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Explanation of Traction Friction of Tires by Ron Kurtus - Succeed in Understanding Physics. Key words: physical science, spinning wheels, torque, friction, treads, water, hydroplaning, snow tires, mud, race cars, slicks, sliding, soft rubber, Ron Kurtus, School for Champions. Copyright © Restrictions

Traction Friction of Tires

by Ron Kurtus (revised 28 March 2008)

Traction is the friction between wheels or tires and the ground that allows a vehicle to move forward. It is the resistance to spinning when a torque is applied to axle the wheel. When a surface is wet, a layer of water can act as a lubricant, greatly reducing the traction and stability of the vehicle. If enough water is under the tire, hydroplaning can occur.

Treads are used to move the water to the sides and increase the traction and ability to stop. When the surface is snow or mud, which is also slippery, deep treads are used to increase traction. In racing where the torque on the tires is high, special rubber is used to prevent loss of traction when tires start to spin.

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Problem of hydroplaning

Because water acts as a lubricant, a wet pavement can reduce the traction of the tires, as well as the ability to stop and avoiding sliding sideways. Still, if you are careful not to increase the torque on the wheels so much that your tires start to spin, and you don't drive so fast that you have problems stopping or going around a curve, you can still operate the vehicle safely.

Layer of water under tire

A major problem occurs when the rain is so heavy that there is a layer of water on the roadway. This can result in an effect called hydroplaning, where the tire is skimming across the water surface with no contact with the road and with extremely low friction.

Tire hydroplaning over layer of water

Tire hydroplaning over layer of water

The problem from hydroplaning is not so much the loss of traction in being able to accelerate but the loss of control in being able to stop or prevent sideways slides.

Solution is treads

A solution to hydroplaning is to add treads to the tire that will channel the excess water out from under the tire. In this way, the rubber can get in better contact with the wet pavement surface, thus greatly increasing friction and traction.

Tire treads help move water outward to prevent hydroplaning

Tire treads help move water outward
to prevent hydroplaning

The disadvantage of having treads automobile tires is that under normal, dry driving conditions, the treads increase rolling friction.

Problem of snow or mud

In soft, slippery surface material like snow or mud, the coefficient of friction of a tire without treads is very low and thus there is little traction to move the vehicle forward. A torque applied to the wheel will just cause it to spin. Tires with special deep, wide treads are used in those conditions.

Special tires have wider treads to improve traction in snow or mud

Special tires have wider treads 
to improve traction in snow or mud

The treads produce a gear-like effect to improve the traction in mud or snow.

Treads increase traction in snow or mud

Treads increase traction in snow or mud

When a tire becomes worn and the edges of the treads become rounded, there can be considerably less traction. The danger then is that the car may go into a skid when going around a corner or may not be able to stop in a sufficient distance in an emergency.

High torque and fast driving

When an automobile engine applies a very high torque to its wheels, the tires will often spin on the pavement. This is common in the rapid acceleration of a drag racer or race car. Once a tire is spinning there is a loss of traction as the friction goes from static to kinetic.

Sticky rubber for traction

Drag racers and many race cars use tires made of soft, almost sticky rubber that provides a good grip on the road, especially when at warm tire temperatures. It uses a form of molecular friction that is related to the material and surface area on the road. The tires are wide to increase the contact to the road and thus the traction.

Treads or no treads

Often the tires have no treads. In vehicle races that are not held in rain or snow, treads to prevent loss of traction due to hydroplaning or driving on a soft, slippery surface are not an issue. Instead, treadless, slick tires are used.

Some race car tires are called slicks because they have no treads

Some race car tires are called slicks
because they have no treads 

Some race car tires have at most an 1/8 inch of tread, primarily to avoid overheating. Excess heating is the major reason for tire failure, particularly at the high speeds attained in the race. Some tires may get so hot that the rubber blisters and the tire blows out.

Summary

Traction is the friction between wheels or tires and the ground that allows a vehicle to move forward. When a surface is wet, a layer of water can act as a lubricant, greatly reducing the traction and stability of the vehicle, including that caused by hydroplaning.

Treads are used to move the water to the sides and increase the traction and ability to stop. When the surface is snow or mud, deep treads are used to increase traction. In racing where the torque on the tires is high, special rubber is used to prevent loss of traction.


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Resources and references

Ron Kurtus' Credentials

Websites

Rolling and Sliding Friction in a Car - Alaska Science Forum

How Tires Work - How Stuff Works

Tyre Rolling Resistance Data - Extensive comparison of different types of tires

Friction Concepts - HyperPhysics

Friction Resources - Extensive list

Books

The following books are available from Amazon.com.

Basic

Complete Idiot's Guide To Physics by Johnnie T. Dennis; Alpha (2003) $18.95

What Is Friction? (Ages 4-8) by Lisa Trumbauer; Children's Press (CT) (2004) $4.95

Advanced

Friction Science and Technology (Mechanical Engineering Series) by Peter J. Blau; Marcel Dekker Pub. (1995) $89.95

Friction and Lubrication in Mechanical Design (Mechanical Engineering Series) by Ali Seireg; Marcel Dekker Pub. (1998) $199.95


Questions and comments

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