Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Traction question -- Best braking strategy in snow and ice

  1. Jan 13, 2017 #1
    When driving on snow and ice does engine braking (by downshifting) provide more traction than applying brakes in a slow and controlled manner?
     
  2. jcsd
  3. Jan 13, 2017 #2

    Nugatory

    User Avatar

    Staff: Mentor

    There are too many variables involved here to give a simple yes/no answer.

    Unless you have AWD or engaged 4WD engine braking is only working through two of the four wheels, and that will make a difference. If you're driving a stick shift, the way you operate the clutch makes an enormous difference. If you're driving an automatic, there are all sorts of complicated and mysterious systems separating your throttle inputs from the torque at the driven wheels; these may or may not be better than you at maintaining traction. The way you operate the brake pedal makes an equally big difference, with even more possible complications - ABS, power assist, traction control all tend to disconnect your operation of the brake pedal from what's happening at the wheels.
     
  4. Jan 13, 2017 #3

    Randy Beikmann

    User Avatar
    Gold Member

    I agree with Nugatory that many factors come into play, especially when you have anti-lock brakes (which any car less than 20 years old will have). With ABS, the standard technique to minimize braking distance is to push the pedal down as hard as you can, let the system control pressure, and just steer the car straight. Still, your technique does make a different on how much the ABS system (and stability control system) will have to do to keep things under control.

    On ice, I actually like to put it in neutral or put the clutch in, so that there is no engine braking - that way the brake pedal alone is controlling the torque on each wheel. Since there is very little traction, which must be shared between braking and steering, you use the brakes (and the steering!) very gingerly. Stay ahead of the steering and use small motions. If ABS or stability control need to kick in, they will, mostly to keep the car pointed straight.

    In deep snow, a car with ABS can be a little aggravating. The fastest way to stop is to lock up the tires, slide, and let snow pile up in front of the tires. Obviously ABS won't allow that, so you get a lot of pulsation of braking force, the system dropping brake pressure to let the tires roll.

    More important than the brakes or technique is having traction itself, so that means better tires. The best car I've ever had in the snow was a 1991 Miata without ABS, but with Blizzak tires. It's a lot of fun to leave a Jeep in the "dust" in a Miata pulling away from a stop light when there's 4 inches of snow on the ground! Plus, with no ABS, I could lock up the tires and stop quick.
     
  5. Jan 13, 2017 #4

    Mech_Engineer

    User Avatar
    Science Advisor
    Gold Member

    Engine braking cannot taking advantage of an ABS system if one of the wheel accidentally starts to slip. Careful, consistent pressure on the brake pedal is your best bet in ice and snow. Let the ABS system do its job.
     
  6. Jan 13, 2017 #5

    Baluncore

    User Avatar
    Science Advisor

    Probably not.
    The brakes apply to all wheels, engine braking only applies to the driven wheels.

    The average driver has quicker and more sensitive control through the brake pedal than through the accelerator, so is less likely to lock up the drive wheels with total loss of directional control when changing down.

    Engine braking is a necessity on long slow controlled descents. If only the brakes are used in that situation, brake temperatures will rise excessively because of the continuous use with poor airflow.
     
  7. Jan 13, 2017 #6
    i dunno about the theories, but im a 4wder, always taking my 4wd in bush. i can tell you from real like experience, engine braking is usually the best. ie, if you are in the habit of trusting your foot brake over the engine brake, you are much more likely to roll your car.
     
  8. Jan 14, 2017 #7

    Ranger Mike

    User Avatar
    Science Advisor

  9. Jan 19, 2017 #8

    Mech_Engineer

    User Avatar
    Science Advisor
    Gold Member

    This is irrelevant to the posed question by the OP. If you're in Snow and Ice, use the brakes.

    This claim is... questionable at best.
     
  10. Jan 19, 2017 #9

    JBA

    User Avatar

    Downshifting is not a good idea because in these conditions you want the minimum amount of torque variation on the wheels while modulating with the throttle or brake and that is best achieved at the minimum engine rpm/torque variation with speed and therefore at the lowest allowed engine speed. Ultimately, engine braking cannot be avoided except by careful throttle modulation so a very controlled easing of the throttle is the first stage of braking on ice. On glaze ice, even the slightest throttle reduction can result in instant total loss of traction and control.
     
  11. Jan 19, 2017 #10
    sorry, i was under the impression op was asking about real life driving, not theoretical traction. my bad.
     
  12. Jan 20, 2017 #11

    JBA

    User Avatar

    While the information in my above post might be construed by some as "theoretical" is the result of many miles of driving in snow and ice conditions and is completely applicable to "practical" handling and braking under those conditions. In reality, the only individuals that are even reasonably safe on ice and snow are those with experience under those conditions, it is definitely a learned art, not a science. Having grown up in South Texas before moving North for college my first act when the first real ice and snow occurred was to first find a large parking lot to learn the basics and then successively go out on the local streets late at night when they were empty and practice, practice, practice at various speeds and maneuvers. I simply like to explain the basics behind my driving recommendations, that is what Engineers do.
     
    Last edited: Jan 20, 2017
  13. Jan 20, 2017 #12

    Baluncore

    User Avatar
    Science Advisor

    An art becomes a science when it is rationalised and reduced to testable fundamentals. An art has no reasoning, code of practice or objective assesment. If it is not a science then there can be no discussion or understanding, only a subjective appreciation.
     
  14. Jan 20, 2017 #13

    JBA

    User Avatar

    Baluncore, I strongly suggest you check with Webster with respect to your limited definition/perception of the the word "art".
     
  15. Jan 20, 2017 #14

    Baluncore

    User Avatar
    Science Advisor

    I interpreted the word 'art' in the way JBA used it, especially as it definitely not being a science. I see JBA's assertion that I have a limited definition/perception of the word 'art' to be a baseless personal insult. JBA should have used a less general word, or better defined the intended meaning before using the word on an engineering forum.
     
  16. Jan 20, 2017 #15

    OCR

    User Avatar

    For 17 years I was employed by MDT, on this section of highways, as a Storm Emergency Temporary (SET) plow driver...
    I can offer only a subjective appreciation of what JBA meant by...
    But jump in one of these and you will, at least, get a better understanding of the issue...[COLOR=#black].[/COLOR] :oldwink:

    DSCF0710.JPG
    DSCF0743.JPG
    DSCF0717.JPG
    DSCF0715.JPG
    DSCF0704.JPG
     
    Last edited: Jan 20, 2017
  17. Jan 24, 2017 #16
    Still no convincing answers here.

    If your're going down a hill that's covered in ice, does engine braking (through downshifting) provide better traction than using vehicle brakes? Let's assume it's a 2wd vehicle, and downshifting controls the speed of two wheels. Braking controls all four wheels slowing/stopping. Which gives better chance of not sliding down the mountain?
     
  18. Jan 24, 2017 #17

    Mech_Engineer

    User Avatar
    Science Advisor
    Gold Member

    The question has already been answered:

     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Traction question -- Best braking strategy in snow and ice
  1. ICE-best book (Replies: 2)

  2. Prony brake question (Replies: 1)

  3. Friction on snow/ice (Replies: 7)

Loading...