QFR18: Suspension design

Queen’s Formula Racing entered uncharted waters last September by assigning the role of suspension design to a third-year project group. As part of their group projects, the six team members were tasked with the design of a suspension system that would improve QFR’s performance at Formula Student UK.

Initial research into QFR17’s performance suggested that understeer had inhibited the team from gaining stronger points in events such as the skid-pad. Working in partnership with the anti-roll bar engineer, a better proportion of roll resistance between the spring-damper and anti-roll bar was achieved.

A suspension design tool was created in Excel, taking calculations from first principles, to evaluate load transfer values, suspension geometries and suspension forces. From this, a greater understanding of kingpin and caster angles were gained as well as scrub radius, mechanical trail, roll centres and anti-dive/squat. Final geometries were selected in a bid to improve the car’s handling. These were passed onto the chassis team who had to accommodate wishbone and rocker hardpoint locations in the spaceframe design.

Front suspension spreadsheet.JPG

One key update in suspension design was the introduction of swept-back rear wishbones, which allowed the removal of several rear chassis members. This mass reduction was complemented further by replacing front and rear push-rods with pull-rods to reduce the car’s centre of gravity.

push v pull.jpg

A limitation of the pull-rod system was the availability of anti-roll bar packaging space. The solution resulted in the anti-roll bar drop-link being attached to the rockers at a plane offset from the other rocker points. The original rocker design incurred a hefty mass increase to provide enough support to the anti-roll bar point, however, following detailed Finite Element Analysis, the mass of the rockers was reduced to a value similar to that of last year’s design.

The FEA study involved a 1D-3D hybrid model which incorporated the 3D rocker model to the 1D wireframe suspension model, giving accurate forces from calculated wheel loads.

The suspension team also designed wishbone bearing housings, rod-ends, top hats, wishbones and pull-rods. The wishbone and pull-rod tubes were spec’d using the forces calculated in the design tool and 1D FEA. Top hats were standardised to suit required bolt sizes, reducing manufacturing time and complexity.

Overall, the suspension has been completely redesigned from last year’s system with the ambition of improved handling. With manufacturing ongoing, it is hoped a greater amount of testing this summer will give the team an opportunity to uncover any teething problems and find the optimum set-up going into July’s Formula Student competition.

Adam Hall


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