## 2 Materials and Methods  
### Support System Requirements and Design  
As stated in Section 1, the support system should leave the motorcycle unrestrained to simulate riding while leaving the vehicle/rider mass and moment of inertia unaltered as much as possible. Such an issue is critical since the rider mass is about 35% of the total vehicle system. Moreover, the rider bends forward to reduce drag along the straights, then raises himself while braking and leans into the corner, thus moving the centre of mass and varying the moment of inertia. The location of the centre of mass was measured for the bike alone, as well as with rider in the straight and braking positions (see Figure 1). The moment of inertia was accurately measured with the well-known pendulum system [16] and, consequently, an equivalent rider mass was devised to obtain a realistic mass distribution. Figure 1: Centre of mass location: motorbike only (Cm), with rider in straight (Cr), and braking position (Cb). Due to a relatively high centre of mass in conjunction with a short wheelbase and high power-to-weight ratio, the interaction between the vertical and longitudinal dynamics is another key parameter to be taken into account. The chain tension in particular can heavily affect the response of the rear suspension under power (see Figure 2) while the front fork can show a tendency to stick up in heavy braking [17], thus jeopardising the ability to cope with small amplitude, high-frequency road bumps. Therefore, the test system should be able to apply power or braking torque while leaving the tyres free to leave the ground as they occasionally do under heavy longitudinal acceleration or on severe bumps. Again, as mentioned in Section 1, a standard four-poster rig has been retrofitted with a custom-designed fixture able to hold the motorcycle in the upright position without adding unnecessary restraints. Three different layouts were considered at first, as shown in Figure 3.  
Layout 2 was discarded due to its asymmetric shape with the resulting lateral scrub between tires and actuator pads during system motion. Layout 3 was discarded for stiffness reasons and the resulting longitudinal movements during system operation. Furthermore, the two long connecting rods can affect system inertia hence dynamics. Therefore, the proposed layout is the first one in Figure 3, since it is able to hold the motorcycle without affecting its dynamic response. Moreover, the connecting rods are very short and stiffness and inertia issues can be avoided. Figure 3: The three considered layouts. Figure 2: How chain force can affect the suspension ride response. Such a layout is based on two low-friction, recirculating ball linear guides along the vertical axis. These are off-the-shelf components deemed to reduce undesired side effects like slip-stick friction, resonances, and additional inertia. Their mass is close to the average rider's weight and can be adjusted by adding or removing ballast. The main structure supporting the twin-sledge system was then designed using commercial extruded aluminium modular components instead of a more traditional welded steel structure. The advantage of this solution is that the structure is lighter and easier to build in-house, and the need for machined components was almost completely avoided. FEM (Finite Element Method) analysis was employed to ensure that natural frequencies of the structure are well beyond the typical frequency range of vehicle ride dynamics. Moreover, bolted junctions can help to dampen vibrations more efficiently due to friction in the contact area. The schematic picture reported in Figure 4 shows the final layout of the road simulator. A pivot mounted on low-friction bushings is placed on the two previously-mentioned sledges. The pivot--whose axis requires careful alignment with the bike centre of gravity--is then connected to the motorbike chassis via three conrods with ball-joint ends to provide an isostatic restraint, so that pitch oscillation around the CG (Center of Gravity) is free (see Figures 4-6).