A lighter agile car is in most conditions likely safer than a heavier less agile one, like for like, same driver.
In the real world there is no substitute for friction, stopping and going.
[/quote]

Hmm.

Not saying you are wrong, but consider the 530i v. 530xi. Both are ostensibly the same car (ceteris peribus, right?) except for the x-drive. Depending on the type of gearbox, the 530xi is about 200-300 lbs heavier than the regular 530i.

Are there any claims -- anywhere, BMW or otherwise -- touting the fact that the 530i is safer than the 530xi?

Regardless of marketing and advertisement, is there even empirical evidence to support this position?
[/quote]I have, of course, seen some assertions both ways. I have never seen anything empirical either way. I think it is safe to say that if one is stuck in the snow, then he or she has a better chance of getting out before freezing to death or expiring from hunger with an all-wheel drive. QED, the all-wheel drive is safer in at least one set of circumstances.

 

A lighter agile car is in most conditions likely safer than a heavier less agile one, like for like, same driver.
In the real world there is no substitute for friction, stopping and going.
[/quote]Are you assuming the lighter one has rear-wheel drive, while the heavier one has all-wheel drive? In other words, are you assuming these cars satisfy the other things equal as far as possible condition? Is that what "like for like" implies? But, regardless, I get confused. If the heavier car has all-wheel drive, then wouldn't it generate the most friction accelerating, cornering, and braking, for example? If so, then wouldn't it be safer in many, if not "most" conditions--assuming it does not have a huge amount of extra weight for some unimaginable reason? And, if you are not assuming satisfaction of the "other things ... condition," then all significant comparability is lost. Please clarify.

Vnod, you are brilliant.

 

My opinion (FWIW)
I tend to drive in a fairly spirited manner on windy roads. When these roads are wet and I'm driving a RWD car, I'll ease off quite a bit for fear of power oversteer coming out of the corners. I'm much less concerned in a AWD car. Does that make it safer? I guess not, just faster.

In snow & ice conditions where basic traction is a major issue, AWD allows more normal acceleration (not a safety issue) and decreases my chance of power oversteer (definitely a safety positive).

Finally, an AWD car is heavier than a RWD car. Much discussion on whether this makes the car safer or not (purely based on weight and not drive wheels). I see two core arguments here:
1. Heavier car handles/brakes worse than light car
2. Heavier car is better in an accident than a lighter car

Both of these arguments are true in isolation (again, ignoring other issues like drive wheels & tire spec). I don't think we need scientific evidence for (1) as there is little to argue. Point (2) has been proven several times - while it isn't very society friendly a heavier car is safer in accidents than lighter cars (just thing of the momentum advantage). More on this below.

Scientific evidence?
I am intrigued by the idea of empirical evidence for AWD vs. RWD/2WD though. Is it really reducing accidents in the real world?

First stop, NHTSA - the USA based agency that investigates car safety. In 2002, they promote AWD as an item to look for in a safer car:
http://www.nhtsa.dot.gov/NCAP/BASC2002/frontpages4.html
However, in 2006, it is mysteriously dropped from a much more comprehensive document:
http://www.nhtsa.dot.gov/ncap/BASC2006/index.htm
I've got to think that this is because they believe it is not reducing accident rates. Still not scientific evidence.

Here is the only scientific study that I could find online relating to safety of 2WD vs. 4WD vehicles. The research was done at the University of Oklahoma Health Sciences Center. The focus here is on the crash issue and not on accident prevention so it is clearly not a complete picture. However I still find the information quite interesting. Two quotes from the study results below.
Research is at this URL which appears to be down right now - I got it on the Google cache:
http://ssml.naoe.pusan.ac.kr/issc/ref/full...sion/col-22.pdf

The first point is interesting since it suggests that AWD will reduce the severity (at least in terms of $$$s) of a crash in the wet/snow/ice which is when it excels.

The second point is clearly a safety plus for the AWD (generally heavier) car but also is at the expense of other drivers which doesn't seem very fair.

