ISSUE 011 Autumn 2021 Candela C-7 hydrofoil speedboat dossier l In conversation: Robert Hoevers l Battery recycling focus l Vehicle dynamics insight l ZeroAvia hydrogen-electric aircraft digest l Motor materials

suspension designs such as those based on a twist-beam axle, forcing a trade-off between battery capacity and range against the cost and complexity of a more compact suspension. With multiple variables such as weight, driveline layout, CoG, tyres, spring/damper tuning and brake design, optimally balancing the lateral, longitudinal and vertical dynamics is a complex task, Karkare emphasises, particularly when it comes to comfort. This is especially true for EVs, he says, as all these factors affect range. “Range is one of the biggest differentiators between EVs,” says Sundstrom. “Most people who don’t want an EV are worried about range, so you need good numbers. The trade-off for that is that you need more battery capacity, and that makes the cars heavier and more expensive, which is the other worry people have.” Packaging suspension Differences in mass distribution affect the suspension designers’ approach to key parameters such as roll centres, pick-up points, geometry and spring/ damper rates, anti-roll bar stiffness and wheel sizes. The biggest change in vehicle architecture is due to the 110-140 mm added in the vertical direction by the flat-floor battery, which also moves the steering wheel centre higher, AVL comments. EVs also tend to have larger diameter wheels and suspension top mounts that sit higher. In the longitudinal direction, the distance between the centre of the steering wheel and the centre of each front wheel tends to be shorter, making the cabin larger and the wheelbase longer compared with IC-engined vehicles of the same length. To maximise space for the battery, AVL continues, the lower suspension link (also known as the comfort link) is oriented towards the front of the vehicle, and the steering rack is positioned in front of the axle rather than behind it. “The generally lower centre of gravity needs to be reflected in the roll centre height as well, influencing the suspension’s hardpoint definition,” AVL says. “Also, in the rear the target is to maximise the space for the battery, so axle types that do not affect battery length tend to be chosen.” Porsche Engineering emphasises that better boundary conditions such as a low CoG and low yaw inertia influence many factors. For example, spring rates, sway bar rates and damping ratios can be set lower than on a comparable vehicle with a higher CoG to achieve the same handling characteristics. Also, roll centres and anti-dive/anti-squat geometry must be adapted to the lower CoG height, otherwise the vehicle could suffer extreme roll or pitch stiffness. “The remaining hardpoints are vehicle- and target-dependent, and not necessarily battery EV-dependent,” Reichenecker says. To some extent, these advantages can compensate for the greater overall mass, which requires increased stiffness in the tyres and/or in terms of higher spring, damper and anti-roll bar rates to handle the extra weight and inertia, Karkare says. He adds that more compact EV driveline components offer designers more flexibility in the choice of suspension geometry and layout, as well as the points at which the suspension connects to the body in order to place the roll centre in the desired position and create the desired anti-squat and anti- dive characteristics. “However, to maximise space for the battery pack, the tyres are pushed as far as possible into the vehicle’s corners,” he says. “Those limitations in packaging space can create additional challenges in achieving the desired suspension geometry.” Skateboard pros and cons The increasingly prominent skateboard architecture is close to ideal in terms of lowering the CoG, centralising mass and allowing maximum flexibility in suspension design. It also allows manufacturers to put any style or configuration of body (the ‘top hat’) on it, just as the ladder frame chassis did in the early days of the car. Examples include Rivian, which offers a pick-up and an SUV on the same skateboard, and Tesla whose Model S and Model X also share a platform, Sundstrom says. “You have all this freedom to place the rest of the parts, seats and bodywork and so on where they make the most sense, because you have a flat floor inside the vehicle.” With a low centre of gravity and polar moment of inertia, skateboard-style architectures are seen as ideal for sportscars and allow for optimised suspension layouts (Courtesy of Porsche Engineering) Autumn 2021 | E-Mobility Engineering 47 Insight | EV dynamics