Why use snow tires on all 4 wheels on 2-wheel-drive cars?

Question

In a car without four-wheel drive—if the car provides drive power on only two wheels—why put snow tires on all four wheels when the freeze sets in? Would it not be sufficient to change just the drive wheels? Is it just a question of use of the wheels and the extra use of two wheels for 12 months, while the other two pairs are being used nine months and three months would complicate things like wheel rotation?

Related: need for matching tires on 4-wheel drive cars

Edit:

About braking: In this question we're more concerned about avoiding to get stuck. It's reasonably clear that having four winter tires offers better braking, but we are in this instant comparing four non-winter tires with two winter and two non-winter. Regardless, after you read Paulster's answer, it will be easy to extrapolate what would happen if your car was not oriented in the direction of motion and you attempted to brake, but someone who brakes during turns over snow or ice, with any kind of tires, is probably in a very bad position anyway.

Context (edit #2, after Harper's answer)

I am asking this question after giving up on moving from a parking lot, which required going up the mildest slope. After many, many, attempts I abandonned the car and used cabs for the day. Since I wasn't sure whether I could be towed from the rear of the car, I asked the question with the objective of buying two winter tires and installing them on the front wheels (on a FWD car).

Curious phenomenon, by the way: if the wheels spin out (as they must if you're stuck), and if the temperature and snow underneath are in just the "right" conditions, the mere heat generated by a little bit of spinning (it's manual-drive and I was careful) will melt the snow and form a nice layer of pure, ultra-slick ice in very little time. But now we're way out of car mechanics and into the physics of water.

In the evening the ice was less hard. Or maybe I managed to get out after pouring an obscene amount of salt, which may have acted as a gravel carpet more so than a melting point modifier.

Answer 1

What you are failing to realize is the non-drive axle provides stability. Yes, the two tires with power going to the ground needs traction to motivate the vehicle down the road, however, you still need to be able to control the vehicle. Regardless of whether your drive axle is up front or in the rear, the opposite axle provides the means to keep the vehicle on the road.

• Front Wheel Drive - Without traction on the rear end, it will tend to slip out from under the vehicle as you go around corners, causing you to spin out.
• Rear Wheel Drive - Without traction in the front end, you'll not have the same steering ability. Your car will tend to keep driving straight as you are trying to turn.

In either case, if you need traction to go, you also need the traction to stop. Not having the traction on both axles severely limits your ability to get a moving vehicle slowed and stopped.

If you need snow tires on one end of your vehicle, you need them on the other.

EDIT:

Special thanks to Kitsunemimi and Bob Cross for finding this video which directly talks to what I've said.

Answer 2

Because the need for traction for movement in the winter is just a small part of the need for traction for BRAKING in the winter.
Also, four winter tires help when cornering.

Answer 3

On a front drive car, the rear wheels are the most important and should always get the best rubber.

This is a really hard concept for people who know enough to realize the rear tires do 0% of steering, 0% of motoring, barely 20% of braking in a hard stop, and they'd last the life of the car if we didn't rotate them. I figured "who cares? The things are just casters." Then, suddenly, I was in a 720 degree flat spin. It was wet, oily road and my rather bald rears had actually become casters. Bad idea.

Actually, rear wheels are this

Ignore the movable rudder, I'm just talking about the fixed vertical stabilizer (fin).

The same is true on rear drive cars -- the best tires need to be on the rear because the rear tires breaking away (spin-out) is much more dangerous than the fronts breaking away (understeer / you go in a straight line).

Everything that needs traction from your tires draws from the same limited supply of available traction.

• braking
• active, commanded steering
• motoring
• passively handling side loads (wind, crowned road, trailing around curves)

If these, stacked, use up more traction than the tire can give, it's gonna slip.

That is how "moonshine turns" work, you yank the handbrake hard to use up all the available traction so the rear wheels "go out", while you swerve a litle to start the tail swiveling around.

Anyway, you can run with 2 winter tires, but they better be on the back.

To your edit:

You are correct that the better rubber on the front would help you climb the hill. But on similarly coated surfaces -- the instant you change from motoring in a straight line to turning or braking, ZOOM your rear end would be out of control and you'd be in a spin-out. So you'd want to swap all 4 tires to put the good ones on the rear, before you attempt to turn or slow down!

In practice I find there are many surfaces no tire will help with. Often in those cases, the smart play is don't try to move the car, lest you crash into something and do some expensive damage. It's not just about physical minimums, it's about personal minimums.

Take these conditions in Seattle. I've been justifiably waved through chain checkpoints wearing performance tires and no chains... But those conditions are way beyond my personal minimums. I would stay put. It would simply not be possible to move a car safely, nevermind the menace from other drivers!

And in a situation like that, the people you're to see will understand.

Driving on ice is hard. If you don't do it every day all winter for the last 10 years, you end up facing a serious skill gap. It is not book knowledge, it must be muscle memory.

Answer 4

Apart from the aspect of controlling the car and braking mentioned, it's also worth to mention that summer tyre's rubber gets very hard during winter and will be much more prone to wear because of usage in conditions they were not designed for.

On the other hand, winter tyres get very soft in summer conditions, also leading to high wear and reduced grip.

However, while winter tyres in summer will usually degrade more evenly (similar to a soft summer tyre, like a semi-slick or rain-grip oriented one), the summer tyre's rubber in winter will behave more like an old rubber and start losing small fragments, usually on the edges of tyre tread pattern.