The second-generation of Ford’s F-150 Raptor, which launched in 2016 for MY17, is available with an exclusive set of front drivetrain hardware that endows it with exceptional off- and on-road versatility. The Raptor’s already rare high-speed off-road prowess also pays dividends on pavement, thanks to a combination of a special transfer case and front differential that give it a unique forward power-distribution configuration.
Following the original Raptor’s mission, the 2nd-gen truck was engineered for Baja-style, fast-desert running, but added capabilities via its new transfer case. The Raptor comes standard with BorgWarner’s Two-Speed ITM (Interactive Torque Management – aka “torque on demand” or TOD) Transfer Case with Clutch Lock. Currently, it’s the only production vehicle offered with this hardware. More atypical, this unique transfer case can route power to the Raptor’s optional 4.10:1 front axle featuring a Torsen limited-slip differential, a component typically reserved for high-performance, rear-wheel-drive (RWD) sports coupes.
Unique transfer case
The Raptor’s transfer case features both a TOD, clutch-based system to dynamically apportion power to the front axle for pavement-based scenarios, as well as a dog-clutch-type setup to mechanically connect the front and rear drivelines for low-traction, off-road duty. According to Pritish Khale of BorgWarner’s Program Engineering, the transfer case was developed for the Raptor after the OEM requested a “mechanical lock/torque-on-demand transfer case.” BorgWarner’s PowerDrive Systems Team worked with Ford over a three-year span (2014-16) for the transfer case’s launch on the 2017 Raptor.
According to Khale, the biggest challenge was packaging the two typically separate systems in a single case. “Due to the mechanical lock, it’s an ‘active’ transfer case that can also act as a ‘part-time’ transfer case,” he said. Beyond packaging, there was the goal of maximizing component commonality to keep the bill-of-material and manufacturing costs down. “Also,” Khale added, “it goes in a truck which is known for its off-roading abilities.”
To shift from the clutch-based TOD mode (“4Auto”) to a mechanically locked mode (“4Hi”; “4Lo”), a servo motor rotates a shift cam, which moves a mode fork that drives a locking collar rearward via a locking hub. “The locking collar engages with the upper drive sprocket and transfers torque to the lower-driven sprocket through a chain,” Khale explained. “The lower sprocket is splined to the front output shaft, and as a result the torque is split between front and rear shafts.”
AWD and 4WD
“Usually the customer has to pick between an ESOF [Electronic Shift On The Fly] electronically engaged 4X4 system, or a TOD,” explained Mark Lecrone, vehicle integration supervisor at Ford Performance. “An ESOF is not all-wheel drive, it's got a dog clutch that engages and you are locked in 4X4. It's not good for on-road, or rain, or patchy conditions because you get too much bind in the driveline.”
“TODs are really good on road, and they can be very capable for high speed, intense desert running, but they have some limitations in terms of the clutch capacity,” Lecrone continued. “You might heat up that clutch, so it's not optimal for super-high performance. The Raptor’s transfer case has both. You've got the clutch-based all-wheel drive, but when you go to 4Hi or 4Lo, it also has the dog clutch that's locking teeth. You're mechanically locked up.”
The Raptor’s pavement-focused 4Auto mode is specifically engineered around the TOD aspect of the transfer case. “The clutch-based system allows an automatic electronic engagement or disengagement of the front driveline based on an algorithm and what the traction demands are. It'll look for rear slip and that will send torque to the front when it's required. It's primarily there for on-road slippery conditions: rain, sleet, snow, patchy ice,” Lecrone said. “With all the power that's going to the rear end of the truck, it's nice to have an all-wheel-drive setup where on a rainy day, you're not spinning up the rear axle just trying to pull out of a driveway.”
Tailored with Torsen
“The main mission of the Raptor is desert running in fast sand. That's what drove most of our technological decisions and how it was optimized,” Lecrone explained. “Of course, it's also a daily driven vehicle, and it needs to be capable in all the enthusiast off-road venues to the extent that its approach and departure angles permit. We tested it in all kinds of terrains – slow, fast, slippery, snow, rock, rock crawl – but the focus is always fast running in the sand. A Baja 1000 type environment.”
During development of the Raptor, high-speed desert running with locked front and rear axles did not provide the necessary steering feel nor front-end directional attributes the team was seeking. “When you have a rear differential lock and a front differential lock like you do for rock crawlers, it binds the truck up and makes it difficult for the chassis to turn. At speed, you still want to be able to turn in, you want the nose to point, and you want to be able to set it up in the corner. For fast, dynamic handling events, and to balance and be able to steer and still have a nimble feel, a locker on the front wasn't the right answer.”
The solution was a limited-slip front differential, which improved steering feel and turn-in, letting the truck more accurately track the desired arc. “It's all about when and how you can get torque and traction to the ground,” Lecrone noted. “With the Torsen, it's the same technology that we use in our Shelby Mustangs, just applied to the front axle. When you're coming off the corner, if you start to lose a bit of traction or you get a little bit of unload on that inside wheel, it's going to transfer that torque to the outside. You've got traction you can use, and because it's mechanical, it's there all the time.”
Tuned for fast grip
“What the development engineers felt when they put that Torsen in the front is you still have that ability to be nimble and turn in and get into the corner, but when you put the pedal down to exit the corner, you've got a lot more drive on the front axle,” Lecrone said. “You've got more balance in the truck and you have more traction capability to shoot out of the corner and get to the next straightaway.”
The Torsen front differential’s 'bias ratio’ (the measure of how much torque it distributes to the opposing wheel), chosen during the Raptor’s development is 2.5:1 under power and 2.1:1 while coasting. “If you make that bias ratio tighter, you're getting closer to the locked differential, so you tend to have more understeer and less nimble feel,” Lecrone said. “As you ease off of it, you get more nimble, but you then don't have as much tractive effort. It's finding that sweet spot and what the chassis likes, how it reacts, and the trade-offs involved.”
“In low-speed, rock-crawl events, the most optimized rock crawler would have the locking differential front and rear, obviously. But the Torsen does help if you get yourself into a situation where you've got very limited traction on one side,” Lecrone said. “If you attack that rock and you get some slippage on the one side, the inertial effect does transfer some torque to the other side. It's obviously not as good as a locker, but it's better than an open diff even in those rock crawling situations, but that's not the main mission of the truck.”
In its Baja Mode, the Raptor uses the transfer case to mechanically lock the driveline into 4Hi and the customer can enable the locking the rear differential at all speeds for added tractive capability. In a deep-sand desert environment, this maximizes traction while the Torsen front diff provides improved steering feel and directional control. “In Baja Mode, when you're transitioning, sliding through sand in turns the goal is to get as much traction as you can to shoot out of the corner,” Lecrone noted.
“That was the primary driver of what the team was trying to do,” Lecrone concluded. “Enhance the performance envelope, make the truck more confident, make it more capable and make it faster. It's a combination of the transfer case and the Torsen front axle which gives you the optimal high-speed off-road capability. It's all about getting maximum performance in the areas where we choose to focus the truck, so you can attack the terrain rather than just survive it.”Continue reading »