Why are turns worse on slippery roads?

posted Jan 4, 2015, 6:06 PM by Patrick Poole   [ updated Jan 5, 2015, 2:42 PM ]
Slow on the turn!
Most people know that it’s more dangerous to drive when the weather is bad—rain, snow, or ice all make roads more slippery. Have you ever thought about why most accidents in slippery conditions occur when a car is turning? Today I’ll talk about the physics that goes into driving on wet roads, and how you can maximize safety during your winter travels.

Friction is the force felt between two objects in contact, and depends on the motion and surface texture of those objects

When two objects are placed in contact and one of them is moved with respect to the other, there is a friction force between the objects that opposes that motion. This force arises from interactions between the molecules of the two objects just where they contact. There are a few different sources of friction that get lumped together into one term called the coefficient of friction, represented by the Greek letter µ (mu)—the higher the coefficient of friction of an object, the more force there will be opposing motion along that object’s surface.


For example, the coefficient of friction between concrete and rubber is 1.0, while the coefficient between wood and wood is around 0.3 (depending on what type of wood), and between Teflon and Teflon it’s 0.1. This is why non-stick pans are coated in Teflon: it’s hard for something to stick to them if the coefficient of friction is low.

There are a number of different types of friction depending on the nature of the motion. For example, the coefficient of static friction comes into play when two objects are being pushed against each other but neither is moving. This is what works against you when you try to push a heavy bookcase across a room. The coefficient of kinetic friction comes into play once one of the objects is moving—you’ll probably note that the bookcase is easier to push once you’ve managed to get it moving, which is because the coefficient is often lower for kinetic friction than static.

Slippery roads have reduced friction, so there is less available to hold car tires on their turning trajectory

Because friction depends on the interaction between two surfaces in contact, it will behave differently if there is something else preventing that contact. For example, a liquid like water will flow between the nooks and crannies of the two surfaces, such that they are no longer rubbing strictly against each other. If there is a full layer of water between the two objects a phenomenon called hydroplaning can occur, which is very dangerous for cars—in this case the tires have lost contact with the road so they cannot grip to control the car’s direction or speed.

Hopefully you aren't Bill Paxton

Even before hydroplaning occurs, though, some amount of water will reduce the effective coefficient of friction. In the case of concrete and rubber, the presence of water drops the coefficient from 1.0 all the way down to 0.3. The coefficient of wet asphalt can vary between 0.25 and 0.75 depending on the quality of the tire and road.

A turning car experiences a sideways force, and friction between the tires and road prevents it from deviating from its path

Here’s the important part: the static friction between your tire and the road is a set amount and will provide up to a set amount of force in response to attempts to push the car away from its current path. For example, if a strong gust of wind comes by to push sideways on your car, there will be a friction force that will oppose that wind force so that your car doesn’t move sideways. If you’re unlucky enough to be in a very strong windstorm, however, that wind force might overcome the maximum amount that can be provided by the tire-road frictional force, and then your car will be scooted sideways. In wet driving conditions this maximum amount of frictional force is diminished, meaning that it takes less force to overcome it and push your car in a direction you didn’t intend on going.

The thing a lot of people forget is what types of forces could try to do this to your car. To have a force you need some sort of acceleration, or change in velocity (that’s Newton’s second law of motion). Anytime your speedometer changes your car is accelerating, whether it’s speeding up or slowing down. These aren’t usually too bad because you’re the one controlling the acceleration, and it’s in the direction the tires are spinning anyway.

Watch that turn!

The problem comes in when you go around a turn: even if you’re turning at constant speed, your car is still accelerating because the direction it’s moving is changing every moment. Imagine you have a rock tied to a string, and you’re swinging this over your head: if you swing at a constant speed but then cut the string at some point, the rock will go sailing straight ahead (its trajectory will be tangent to the arc it was moving in), and if you cut the rock a bit later instead this direction would be different.

Changing direction is acceleration too—in fact you feel this every time you go around a corner sharply or quickly enough in your car. You will feel yourself be pushed away from the direction the car is turning, or away from the center of the circle your car is turning on. This is called centrifugal force—as the car started turning your body continued going straight, and has to be pulled back along the car’s circular path (usually by your seat and seatbelt).

This centrifugal force acts on your car wheels, pushing them perpendicular to the way the car is moving. That force is opposed by friction, but this can be overcome just like from a strong enough gust of wind—in this case the strength of the centrifugal force depends on how fast you’re going, and how tight a turn you are making. This is why interstate off-ramps have speed limitations, and why race tracks often have banked turns—both of these help ensure that the friction from your tires can overcome the forces endured while turning.

Slippery roads decrease the amount of friction available to your tires, and turning, breaking, and speeding up all utilize this friction—so don’t do two at once!

Unfortunately, many people go into these turns a bit too fast, and end up braking part way through. This is dangerous because braking is yet another deceleration—slowing down helps reduce the centrifugal force, but it also adds its own force, and if you aren’t careful it can be enough to overcome the tires’ friction before it starts to help.

The safest thing is to enter a turn slower than the recommended speed if the road is slippery, and then try not to break or accelerate at all while you’re turning.

Bonus physics—Friction and the Greeks

Before Newton came along in the 18th century and set things straight with his Laws of Motion, Aristotle had his own ideas about how forces worked. When he pushed a block along the ground it always stopped eventually, so he concluded that all objects in motion eventually cease to move. This is because he did not consider that another force, in this case friction, was acting on the block to stop it. If you have a frictionless environment (such as a block moving on perfect ice, or better yet in space) then a pushed block will never stop if it never encounters another force to slow it down.