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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.

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    That's a really good question and unfairly under-voted: You can't imagine how many times I've heard that: You only need winter tires on the driving axle. – Stelios Adamantidis Feb 14 at 18:17
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    I only had a pair of snow chains, and the advise was to put them on the steering axle (ie the front) in preference to the rear/driven axle. Absolutely 4 chains would have been better but chaining the front made the best use of what I had. Steering and stopping is more important than getting going, cos if you can't go you don't need to stop. – Criggie Feb 15 at 7:04
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    You're confusing "traction" with "drive". You get winter tires for the explicit purpose of ensuring traction on the wheels, whether you put them on drive wheels or not. To have traction means for the wheels to have sufficient friction against the ground to be able to affect the vehicle's handling. If your front wheels don't have traction you won't be able to steer. If your drive wheels don't have traction you won't be able to accelerate. Any wheel without traction will be unable to contribute to breaking. – Kapten-N Feb 15 at 11:56
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    @Criggie - I grew up in the mountains of Montana (USA). In Montana, chains are a way of life in the winter. I never saw anyone put chains on the non-drive axles. Chains are for traction to get you going, not for turning. Also, while you are using chains, you aren't supposed to go over a certain speed (IIRC it's 30-35mph). Going faster causes instability and the chains will beat your car to death. Due to this, stopping/turning is a lot less hazardous. If you doubt what I'm saying, read this article. – Pᴀᴜʟsᴛᴇʀ2 Feb 15 at 13:43
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    @Johnny - I wouldn't be surprised that all of the axles on the fire truck are actually drive axles, but I wouldn't know that for sure. You could do the same thing on 4x4 vehicles as well. Chains truly are made for the drive axles, though. Even semi-trucks which have to go over mountain passes only use them there (back duals). However, I do take your point and appreciate you pointing it out. – Pᴀᴜʟsᴛᴇʀ2 Feb 16 at 5:30
55

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.

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    I can vouch for the behaviour of a car with winter tires on the front drive wheels and summer tires on the rear... You have to be VERY quick with the opposite lock to catch the back end ... A good exercise to practice your skills when no-one else is around... :) – Solar Mike Feb 14 at 14:07
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    @SolarMike Relevant: youtu.be/A5aMnmekA38 . There's just something hilarious about seeing this from a FWD car! – Kitsunemimi Feb 14 at 15:16
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    I would explicitly call out the fact that all four wheels have brakes. The sort of people who don't implicitly understand the value of non drive wheel snow tires probably won't realize that either. – wedstrom Feb 14 at 18:28
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    Having mismatched tires can be dangerous under any circumstances. I once had winter tires only on the rear of my RWD car, which had a tendency to follow road grooves more than the front. On a long highway trip in dry weather, I found that truck ruts would make whole car oscillate as the rear pulled to one side, making the car point the opposite direction, back and forth. At 60 MPH it was sometimes hard to stay in my lane, and I could have lost control if I needed to stop hard or swerve to avoid something. The constant attention took also focus away from my surroundings and was fatiguing. – mbmcavoy Feb 14 at 21:25
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    Another interesting reference video is what in Dutch is called "ice driving". They take a FWD car and put its rear axle on swing wheels to mimic what it's like to have no sideways traction on your non-drive axle. It's really hard to steer this (I did this exact thing in Zandvoort - it was fun but hard). – Flater Feb 15 at 8:24
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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.

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    Paulster's answer already says all of this. – David Richerby Feb 14 at 17:18
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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.

  • Then you should also mention that the rubber used for winter tires degrades very rapidly in summer and they are not designed for use in the summer temperatures... – Solar Mike Feb 15 at 9:59
  • @SolarMike They of course do, as the rubber gets very soft, however the question was regarding using summer tyres in winter, which is why I didn't mention the summer. I will edit the answer though. – Rachey Feb 15 at 10:03
  • I thought the question was about having 2 or 4 winter tires.... – Solar Mike Feb 15 at 10:08
  • And if you have two winter tires on, it follows that you have two all seasons or summers on as well. It's relevant to the question, imo. – Adonalsium Feb 15 at 18:07
2

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

enter image description here

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.

  • Your answer may be perfectly rational for some scenarios, but it's my mistake. I hadn't given enough context. Please see "Context (edit #2)" in the question. I am reasonably certain that winter rubber on the non-drive axle would have been worthless for the predicament I was in. – Calaf Feb 17 at 19:58
  • @Calaf that is correct. The best tires on your front drive axle would have helped you motor up that hill. However at the very next curveor a – Harper - Reinstate Monica Feb 17 at 20:17
  • Very clever analogy with airplane stabilization, but the part in an airplane that provides stabilization similar to winter tires on the non-drive axles would be the fin (the part that doesn't move), not the rudder (the part that rotates); is that right? – Calaf Feb 17 at 20:37
  • @calaf that is correct, fin=vertical stabilizer. – Harper - Reinstate Monica Feb 17 at 21:09

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