I think this issue is still open to general argument since there is no conclusive evidence I can find to prove on side or the other. Can anyone else find anything? I'd really like to see a general study that compared accident rates in general between 2WD and 4WD or more specifically RWD/AWD.

This topic intrigues me.

 

A lighter agile car is in most conditions likely safer than a heavier less agile one, like for like, same driver.
In the real world there is no substitute for friction, stopping and going.
[/quote]Are you assuming the lighter one has rear-wheel drive, while the heavier one has all-wheel drive? In other words, are you assuming these cars satisfy the other things equal as far as possible condition? Is that what "like for like" implies? But, regardless, I get confused. If the heavier car has all-wheel drive, then wouldn't it generate the most friction accelerating [yes], cornering [it would depend on the circumstances], and braking [not--my bad] ....? If so, then wouldn't it be safer in many, if not "most" conditions--assuming it does not have a huge amount of extra weight for some unimaginable reason?

Vnod, you are brilliant.
[/quote]

Now, now. You were asked to play nicely. And, yes, I was completely off on one thing I said--braking safety. Silly thinking. But, no reason, for what I take to be sarcasm. If I am wrong, then I apologize.

Roughly speaking, in the context of autos, isn't friction the force that would keep an auto's tires from slipping on the road--the more the friction, the less the slippage? If so, then given sufficient power, a rear-wheel drive car would overcome this force more readily when accelerating than a comparable, but slightly heavier, all-wheel dirve car. Thus, I would think that as horsepower increases, the all-wheel drive car would become and safer upon acceleration. Given lower horsepower, I doubt there would be much of a difference since both would get adequate traction. Braking would be the other way around since again the heavier car again overcome the force more readily. To me, the jury is out on what happens in all cornering situations. I think it would depend on the circumstances.

 

Are you assuming the lighter one has rear-wheel drive, while the heavier one has all-wheel drive? In other words, are you assuming these cars satisfy the other things equal as far as possible condition? Is that what "like for like" implies? But, regardless, I get confused. If the heavier car has all-wheel drive, then wouldn't it generate the most friction accelerating, cornering, and braking, for example? If so, then wouldn't it be safer in many, if not "most" conditions--assuming it does not have a huge amount of extra weight for some unimaginable reason? And, if you are not assuming satisfaction of the "other things ... condition," then all significant comparability is lost. Please clarify.
[/quote]If the only difference between the two cars is RWD & AWD (e.g. 530i vs. 530xi), it really boils down to traction (drive friction).

1. Acceleration: A RWD car in this power/weight band will always out accelerate an AWD car on dry roads due to the weight transfer to the rear on acceleration. Just compare the 530i and 530xi acceleration times for proof. In slippy conditions, the rear wheels will have more power than can be applied through the tires (traction) so having front wheel drive in addition will help acceleration to get power down "to the road".

2. Cornering: If we limit this to neutral throttle cornering (not accelerating), AWD has no advantage over RWD in the dry. Perhaps the additional weight makes a slight negative for AWD. However, in slippy conditions and driving reasonably fast (near the edge of traction), AWD will reduce the likelihood of power oversteer. This is very evident in ice/snow conditions.

3. Braking: This boils down to pure momentum - AWD loses every time due to additional weight.

I think AWD value is very location dependent (just look at where people are located and their positive/negative comments!). If you rarely see rain and never see snow/ice, I don't see any reason to have AWD. However, if you see lots of rain and/or snow/ice, it makes sense.

The argument about snow tires is mute to me. Snow tires can be added to a RWD or AWD car. If the subject is whether AWD is better than RWD with snow tires, that is a very different subject and very driver dependent.

 

If the only difference between the two cars is RWD & AWD (e.g. 530i vs. 530xi), it really boils down to traction (drive friction).

OK, the topic is not my strong suit, but I'll try anyway.

1. Acceleration: A RWD car in this power/weight band will always out accelerate an AWD car on dry roads due to the weight transfer to the rear on acceleration. Just compare the 530i and 530xi acceleration times for proof. In slippy conditions, the rear wheels will have more power than can be applied through the tires (traction) so having front wheel drive in addition will help acceleration to get power down "to the road".

