Usually, open-wheelers, prototypes and GT racers utilise a double wishbone arrangement, whereas saloon, touring and WRC-style cars often run a MacPherson strut at the front axle.
Compliance and flex
Double wishbone systems often have chassis-mounted spring-damper units, which must include a linkage to enable their actuation. This additional structural member is known as a pushrod (loaded in compression in bump travel) or pull rod (loaded in tension in bump travel). The pushrod/pull rod actuates a component known as a bell crank, which pivots around a fulcrum to exert a force on the damper. Running through the load path from the wheel upright to the damper in this inboard arrangement there is an array of bearings, fasteners and joints the force must journey through. These components help designers to tune the inherent compliance and flex of such a system and its influence on the kinematics.
Pushrods, in particular, are susceptible to bending modes, control arms and bell cranks deform, while fasteners and bearings introduce backlash and flex. MacPherson strut configurations, on the other hand, don’t have pushrods or bell cranks as the spring-damper is mounted directly to the wheel upright. It does, however, mean the damper unit essentially acts as the upper wishbone and therefore must react to significant loads during cornering, introducing flex into the system.
Understanding the effects of flex and deflection, and their influence in a suspension system, is known as a compliance study.
Compliance is, essentially, understanding the geometric effects of the forces a suspension system is subjected to during accelerative loadings, and how these forces cause the suspension to deform and influence the orientation of the tyre’s contact patch.
Kinematics characterises two degrees of freedom (DoF) – vertical translation and rotation around the steering axis. Compliance focusses on understanding the movement of the wheels in the remaining three DoF of which they are not intended to operate in – longitudinal translation, lateral translation and lateral rotation. Here we’re considering rotation around the transverse axis separately, as it is not a function directly controlled by suspension. In short, kinematics is the study of intended wheel movement, while compliance is the study of unintended wheel movement.
Kinematic and compliant behaviour are studied through virtual assembly and simulations, and multi-body analysis software is used during the design phase. As ever with simulation, it is not accurate enough on its own to trust without verification. The mechanical properties of every link, member, fastener and part must be accurately understood and put into the software to simulate compliant behaviour, while relative movement and clearances in bearings and other hardware must be precisely modelled to understand the relative movement of components in kinematic simulations. Quite a task.
In fact, the hand-built nature of racecar parts can lead to tolerance stack ups and dimensional errors, which can result in the K&C (kinematic and compliance) behaviour of the suspension producing some unwanted effects. Therefore, teams need to assess the K&C qualities of their racecar in a controlled environment.
Horiba MIRA has a special K&C rig for exactly that. Clamping the chassis to a large, rigid bed plate and supporting the wheels with four pads, the car can be actuated in pitch, heave and roll modes to assess kinematic behaviour. Longitudinal, lateral and rotational forces can be put through the wheels to assess compliant behaviour, while steering and brakes can also be actuated by the rig control system to offer full and repeatable control of all dynamic situations a racecar may be exposed to.
The wheel pads can be moved with a lateral and longitudinal offset. This allows the wheel to be moved left / right or fore / aft of the wheel centre respectively. In this manner, a whole range of potential loading scenarios can be simulated. Tests in a controlled environment such as at Horiba MIRA are repeatable, but notably are also quasi-static, preventing frequency response related noise / variables from the data that would be experienced with measurements gathered from track running.
Rigged to win
Generally, a K&C rig is used by race teams in one of three different ways: to verify / correlate initial CAE (computer aided engineering) models before proceeding with further optimisation; to gather information in order to generate an initial model or – without CAE – to understand the characteristics of a previously unknown suspension system.
With kinematic assessment, a range of inputs can be applied to the chassis and wheels. Data for camber gain, bump steer and caster angle, to name just a few, is collected and fed back to the controller for analysis. Monitoring relative wheel movement to generate the data are six DoF displacement sensors (three degrees of translation and three degrees of rotation) mounted to the wheel centres.
‘Differences and discrepancies from design intent usually present themselves through the assumptions made in the CAE model inputs,’ says Luke Cosgrove, K&C facility team leader at Horiba MIRA. ‘The model can only be as good as the boundary conditions that are provided, so we see common sources of error from play in joints, flex in the chassis and so on.
‘Teams usually arrive here with results that they are expecting to see, so it is mostly a process of verifying those results.’
Gathering reliable information on how the vehicle performs with the tyre is especially important, because it has a significant influence on the suspension due to its own frequency response. Tyres are also particularly difficult to model due to their non-linear behaviour.
Suspension designers use various approaches to minimise and constrain unwanted DoF, yet no system can be infinitely stiff – the approach to understanding compliant behaviour of a system is slightly different.
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