Student Formula EV Space-Frame Chassis: Design and Torsional Stiffness

The problem

Student Formula teams need a chassis that protects the driver, meets competition rules for tubes and the accumulator container, and is stiff enough in torsion for the suspension to work as designed. Teams often guess tube sizes and only find stiffness problems on track. A rules-aware, measured design process saves a season.

Objectives

• Package a driver manikin, accumulator container, motor and suspension pick-up points.
• Model the space frame with Weldments using rule-compliant tube sizes.
• Measure torsional stiffness with a Simulation twist test.
• Iterate triangulation and tube sizes for stiffness per kilogram.
• Produce a frame drawing, cut list and a stiffness report.

What you need

Key concepts

Torsional stiffness

The torque needed to twist the chassis by one degree between front and rear axles, in N·m per degree. Teams set a target relative to the roll stiffness of the suspension.

Triangulation

Arranging tubes so every bay forms triangles. Loads then travel as axial forces in the tubes, which is much stiffer than bending.

Primary structure

The tubes the rules define for driver protection, such as main hoop, front hoop, side impact members and bulkhead. They have minimum size and material requirements.

Node

A point where tube centre lines meet. Loads should enter the frame at nodes; a suspension pick-up mid-tube puts the tube in bending.

Step-by-step guide

1. 1

   Collect rules and targets

   Read the current rule book and list primary structure tube minimums, driver template positions and accumulator container requirements. Set example targets: wheelbase 1550 mm, track 1200 mm, frame mass under 35 kg and torsional stiffness above an example 1500 N·m per degree. Record the steel used by the team, for example AISI 4130, and its properties from the SOLIDWORKS material library.

   Checkpoint: A target sheet listing every rule-driven dimension and the stiffness and mass targets.

2. 2

   Fix hard points

   Get suspension pick-up coordinates from the suspension team, or a kinematic sketch, and place them as 3D Sketch points. Add points for steering rack mounts, motor mounts and the accumulator container corners. Insert the driver template or a manikin built from blocks. Define main hoop and front hoop positions from the driver template and the roll-over envelope rules. Save the hard point sketch in its own part or layout so suspension changes can be pushed into the frame without redrawing it.

   Checkpoint: All hard points are fixed in a 3D Sketch and the driver template sits inside the planned hoops with required clearances.

3. 3

   Draw the node skeleton

   Join hard points with 3D Sketch lines to form the frame, aiming for triangulated bays in the side pods, floor and front box. Make sure every suspension pick-up lands on a node, with two or more tubes meeting there. Use Coincident and Symmetric relations about the centre plane. Avoid nearly parallel tubes meeting at shallow angles, which are difficult to weld.

   Checkpoint: Every suspension pick-up sits on a node with at least two tubes, and the sketch is fully defined.

4. 4

   Place tubes with Weldments

   Use Structural Member with round tube profiles sized for each group: primary structure at the rule minimum or larger, secondary bracing smaller. Use separate groups for hoops and bracing so trims work. Model the bent main and front hoops by sweeping the profile along a sketch with bend radii. Apply AISI 4130 and check the cut list and total mass.

   Checkpoint: The frame is built without errors and its mass is recorded for comparison with the target.

5. 5

   Check packaging

   Assemble the frame with driver manikin, accumulator container, motor, inverter blocks and steering rack. Run Interference Detection and check driver template clearances, head clearance under the hoop line and pedal box space. Use Mass Properties to locate the centre of gravity with driver and container installed and check its height, since a low centre of gravity matters for handling. Check access for the accumulator container removal and the steering column so the frame does not block service tasks.

   Checkpoint: Interference Detection passes and the driver template clears every rule-defined line.

6. 6

   Set up the torsion test

   In a Static study with beam elements, restrain the rear suspension pick-up nodes with Fixed Hinge restraints. Apply equal and opposite vertical forces at the left and right front pick-up nodes to create a known couple, for example 1000 N each at 1.2 m apart. Add a small restraint at the front centre if needed to remove rigid-body motion without adding stiffness. Run the study.

   Checkpoint: The deformed shape shows a clean twist about the longitudinal axis and the reaction forces balance the applied couple.

7. 7

   Calculate stiffness

   Probe vertical displacement at the two front loaded nodes, compute the twist angle as the arctangent of their difference divided by the track, and divide the applied torque by the angle. Repeat with twice the load to confirm the response is linear. Export the twist distribution along the frame by probing nodes at each bay, which shows which bay is softest.

   Checkpoint: A torsional stiffness value in N·m per degree that is identical at two load levels, plus a twist-versus-position plot.

8. 8

   Iterate for stiffness per kilogram

   Create configurations with added diagonals in the softest bay, changed tube wall thicknesses or a moved node. Use a Design Table to switch profiles and record mass and stiffness for each. Rank changes by added stiffness per added kilogram. Keep primary structure at or above rule minimums and avoid secondary tubes that only add mass. Re-run Interference Detection after each change, since a new diagonal can block the driver, accumulator or harness routes.

   Checkpoint: An iteration table with at least four variants and the final variant meeting both stiffness and mass targets.

9. 9

   Check other load cases

   Run braking, cornering and bump cases using suspension loads from the vehicle dynamics team or example values, such as 1.5 g lateral and 1.5 g braking at the contact patches transferred into the pick-up nodes. Check Upper Bound Axial and Bending stress and Factor of Safety. Hand-check the axial force in one diagonal using a simple truss balance at a node.

   Checkpoint: Factor of safety above 1.5 in every case, and the hand truss check agrees with Simulation within about 15 per cent.

10. 10

    Drawings, jig data and report

    Produce a frame drawing with a cut list and a table of node coordinates for the welding jig. Show tube coping angles from the cut list. In the report, include the rules checklist, packaging, torsion setup and results, iteration table and load cases. Plan a physical torsion test on the welded frame with dial gauges, and state that Simulation does not include weld or joint flexibility so the real frame will usually be less stiff.

    Checkpoint: A drawing pack, a node table and a report with a planned physical test.

Testing & validation

• Every primary structure tube meets the current rule minimum.
• Torsion test gives the same stiffness at two load levels.
• Hand truss check agrees with the beam result.
• Packaging passes Interference Detection and the driver template rules.
• A physical torsion test plan is included for comparison.

Expected results

A rules-aware student Formula EV space frame with packaged driver and accumulator container, a measured torsional stiffness, an iteration record ranked by stiffness per kilogram and a drawing pack with jig node data.

Take it further

• Weld the frame and compare measured and simulated torsional stiffness.
• Model nodes as shells to study local stiffness loss at joints.
• Run a Frequency study to compare frame modes with motor excitation.
• Compare the space frame with a hybrid frame plus bonded floor panel.

Viva and review questions

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