From Metal Flexures to Carbon Fibre Flexures

In Part 1 of this series, I looked at the thinking behind the F1 suspension flexure — replacing a conventional spherical bearing with a component that allows suspension movement through controlled elastic deformation.

At Prost Grand Prix, the flexures I first encountered were beautifully machined titanium components bonded into the carbon-fibre suspension wishbones. They eliminated the conventional rotating bearing at the chassis end of the wishbone, providing an elegant solution to the problems of friction, wear and clearance.

But there was an obvious next question.

If the wishbone was already made from carbon fibre, could a carbon fibre flexure be integrated as part of the composite structure?

That is where the carbon fibre flexure becomes particularly interesting.

Designing Carbon Fibre to Flex

Carbon fibre is normally associated with stiffness. In motorsport, aerospace and other high-performance applications, much of the design effort goes into controlling fibre orientation and laminate construction to create a lightweight structure that resists deformation under load.

A carbon flexure turns that thinking slightly on its head.

Part of the structure still needs to be extremely stiff and accurately locate the suspension, while another carefully defined region must be capable of repeatedly flexing through the required suspension movement.

This doesn’t mean simply making part of the wishbone thinner. The flexing region has to be considered as part of the complete load path through the component.

Geometry, laminate thickness, fibre orientation, transition regions and the loads acting through the wishbone all influence how the flexure behaves.

The objective is controlled compliance: flexible where movement is required, stiff where it isn’t.

Why Composites Make This Possible

One of the great advantages of composite materials is that their mechanical behaviour can be engineered.

With a conventional isotropic material such as aluminium or titanium, the material properties are broadly the same in every direction. A carbon-fibre laminate can instead be constructed with fibres orientated to provide different properties in different directions.

This gives the designer another set of variables to work with. Geometry is no longer the only means of controlling stiffness — the laminate itself becomes part of the engineering solution.

That is why good composite design isn’t simply about recreating a metal component in carbon fibre. The component, laminate and manufacturing process need to be considered together.

Designing for Repeated Movement

A suspension flexure isn’t subjected to a single deflection. It must move repeatedly as the suspension operates.

That introduces considerations beyond simple static strength.

The designer needs to understand the strain generated through the required range of movement, the loads transmitted through the wishbone and how those loads interact with the flexible region.

Fatigue, local stress concentrations and the transition between flexible and rigid areas therefore become important parts of the engineering problem.

Analysis can help understand that behaviour before physical testing, but in an application such as this, simulation and testing ultimately complement each other.

Manufacturing a Composite Flexure

Designing the flexure is only part of the challenge. It also has to be manufactured consistently.

The original carbon-flexure wishbone was produced using a closed-mould process with an internal core, allowing the composite laminate to be consolidated during curing.

Maintaining control of the flexure region is particularly important. Small changes in geometry or laminate thickness can influence the stiffness and therefore the behaviour of the flexure.

Tooling, core design, laminate placement and processing therefore become part of the component design rather than separate manufacturing considerations.

This relationship between component design and tooling is something we still encounter regularly in composite engineering today: the way a component will be manufactured needs to influence the design from the beginning.

The Original Carbon Flexure Wishbone

More than two decades later, we now have the original carbon-flexure test wishbone at Plan D.

For me, that makes this project particularly interesting. It isn’t simply an old Formula One component sitting on a shelf; it represents a particular engineering idea and a stage in the development of composite suspension technology.

We’ve since 3D scanned the wishbone, allowing us to preserve its geometry digitally and examine the component in considerably more detail.

The scan also gives us the opportunity to use the component educationally. One of the reasons for sharing these F1 Tech Tales is to give younger engineers and students some insight into engineering solutions that they might otherwise only ever see as photographs of old Formula One cars.

The Story Behind the Carbon Flexure

What I’ve deliberately not covered here is how the carbon flexure was actually developed.

That’s because the engineer who developed it can tell that story himself.

My Plan D colleague Dan Fleetcroft worked on the development of the carbon flexure during his Formula One career, and the process involved considerably more than simply deciding to replace titanium with carbon fibre.

In Part 3 of F1 Tech Tales, we’ll let Dan tell the complete story — from the original engineering design challenge through the development, manufacture and testing of the carbon flexure, including the decisions and lessons that ultimately led to the component we have today.

In order to communicate the concept better, for young engineers looking for a career in motorsport, we have 3D scanned the component to make it available to investigate. If you’d like to have this scan data file, please contact us.