Heresy Unsprung, Lotus Engineering: Unsprung Weight Doesn't Really Matter Much

Protean Electric In-Wheel Electric Motor

It’s April, when automotive engineers from all over the world gather in Detroit for the SAE World Congress. Protean Electric, which has been promoting the electrification of cars with their in-wheel mounted direct drive motors for the past few years, used the occasion of the World Congress to introduce the production version of their motor, which will start being assembled in a Protean owned factory in China next year.

Protean is also looking to license their technology to OEMs. The Protean motor can fit on standard hubs inside wheels that are 18 to 24 inches in diameter and the power units are also compatible with disk brakes, with mounting bosses for calipers. They can be used as part of a hybrid system, or since they have a continuous output rating of 86 HP per motor, they are powerful enough when used in pairs to drive pure battery electric vehicles. A self-contained unit with integral electronics and controls, the Protean motor can even be retrofitted to used cars. Protean is selling the fact that it’s a bolt on solution to turn a conventional combustion powered car into a hybrid. It’s actually pretty nifty and the motors allow for reverse torque so with the proper electronic controls torque vectoring is possible, allowing for very sophisticated traction and stability control. Theoretically, with a motor at each wheel it could also make parallel parking a snap by turning on the car’s own axis. Their in-wheel motor, though, is not why I’m writing about Protean. The real reason why I’m writing about Protean is a study the company commissioned Lotus Engineering to do on the real world impact of unsprung mass. A study with a somewhat counter-intuitive conclusion.

Surprisingly, Lotus, which knows a thing or two about chassis dynamics, said that tires and suspension tuning have a greater impact on ride and handling than adding the 68 lbs per wheel that the Protean system weighs. That sounds like heresy in an automotive world that has chanted “reduced unsprung mass” as a mantra for the past 50 years, at least. Reducing the mass of the parts of the car not supported by the suspension, in other words the mass of the suspension itself, has long been considered essential to better handling. Since they’re mounted in the wheel, hub motors are on the wrong side of the unsprung mass equation. Lotus says that while adding weight to the wheel has an effect on ride and handling, that effect can be offset by normal suspension tuning procedures.

The choice of Lotus Engineering to do the testing was not coincidental, because of Lotus Cars’ reputation for making what are arguably the best handling cars in the world. Cars that have supple road manners in addition to their cornering grip. The fact that Lotus cars can deliver that kind of cornering performance while still maintaining a comfortable ride on real world loads has not been lost of reviewers and car enthusiasts. Essentially, Protean was thinking that if Lotus gives them a pass, so will all the car guys who are asking, “But what about unsprung weight?”

Here’s what Steve Williams of Lotus Engineering said:

Whilst it is true to say that the vehicle dynamic performance was degraded by the increase in unsprung mass, the degree to which this was noticeable was small and could be said to have moved the overall dynamic performance of the test vehicle from class leading to mid class. Further more, the understanding gained from this study has led Lotus to believe that the small performance deficit could be largely recovered through design changes to suspension compliance bushings, top mounts, PAS characteristics and damping, all part of a typical new vehicle tuning program.

Add the powerful benefits of active torque control and Lotus’s finding make a strong argument for the vehicle dynamics benefits of hub motors as an EV drivetrain.

White paper (PDF) on the study here. Powerpoint presentation here.

Ronnie Schreiber edits Cars In Depth, a realistic perspective on cars & car culture and the original 3D car site. If you found this post worthwhile, you can dig deeper at Cars In Depth. If the 3D thing freaks you out, don’t worry, all the photo and video players in use at the site have mono options. Thanks – RJS