From the October 2015 issue

The Japanese say torque vectoring reduces steering effort. The Germans will tell you that these trick differentials increase dynamism (whatever that means). And the Americans—just now getting started with the upcoming Ford Focus RS—claim it provides for “unmatched agility.” We’ve quaffed the cherry Kool-Aid ourselves, praising the technology when we’ve driven cars equipped with it, so it appears that everyone is in agreement: The ability to apportion torque between a car’s right and left drive wheels does super­natural things for vehicle dynamics.

Yet when we pressed for the hard evidence, no one in the industry could provide data showing that a torque-vectoring differential measurably improves performance. To ­venture beyond manufacturer claims, we slapped our instruments on two cars—one with a torque-vectoring differential, one without—to collect our own data.

Where the Magic Happens: Bolt-on torque-vectoring modules from British driveline specialist GKN transform this otherwise-normal open differential into a corner-carving tool. ROBERT KERIAN, MARC URBANO

Like all differentials, the torque-vectoring unit’s first responsibility is to reconcile speed differences between the drive wheels. As a car rounds a corner, its tires trace four distinct arcs with four different radii, so each wheel turns at a different speed. Whether it’s an open, a limited-slip, or a torque-vectoring example, a differential allows any wheel that’s connected to the powertrain to rotate independently.

Torque-vectoring differentials are capable of much more, though. Controlled by sophisticated electronics and fitted with complex gearboxes, these high-tech diffs are placed on the drive axle to regulate thrust between the left and right wheels. With unequal torque between the two sides, the resulting yaw moment (torque about a vertical axis) either encourages turn-in or stabilizes and straightens the car, depending on how the torque is distributed. In theory, torque vectoring helps a car corner with reduced steering lock and less understeer. Those attributes should translate to a more controllable car, higher speeds in corners, and faster lap times.

View Photos ROBERT KERIAN, MARC URBANO

We should note the distinction between the differentials we’re discussing here and brake-based torque vectoring, the dollar-store variety that’s increasingly found on economy cars and crossovers. Brake-based systems selectively squeeze individual brake calipers to slow the inside wheels and increase torque to the outside wheels in turns. Automakers use this setup because it’s lighter and cheaper than ponying up for the more complex hardware while still creating a useful yaw moment. But it doesn’t take a race engineer to recognize the paradox in using the brakes to go faster.

Proper go-fast torque vectoring requires at least one overdrive gear in the differential (though often there are two) capable of spinning the wheels faster than if they were driven through a conventional diff.

When the computers decide to divvy up the torque, clutch packs connect the overdrive gears to the differential output, varying the clamping force to adjust thrust between the left and right wheels. Because the clutches don’t fully engage, the outside wheel doesn’t actually spin faster. Instead, the partially meshed overdrive gears provide a push, like paddling harder—not faster—on one side of a canoe.

Torque vectoring came to production cars via rally-bred racers such as the Mitsu­bishi Evo, but today it’s most commonly found in expensive, overfed, four-wheel-drive vehicles with a performance pretense. The torque-vectoring differential is why the massive BMW X5 M and X6 M are more ridiculously entertaining than they have any right to be. Audi’s Sport Differential helps the nose-heavy, four-wheel-drive S4, S5, and S6 turn in like rear-drivers.

Automakers insist that torque vectoring will always be a niche offering, but as the technology has matured, it’s found its way into more diverse applications such as the four-wheel-drive Nissan Juke and the aforementioned Focus RS. When the 467-hp Lexus RC F launched last year, it was the first front-engine, rear-drive production vehicle to offer torque vectoring. At more than 4000 pounds, though, the RC F weighs as much as some four-wheel-drive cars.

The torque-vectoring differential, cleverly called TVD by Lexus, is optional on the RC F, while a Torsen limited-slip differential comes standard. That makes the RC F the perfect test bed for our examination.

C/D Test

Results 2015 Lexus RC F with

Torsen Limited Slip 2015 Lexus RC F with

Torque Vectoring Weight Curb 4016 pounds 4057 pounds %Front/%Rear 54.4/45.6 53.5/46.5 CG Height 20.0 inches 19.5 inches 300-ft Skidpad Roadholding 0.91 g 0.94 g Average Steering Angle 135 degrees 99 degrees Average Drift Angle 0.6 degree 2.3 degrees 610-ft Slalom Average Speed 42.2 mph 42.7 mph 1.7-mi Road Course Lap Time 1:19.1 1:18.7

ROBERT KERIAN, MARC URBANO

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