While the term "push-pull-rod system" might suggest a single mechanism, it typically refers to two distinct but related suspension designs: push-rod systems and pull-rod systems. These advanced suspension configurations are predominantly found in high-performance vehicles, especially racing cars like those in Formula 1, where precise control over suspension geometry and aerodynamic efficiency is paramount. Both systems are designed to package the main suspension components – the coil spring and damper unit – inboard, away from the airflow, for better aerodynamics and a lower unsprung mass.
Understanding Push-Rod and Pull-Rod Systems
These systems utilize a connecting rod to transfer vertical wheel movement to an inboard-mounted rocker arm. This rocker arm then actuates the spring and damper unit. The key distinction lies in the direction of force the connecting rod exerts on the rocker arm.
What is a Push-Rod System?
A push-rod system operates by pushing the connecting rod upwards to actuate the rocker arm. In this configuration, the rod is positioned such that as the wheel moves up (compressing the suspension), the rod is put under compression.
- Mechanism: In a push-rod system, the rocker arms are strategically placed at the highest point in the assembly, often above the main chassis pick-up points. As the wheel travels upwards, the push-rod transmits this movement by pushing upwards into the rocker arm. This means the rod is under pressure as it transfers compression forces upwards into the rocker arms, which then act on the spring and damper unit.
- Advantages:
- Lower Center of Gravity (CoG) of Sprung Mass: By moving heavy spring and damper units from the wheel hub area to an inboard location, the overall CoG of the vehicle's sprung mass can be lowered.
- Aerodynamic Benefits: Outboard suspension components create significant aerodynamic drag. Inboard mounting allows for cleaner airflow around the wheels and suspension uprights.
- Packaging Flexibility: It can allow for more compact packaging around the wheel assembly, benefiting brake cooling and steering geometry.
- Easier Adjustment: The inboard-mounted dampers are more accessible for quick adjustments during pit stops or tuning sessions.
- Disadvantages:
- The push-rod itself is under compression, which requires it to be robust enough to resist buckling, potentially adding weight.
- Packaging constraints might still exist for the rocker arms at the high point.
- Applications: Push-rod suspension is commonly seen in the front suspension of many modern Formula 1 cars. This allows for the nose cone and front wing to be placed lower, aiding aerodynamics.
What is a Pull-Rod System?
Conversely, a pull-rod system operates by pulling the connecting rod downwards to actuate the rocker arm. In this setup, as the wheel moves up, the rod is put under tension.
- Mechanism: In a pull-rod system, the rocker arms are typically located lower down in the assembly, often between the upper and lower control arms, at the centre of the assembly. As the wheel travels upwards, the pull-rod transmits this movement by pulling downwards on the rocker arm. This means the rod is primarily under tension as it transfers pulling forces into the rocker arms, which then act on the spring and damper unit.
- Advantages:
- Even Lower Center of Gravity (CoG) of Suspension Components: Placing the rocker arms and damper units even lower in the chassis compared to a push-rod system further lowers the vehicle's CoG, enhancing stability and handling.
- Aerodynamic Integration: The lower positioning can lead to even better integration with the vehicle's underbody aerodynamics, especially around the diffuser area.
- Lighter Rod: A rod designed primarily for tension can often be lighter than one designed for compression (which needs to resist buckling).
- Disadvantages:
- More Complex Packaging: Integrating pull-rod systems can be more challenging due to the lower placement of components, potentially interfering with other chassis elements or the driver's feet in some applications.
- Accessibility: Inboard components might be harder to access for adjustments compared to some push-rod setups, depending on the specific design.
- Applications: Pull-rod suspension is frequently used in the rear suspension of Formula 1 cars. This helps in packaging the complex rear-end components (engine, gearbox, diffuser) and lowering the rear CoG.
Key Differences and Design Considerations
The choice between a push-rod and a pull-rod system is a complex engineering decision influenced by factors like weight distribution, aerodynamic goals, packaging constraints, and desired suspension kinematics.
| Feature | Push-Rod System | Pull-Rod System |
|---|---|---|
| Rod Force | Compression (pushing) | Tension (pulling) |
| Rocker Arm Location | Highest point in the assembly, often above chassis | Lower in the assembly, between control arms, at center |
| Spring/Damper Location | Inboard, generally higher | Inboard, generally lower |
| Primary Advantage | Aerodynamic cleanliness, packaging flexibility | Lowest possible CoG, potentially lighter rod |
| Common Application | Front suspension of F1 cars | Rear suspension of F1 cars |
Practical Implications and Examples
The strategic use of push-rod and pull-rod systems is a hallmark of cutting-edge vehicle design. For instance, Formula 1 teams meticulously choose their suspension layout to gain a competitive edge.
- Front Suspension (Push-Rod Emphasis): In F1, a push-rod front suspension allows for a lower nose and front wing assembly, which is critical for generating downforce. The upward push of the rod neatly packages the springs and dampers into the upper section of the chassis.
- Rear Suspension (Pull-Rod Emphasis): A pull-rod rear suspension helps to keep the entire rear of the car as low as possible. This is crucial for optimizing the performance of the diffuser, a key aerodynamic device that generates downforce from the underbody. The lower packaging also allows for more aggressive engine and gearbox integration.
- Engineering Choice: Designers weigh the benefits of a lower CoG (pull-rod) against packaging ease and accessibility (push-rod). Often, the choice depends on specific car architecture and aerodynamic philosophy. For example, some racing cars might even opt for a pull-rod at the front if the aerodynamic benefits outweigh the packaging challenges.
While complex, understanding these systems highlights the intricate engineering involved in modern high-performance vehicle design, where every component's placement and function are optimized for ultimate performance.
