As drivers steer a road vehicle along the desired trajectory, they will experience resistance from the steering wheel that they must overcome to maintain a constant steer angle.

That resistance is known as the aligning moment and acts to return the steering alignment to the centre. Understanding and controlling the aligning moment in a race car is essential to maintain balance and drivability.

This Tech Explained article will cover the different sources of the aligning moment, its effect on vehicle balance and performance, and how it shapes the subjective driving experience, with some general design considerations for the race car engineer.

How aligning moment is generated.

The aligning moment has four primary sources. Three relate to tyre forces, which you can review in this previous Tech Explained article. The first source is a direct function of the tyre construction. Figure 1 below depicts the tyre deformation present when rolling at a slip angle.

Figure 1


The tyre’s structure resists this twisting deformation, generating an Aligning Moment acting to restore the tyre to a free-rolling orientation. The second source is a function of the tyre contact patch in cornering known as the pneumatic trail. See figure 2.

Figure 2

When a tyre is generating lateral force under a slip angle, that force unequally distributes along with the contact patch. Instead, lateral force gradually increases rearward until the tyre can no longer sustain the slip angle, at which point the tread returns to free-rolling orientation.

The resultant sum of the lateral force generated concentrates behind the tyre contact patch’s centre, creating a moment arm referred to as the pneumatic trail. The resulting aligning moment is the multiple of the total lateral force and the pneumatic trail.

The third source of the aligning moment is suspension kinematics, known as the mechanical trail. Most road vehicles feature an inclined steering axis like that seen in figure 3, creating an offset between the contact patch’s centre and its point of rotation on the ground.

Figure 3

Tyre forces act on this moment arm adding to the aligning moment, like the pneumatic trail above.

The fourth source of the aligning moment is also a function of steering axis geometry. When a steering lock is applied, any inclination in the steering axis will result in some vertical tyre motion, which can raise or lower that corner of the suspension. This vertical motion creates an equal and opposite jacking force reaction to raise or lower the chassis, creating an extra steering effort from the driver.


The rear tyres also generate aligning moment under similar mechanisms, even if the driver does not directly experience them.

Effects on Vehicle Performance

Accurately quantifying the aligning moments at each corner during a vehicle manoeuvre is critical to achieving target vehicle balance and performance.

When a vehicle turns into a corner, all four tyres generate lateral force towards the vehicle’s centre of rotation. However, the aligning moments work in the opposite direction, working to rotate the car out of the corner.

This force increases the understeer and creates a slower yaw response into the corner. These moments must be accounted for to predict vehicle balance and dynamic behaviour accurately.

A previous Tech Explained article discusses the importance of quantifying compliance effects on kinematic performance. Steering component deflection is one of the most critical sources of compliance to manage, as steering is a direct source of control for the driver.

Sizing suspension components involves a delicate trade-off to achieve the required stiffness without incurring unnecessary weight, making it necessary to accurately predict the aligning moments that the steering system will have to withstand.

Effects on Driving Experience

Steering wheel feedback via the aligning moment is an essential part of the subjective driving experience. It transmits information about the road surface and the forces generated at the tyres to the driver. Too little steering effort will lead to complaints about “numb” steering, while too much steering effort will wear out the driver.

The steering ratio, defined as the ratio between the steering wheel’s input angle and the front tyres’ output angle, plays a vital role in this system. A higher steering ratio indicates a higher mechanical advantage for the driver, which reduces steering effort but requires more steering input to achieve a similar vehicle response.

For racing series where power-assisted steering is not available, a designer will compromise between desired steering effort and steering responsiveness when selecting a steering ratio.

Steering feedback also helps alert a driver when they are approaching the adhesion limit of the tyres. Figure 4 compares the lateral force and the aligning moment generated by a Formula Student racing tyre as a slip angle function. Values are non-dimensional to present a generic case.

Figure 4


As slip angle increases, lateral force (blue) continues to increase, albeit with diminishing returns, until a maximum force is achieved. On the other hand, aligning moment (orange) peaks much earlier and begins to decrease as the tyre approaches the limit. The driver experiences this as the steering effort dropping off at higher cornering forces, which serves as a warning that they are approaching the vehicle’s limit.

The designer’s challenge is to fine-tune when and how severely the aligning moment should drop off. If the aligning moment peaks too late, the driver will not have enough warning when they are close to the cornering limit and risk losing control of the vehicle.

If it peaks too early, the resultant reduction in steering effort at high lateral acceleration could affect driver confidence and undermine their ability to exploit the vehicle’s full performance.

Figure 5 illustrates a simplified example of aligning moment breakdown at the front axle of a vehicle as it approaches its cornering limit. The total aligning moment is the sum of the pneumatic effects (tyre construction and pneumatic trail) and mechanical effects (mechanical trail and jacking).

Figure 5


As the lateral acceleration increases, the pneumatic contribution (blue) begins to fall off, while the mechanical contribution (red) increases steadily. The combined total aligning moment peaks at around 1.1 g’s, signalling to the driver that they are approaching the vehicle’s cornering limit.

A further challenge for the race car designer lies in finding the right balance between mechanical and pneumatic contribution to the total aligning moment. Increasing the mechanical contribution can help raise the point at which peak aligning moment arrives.

However, suppose the mechanical contribution is too high. In that case, there is a risk that it dominates the total response, so when the pneumatic aligning moment falls off, the driver will not notice. Also, adjusting the steering axis geometry will have other effects that designers must manage, such as changing the camber kinematic characteristics. As with any vehicle design, the goal is to find the right compromise based on the team’s priorities.