Circle of forces | Understanding tire dynamics

Because no one knows your vehicle better than its manufacturer, SimpleTire.com recommends Original Equipment (OE) tires.

How do manufacturers know which tires are right for your vehicle? They test tires. Every vehicle manufacturer sets requirements for the tires that can be put on each of its models. Although tires are highly specialized, there are a couple of constant performance characteristics that affect nearly all vehicles: handling and cornering. These two attributes are measured in the testing that determines whether tires are good enough to be put on certain vehicles. To do that, these tests measure a tire's circle of forces or traction circle.

The circle of forces is a way of understanding all the different forces that tires exert while they turn. If you understand a little bit about all the physical forces that impact your tires, you can make yourself a better and safer driver. You can also learn about all the work your tires do to deliver excellent handling and cornering to your vehicle.

What Is the Circle of Forces?

Imagine drawing a circle on the road around your vehicle's tire. Within this circle is every direction that your tire can move as it turns. This is the circle of forces. As your tires maneuver in response to your steering, different things within the circle of forces affect your tires.

Lateral Force

Lateral force is the sideways motion the tire generates as it turns. When you steer your vehicle, you're forcing your tires to focus their motion in another direction. This is what causes lateral force, and it's part of what keeps your vehicle on the road as you turn.

Vehicle Weight

Tires need to support the weight of the vehicle at all times. Much of the circle of forces works to do just that. The weight is a constant force on the tire no matter which direction it is facing. As tires turn, they need to grip the road while shifting the direction of all the vehicle's weight. There are a number of tire dynamics that support and shift vehicle weight: slip angle/side slip, pneumatic trail, cornering force, cornering power, relaxation length, and self-aligning torque.

Through all these components, the circle of forces keeps track of how well a tire maneuvers and takes corners. These results are easy to see when they're measured in a controlled test. In the real world, though, results may vary.

What Affects the Circle of Forces?

Tire construction, tire material, and tire design play a key role in the circle of forces. Some tires are manufactured to offer a rapid response to the road. Generally, these high-performance tires are sturdy yet manufactured with soft materials and unique tread design. The soft materials allow the tread to stick to the road helping the vehicle turn more quickly. Tires that offer a touring or passenger ride focus more on comfort and low noise than on quick turns. The circle of forces in these tires produces a more gradual turn. Other variables are vehicle weight, drag, acceleration, deceleration, and driver response. The vehicle suspension, drive train, and complete chassis design also affect the circle of forces.