Special Bonus Dear Science: Why is My Car Shit in Snow?

Dec 21st, 2008 | By | Category: Dear Science Column

This just in to the Dear Science SNOWPOCALYPSE 2008 ™ (Hannukah eve again, bitches) crisis center war room control:

Dear Science,
Do AWD or 4WD help me STOP my vehicle faster in inclement weather? I’ve always assumed that even with my extra weight and wider tires this was true… hence my absent mindedness when tailgating other drivers and driving 5mph above the posted speed limit (cops are too busy dealing with traffic accidents to be shooting a radar gun anyway) when it snows.

Thanks Dear Science!

Your all-wheel or four-wheel drive does not help you stop or steer; such devices only help you get your vehicle more readily achieve a speed at which you will be unable to control it. So, stop tailgating. Stop now. Stop. Park your car and stop. Stop. Do not drive. Stop. Stop. Go home and eat soup. Stop.

Why? The frictional force generated by your vehicle’s tires keeps your car in place, allows you to steer, stop and start. It’s your tires, and the weight of the vehicle, that really determine how well you can steer and stop–not how many wheels the motor turns.

This is what your car looks like where it touches the ground, four little patches of rubber.

patches

When the tires are rolling down the road, these little patches should not move. The moment the other forces you’re applying to these patches exceed the frictional force, the patches start to move relative to the ground. When they do, you’re skidding, i.e. fucked.

What kind of forces do you apply to tires, when driving? Well, when you try to stop, you apply a force going backwards to to your tires:

stop

When turning, you apply a force perpendicular to the motion of the car:

turning

When accelerating, you’re applying a forward force to your tires. (This is where four-wheel or all-wheel drive helps. This forward force is spread over all four tires, rather than just two. So you can apply, roughly, twice as much accelerating force to the ground before breaking above the frictional force.)

accelerate

If this reads as gibberish to you, here’s what you need to know: The amount of frictional force generated by your tires determines how fast you can change the speed of your car (speeding up or down) and how fast you can turn. The less friction, the slower you can make your car change speed or direction.

Friction is a nifty little force, with a fantastically simple formula, filled with eeevil, to calculate it:

F = μ*N

Let’s talk about N first. N is equal to normal force–the force shoving your car into the pavement, generally gravity. The bigger your car, the mass gravity has to grab and pull to the center of the earth. On flat ground, that means more normal force, and thus more frictional force.

μ is a bit trickier. μ is how sticky the meeting between the tire and the road is. The wider your tires are and the stickier the rubber they’re made of, the higher your μ. The snowier and icier the road is, the lower your μ becomes, regardless of your tires.

μ comes in two flavors, static (when stuck) and kinetic (when slipping.) When the tires are still stuck to the road–when the sum of the forces applied to the tire is less than the frictional force, it’s static μ that counts. Once things start slipping, kinetic μ takes charge. Here’s the evil: kinetic μ is always less than static.

Let’s play this out. You’re attempting to go up Denny Way, despite the road closed sign. Your (idiotic) strategy? Floor it, fuckers!

Just before you start, your tires are still stuck to the road–static friction is in charge. You press the gas all the way down, causing the tires to apply a huge force to the road, speeding you up a bit. Pretty quickly, this force exceeds the modest static frictional force your tires are producing. They start to spin. Now it’s kinetic friction’s turn! Not only is your car not going forward, now you cannot steer or stop as you slowly drift into a pole. You panic and slam on the brakes, figuring you should at least be able to stop since the brakes just worked a few seconds ago. But they don’t. Because the kinetic friction generated by your tires is so much less than the static friction you had to work with before, even the modest force generated by braking exceeds it.

So, once your tires start slipping, it’s really difficult to get them stuck to the road again.
The solution? Do things slowly. Accelerate slowly. Turn slowly. Brake slowly. Go unbearably slow, slow enough that the forces you’re applying to turn, accelerate or brake are less than static friction.

If you do start to skid, the best thing to do is get your tires moving at about the speed your car is moving relative to the road. Take you feet off the gas and brakes. Turn into the skid. Wait, and you’ll soon feel the shift from kinetic friction to static. Then, you’re back in charge, and can start steering.

And this is why tailgating is such a profoundly bad idea. You simply cannot stop quickly. If you try to stop too quickly, you’ll totally lose control and fuck over someone more responsible than you. Stop.

Double Bonus Time: Why are hill such a pain in the ass in the snow?

Normal force is always perpendicular to the surface. When you’re on a flat, all of gravity goes to make normal force. When going up a hill, it’s split, part stays as normal force, part becomes a new pull-you-down-the-hill-force. This force is not your friend:

hill

The steeper the hill, the more you shift from normal force to this evil force. If the hill is steep enough, this force can all by itself exceed the dwindling static friction generated by your tires–dwindling because the normal force is declining. Hence the “road closed” signs on Seattle’s steepest hills, that you should heed.

Bonus bonus bonus: Why do chains work? By dramatically increasing μ. Put ’em on your front tires to help you stop and steer.