Vehicle and Frame Design

Vehicle Layout

The vehicle is a rear-mid engine layout, with two occupants sitting side by side. The following images highlight the relative locations of the main systems, click to enlarge.

• Powertrain: engine, exhaust, and differential.
• Twin radiators and air ducts
• Occupant seats
• Fuel storage and fire suppression systems (yellow)

As shown in the images below, the engine, exhaust, differential, and other powertrain components are all mounted to a separate sub-frame as compared to the rest of the vehicle systems. The red circles show the 4 connection points to the frame. Again, click to enlarge.

​References:

1. SCCA 2013 GCR - January.pdf
2. MMPDS-06; Metallic Materials Properties Development and Standardization, April 2011
3. ArcelorMittal_DOMSpecs
   
4. The Equivalent Analysis of Honeycomb Sandwich Plates for Satellite Structure; X. li-juan, J. Xian-ding, W. Yang-bao
   
5. Stress Concentration Factors; R.E. Peterson
6. Determination of the Johnson-Cook material parameters using the SCS specimen; A. Dorogoy, D. Rittel
   
7. IMPAXX700 CAE cash analysis and validation; Dow Chemical
   
8. FEA Software User Documentation

Frame Improvements

Clearly the initial design was inadequate and design changes were made throughout the analysis iterations, this section presents an example of that. Focusing on the two cases presented above, these cases together lead to the addition of the main roll hoop center support. Initially the main roll hoop cross section was increased from 0.090in to 0.120in wall thickness to prevent failure in bending, but this proved insufficient for both SCCA cases. As shown below the final solution incorporates 2 center supports for the main roll hoop. A forward pointing support going to the floor is required to prevent vertical and forward bending for the SCCA fwd case, while a rearward pointing support going to the shoulder harness tube is required to prevent vertical and rearward bending during the SCCA aft case. Both the supports provide additional strength for the restraints roll over case between the roll bar and the restraints attachment points.

​Analysis Results

The results shown here are only the final results, for the selected load cases, following over 100 iterations of 6 loads cases. Throughout these iterations, changes were made to:

• tube cross sections
• sheet metal thickness
• material application for tubes and sheet metal
• tube connection location

These changes were in large part done to successfully meet the pass criteria for each load case. Once a design was found that passed all load cases, some additional changes were made for weight reduction and component mounting placement. Of course, each time a change was made, it was verified against all load cases. Again, the complete report can be requested using the Contact page.

SCCA Roll Bar (forward)

The first image below shows the constraints at the suspension mounting points (blue triangles), the distributed load on the roll bar (pink arrows), and the gravitational loads of the passengers and powertrain (yellow arrows). The 519 lbf shown is the resultant load vector from the 3 components specified by the SCCA case. Here the load is distributed over 26 elements, so 519 x 26 = 13504 lbf total. Finally, in the second image we see stress results showing some failure in the center support floor tube; this is limited to a single node adjacent to an MPC and therefore meets the pass / fail criteria.

Frame Material Application

The below table and supporting figures present the design of the frame and material application. Note that the colours of the cross sections in the table, correspond to the colours of frame elements in the figures. Also, for clarity, not all frame elements are coloured in all figures.​

FMBA Frame Structural Analysis

Intro

This page provides a summary of a report made to describe the design and analysis steps which lead to the final structure planned for FMBA vehicle. Not all the work presented in the complete report is covered. A complete copy of the report can be requested using the Contact page of this site. The FMBA vehicle is classified by the CVSE as a U-built.

The work presented in the complete report aims to provide supporting information to assist inspectors and Auto body Technicians with a structural integrity assessment and required documents as defined in section 8 of the Vehicle Safety and Inspection Manual published by (Government of British Columbia), and CVSE Notice #04-13. Specifically CVSE Notice #04-13 requires that the following two reports must be submitted for this type of vehicle inspection.

• Body Integrity Inspection Report (CVSE0031)
• The Structural Integrity Declaration Report (CVSE0032)
  

Concept

The basic shape of the vehicle was chosen by reviewing existing vehicles of similar design, namely the Ariel Atom. Track width, wheel base, and frame design are all closely linked to the design of the Atom. The Radical SR3 was also strongly considered but abandoned due to the seemingly more complex body work required. Both the Ariel Atom and Radical SR3 are street legal in the UK.

Figure 1: Left to Right: FMBA concept, Ariel Atom, Radical SR3

With the basic shape in place, the aerodynamic envelop for the vehicle was tuned through iteration of CFD analysis and conceptual design of vehicle systems and structure. The result of this work was the completion of the conceptual design phase.

Material Selection

Initial material selection was based on the SCCA guidelines for roll cages for sports racing cars. At approximately 1400 lbm including both occupants, the FMBA falls into the lowest weight category of the guidelines, see figure below. Initially material was selected from the heaviest weight category, but through the work done for the report, the material from the middle category became the final selection.

Figure 2: SCCA Regulations, Sections 9.4.5.E.4, a) and b)

Manufacturing Strategy

The general manufacturing strategy for the structure is outlined below.

• Primary frame elements are single continuous bars.
• Secondary bracing elements are profiled at the ends to mate with primary elements.
• Tube notcher is used to create profiles on secondary elements.
• Full frame jig (plywood) used to support structure throughout the tacking and welding processes.
• All joints TIG welded according to recommended practices.

Figure 3: Frame shown inside the plywood welding jig.

