In-Wheel Motors: Unsprung-Mass Effects on Ride and Handling

The problem

In-wheel motors free space and enable independent wheel control, but they add significant mass to the wheel, which is unsprung. Higher unsprung mass is widely understood to degrade ride and road holding, yet the size of the effect depends on road, speed and suspension tuning, and published conclusions vary. A systematic multi-fidelity study with mitigation options can clarify when in-wheel motors are acceptable.

Objectives

• Build quarter-car, half-car and multibody models with variable unsprung mass.
• Evaluate comfort, road-holding and travel metrics on ISO 8608 roads and speed breakers.
• Quantify handling effects such as transient tyre load variation during cornering.
• Evaluate mitigation by suspension retuning, a dynamic-damper motor mount and active damping.
• Compare model fidelity levels and identify what each adds.

What you need

Key concepts

Unsprung mass

Mass not supported by the suspension spring: wheel, tyre, hub, brake and, with in-wheel motors, the motor. It strongly influences wheel-hop frequency and tyre load variation.

Dynamic damper motor mount

Mounting the motor on its own spring and damper inside the wheel so it acts as a tuned mass absorber, reducing the effective unsprung mass at critical frequencies.

Road holding

Measured by dynamic tyre load variation. Higher variation means moments of low contact force and less available grip for traction, braking and cornering.

Multi-fidelity modelling

Using simple and complex models together, with the simple model for broad sweeps and the complex one to check where simplifications break down.

Step-by-step guide

1. 1

   Define the study and baseline

   Choose an example compact EV with central motor as baseline, and in-wheel variants adding an example motor mass per wheel, for example 15, 25 and 35 kg. Define roads (ISO 8608 classes and speed breakers), speeds and manoeuvres. Define metrics: weighted RMS body acceleration, dynamic tyre load coefficient, suspension travel and handling indicators. Write research questions and hypotheses explicitly. Keep sprung mass the same across all variants.

   Checkpoint: A study plan with variants, scenarios, metrics and hypotheses.

2. 2

   Quarter-car sweep

   Build a quarter-car model and sweep unsprung mass across the variants for each road class and speed. Compute metrics and frequency responses. This gives the first-order picture: wheel-hop frequency falls and tyre load variation increases with unsprung mass. Verify the model against analytical natural frequencies. Keep spring and damper fixed in this step so the effect of unsprung mass alone is isolated. Present results as percentage change from the baseline, and plot frequency responses for all variants on one graph.

   Checkpoint: Quarter-car frequency responses and metrics showing the trend with unsprung mass, with natural-frequency verification.

3. 3

   Half-car model

   Build a half-car with pitch and two axles so that speed-breaker crossings and wheelbase filtering are captured. Use correlated front and rear road inputs with a time delay of wheelbase over speed. Compare metrics with the quarter-car results. Use the same road profiles and speeds as the quarter-car sweep. Speed breakers crossed at different speeds will show pitch effects that a quarter-car cannot capture. Record body pitch acceleration and front and rear tyre load variation separately, and explain differences from the quarter-car results.

   Checkpoint: Half-car results consistent with quarter-car trends, with additional pitch metrics.

4. 4

   Multibody model

   Build a Simscape Multibody model of one suspension corner, such as a MacPherson strut or double wishbone, with wheel, motor mass and a tyre force element. Include kinematic effects such as camber change. Verify the model's vertical stiffness and natural frequencies against the quarter-car model. Add the motor as a rigid body attached to the wheel carrier with its own inertia. Check that suspension travel, camber change and spring force are physically reasonable during a bump input, and record the model's run time for comparison.

   Checkpoint: Multibody wheel-hop and body frequencies match the quarter-car model within a few per cent when kinematics are linearised.

5. 5

   Ride and road-holding results

   Run all variants on the road set with each model. Compile metrics and their percentage change from the baseline. Identify conditions where the in-wheel penalty is large, such as rough roads at moderate speeds near wheel-hop excitation, and where it is small. Present the results as a table of percentage changes and as heat maps against speed and road class. State which changes are large compared with differences between models, and link the largest effects to wheel-hop frequency shifts.

   Checkpoint: A results matrix of metric changes for each variant, road and speed, with significant effects highlighted.

6. 6

   Handling effects

   Simulate cornering on a rough road, where tyre load variation reduces available lateral force, using a simplified vehicle with load-dependent tyre forces. Quantify loss of lateral grip margin or path deviation for each variant. Discuss how in-wheel motors' torque-vectoring capability could compensate. Use a constant-radius turn at a fixed speed with rough-road input, and record the minimum tyre load and the lateral force margin. Compare variants and explain how load variation reduces cornering capability even when average load is unchanged.

   Checkpoint: Handling metric changes for each variant on rough and smooth roads.

7. 7

   Suspension retuning

   For each in-wheel variant, optimise spring and damper settings using a weighted objective of comfort and road holding with a travel constraint, using a parameter sweep or fminsearch. Compare the best retuned in-wheel design with the baseline to see how much penalty remains. Use the same objective and constraints for all variants and the baseline so comparisons are fair. Report the optimised parameters, the metrics and whether retuning brings the in-wheel design close to the baseline or only partly recovers the loss.

   Checkpoint: Retuned designs and the remaining penalty relative to the baseline for each variant.

8. 8

   Dynamic-damper motor mount

   Model the motor on its own spring and damper inside the wheel. Tune mount stiffness and damping to place an absorber near the wheel-hop frequency. Compare tyre load variation and comfort with the rigidly mounted motor across roads. Check that mount travel stays within realistic space inside the wheel, and discuss the effect on the motor air gap qualitatively. Compare results across road classes and speeds, since a tuned absorber works best near its design frequency and may help less elsewhere.

   Checkpoint: Tuned mount parameters and reduction in tyre load variation compared with a rigid mount.

9. 9

   Active damping

   Design a skyhook or groundhook active or semi-active damper controller using body and wheel velocities. Evaluate its benefit on each variant with realistic actuator bandwidth and force limits. Compare energy demand of the actuator with the ride improvement. Implement the controller in Simulink with a first-order actuator model and force saturation. Compare skyhook, which targets comfort, with groundhook, which targets road holding, and report which suits in-wheel variants better. Estimate actuator energy over each road profile.

   Checkpoint: Metrics with active damping for each variant, including actuator force and power.

10. 10

    Fidelity comparison and report

    Compare quarter-car, half-car and multibody predictions for the same cases and identify where simple models are adequate. Present all results with uncertainty from parameter variation. State limitations: tyre models, absence of measured data and generic suspension geometry. Present a table of metric differences between model levels and identify where the quarter-car model is sufficient for design decisions. Release the models and scripts for reproducibility, and state which conclusions need experimental confirmation before use in vehicle design.

    Checkpoint: A thesis chapter with multi-fidelity results, mitigation comparison and clearly stated limitations.

Testing & validation

• Analytical natural frequencies match quarter-car and multibody models.
• Half-car results consistent with quarter-car trends.
• Metric changes across variants, roads and speeds.
• Mitigation effectiveness for retuning, motor mount and active damping.
• Fidelity comparison between models.

Expected results

A multi-fidelity quantification of the ride, road-holding and handling cost of in-wheel motors on Indian roads, and of how much retuning, a dynamic-damper mount and active damping recover.

Take it further

• Add an electromagnetic model of the motor air gap under wheel vibration.
• Combine with torque vectoring to see if handling losses are offset.
• Calibrate against measured data from a vehicle with in-wheel motors.
• Study durability loads on the motor bearings from road excitation.

Viva and review questions

Learn it at DIYguru

These DIYguru courses and programmes teach what this project needs.