Tuesday, January 22, 2008

How (not) to drive in the snow

Provo, Utah is an interesting place to live because:
  1. It snows a lot.
  2. There are a lot of people here from places where it doesn't snow a lot.
  3. These people drive.
As I was walking home from school tonight, my path was blocked by a car with one of these people in the driver's seat. Katherine (maybe—I think I might have forgotten her name already) was spinning her tires on ice, futilely trying to get out of the driveway. I stopped to help her get unstuck, and after a few minutes she was on her way.

Unfortunately, there are many more people in Provo like Katherine. If you are one of them, count your lucky chickens, because what I'm about to tell you may:
  • save your life.
  • save you time.
  • save you money.
  • save you from embarrassing yourself in front of your friends (or worse, enemies).
  • bring you a lifetime of happiness. *
Paying attention now? OK. I'm going to give you a few practical tips on winter driving, but first, I'm going to give you a physics lesson.

The key physical principle that governs winter driving is friction. Your car runs more or less the same in the winter as it does in the summer (except for the occasional dead battery), but the way that it interacts with the road is quite different when there's snow or ice on the ground. Your goal while driving is to maintain maximum friction in the few square inches where your tires touch the road.

Friction comes in two flavors:
  • static friction
  • dynamic friction
(There's actually a little more to it than that, but we'll stick with the simple version for now.)

To illustrate the difference, think about the old pull-the-tablecloth-out-without-making-everything-fall-on-the-floor trick. Why does the stuff fall on the floor when you pull slow, but it stays put when you yank? Because of static and dynamic friction, of course! Static friction applies when two surfaces are touching without slipping (like a bowl on a tablecloth that you're slowly pulling); dynamic friction applies when they are slipping (like when you yank the tablecloth). The key point here is that for most surface combinations, including ice on rubber, dynamic friction is significantly less than static friction.

You can wake up from your bad dreams of high school physics class now. On to the practical tips for increasing the friction between your tires and the icy road.

Bruce's first tip for winter driving: no spinning allowed!

You get up in the morning, get ready for the day, and open the front door to a cold, wintry day. You go scrape the ice off your windshield, hop in the driver's seat, and fire up the engine. After a few sluggish cranks, it rumbles to life and you put it in gear. You press on the gas and the speedometer needle jumps forward, but the car doesn't move. What do you do?
  1. Press on the gas harder.
  2. Swear vehemently, and then press on the gas harder.
  3. Take a deep breath, and then press on the gas very gently.
If you chose option A, you might be from Arizona. If you chose option B, maybe you're from Las Vegas. If you chose C, you get a gold star.

What's so bad about spinning your tires? After all, it makes you feel powerful, right? Well, the problem is that as soon as your tires start slipping, you've left the sunny, rolling hills of static friction for the hellish, fiery pit of dynamic friction. Once your tires start slipping, you have pretty much no friction. (Ever wonder why antilock brakes were invented?) Spinning them faster won't fix that. Instead, try pressing the gas pedal very gently until the car just barely begins to move, then slowly increase your speed. It helps to open your door and watch the tire; if it starts slipping, stop (to get back into happy static friction land) and then try again, a little slower.

Corollary to Bruce's first tip for winter driving: don't slam on the brakes (unless you have antilock brakes).

Lots of college students in Provo have older cars without antilock brakes, and when you slam on the brakes, your tires slip, just like when you stomp on the gas. The ideal situation is to brake as hard as possible without the tires slipping. Unfortunately, you don't really know how hard that is until it's too late and you're already sliding. If you start to slide, let up on the brakes until you regain traction (switching from dynamic back to static friction), and then start braking again.

If you have antilock brakes, there's a computer in your car that will do that for you much better than you can, so just stomp away. If you're not sure if you have antilock brakes, find an empty, icy parking lot, accelerate to 10 MPH or so, and stomp on the brakes. If you can hear and feel the brakes pulsing, you have antilock brakes; If the car just smoothly skids, you don't.

Bruce's second tip for winter driving: avoid braking (or accelerating) and turning at the same time.

Why? Because there's only so much friction available between your car and the road. If you try to turn and accelerate at the same time, part of the friction helps slow you down, and part of it helps you turn. The problem is that there might not be enough friction to do both, and your tires could start slipping. Next time you're stuck, turn the wheels straight and try to get out.

Bruce's third tip for winter driving: get down and dirty.

If you're really stuck (like on polished ice), there's a good chance that your tires will slip even if they're turned straight and you're really gentle with the gas. That's the situation that the girl I helped tonight was in. What do you do? One easy option is to put something between the tire and the ice to increase the friction. Snow plows often spread sand or ground rock behind them to make surface of the ice rougher so that your tires will have something to grip.

You can do a miniature version of that when your car is stuck. Tonight Katherine grabbed a flower pot off her front porch, and I got down on the ground and spread the dirt behind her front tires, making sure to push it as far as I could into the space between the tires and the ground. In combination with tips one and two, the dirt added enough friction to get her out of the driveway. Some people keep a small bag of kitty litter in their trunk to get them out of similar (un)sticky situations.

I was going to stop with three tips, but I'm feeling generous tonight, so I'll give you one more.

Bruce's bonus tip for winter driving: rock it, baby!

Sometimes you get stuck in loose snow. When that happens, try rocking the car back and forth, quickly switching from forward to reverse, but be careful to not damage your transmission by switching while you're still moving. By doing so, you'll compact the snow and go a little bit further each time you switch directions. If things go well, you'll eventually make it out.

Have fun out there—and don't hit me!

* Uh, yeah. Maybe. But no guarantees.


Alex said...

Alex's Corollary to Bruce's laws of driving in the snow:

If you're in loose snow with tires that have any tread, don't be afraid to spin them a little bit, it will help you get down to a harder surface.

tbone said...

NOOOOooooo! Anything but the fiery pit of dynamic friction! What was this woman thinking?!!?!?!?!??!

Cindy said...

The other day I woke up to the sound of someone spinning their tires in our parking lot. All I could think of was the accident waiting to happen if their tires ever actually caught, resulting in their car flying forward and my car getting smashed. Lucky for them they just stayed stuck. They wouldn't like to deal with a grumpy morning Cindy.

jacob said...

I guess you're right, we did write on the same subject. Maybe it is just that important.

When I was in Provo just before Christmas, I was driving to go visit Matt Asplund's home, and there was this truck trying to get up the hill, and it was just spinning its tires and not going anywhere until it slide to the side and got stuck in some snow, at which point I casually drove by in my little Ford Focus that I was renting.