Skateboard EV Platform: Battery-Integrated Floor and Crash Rails

The problem

Dedicated EV platforms put the battery in the floor between the axles, which lowers the centre of gravity and frees cabin space but makes the battery part of the structure. The side rails must protect the cells from side loads, and the floor must be stiff without excess mass. Students rarely get to reason through these trade-offs on a full platform.

Objectives

• Package the battery, front and rear drive units and suspension mounts on a flat-floor platform.
• Model extruded multi-cell side rails, cross members and a battery tray integrated with the floor.
• Evaluate global bending and torsional stiffness of the platform.
• Assess load paths and intrusion into the battery zone for a quasi-static side load.
• Compare two cross-member strategies for stiffness, mass and battery volume.

What you need

Key concepts

Skateboard platform

A flat chassis that holds battery, drive units and suspension, onto which different bodies can be mounted. The battery sits in the floor between the axles.

Multi-cell extrusion

An aluminium profile with internal webs, used for side rails. It is stiff in bending and absorbs energy by progressive folding when crushed.

Midsurface shell model

Thin-walled parts are modelled as shells at their midsurface, which is much more efficient than solid meshing and represents bending well.

Battery zone intrusion

How far a side load pushes the rail into the space where cells sit. Keeping it small protects the cells from crushing.

Step-by-step guide

1. 1

   Set platform targets

   Choose an example compact EV: wheelbase 2700 mm, track 1550 mm, a 50 kWh pack in the floor, kerb mass around 1500 kg and front and rear drive-unit blocks. Set example targets for bending stiffness, torsional stiffness, battery floor deflection under a distributed cell load and a maximum battery-zone intrusion under a side load. Note that these are example targets for a study, not values for any real vehicle.

   Checkpoint: A target table with platform dimensions, masses and four stiffness or intrusion targets.

2. 2

   Lay out hard points and battery volume

   In a layout part, sketch wheel centres, suspension mount points, drive-unit envelopes and the battery volume between the axles. Link wheelbase, track and battery length to Global Variables. Model battery modules as blocks with the example cell mass. Check the floor height and ground clearance and keep a crush gap between the rails and modules for later side-load study. Keep the layout in a separate part referenced by every component, so a change in battery length moves rails and cross members together.

   Checkpoint: Layout rebuilds from Global Variables and the battery volume and crush gap are dimensioned.

3. 3

   Design the side rail extrusion

   Sketch a multi-cell rail profile, for example a 120 × 160 mm section with two internal webs and 3 mm walls, and extrude it along the side of the platform. Use a Design Table to try wall thicknesses and web counts. Compute area and second moments with the Section Properties tool. Choose an example 6082-T6 aluminium alloy, and record a reduced strength for heat-affected zones near welds.

   Checkpoint: A table of section properties for at least three rail variants with mass per metre.

4. 4

   Model cross members and the battery tray

   Add front and rear torque boxes, a set of cross members across the floor and a battery tray with a bolted lid that also acts as floor. Model thin parts as surfaces or sheet metal so they can be meshed as shells. Use the Hole Wizard for the tray-to-rail bolt pattern. Create two configurations: a few deep cross members, and many shallow members integrated in the tray.

   Checkpoint: Both configurations rebuild and Mass Properties gives mass and battery volume for each.

5. 5

   Check packaging

   Assemble platform, battery modules, drive units, suspension blocks and a simple body-mount frame. Run Interference Detection and check the battery-to-rail crush gap, ground clearance and the service access to battery bolts. Use Mass Properties for the centre of gravity height, which should be lower than an equivalent car with a tunnel battery. Record the centre of gravity height in the target table, because it feeds later handling and rollover estimates. Fix any clash before meshing, since overlapping bodies cause bonded contact errors.

   Checkpoint: Interference Detection passes and the crush gap and ground clearance meet the target values.

6. 6

   Prepare the Simulation model

   Use midsurface shells for rails, cross members and tray, with thickness assigned from the part. Use bonded contact for welded and adhesive joints and Bolt connectors for the tray bolts. Represent battery modules as distributed mass or Remote Mass on their mounts. Apply mesh control at joints and run a quick mesh convergence on one rail joint, refining until peak stress changes by less than about 5 per cent.

   Checkpoint: A mesh convergence plot at one joint and a mesh that passes the quality check.

7. 7

   Run bending and torsion cases

   For bending, support at suspension mounts with hinge and roller restraints and apply a central vertical load; compute stiffness from the mid-wheelbase deflection. For torsion, restrain rear mounts and apply a couple at front mounts, computing N·m per degree from the twist. Run both configurations and compare with targets. Check the bending result against a simple beam estimate using the combined rail section.

   Checkpoint: Bending and torsional stiffness for both configurations, with the bending result within about 20 per cent of the beam estimate.

8. 8

   Run the battery floor and side-load cases

   Apply the battery mass at 3 g vertical to check tray deflection between cross members. For the side case, apply a quasi-static distributed load over a short length of one rail, such as an example pole contact patch, reacted at the opposite rail. Measure rail inward displacement against the crush gap. Remember that a linear static study cannot predict crushing; it shows stiffness and load sharing only.

   Checkpoint: Tray deflection under the limit and a side-load displacement that is smaller than the crush gap with a margin.

9. 9

   Compare and optimise

   Tabulate mass, bending and torsional stiffness, tray deflection, side-load displacement and battery volume for both configurations. Run a small Design Study varying rail wall thickness and cross-member depth to find the lightest design meeting every target. Optimisation with Simulation sensors in Design Study needs Simulation Professional; a manual sweep with configurations works otherwise. Check the final design again for packaging and battery volume, because thicker rails may eat into the crush gap. Re-run the side-load case on the final design to confirm the margin.

   Checkpoint: A comparison table and a final design meeting all targets with the lowest mass found.

10. 10

    Report and drawings

    Produce an assembly drawing with key sections through the rail and tray and a bill of materials. The report covers targets, packaging, model assumptions, mesh convergence, all load cases, the comparison and design choice. State that real side impact needs nonlinear crash simulation and a physical test, and that battery safety must be checked against current standards such as AIS-038 Rev 2 for M and N categories.

    Checkpoint: A report with clear assumptions and limitations, and drawings showing the final rail and tray sections.

Testing & validation

• Mesh convergence at a critical joint.
• Bending stiffness compared with a beam estimate.
• Reaction forces balance applied loads in every case.
• Side-load displacement compared with the designed crush gap.
• Two configurations compared on the same load cases.

Expected results

A skateboard EV platform model with an integrated battery floor and multi-cell side rails, measured bending and torsional stiffness, battery floor deflection and side-load response, and a justified choice between two cross-member strategies.

Take it further

• Run a nonlinear crush study on a short length of side rail and compare energy absorbed for different web layouts.
• Add a Frequency study and compare the first torsion mode with a target.
• Study a cell-to-pack variant where modules are removed and cells bond to the tray.

Viva and review questions

Learn it at DIYguru

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