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###1. General concept

The Riversimple technology demonstrator has been designed with “maximum overall efficiency” in mind and the rear suspension follows the same philosophy. The primary target was minimal weight commensurate with good handling and roadholding.

The decision to use hub motors rather than inboard motors and driveshafts placed extra emphasis on suspension performance; the combination of increased unsprung weight and very low sprung weight compounds the reduction in sprung to unsprung mass ratio. As the hydrogen fuel tank is located in the back of the car, the suspension had to be very compact and give a degree of camber change with minimal bump steer and tyre scrub in bump. Finally, in the interest of keeping weight down yet achieving good torsional rigidity of the chassis, the location of all chassis pickups on a bulkhead forward of the axle line, rather than much further rearwards, was to be preferred. For all these reasons, a semi-trailing arm configuration has been chosen (see Figure 1 and Figure 2). \ \ [{Image src=’rear_susp.png’ align=’center’ width=’400’ caption=’Figure 1’ alt=’figure1’ }]

###2. Assembly design

As seen on Figure 1, with a track of 1300mm and a fuel tank length of 915mm, the choice of semi-trailing arm is optimum for packaging. The only other layout that could come close to satisfying these requirements was a single trailing arm wrapping round behind the hydrogen tank that would have been in effect both a dead axle and anti-roll bar. However, this would have been heavier, would not allow camber change and anti-roll would not be desirable anyway. The suspension uses a lightweight steel tubular subframe (1.9kg) fixed to the car’s composite monocoque. It provides pick-ups for both the semi-trailing arms and the shock absorbers. \ \ [{Image src=’Picture 2.png’ align=’center’ width=’400’ caption=’Figure 2’ alt=’figure2’ }]

Figure 3 shows a more detailed view of a “quarter-car assembly”. The semi trailing arms each comprise two tubular steel wishbones, sharing common inboard pivots, connected to the wheel axle. The lower wishbone has a mass of 0.6kg and the upper wishbone 0.5kg. The adjustable rear toe link is also a steel tube. On the demonstrator, all suspension components are connected using rod ends and spherical joints and the result is a very light yet strong suspension system. \ \ [{Image src=’Picture 3.png’ align=’center’ width=’400’ caption=’Figure 3’ alt=’figure3’ }]

Figure 4 and Figure 5 are top/back views of a quarter-car assembly without the wheel. It exposes the wheel axle, which holds the trailing arm pick-up points, toe link and calliper mount. It is a steel assembly and has a mass of 1kg.

[{Image src=’Picture 4.png’ align=’center’ width=’400’ caption=’Figure 4’ alt=’figure4’ }]

The trailing angle is 19 ° to the transverse axis (see Figure 4) in the horizontal plane. In the vertical plane, the inclination is 11.5 ° to the horizontal (see Figure 5). The orientation of the pivot axis has been determined to control camber change. In the current configuration, the instantaneous camber change at ride height is 0.45° per 10mm of vertical wheel travel.

Figure 6 is a side view of the assembly, on which the adjustable toe link can be seen. It has been carefully calculated to minimise bump steer.

The damper and spring had to be selected in accordance with the characteristics of the car. The overall mass of the car is 380kg and the unsprung mass is 16.2kg per corner, excluding contribution from the suspension links. To provide a comfortable and safe ride, the damper selected on the prototype is a adjustable Koni 26series, and the spring rate is 300lbf/in

As we can see from this short description, the suspension design was mainly driven by weight and volume constraints, but will still offer good performance. \ \ [{Image src=’Picture 5.png’ align=’center’ width=’400’ caption=’Figure 5’ alt=’figure5’ }] \ \ [{Image src=’Picture 6.png’ align=’center’ width=’400’ caption=’Figure 6’ alt=’figure1’ }]

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