In-Wheel Motor Packaging with Brakes and Suspension

The problem

In-wheel motors free space in the body and allow torque vectoring, but they put the motor inside a wheel that already contains the brake, hub bearing, upright and suspension joints. They also add unsprung mass, which can worsen ride and road holding. Research needs a packaging method that checks geometric feasibility through suspension and steering motion and quantifies the dynamic penalty honestly.

Objectives

• Define a wheel envelope with rim, tyre, brake, bearing, upright and suspension hardpoints for an example car.
• Package two motor concepts (direct drive and geared) with brake and cooling inside the wheel.
• Animate suspension and steering in SOLIDWORKS Motion and check clearances throughout.
• Estimate unsprung mass and evaluate ride and tyre-load metrics with a quarter-car model.
• Propose mitigation, such as lighter components or a different suspension, with evidence.

What you need

Key concepts

Unsprung mass

Mass between the springs and the road, including wheel, tyre, brake, upright and in-wheel motor. More unsprung mass raises wheel-hop amplitude and tyre load variation.

Quarter-car model

A two-mass model of sprung and unsprung mass with suspension and tyre springs and a damper, used to compare ride and road-holding metrics.

Wheel envelope

The volume swept by the wheel and tyre through bump, rebound and steering. Every in-wheel component must stay inside the rim and clear the body.

Direct drive versus geared hub

A direct-drive motor is simple but large and heavy for the torque; a geared hub uses a smaller, faster motor with a reduction, saving mass but adding complexity.

Step-by-step guide

1. 1

   Set targets and hardpoints

   For an example small car with 15-inch wheels, 1000 kg kerb mass and a MacPherson front suspension, define hardpoints, wheel travel (for example 80 mm bump and 70 mm rebound), steering lock and rim inner dimensions. Record example spring rate, damper rate and tyre vertical stiffness. Note the source of each value, whether a measured car, a textbook example or a published paper, in a source table.

   Checkpoint: A layout sketch with all hardpoints fully defined from global variables and a source table.

2. 2

   Model the baseline corner

   Build a baseline corner without the motor: rim, tyre, hub bearing, upright, brake disc and caliper, lower arm, strut and tie rod. Connect them with concentric, coincident and limit mates that reproduce the suspension's joints, using simple pin-and-socket geometry for ball joints. Drag the wheel through travel and check that camber and toe change along a plausible path for this suspension type.

   Checkpoint: The baseline corner moves through bump, rebound and steer without over-defined mate errors.

3. 3

   Package two motor concepts

   Create configurations for a direct-drive outer-rotor motor and a geared hub with a smaller, faster motor and a planetary reduction. Fit each inside the rim with the brake, which may need to shrink or move inboard. Include coolant connections and cable exits routed along the upright with sensible bend radii. Use mass overrides for bought-in parts based on literature estimates.

   Checkpoint: Both concepts fit inside the rim at static position with Clearance Verification above an example 5 mm minimum.

4. 4

   Set up the motion study

   Create a Motion Analysis study, which needs SOLIDWORKS Premium or the Motion add-in. Add a linear motor to drive the wheel centre vertically through full travel and a rotary or linear motor on the steering rack or tie rod for lock-to-lock steer. Combine the profiles so extreme corners such as full bump at full lock are covered, and add springs and dampers with the example rates.

   Checkpoint: The study runs through combined bump and steer and plots wheel centre travel and steering angle as expected.

5. 5

   Check clearances in motion

   Run interference detection over the motion study, or sample key frames and run Interference Detection and Clearance Verification at each. Check motor and brake against the lower arm, strut, tie rod and wheel arch, and check cable and hose lengths and bend radii through the motion. Record the minimum clearance for each pair and the wheel position at which it occurs.

   Checkpoint: Minimum clearance through the whole motion stays above the target for at least one concept, with the location of each minimum recorded.

6. 6

   Compute unsprung mass

   Use Mass Properties on the unsprung group for each concept, counting half the mass of links that move partly with the wheel, as is common practice, and state that convention. Compare with the baseline corner. Also record polar moment of inertia of the rotating parts, which affects wheel acceleration and braking, and the change in mass on the steering axis, which affects steering feel.

   Checkpoint: A table of unsprung mass and rotating inertia for baseline and both concepts.

7. 7

   Build a quarter-car model

   In Simulink or Python, build a quarter-car model with sprung mass, unsprung mass, suspension spring and damper and tyre spring. Run it on a random road profile generated from a road roughness class and on a single bump. Compute RMS body acceleration, RMS suspension travel and RMS dynamic tyre load as a fraction of static load, which are standard ride and road-holding metrics.

   Checkpoint: The baseline model gives body and wheel-hop natural frequencies in typical ranges of about 1 to 1.5 Hz and 10 to 15 Hz.

8. 8

   Compare concepts dynamically

   Run the quarter-car model for both concepts and plot the three metrics against the baseline. Then retune damping and spring rate for the heavier corner to see how much of the penalty can be recovered without hurting the other metrics. Be clear that a quarter-car model ignores pitch, roll, tyre nonlinearity and suspension geometry, so it compares concepts rather than predicting full-vehicle behaviour.

   Checkpoint: A comparison table and plots showing the change in each metric for both concepts, before and after retuning.

9. 9

   Check structural loads

   Run a Static study on the upright with the added motor mass, combined braking and cornering loads at the contact patch and the motor torque reaction. Converge stress at the strut clamp and the lower ball-joint boss and check deflection at the bearing seat, since the motor air gap is sensitive to it. Note any upright changes needed because the motor alters the load paths.

   Checkpoint: Upright factor of safety above 2 and bearing-seat deflection within an example limit.

10. 10

    Propose mitigation

    Evaluate options such as an inboard brake, a lighter geared motor, a larger rim with more internal space or a different suspension type that frees room near the wheel. Apply the same motion and quarter-car checks to each option rather than judging by eye. Score each one on clearance, mass penalty, ride metrics and complexity in a matrix, with weights you justify.

    Checkpoint: A scored mitigation matrix with the evidence for each score.

11. 11

    Report

    Present the method, hardpoints, both concepts, motion and clearance results, unsprung mass figures, the quarter-car comparison, the structural check and the mitigation study. State limitations clearly, including simplified joints, linear springs and the quarter-car scope. Propose full-vehicle multibody simulation, or measurement on a quarter-car rig, as the next step to test the conclusions. Archive the motion study, mass tables and quarter-car scripts with the report so another student can rerun every result.

    Checkpoint: A report that ties every conclusion to a model result and lists its limits.

Testing & validation

• Baseline suspension kinematics look plausible through travel and steer.
• Minimum clearance through combined motion meets the target for the recommended concept.
• Quarter-car natural frequencies fall in typical ranges for the baseline.
• Unsprung mass is computed with a stated, consistent method.

Expected results

A motion-checked packaging study of two in-wheel motor concepts with brakes and suspension, unsprung mass figures, a quarter-car comparison of ride and road-holding metrics and a scored set of mitigation options.

Take it further

• Extend to a full-vehicle model with pitch and roll in a multibody tool.
• Add a thermal study of the brake heating the motor.
• Study torque vectoring effects on upright loads.
• Compare with a measured quarter-car rig if available.

Viva and review questions

Learn it at DIYguru

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