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Designing a Lightweight Principle Roll Structure for a Formula 1 Car

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The following case study is extracted from a presentation given by Renault Sport F1 at OED2016 (www.grm-consulting.co.uk/presentation-renault)

SUMMARY

As most formula one fans will know the roll hoop is integral to driver survivability in the event of a crash in which the car was to “roll.” It also acts as an air intake to the engine and can influence significant aerodynamic gains. Not only are its functions important but the weight of it is also extremely important. Weight of the roll hoop is important because the lower the CoG of the vehicle, the better it handles. Lower CoG means less weight transfer, more grip on the inside wheels and therefore more stability when braking and cornering. Renault Sport F1 utilised TruForm Abaqus to design a lightweight roll hoop on their 2016 car, thereby lowering CoG and improving handling. GRM Consulting then utilised a multi-method optimisation approach in GENESIS to see if further weight savings could be achieved.

Key Highlights

Industry

Formula 1

Challenge

Design a lightweight roll hoop in a limited time

frame without compromising on performance

Renault Solution

Topology optimisation using TruForm Abaqus

GRM Consulting Solution

Multi-method optimisation using GENESIS

Outcomes

Two optimised engineering design processes

TruForm Abaqus

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What Do We Know? (The Problem) Formula one cars need to be light. A reduction of even a few grams in a component can reduce lap times by that crucial 10th of a second. For the Renault Sport Formula One Team, operating at the pinnacle of the sport, weight is absolutely critical. This, coupled with the compressed timescales that the team needs to deal with, means that they need to find a simple, easy-to-use solution, which can guide them towards the lightest component, in the shortest timescale. Enter TruForm Abaqus. TruForm Abaqus is an integrated topology optimisation solution for Abaqus/Standard users, powered by an embedded version of the industry leading optimisation solver, GENESIS. TruForm Abaqus provides a powerful optimisation method in a simple and straightforward user interface.

The roll hoop, much like the rest of the vehicle, needs to be lightweight. Not only does it need to be lightweight but it needs to be stiff and strong to meet the FIA legislative requirements for roll hoop structures (see above). These considerations are taken into account throughout the development process. For example, the material for the roll hoop was chosen to be Titanium because it provides the best stiffness to weight and strength to weight ratio, despite being expensive.

The first step in any optimisation process is to define the available design space. In the case of the roll hoop this was defined by the aero package and the section of chassis which the roll hoop would bond to. Once this was done, the load cases defined by the FIA were assessed using Abaqus/Standard with the setup shown in Figure 1 below. These load cases were then used as the basis for the subsequent topology optimisation which was conducted using TruForm.

Article 17.2 Principle roll structure test:

‘A load equivalent to 50kN laterally, 60kN longitudinally in a rearward direction and 90kN vertically, must be applied to the top of the structure through a rigid flat pad

which is 200mm in diameter and perpendicular to the loading axis’

Figure 1 – Abaqus/Standard Load Case Setup

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The Renault Solution Before running any optimisations a design limit needs to be defined. Article 17.1.3 stipulates that ‘deformation must be less than 25mm’ and ‘structural failure limited to 100mm below the top of the rollover structure when measured vertically’. Therefore the optimum solution would ideally fail at 121kN but in reality a safety factor is required. The design criteria that was chosen for the roll hoop is detailed in Figure 2. Therefore the optimisation was setup in TruForm to minimise mass, while limiting plastic strain to 6%. The results are shown in Figure 4 below.

These results highlight the minimum mass required to deliver a component that does not fail, however, they require interpretation to turn into a feasible design that can be manufactured. Renault’s engineers use the

results from TruForm as a guide to efficiently develop light-weight solutions to their engineering problems, understanding that not all of the features can be designed in. In the case of the roll hoop the result was turned into a design that could be cast using titanium and then verified in Abaqus/Standard (see results below) before production into a physical part. This finished component then featured on Renault’s Formula 1 car throughout the 2016 season.

Design Criteria

Target safety factor

Max elongation target assumed

Must not exceed deformation limits

Safety : mass saving

Figure 5 – Abaqus/Standard Verification of Interpreted Design

Figure 3 – Titanium Material Properties

Figure 2 – Roll Hoop Design Criteria

Figure 4 – TruForm Topology Results

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The GRM Solution GRM took a slightly different approach. The thinking was to utilise Genesis which has increased optimisation capabilities over TruForm but, the trade-off is that it is a separate stand-alone software, not integrated into Abaqus, which is Renault’s main analysis solver. The objective of the Genesis study was to engineer a roll hoop with a 15% weight saving over the already lightweight, optimised design. The first thing to do was run a solid topology using the same package space and model used

for the TruForm optimisation. The initial topology optimisation was setup to minimise mass with stress limits applied to the designable region. The result of this optimisation is shown in Figure 6. While it clearly provides design clues it hasn’t produced the cleanest solution reflecting the difficult loading conditions. The topology results were then used as a guide and the areas highlighted as important were designated non-designable for a more refined shell and solid topology for the remaining structure. The results of this refined topology are shown in Figure 7. This result was then interpreted into a roll hoop design that could be cast using Titanium, shown in Figure 8. The design illustrated above already represents an optimised design, however, Genesis allows the user to go beyond topology optimisation and experiment with a plethora of other optimisation methods. One such method is Topometry optimisation which can optimise the thickness of a shell. A Topometry optimisation was therefore setup with the objective of maximising stiffness using a fixed mass of material, while also keeping stress levels below key design limits. The results of this Topometry are shown in Figure 9. These results were again used as a guide and the thicknesses mapped to a design which could be cast, as

Who are GRM Consulting?

GRM Consulting is an engineering consultancy founded in 2003 on the basis of CAE-led design. Within its core is the development and application of design optimisation methodologies to solve complex engineering problems in the most efficient, cost effective way. GRM have been supplying software to Formula One teams for over 10 years and have supplied software to the championship winning team 11 out of the last 12 seasons.

Figure 6 – Initial Topology Results

Figure 7 – Refined Topology Results

Figure 10 – Interpretation

Figure 8 – Topology Interpretation

Figure 9 – Topometry

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shown in Figure 10. Following this interpretation phase, much like with Renault’s TruForm methodology, the design needed to be verified in a non-linear solver, in this case LS-Dyna, to ensure the design was robust to the safety requirements of the FIA. The non-linear verification results can be seen below in Figure 11.

At the end of GRM’s optimisation-led design process they had designed a component that was 16.5% lighter than Renault’s already light weight 2016 roll hoop.

The Outcome The key objective of this work was to demonstrate two different, but equally effective, approaches to design. The different methodologies were linked by the common thread of optimisation with both methods using simulation results to guide the design process. The optimisation results still need to be reviewed, understood and interpreted with good engineering practice but both approaches show that using a simulation-led process leads to greater weight savings. Using more optimisation methods results in greater weight savings but the key is to apply each particular method at the appropriate time in the design process and then use sound engineering judgement to turn the results into feasible designs. This is just two possible approaches, there could be many more!

Figure 11 – Non-linear verification of GRM’s Design

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