Friction - Static can become Kinetic

Written by Ulrik Andersson,

We all know the feeling when tires lose grip, and the vehicle starts skidding. Controlled skidding in a go-kart could be fun, but a heavy vehicle unexpectedly skidding on a road could cause a very dangerous situation. The skidding is due to the vehicle losing road grip and obviously, this has to do with tires and road surface and how the friction is affected.

All about Friction

First, let’s dive into the physics of friction and add some rubber and asphalt. Friction as such doesn’t move the vehicle forward. Friction is a resisting force that resists the relative motion of two surfaces. Simply put, when driving, the engine generates a force on the driving wheels that moves the vehicle onwards. Friction is the force that opposes the tire rubber from sliding on the road surface. However, things are not really that simple – we have two different frictions to consider: static and kinetic friction.

Static friction – the frictional force between surfaces that are NOT moving relative to each other.

Kinetic friction – the frictional force between surfaces that ARE moving relative to each other.

But when the wheels are rolling, isn’t it kinetic friction? No. When driving on a dry road, irrespective of vehicle speed, it’s the static friction that keeps the vehicle steady on course. If you look at it in a microscope – and in very slow-motion – the contact area of the tire doesn’t move relative to the road surface. It’s just that continuously new parts of the tire come in contact with the road as the wheel is rolling.

In certain conditions, there may appear something else between the tire and the road surface – rainwater for example. The water works as a lubricant between the rubber and the asphalt, and the static friction is reduced as a result. Even worse, the road could be icy.

When accelerating on ice, if the applied force (the driving force on the wheels) exceeds the static friction, the wheels will lose grip and spin.

When turning or cornering, if the centrifugal force exceeds the static frictional force, the wheels will lose grip and the kinetic energy will make the vehicle slide straight onwards, despite you’re turning the steering wheel.

What happens here is that, when static friction is exceeded, another kind of friction takes over; the kinetic friction, which is also known as dynamic or sliding friction. The vehicle will slide until this kinetic friction eventually makes it stop.

In the situation of spinning wheels, they will spin until the static frictional force exceeds the kinetic frictional force (it’s achieved by throttling down) – then tires will grip.

To really increase traction, you need to physically introduce something with a higher coefficient of friction under the tires, or simply put, add a variable that increases more friction than the tire and road surface can provide to each other. This is what you do when you sand an icy road or use snow chains – you increase the coefficient of friction. At the end of the day, it’s all about friction in that small area of contact between the tire and the road – and it’s all pure physics.

Traction aid for heavy vehicles

icy conditions Traction aid for heavy vehicles friction

Ulrik Andersson

About Ulrik Andersson

Ulrik is Global Brand Manager - Onspot and is located at the headquarters in Vänersborg, Sweden. He has been with the company for over ten years and have been working with sales, marketing and customer relations all of the time. Ulrik enjoys spending time with his wife, family and friends. On the weekend, you might find him with a camera taking pictures since that has been a hobby of his for a long time.


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