discussion Starter #20
Paul Frere, since departed unfortunately, gives a very good description of the differences between grip and steering behaviour of awd, fwd and rwd in his book Sports Car and Competition Driving in Appendix III. Having driven them all in winter I can assure you that the safest is fwd and the most difficult to drive is rwd. However, at the limits of traction the most dangerous is awd unless you know what you are about.

The limits of traction for awd are no higher than for the other forms of drive as the four contact patches determine that. The balance of the car is different and it is correct that awd gets you more traction in a straight line.

However, and this is a big "however", in corners the awd uses up some of the cornering grip at all four wheels to drive the car forward. The awd car does NOT give you more grip in cornering. It is this effect that makes it so dangerous for the inexpert driver. The amount of grip consumed by the driving force at each end of the car is not under the driver's direct control as it is in rwd or fwd. This means that awd cars rely upon the set up designed in by the engineers as to how the torque is split and in what proportions. Unless you have a Subaru WRX there is no driver adjustment for the torque split or slip rate front to rear.

In consistent conditions this is not a big problem but when the going gets slippery it can be a big problem.

Old style 4x4 used locking hubs which you could unlock if you were driving fast. This essentially gave you two wheel drive. With the capability of locking the rear diff you could get three wheel drive with one front wheel joining in if you also locked the front hubs.

With proper traction and stability control "on" awd is reasonably safe but then so is rwd and fwd....
 
1 - 20 of 20 Posts
Top