Brake Cooling Airflow and Wheel Design Trade-offs

The problem

Regenerative braking takes much of an EV's braking load, but friction brakes still face hard stops, long descents when the battery is full, and corrosion from disuse. Meanwhile, EV designers close up wheels to cut drag, which also cuts brake cooling. Students need to quantify that trade-off for a design decision rather than assume either side wins.

Objectives

• Estimate brake heat input for a repeated-stop and a downhill case with limited regeneration.
• Model a ventilated disc, calliper and wheel with configurable spoke openness and a brake duct.
• Use Flow Simulation with a rotating region and heat sources to predict disc temperatures.
• Quantify the cooling benefit and drag penalty of each wheel and duct option.

What you need

Key concepts

Brake energy

Each stop converts kinetic energy 0.5·m·(v1² − v2²) into heat, minus what regeneration recovers. Most of the heat goes into the disc at first.

Rotating region

Flow Simulation can model rotating parts with a rotating region, using an averaging approach for steady runs or a sliding mesh for transient runs, so the disc vanes pump air.

Ventilated disc pumping

Vanes between the disc faces act as a centrifugal pump, drawing air from the hub side and throwing it outward, which raises the convective coefficient.

Wheel drag

Open wheel spokes churn air and add drag; closed covers reduce it but block cooling air reaching the brake.

Step-by-step guide

1. 1

   Define vehicle and duty cases

   Use an example 1,700 kg electric car. Define two cases: repeated stops from 100 km/h to rest with regeneration limited, as with a full battery, and a long descent at constant speed with a stated braking power. Assume a front axle brake share and a split of heat into the disc, for example 90 per cent, as stated assumptions. Set an example disc temperature limit that your pad material allows, and a drag-change budget for the wheel design.

   Checkpoint: A duty table with energy per stop, braking power and heat per disc for both cases.

2. 2

   Model the disc, calliper and hub

   Model a ventilated disc with two friction rings joined by radial vanes, with vane count and angle as parameters, and a hat section to the hub. Use grey cast iron from the material library. Model a simplified calliper body and pads as blocks and the hub as a cylinder. Make a curved-vane configuration as an alternative. Check that vane passages are open from the inner to outer diameter so air can flow through them.

   Checkpoint: A disc part that rebuilds when vane count changes, with clear passages visible in a section view.

3. 3

   Model wheel and duct variants

   Model a wheel with spokes whose opening area is set by a design table: open, half closed and almost fully closed aero cover. Add a tyre profile and a wheel arch around it in a simple body section. Create a brake duct configuration feeding air from the front of the arch towards the disc centre. Assemble everything with concentric and coincident mates, keeping the arch and body section the same for every variant.

   Checkpoint: Every wheel and duct configuration builds without interference, checked with Interference Detection.

4. 4

   Set up the flow and rotation

   Run the Wizard as External with Heat Conduction in Solids. Set free-stream velocity at the test speed and a moving ground plate. Create a Rotating region around the wheel and disc at the matching wheel speed, using the averaging option for steady runs. Make sure the rotating region is a body of revolution that does not cut through stationary parts, which is a common set-up error.

   Checkpoint: A steady cold-flow run converges and a cut plot shows air pumped outward through the disc vanes.

5. 5

   Steady convective cooling check

   For the descent case, apply a Volume Source in the friction rings equal to the steady braking power for that disc. Run steady state and read disc temperature and the heat carried away by air. Add Surface Goals for heat flux from disc surfaces and a Global Goal for drag force on the wheel. Run a mesh study around the disc and vanes and confirm disc temperature changes under 5 °C.

   Checkpoint: A converged steady case with disc temperature, convective heat flow and wheel drag.

6. 6

   Transient repeated-stop case

   Clone the project as a transient analysis. Define heat input as a time-dependent table: a pulse during each braking period followed by a cooling period at cruise speed. Use a sliding rotating region if your set-up supports it, or keep averaging with care. Run several cycles until peak disc temperature stops rising much between cycles. Record peak and end-of-cycle temperatures.

   Checkpoint: A temperature-time plot showing disc temperature levelling off over the repeated stops.

7. 7

   Hand check with a lumped model

   Build a lumped-capacitance disc model in Excel: disc mass and specific heat, heat input per stop and a convective coefficient back-calculated from the steady CFD heat flow. Step it through the same stop cycle and compare peaks with the transient CFD. Agreement within about 15 per cent shows the CFD is behaving sensibly; larger gaps usually mean the mesh, time step or heat input definition needs checking.

   Checkpoint: Lumped and CFD peak temperatures within about 15 per cent.

8. 8

   Compare wheel and duct variants

   Run the steady descent case and the cold-flow drag case for every wheel opening and duct option, and the transient case for the best and worst. Tabulate disc temperature, convective heat flow and wheel drag. Plot disc temperature against drag change. Identify options that meet the disc limit within the drag budget. If none does, note whether a brake duct alone closes the gap, since it adds less drag than opening the spokes.

   Checkpoint: A trade-off plot of temperature against drag, with every option plotted and the feasible region marked.

9. 9

   Refine the disc vanes

   For the chosen wheel, compare radial and curved vanes and two vane counts. Curved vanes can pump more air when rotating in their design direction, but need left and right handed parts. Check disc mass with Mass Properties, since a heavier disc stores more heat but adds unsprung mass. Pick a disc and wheel combination with a short justification that names the governing duty case.

   Checkpoint: A final disc and wheel choice that meets the temperature limit with mass and drag noted.

10. 10

    Report

    Write up duty cases, assumptions, geometry, rotating region set-up, mesh and transient settings, lumped-model check and trade-off results. Discuss the effect of regeneration share on the conclusion, since more regeneration reduces friction brake heat. State limitations: no radiation if not modelled, simplified calliper and assumed heat split. Recommend a vehicle test with disc thermocouples or a rubbing thermocouple on a repeated-stop route.

    Checkpoint: A report with a recommended wheel and duct design, its drag cost and a validation test plan.

Testing & validation

• Mesh study on disc temperature within 5 °C.
• Transient peaks agree with a lumped-capacitance model within about 15 per cent.
• Steady heat carried by air equals braking power input at steady state.
• Wheel drag differences exceed mesh uncertainty.

Expected results

A parametric brake and wheel model, steady and transient disc temperature predictions for two duty cases, drag for each wheel option, and a trade-off plot leading to a recommended wheel, duct and disc design.

Take it further

• Add radiation from the disc if your licence supports it, and quantify its share at high temperature.
• Study pad and calliper temperatures for brake fluid boiling risk.
• Vary regeneration share to find when friction brakes become critical.
• Run a thermal stress study on the disc in SOLIDWORKS Simulation.

Viva and review questions

Learn it at DIYguru

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