Types of Suspension Systems
There are many types of suspension systems used within motorsport; the most common being a spring and damper set up. The popularity of the spring and damper set up is mostly due to its compact size and its easy, fast and direct tune-ability.
There are multiple ways of using a spring and damper within motorsport today. Examples of these are:
MacPherson strut suspension systems are predominantly used at the front end of a car for the steering system. The design uses a telescopic damper with a spring mounted to it and is mounted between the top of the hub and the chassis and is used as a steering pivot.
The near vertical angle of the damper is possible as there are no other suspension components to avoid such as an upper wishbone. This means that there is more room available within the engine compartment of a car which is particularly useful for a front wheel drive car. Furthermore, the reduction in components to create this system means that is cheaper to produce and lightweight. These reasons made it a very popular choice for the vast majority of mainstream car manufacturers.
One downside to a MacPherson strut design is that the quality of the ride and the handling characteristics are impaired. Also, the wheel cannot move vertically without moving the wheel backwards and forwards in the wheel arch to some degree as well as giving the system some camber gain. Due to this it is not known to give as good handling as a double wishbone or multi-link suspension system.
A double wishbone suspension system is an independent suspension system that has two wishbones that mount to the top and bottom of the hub and are often parallel to each other and are different lengths. The coilover then mounts to the bottom wishbone, controlling the vertical movements of the system. This set up also allows the use of a push or pull rod suspension system.
A double wishbone set up is advantageous for a few reasons which has led to its popularity within motorsport and is broadly used. First of all, the fixed nature and predictable movement of the system makes it easier for geometry to be designed into the system such as camber gain, caster and bump steer. A double wishbone set up also provides continuous negative camber gain all the way to full compression which is useful during hard cornering, whereas a MacPherson strut set up only gives negative camber gain to a certain point then reverts to positive camber gain. Another benefit of running a double wishbone set up is that it is easier to calculate the forces through each component and therefore easier to optimise the size and mass of each part, reducing the mass of the system.
There are some disadvantages to a double wishbone set up. Due to the increased components of the system, it is a heavier system than a MacPherson strut set up. Furthermore, the increased amount of components means that there are more joints to service and makes it a much more expensive system to manufacture.
Push rod suspension is where a rod is put in place of where the damper would traditionally mount in a double wishbone set up. Therefore the damper and spring is mounted in-board of the chassis, operated via a bell crank which is activated by the push rod, which is bolted to the hub.
The main benefits of this set up and the reasons for its popularity, especially within open-wheel motorsport, are due to mass, aerodynamics and design freedom.
The main reason push rod suspension has become one of the most popular suspension systems within motorsport is due to its benefits on mass. A push rod weighs much less than a damper and spring and having the ability to move all of this inboard as close to the centre line of the chassis as possible has great effects on the centre of gravity position of the car helping to keep the mass inboard and therefore improving the handling characteristics and capabilities of the car.
Push rod suspension improves aerodynamics as the damper is replaced by a much thinner, composite or metal rod. In an open wheel application, this is paramount as the airflow flows between the wheel and the chassis; meaning that any downsizing of an obtrusive object in the airflow has a great aerodynamic advantage. Particularly at high speeds within motorsport such as F1 or Indy Car.
The design freedom is available due to the use of a bell crank. A bell crank is a pivoting mounting point that transfers the force from the push rod into the damper. The distance between the pick-up points can be increased or decreased to either raise or lower the effect/force that the coil over has on the wheel.
Push rod suspension is also a benefit as an anti-roll bar link can also be connected to be bell crank at a designed point, allowing the effects of the coil over and roll bar to be designed accordingly.
Pull rod suspension is similar to push rod suspension in many ways. It shares many of the same benefits, mounting the spring and damper inboard, thereby improving the mass distribution of the car. It places a thinner, more aerodynamic pull rod in the airflow reducing the drag of the car through the air, improving acceleration and top speed. It also utilises a bell crank to operate the spring and damper. All of these benefits are discussed within the push rod section and are the same for pull rod.
One area where pull rod suspension has the advantage is that the spring and damper can be mounted low at the bottom of the chassis, lowering the centre of gravity of the car. This is because the pulling action means that the pull rod needs to be mounted to the top of the hub so it pulls on the bell crank mounted low down under compression of the wheel.
However, having a coilover mounted this low can be disadvantageous for a few reasons. Firstly, it can mean that the damper is liable to impact with the ground or a kerb on a racing track that could cause severe damage. It’s also more difficult to surround with body work to improve the airflow over the spring. Finally the low position of the damper can be almost inaccessible quickly which in racing conditions could be fundamental if a part needs to be adjusted or replaced quickly.
A rocker arm suspension system is not used much in modern day motorsport and is more of a predecessor to push rod suspension. Rocker arm suspension used an upper arm, pivoted at a set length, which is mounted to the top of the hub and to the top of a coilover. Under compression, this arm would then pivot and compress the coilover. The wheel rate could be set by altering the position of the pivot point in the arm at the design stage. However, once on the car this was not tuneable.
Also, due to the bending forces going through the arm, it had to be much thicker and stronger than a push rod meaning that the system weighed more and created more drag due to the increased size. This system did successfully move the damper in board of the chassis and remove the coilover from the airflow.