Mention the words “unsprung mass” to Tom Prucha, and he responds with reasoned vigor. As the Principal Applications Engineer in the U.S. for Protean Electric, a U.K.-based developer of in-wheel traction motors (IWM) for electrified vehicles, Prucha is an advocate for what he believes are the technology’s many benefits. They include packaging that helps optimize passenger and cargo space, a reduced bill of materials, lower complexity, and enhanced vehicle performance including more effective regenerative braking.
IWMs have been a potential engineering solution for hybrids and EVs since they were first used on the Lohner-Porsche in 1899. But critics cite the technology’s challenges. They say adding the weight of an electric machine (albeit a compact one) and its integrated power electronics into the aggregated mass of a vehicle’s road wheels, tires, brake calipers and rotors, suspension knuckles, etc., at each corner will negatively affect vehicle dynamics—particularly grip, active safety, and ride quality.
Indeed, at SAE International’s recent 2011 Hybrid and EV Symposium, an expert panel of OEM engineers was asked whether IWMs (also known as wheel-hub motors) were a viable option for near-term production vehicle applications. No one replied affirmatively. Some cited unsprung mass as a significant reason. And in a 2009 interview with AEI, Tesla Motors Chief Technology Officer JB Straubel noted that while his engineers considered IWMs, they instead chose a single-motor layout for the company’s initial vehicle programs.
The industry has not yet been hot for IWMs in its early round of electrified vehicles, and that gets Prucha’s own electrons stirred up. He says the penalty of excessive unsprung mass has been perpetuated so often, it’s become almost a default response.
“In reality, the unsprung-mass penalty related to wheel motors is a myth,” Prucha asserted recently. And it’s about time chassis and powertrain systems engineers heard the full story, he said.
That’s why Protean Electric decided to tackle the topic head-on with a technical session at the 2011 SAE World Congress. Their presentation (oral only) titled “Unsprung Mass with In-Wheel Motors; the Myths and Realities – Closing the Circle” aims to address the skeptics. It is scheduled to be delivered by company Chief Technology Officer Andrew Watts, in collaboration with Martyn Anderson, Chief Engineer at Lotus Engineering, and Damian Harty, a technology consultant with Dunamos Ltd. The session will discuss the results of Protean Electric-commissioned studies recently conducted by Lotus Engineering and Dunamos. The studies investigated the dynamic effects of unsprung mass related to IWMs in mainstream hybrid and EV applications.
Anderson and Harty carried out the studies, which involved theoretical analysis, test-rig work, and actual vehicle driving experiments.
A stock 2007 Ford Focus was compared with an identical vehicle modified with 66 lb (30 kg) of ballast fitted to each wheel. The weight was distributed between rotating and nonrotating unsprung masses as to broadly replicate Protean Electric’s PD18 (18-in diameter) wheel-hub-motor unit. The project plan included three phases of analysis and testing. Phase 1 focused on modeling of different modifications, including suspension spring, bushing, and damper rates, and different tires and pressures, and their effects on the IWM-equipped vehicle. It was determined that simply fitting a standard Focus ST suspension (an upgrade on the stock base car) would be a good practical solution.
In phase 2, the stock vehicle was modified with the Focus ST suspension. This setup included revisions to the front and rear spring rates, dampers, and the rear antiroll bar. In phase 3, the Focus with the modified ST suspension was retested. The process included a subjective vehicle assessment, objective ride and handling tests, on-road shake measurements, and two-post shaker rig measurements.
The studies concluded, and the presenters argue, that while the vehicle carrying the greater unsprung mass at each wheel did display perceptible differences compared with the stock vehicle, those differences were minor and can be mitigated using “normal engineering processes within a product development cycle.”
By fitting the upgraded ST-level suspension to the car replicating one equipped with Protean PD18 in-wheel motors, the vehicle’s handling and on-center tracking were improved back to reference. Overall, the effort conducted by Protean Electric, Lotus Engineering, and Dunamos may help convince skeptics that the addition of 30 kg of unsprung mass per corner will not adversely impact overall vehicle dynamics and can be addressed fairly easily with cost-effective countermeasures.