Everything about Ackermann Steering Geometry

The Ackermann steering geometry is a geometric configuration of connections in the steering of a car or other vehicle created to address the issue of wheels needing to trace out circles with differing radii on the inside and outside of a turn.

The Ackermann steering is the invention of Georg Lankensperger, a German carriage maker. He created it in Munich in 1817, and his representative in England, Rudolph Ackermann, obtained a patent for it in 1818 for horse-drawn carriages.

In a vehicle's steering geometry, the Ackermann level is expressed as a percentage, with 100% Ackermann meaning that the difference in steer angle between the inside and outer tyres matches the geometric low-speed turn centre.

How Does the Ackermann Steering Concept Work?

The steering column transmits the action from turning the steering wheel to the steering axis. The moving link of this steering mechanism is located in the middle and connected to the steering axis. 

By imparting motion to the link that is attached to the wheels, the moving link, in turn, causes the wheels to shift in direction. This is how steering kinematics via Ackermann functions.

Because Ackermann's steering concept is unaffected by outside forces, it is known as kinematics. According to the Ackermann steering geometry, the two front wheels are always oriented toward the exact centre of the turning radius.

What Is the Ackermann Steering Geometry Formula?

The final equation that is employed to calculate steering angles using the Ackermann mechanism is

δR = δL = δin/ γ

Here, δR and δL are right and left steering angles, respectively, while δin is the steering wheel angle.

In contrast, γ is the steering ratio or the Ackermann level that needs to be fine-tuned by the automotive engineers for the smooth functioning of cars.

Adjusting the left front tie rod end in a slotted spindle arm can often alter the Ackermann steering geometry calculations.

More Ackermann will result from moving the tie rod end closer to the ball joint. To make the change, certain cars use an offset slug design. Engineers must conduct multiple checks concerning the Ackermann settings at minimum and maximum levels.

Why Does Ackermann Steering Mechanism Stand Out from the Davis Steering Gear Mechanism?

The Ackermann steering mechanism is much simpler than the Davis Steering Gear mechanism. The Dais steering mechanism deploys sliding pairs, which is prone to wear and tear.

Applicability of Ackermann Steering in Present Times

Car steering systems are generally made to function between low-speed Ackermann and parallel steer. Sports vehicles typically have parallel steer configurations, whereas sedans typically have pure Ackermann configurations. 

Tire toe-in for stability is present on both vehicle platforms. On the other side, race cars may use parallel steer, reverse steering, or anti-Ackerman steering with toe-out to maximise tyre scrub and steering. 

It is hypothesised that steered wheels that work with reverse Ackermann or remain parallel during cornering would cause extra lateral scrubbing to the outside tire-wheel assembly, further improving handling. The price is rapid tread wear.

On the other hand, vehicles with either four-wheel steer or rear-wheel steering systems (like forklifts) will likely stay confined. Both steering modes tend to overseer at moderate to higher speeds, but they work effectively when agility is needed at low speeds.

How Is Ackermann's Principle Applied to Different Cars?

Due to the placement of the steering rack, Ackermann's geometry is not linear, as most drift cars are modified production vehicles. One car model may exhibit reverse Ackermann at 30 degrees of lock, parallel steering at 50 degrees, and positive Ackermann at 65 degrees. It can alter more slowly or in the opposite direction on a different car model. 

A driver may use an average of 40 degrees of lock, and another may use an average of 55 degrees of lock. Because the Ackermann is different for both angles, there is no ideal setup, not even on the technical side.

What Are the Advantages of Applying Ackermann Steering?

An Ackerman steering system has several advantages compared to other steering mechanisms.

• As the car is being driven, it prevents the tyre from slipping outward.
• As the tyre rods are shorter, the controllability of the vehicles is improved.
• They can readily navigate steep moves thanks to the tyre toe-out caused by the Ackerman steering geometry.

What Are the Disadvantages of Pure Ackermann Steering and How to Overcome Them?

Pure Ackermann steering's primary disadvantage for modern automobiles is that it is founded on low-speed turning or quasi static motion. When travelling over a curve at a moderate or high speed, a centrifugal force is generated, countered by a cornering force acting on each tyre. The laterally flexible tyres provide greater slip angles due to these balancing forces.

An expanded "dynamic Ackermann equation" can accommodate these angles, averaged at the front and back, with some simplifications but fair assumptions. Notably, the cornering-force-induced slip angles help with vehicle handling by reducing turn radii. Although it may not be immediately obvious, rear tyres also steer through these angles, but in a passive manner.

Conclusion

Ackermann steering mechanisms were used in several robotic vehicles explicitly designed for the experiment. 

In addition, there are several other steering mechanisms, including recirculating balls, parallelograms, and rack and pinion. Enrolling in a mechanical engineering course at Skill-Lync will help you learn more about the automobile industry and its fundamental concepts.