Pull-Rod vs Push-Rod: F1 suspension kinematics compared amid downwash and reduced aero block

In F1, the front pull-rod trend is gaining momentum. We are witnessing a shift towards this front suspension system, which contrasts with the “classic” front push-rod. The pull-rod involves the transverse element, namely the strut, anchored to the lower part of the chassis and the upper part near the hub. In the winter preceding the 2022 season, it was announced that some cars would adopt this concept for various reasons. The Red Bull RB18 set a precedent in the wing car era, making the front suspension effective.

McLaren also adopted the front pull-rod, although it took more than a year to make this feature effective. Observing the initial presentations of the 2024 cars, the decision of the Saber C44 to follow this philosophy by abandoning the push-rod is evident. Simultaneously, in a few hours, the Visa Cash App RB will be unveiled. This is a team that has undergone significant technical and managerial changes and, apparently, like its big sister RB20, will adopt the front pull-rod suspension.

F1, Pull-Rod vs. Push-Rod: Kinematics Compared

The pull-rod system brings some advantages, but it’s crucial to understand that the choice between “push” and “pull” alone does not determine the overall dynamics of the car, as external suspension components are often subject to aerodynamics.

This trend is declining compared to cars of the previous generation, as suspension kinematics are crucial for optimizing the underbody and obtaining a stable platform during various vehicle phases. For this reason, overlapping triangles often have a certain degree of inclination, while in the previous era, they tended to be parallel to the ground to minimize losses. However, the aerodynamic aspect remains valid.

From a kinematic perspective, the pull-rod works in traction, while the push-rod works in compression. The responsibility for dynamic behavior is mainly entrusted to the spatial arrangement of the two overlapping triangles. The pull-rod system offers a lower center of gravity because all internal components, such as the rocker, anti-roll bars, and torsion bars, are positioned lower.

It should be noted that such a choice is not made solely based on the center of gravity, although it can undoubtedly be a significant advantage, contributing to further lowering the suspended masses. One reason for choosing the pull-rod involves the different shape of the chassis, which, in the new generation of cars, has been lowered further by a few millimeters vertically.

The allocation of internal components is thus facilitated by a pull-rod suspension. The choice depends on the project’s goals and the “secondary benefits” that can be obtained by positioning mechanical components higher or lower. Since the pull-rod involves a tie rod rather than a strut, the stress it will have to withstand will be lower, and consequently, it can be thinner, offering less aerodynamic blockage.

Moreover, each arm has a specific aerodynamic function, helping to direct a larger quantity of fluid to the inlet section towards the Venturi channels. The triangles have a precise orientation in space. The upper wishbone is inclined towards the rear for dynamic reasons, but the concept aligns well with the goal of generating the downwash effect.

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Exiting the front wing, the flow is upwash, as it has a high Z-component given by the flap’s incidence. The direction of the flow must then be manipulated again. This is the general concept; each arm has a particular task that varies depending on the car. The fluid field can vary significantly between one car and another. Efforts are also made to avoid flow separation on various suspension links to minimize losses.

Because the flow that hits the suspension arms is “upwash,” these elements cannot be shaped to have a high incidence, otherwise, the detachment of the fluid vein increases the chances of being present. In terms of areas, the push-rod divides the frontal space between the two overlapping triangles. In the previous comparison, we notice the two opposite solutions, which, as mentioned, are effectively equivalent in terms of occupied areas.

It is therefore challenging to fully understand the aerodynamic advantage that can be gained. However, we can say that the more internal area, adjacent to the chassis, is more organized with the pull-rod, and thus the fluid remains cleaner. The mass of fluid flowing first over the nose and then over the chassis is clean and must be preserved and lowered (downwash) to feed the underbody. On the wheel side, we have a more complex situation with more turbulence and various recirculation zones.

Furthermore, the pull-rod has a slightly lower inclination of about 4° compared to the push-rod solution (a variable factor from car to car). It’s not much, but it certainly contributes to reducing the impact on the fluid field in this area. Along with these considerations, the trend that is proving effective is to increase the “spacing” between the two triangles to achieve good structural rigidity on the suspension and reduce the thickness of each arm. To do this, the upper triangle is attached to the highest limit of the chassis.