Joint Design

Joints are all fitted with profiled tubes. As mentioned above, a tube notcher is used to create the exact profile on the end of the mating tube. Joints are designed to provide adequate access for welding. Gussets have been designed with approximately the same thickness steel as the mating tubes. Gussets are formed sheet metal making C shape profile, and fitted to both adjacent tubes. Gussets like this are used to strengthen joints at the following locations:

• Upper main hoops to main roll bar
• Upper main hoops to front plane structure
• Upper main hoops to should harness tube
  

​Passenger Restraints Roll Over Strength

Recall the goal of this case is to verify the attachment points for passenger restraints, and as a byproduct, the powertrain attachments as well. Although it’s not perfectly clear, the image below shows that the two roll bars are fixed, and 40G loads are applied to the center of gravity of both passengers (7900lbf each) and the powertrain sub-system (12560 lbf). The results for the steel parts show areas of yielding in the floor center support and shoulder harness tubes but otherwise there is no failure of any tubes at, or near the restraint connections points for this case.

Material Properties

For details regarding material properties used for isotropic and non-isotropic materials, please request a copy of the complete report using the Contact page of this site.

Finite Element Model (FEM)

The FEM has all the same frame members as the frame. Tubes are modeled using 1D elements, while sheets and plates are modeled using 2D elements. 3D element were not used. The joints between tubes are typically made at common nodes. Tubes joints where center lines don’t cross, are joined by multi-point constrains (MPC), which are rigid connections between two nodes. MPCs are also used to join the sheet metal and sandwich to the frame tubes. Finally, MPCs are also used to join the center of gravity of the passengers and powertrain, to the appropriate connection points on the frame. The Powertrain MPCs make up a rigid structure, while the passengers MPCs allow more freedom similar to how a seat belt can only apply loads in certain directions (i.e. no torque can be applied by a seat belt, to the frame).

Closing Remarks

The work presented here shows that the design of the structure has been given careful attention to ensure structural integrity for some common crash scenarios, again a fair amount of work is not presented here, but the complete report can be requested using the Contact page
Section 8 of the Vehicle Safety and Inspection Manual, and the required reports describe the structural integrity assessment process as well as some vague guidelines, but they do not define any un-ambiguous requirements that can be used as design goals. Therefore, as mentioned in the introduction, the complete report remains open ended as a source of supporting information for the assessment and required reports.

Frame Structural Analysis

The structural analysis is concerned with the crash integrity of the frame and passenger compartment. The goal was to determine static loading conditions which represent the worst case loading during an impact, and verify that the structure does not fail. Impact attenuators are placed in front and behind the vehicle, relieving the need for the structure to absorb impact energy.
Two methods were used to simulate these load cases.

• Method 1. For some cases, the structure was constrained at the impact area, and loads were applied at center of gravity of the large masses inside the vehicle: driver, passenger, and powertrain.
• Method 2. For other cases, the structure was held fixed at appropriate location, and loads were applied to the frame elements being analyzed. This second group applies to the roll bar and lateral strength test.

The table below shows a breakdown of the sub-system weight. Given that the passengers and powertrain are approximately an order of magnitude heavier than the rest, only the driver, passenger, and powertrain are included for analyses performed using Method 1.

Table 3: Sub-system weight breakdown.

A total of 6 loads cases were analyzed, these are listed below. Only cases 4 and 6 are presented in detail here, for the full report, please use the site Contact page.

1. Front Impact
2. Rear Impact
3. Lateral strength
4. SCCA Roll bar (rear direction loading)
5. SCCA Roll bar (front direction loading)
6. Passenger Restraints Roll over strength

Load Case, SCCA Roll bar (front direction loading)

The Sports Car Club of America specifies 2 load cases for sports racing cars. (Sports Car Club of America, 2015); Section 9.4.5.F reads:

F. Exceptions for Formula Cars and Sports Racing cars
Any roll hoop design which does not comply with the specifica­tions in 9.4.5, will only be considered if it is accompanied by engi­neering specifications signed by a registered engineer. No alternate roll hoop will be considered unless it contains a main hoop having a minimum tubing size of 1.375” x .080” wall thickness. The roll bar must be capable of withstanding the following stress loading applied simultaneously to the top of the roll bar: 1.5 (X) laterally, 5.5 (X) longitudinally in both the fore and aft directions, and 7.5 (X) vertically, where (X) = the minimum weight of the car.
Pass / fail criteria: no wide spread failure of any frame part. Localized failure restricted to a single FEM element, located near a discontinuity is permitted. Failure is defined total combined stress exceeding the material ultimate stress.

This pass fail criteria is self imposed by engineering judgement, and experience.

Load Case, Passenger Restraints Roll Over Strength

The goal of this load case is to verify the integrity of the connection points for passenger restraints and powertrain sub-assembly. This case is required in addition to the SCCA cases since those cases do not validate these connections points relative to the potential inertial loads which would be applied in the event of a rollover. For this case it is assumed the car is placed upside down resting on the front and main roll bars, with the inertial loads pointing downward perpendicular to the ground.
The load magnitude for this case is 40 G for each sub-system. Although only one passenger load is visible in the description figure below, the load was applied at both the passenger and driver center of gravity.
Pass / fail criteria: no wide spread failure of any frame part. Localized failure restricted to a single FEM element, located near a discontinuity is permitted. Failure is defined total combined stress exceeding the material ultimate stress.