Makes sense to me. You did specify in "this power/weight band." The dry results may flip flop as power increases significantly or weight decreases significantly, I think. In this regard, the extra weight becomes less and less significant, and the rear-wheel drive car has more trouble getting traction.

2. Cornering: If we limit this to neutral throttle cornering (not accelerating), AWD has no advantage over RWD in the dry. Perhaps the additional weight makes a slight negative for AWD. However, in slippy conditions and driving reasonably fast (near the edge of traction), AWD will reduce the likelihood of power oversteer. This is very evident in ice/snow conditions.

OK, so plus one, I guess, for all-wheel drive safety under the conditions you specify. That's two total for all wheel-drive safety given the one I mentioned earler. I am not sure if any specific safety points have been yet accumulated for rear-wheel drive. Maybe 1/2 from what you say above.

3. Braking: This boils down to pure momentum - AWD loses every time due to additional weight.


Makes sense to me. Plus one safety point for rear-wheel drive.


I think AWD value is very location dependent (just look at where people are located and their positive/negative comments!). If you rarely see rain and never see snow/ice, I don't see any reason to have AWD. However, if you see lots of rain and/or snow/ice, it makes sense.

Right. Definitely easier to get places (safely?) than with rear-wheel drive under certain conditions. Do we have a third point for all-wheel drive safety?

The argument about snow tires is moot to me. Snow tires can be added to a RWD or AWD car. If the subject is whether AWD is better than RWD with snow tires, that is a very different subject and very driver dependent.

Right snow tires, chains, etc. are not part of the fundamental issue. Once we have figured the first one out definitely , then we can add snow tires etc., to the equation. I think all-wheel drive is ahead three to one on the safety dimension--at worse it is three to 1.5.
[/quote]

 

 

I can't speak for all scenarios, but from what I know:
Disregarding the harcore off-road vehicles, 4WD is just a marketing gimick. It is totally unecessary if you understand vehicle dynamics and traction.

I grew up in NY and in my experience:

RWD BMWs and Porsches + Snow Tires + Additional Weight Over The Drive Wheels (ie. sand bags in the trunk)

...yielded greater traction (ie. not getting stuck in the snow) than...

4WD/AWD SUVs + Snow Tires


My 2 previous cars were a '98 328is and a '99 Jeep Grand Cherokee Limited. The Jeep had the top of the line Quadra Drive AWD system, yet it performed the worst compared to all the RWD german cars I've ever driven in the snow. Go figure...

I also think the E60 is so good at preventing spins and drifts because of DTS/DSC. The Northeast has been getting an inch or two of snow over the past few days. Well I thought this past Monday was a great oppurtunity to drive up to Bear Mountain and have some slippery fun . Roads were windy, disserted, and pretty slick. Around the hairpin turn right before the main parking lot, I had all electronic ON and no matter how hard I tried, I could NOT perform a power slide and get the rear to drift. The computer just refuses to do it, even though the road was pretty icy.

So what are the limitations? Stopping in slippery conditions. This is where any vehicle, no matter 4WD/AWD or RWD, will have a problem. And in this case, 4WD/AWD doesn't help. 4WD/AWD only helps in accelerating in slippery conditions, but that's when I put the sandbags in my trunk.

So the only advantage of 4WD/AWD is it saves me money in that I don't have to buy sandbags to put in the trunk.

 

I am always will to add my $.02 - And here is my take which has been mentioned multiple times. WEIGHT is the most important factor, then WEIGHT, then WEIGHT. My wife has a Ford Excursion 4WD - the curb weight is not published - It is considered a Truck so it is over 8000 lbs. Tried multiple times to see the difference in all conditions rain, snow, ice, deep snow, etc. and the only time AWD helps is going up a hill.
The Problem is that when you do loose control of an 8000+ lb vehicle it keeps going and going and going and going. Momentum is a nifty thing.

Anyway, I got an xi.

 

Good one. Obviously, you have had much experience in the snow. I would imagine the advantages of different things would depend a great deal on the driver and how he or she wants to equip a car--e.g., with or without snow tires, chains, and sandbags, etc.