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technical evolution of the car from previous season. Focusing to S.C.R. project, gear-box especially represents an important evolution aimed to upgrade of the sprocket-chain transmission that equips S.C.12e.
3.4. High level of integration.
A F.S.A.E vehicle that features a wheelbase of about 1600 [mm] is a very tight ambient of work. For this reason, different systems which are essential for the operation of the car, have to be accurately positioned and integrated. The typical example on S.C.R. is described in Chapter 1.2, on-board rear brakes, which integrates the transmission with the components of the braking system. Another important example is described at Chapter 1.4.3. The “corner” of S.C.X.V. integrates functions typical of un-sprung mass components and power-train components. That represent a an important save of weight and money due to the reduced number of components.
Picture 3.4-1: S.C.X.V. un-sprung weight.
3.5. Higher gear ratio.
As revealed in advance at Chapter 1.4.2, tests and races shown that torque discharged from S.C.12e to the ground wasn’t enough to exploit good acceleration performances. For this reason, one of the target in S.C.R. design is the increase of gear ratio from 6:1 to a value around 9:1. That is a quite complex goal which cannot be reached by a simple chain-sprocket system or by a single stage gear-box.
3.6. Best weight stiffness trade off.
It’s simple to image that main requirement of a race car is the save of weight. In addition F.S.A.E.
official regulations don’t impose a minimum weight.
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Extreme research of lightness is congruent but the light weight isn’t the only requirement to fulfil.
Thinking to the entire vehicle, the main requirement it’s, of course, driver safety but that’s not all. Then It’s important reflect on operation of the components. Considerably lightened components can operate differently from what intended by the designer, that’s due basically by displacements. For example, a suspension arm, like many other components, cannot be considered infinitely stiff. It deforms in operation, but if it isn’t stiff enough it may deform more than necessary. This condition may produce undesired displacement of suspension hard-points housed on the un-sprung mass.
Precise position and bounded movement of suspension hard-points is essential for the good operation of the entire suspension. Undesired displacements on the suspension can dangerously affect dynamic performances of the car causing loss of driver feeling and worsening of handling. Another example can be applied to transmission, a poor matching between bearings and case can cause a too high displacement of gearwheels. Such issue can drive to severe problems in reliability, efficiency and noise.
By virtue of that, designer haven’t to consider weight of components only, but he must take into account stiffness too. Therefore, the target of a good design have to be set on the “best trade-off between weight and stiffness”.
Picture 3.6-1: Dimensional checks on S.C.R. gear-train components.
3.7. Easy and accurate assembly.
Transmissions, gear-boxes in particular, are a very delicate topic in mechanic field. In the entire range of the vehicle, in a electric car especially, transmission is the most complex system by mechanical point of view. Usually transmission exploits a large range of mechanical components like gears, bearings, gaskets springs, levers and pneumatic actuators. In addition, a wider range of electronic components like sensors, control units, looms are integrated in order to increase performances and reliability of the system. How It’s widely described by “L. Morello – Progetto della Trasmissione Meccanica” Ref.[6], good operation of a transmission depends on a wide range of issues like thermal dissipation, displacements, backlashes and wear.
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Anyway, transmission designers haven’t to care about operational problems only. First of all they need to concentrate on manufacturing issues too. A well engineered transmission layout have to be envisioned caring about assembly and tuning issues. A transmission such that of S.C.R. is based on bevel gears and angular contact bearings. By assembly and manufacturing point of view, these kinds of components needs to be installed on accurate machined housings. Furthermore, gears and bearings need a proper set preload in order to achieve an efficient and reliable operation. By virtue of that S.C.R.
gear-box case needs to be designed ensuring measurement ability and needs to be manufactured following highest standards of accuracy.
On the other hand S.C.R. gear-box needs to be simple enough to be assembled quickly using the minimum number of tools and custom devices.
Picture 3.7-1: Squadra Corse crew making interventions on S.C.X.V. before a performance test. Lingotto 2015 (https://www.facebook.com/SCPolito/).
3.8. Fast maintenance and tuning.
Unfortunately race cars, prototypes especially, needs interventions of maintenance and tuning any time. During tests and races, events are more dense and times are very tight. For this reason, times devoted to maintenance have to be shorter as possible.
Systems need to be simple enough to ensure the ability of intervention on the track too, where workshop equipments are more lacking and where multiple operators need to work on the car at the same time, as depicted by Picture 3.7-1. S.C.R. is therefore designed trying to match different maintenance requirements. With reference to Chapter 1.3, suspension are one of the most effective examples of easy and fast intervention. Such system is designed to ensure wheel and brakes quick replacement. In addition, few dozens of minutes are enough to regulate set-up parameters of the vehicle: camber, toe, ride height, A.R.B. stiffness and hydraulics. About transmission, all the system can be disassembled from the vehicle in ten minutes only. These features ensure a strong ability of tuning and maintenance of the car.
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