Discussion Starter · #1 ·
By: Marc Noordeloos, 4 November 2015
I can’t hide my enthusiasm for the new, 2016 Ford Focus RS, its 350-hp, 350-lb-ft turbocharged 2.3-liter engine, and its all-wheel-drive. I thought I had disseminated everything you need to know about the car in my September 24 column.
But there’s more. I recently sat down for separate interviews with Jim Fritz, Ford’s AWD systems engineer, and with Ray Kuczera, vice president for global product technology at GKN Driveline, to get more details on the trick new traction-enhancing drivetrain on the hot Ford hatch.
Ford buys the Twinster system, which has two wet clutches in the rear driveline module in place of the standard front-wheel-drive Focus’ rear differential, from GKN, which developed it at its Auburn Hills, Michigan, tech center. Its core software and hardware were developed in Lohmar, Germany.
How the system works
Valves independently control the hydraulic torque pressure to the rear and to each rear wheel, Fritz explained. A GKN electronic control unit attaches to the top of the rear-drive unit and communicates with other ECUs, such as the engine controller and brake controller.
“When the clutches are completely open, the rear of the vehicle is receiving zero torque,” Kuczera said. “The clutches are constantly moving to controlled states between full open and full lock as determined by the torque control portion of the AWD software. The hydraulic controls are updated 500 times per second to provide quick, accurate torque control.”
Different drive modes affect the system’s torque control via a couple of unique mode-dependent subsystems and a bunch of multi-axis calibration tables.
“When the driver selects a new mode, the software will enable some new subsystems, disable others, and reference new columns and rows in calibration tables, which change the amount of torque, rate at which torque is increased or decreased, and how the torque is biased left and right,” Kuczera said.
“In the Normal and Sport mode, we’re still trying to provide full function all-wheel-drive capability,” Fritz added. “You still have good handling capabilities. Moving to the Track mode, the goal is to get the best lap times possible. We’re trying for neutral performance without sending too much torque to get into oversteer unnecessarily. Drifting too much on the track is fun, but it’s scrubbing speed each time you oversteer. The Drift mode is all about trying to get the car to drift -- we’re trying to create as much oversteer as possible.”
Range Rover Evoque also has Twinster
The Range Rover Evoque is the only other production model with the GKN Twinster system, though other manufacturers will add it in the next year, Kuczera said.
“The system for the Focus RS has additional torque capacity and has a gear ratio offset,” he said. The rear has a taller gear ratio than the front, which allows overspeeding the rear wheels, which, in turn, allows “true torque vectoring,” Kuczera said.
But the Focus RS does not have the Evoque’s decoupling driveshaft, Fritz said.
“It’s permanently attached. The front/rear ratio mismatch is unique to the Focus RS -- to get more torque to the rear. During normal operation or straight-line driving, the overspeed isn’t really doing anything. The clutches slip to take up the difference in the ratio mismatch. But during handling events, we try to vector or send torque to one rear wheel -- the outside rear wheel in a turn. We take advantage of the ratio mismatch to get more torque to the rear axle than without a ratio mismatch,” which Ford says is about 1.8 percent.
Avoid talking torque-split
Twinster can immediately send torque to the rear without spinning the front wheels.
“It has a pre-emptive torque control, just like a normal on-demand system,” Fritz said. “There is no center differential so we tend to avoid talking about torque splits, because it confuses people when they’re thinking about a differential. Our active, on-demand system can send up to 100 percent of the available torque to the rear axle. Once it hits the rear axle, it can send up to 100 percent of that torque to individual left or right sides. If you were [in a scenario where] the front wheels were losing traction, then 100 percent of the torque to drive the car is going to the rear axle.”
But the Ford press release says that only 70 percent of torque can go to the rear axle
“That’s a bit misleading,” Fritz said. “It’s potentially 50 to 70 percent, but in other scenarios where you have no traction with the front wheels, then it’s 100 percent to the rear wheels. I’ve tried to clarify that, but it gets confusing.”
How does Twinster deal with heat from driving hard on the track or rallying?
Software algorithms control the temperature, Kuczera said, and they can detect issues when the car is driven aggressively.
“The software determines when the system risks going over temperature thresholds,” he added.
“For overheating conditions, we will have a message in the cluster,” Fritz added, “like our other all-wheel-drive systems … All-wheel drive off. If the system sees extended periods of high-temperature operation, a service message will tell the customer to change the fluid -- a hydraulic fluid for the clutches, and a gear lube -- in the rear-drive unit.”
No snow or rain mode
“With the vehicle being sold with summer-only (Michelin Pilot Super Sport) and R-compound (Pilot Sport Cup 2) tire options, it would seem Ford was much more focused on the high-grip portions of performance driving,” Kuczera said.
But low-friction testing is a vital portion of any vehicle development process. The Focus RS is certainly fun to drive in the snow, especially when fitted with winter tires, he said. While Drift mode might be the best for having fun in a snow-covered parking lot, the car may exhibit better overall behavior in Normal mode due to steering, brake control, and damper settings.
“We worked closely with the brake controls group,” Fritz said. “They did a really nice job with the tune of the ESC [stability control] system for the Sport mode. So if you’re someone who wants to have a bit more fun with the Focus RS, you can put the ESC in Sport mode and that will allow the yaw of the vehicle to open up a bit. When ESC comes in and provides corrections, the corrections are more damped compared to the default mode.”
AWD means more power
The first two generations of Ford Focus RS were FWD cars only. Going all-wheel drive allowed for more power.
“If we just had front-wheel drive, we’d have to make more compromises with how we’d deliver the performance -- 350 hp is really going beyond the capabilities of what the [front-wheel-drive] hardware can support,” Fritz said.
The most exciting aspect of the GKN’s new Ford Focus RS AWD system is its potential to bring a new level of driver involvement and outright fun to the segment. The Mitsubishi Lancer Evolution features the highest-tech system in the segment with its S-AWC (Super All-Wheel Control), but that car has been discontinued.
The other rally-bred Japanese entry, the Subaru WRX STI, has an electronically controlled center differential along with a helical limited-slip front differential and a mechanical Torsen rear differential. The much-loved Volkswagen Golf R makes do with a rather simple Haldex all-wheel-drive system with open front and rear differentials. The Focus RS promises the ability for proper lunatic behavior with the ability to send most of the 350 lb-ft of maximum torque to a single rear wheel. I can’t wait to see if both GKN and Ford got it right when we finally get to drive the car in January.
The 2016 Ford Focus RS Gets an Advanced Torque-Vectoring AWD System