5.2.1 Decision Making (Space frame vs. Monocoque)
The design process began by looking on the formula student website and comparing all the designs from previous years, directing the majority of focus on the winner from last year as it is assumed that their design was the best across the board to end up winning the whole thing as the ability of the drivers cannot be easily compared. Before proceeding, a decision had to be made as to what type of chassis to make. This brought up three options: a tubular space frame, a full monocoque made up of composite material and a half monocoque made up of a composite material majority attached to a rear tubular space frame. Multi Criteria Analysis was used.
Table 5.1 Multi Criteria Analysis for the Chassis
Cost Mass Manufacturing
Process Design/Analysis
Process Torsional
Stiffness Total
Weighting 2 5 2 3 4
-Tubular Frame 5 1 4 5 3 50
Half Monocoque 2 3 2 1 4 42
Full Monocoque 1 5 1 2 5 54
The criteria considered were weighted according to their relevance to both this particular project and the group aims. So mass was decided to be the most crucial, whereas the manufacturing process was not of great importance due to this only being a design project and not actually constructing the car in real life. The design and analysis process represents how easy it would be to model the chassis using Solidworks, as this is the tool that has been made available for us to use and that we have previous experience in using. A full monocoque was the resultant winner from the Multi Criteria Analysis. The flow chart in Figure 5.1 represents the step-by-step approach used when designing and optimising the vehicle.
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Figure 5.1 Design Flow Chart 5.2.2 Regulations and Dimension Specifications
As mentioned previously, the design of the vehicle needs to meet the specifications stated in the
‘2015 FSAE Rules’. These vary from the minimum length of the wheelbase, being 1525 mm, to the need for the vehicle to be an open-wheeled and open-cockpit with four wheels not in a straight line.
The definition of open-wheeled is defined in the FSAE Rules in Article 2: T2.1 where the wheels cannot be obstructed from the side view and when viewed from vertically above 180˚ of the wheels must be unobstructed. Figure 5.2 shows the ‘keep out’ zone which is defined by extending two vertical lines 75 mm both in front and behind the outside diameters of each of the tyres.
Figure 5.2 Open-wheeled visualisation [1]
In order to ensure the cockpit opening is of adequate size, a template defined in Article 4: T4.1 Cockpit Opening, shown in Figure 5.3, needs to be inserted vertically into the cockpit to a height of 350 mm above the monocoque floor, this essentially for safety as it represents the ease of escape for the driver in case of an incident.
While the cockpit opening needs to be of adequate size for a driver to get in and out easily, the driver’s cell needs to be large enough to fit a 95th percentile male. The dimensions for this are specified to the following: a circle of diameter 300 mm will represent the head, the centre of which will be positioned at 280 mm away from the centre of a circle of 200 mm to represent the shoulder
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area. The hip region will again be represented by a circle of 200 mm where the centre of which will be located at 490 mm away from the circle representing the shoulders. The rearmost point of the pedal is to be positioned at a minimum of 915 mm away from here. This configuration is shown, in blue, fitting into the driver’s cell in Figure 5.4.
Figure 5.3 Cockpit Opening Template
The chassis, according to section T3.10, must include both Main and Front Roll Hoop structures securely integrated into the primary structure so that the driver’s head and hands must not contact the ground in any rollover situation, each has to be made up of a single piece of uncut, continuous, closed section steel tubing. The main hoop must have bracing supports at a minimum of 30˚ to the vertical and attached at a minimum of 160 mm from the top-most surface of the main hoop. The Front Hoop bracing must protect the driver’s legs and extend beyond the driver’s feet and be attached at a minimum of 50.8 mm below the top-most surface of the Front Hoop. The construction of the Roll Hoops is to protect the driver’s head in case of a roll over incident. This is proven by there being a gap of at least 50 mm between the driver’s helmet, and a line projected from the top of the Front Hoop to the top of the Main Hoop and another line from the top of the Main Hoop down to the rearmost point of the Main Hoop Bracing. These are all shown in Figure 5.4, outlined with the template of a 95th percentile male in position.
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Figure 5.4 95th percentile male dimensions in driving position with roll bar clearances
One of the many safety requirements specifies a firewall separating the driver compartment from all components of the fuel supply, engine oil and liquid cooling system is necessary. This firewall must have no gaps, made from a non-permeable, rigid, fire-resistant material and extend high enough to protect the neck of the tallest driver defined by extending high enough upwards and/or rearwards so that the bottom of the helmet isn’t in direct line of sight with any of the necessary components. Both the main bulkhead and the seat of the car make up the firewall in this design, which even though aren’t made of a fire resistant material, apply the rigid surface onto which to apply Teknofibra [2] heat resistant tape. This tape made of a material derived from carbon fibre with very high insulating properties, along with an adhesive layer that increases its strength with temperature. The conductive properties of this tape range from 0.026 W/mK at 0 ˚C to 0.045 W/mK at 400 ˚C. This tape has a special surface on one side of embossed aluminium to reflect heat.
Figure 5.4 shows the driver’s seat extending high enough to protect the neck of a 95th percentile male, adhering to Article 4: T4.5 in the FSAE Rules.
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