The bad thing about torque is that it often isn't where you want it when you need it. Just ask Prodrive (Banbury, UK) Dynamics specialist Damian Harty. A self-proclaimed "big proponent" of rear-drive handling dynamics, he says he's prepared "to suffer through its bad weather disadvantages in exchange for the amusement it can deliver." Harty came to Prodrive eight years ago, about the same time the company was winning the World Rally Championship as the Subaru factory team. Intrigued by Imprezas success, and the media's effusive praise for the abilities of the road car's all-wheel-drive system, Harty gave it a go, only to find himself "cursingly disappointed" that it behaved like an overgrown front-drive car.
Undeterred, he looked at engaging the differentials via an on-board controller to enhance handling. Mitsubishi's Lancer Evo and Acura's RL already do this via mechanical means that add weight, cost and complexity. Harty was curious whether vehicles–of all drive configurations (see "ATD Development")–fitted with electronic stability control and limited-slip differentials could be made to approach this ideal without adversely affecting cost, content, or packaging. It can, "All of the hardware is pretty much off-the-shelf technology," he says. "We add a small amount of hardware and proprietary software that works with the stability control system's sensors to determine when and for how long to engage the differential."
Because the technology is relatively commonplace, Harty and his team expect that implementation can be easily accommodated within a normal product development cycle. Though many OEMs are looking to apply ATD to a premium product first, then add it across their vehicle range, Harty insists they need not wait for the next design cycle. "You're basically talking about 3 to 4 months to implement it, followed by a normal 18-month sign-off cycle, making it possible to add Automatic Torque Distribution (ATD) to a platform within two years of making the decision to proceed." With greater familiarity, that time could be reduced even more.
It should be noted that Prodrive prefers to use multi-plate wet clutch differentials because it considers them to be essentially bullet proof. Says Harty, "You can abuse it and abuse it and abuse it, and it will still work once it cools down to normal operating temperatures. Dry clutches, on the other hand, don't have to be abused for long before they decide they don't want to play again–ever."
And while Harty claims ATD could be used to decontent Mitsubishi's and Honda's elaborate AWD systems without harming their performance, some entry-level vehicles would require additional pieces (multi-plate limited-slip differential, steering sensor, infrastructure to support its actuation, the Prodrive ECU) to reach the required baseline. From so low a starting point, the ATD system would add approximately $200 to the cost of the car, but could be sold as part of a larger safety package, Harty suggests. "If it was standard fitment, however, the cost would be closer to $50 per car in high volumes."
Seven of Prodrive's test vehicles so far have AWD or 4WD drive systems, and range in complexity and functionality from a Subaru Impreza to a Ford Expedition. "ATD does pretty much the same thing in all of them," he says, "and we dial it up or down by varying degrees depending on what the customer has in mind." With platforms that have both front- and all-wheel-drive variants, Harty says, adding the system benefits both, but gives the AWD version a bit more functionality than it otherwise would have. "From moment-to-moment, it can shift from front- to rear- to all-wheel-drive as necessary, and without any intervention by the driver." Rear- and front-drive prototypes also exist, and are currently under evaluation by OEMs. "Everybody gets out of the cars grinning," he says. Now all Prodrive has to do is translate those smiles into sales..
Intrigued by the AWD prototypes, customers began asking Harty and his team for further permutations, starting with rear-drive. Aware that ATD couldn't overcome the laws of physics, or make a rear-drive car the equal of an AWD vehicle in the snow, they used it to manage torque from side-to-side as a way to enhance vehicle stability. What they discovered was that, on a medium- to high-grip surface, a less-skilled driver could take a rear-drive vehicle closer to its limits, stay at those limits longer, and recover from situations leading up to a spin. "You can spend much more time smiling and much less time worrying about whether or not it's going to go wrong," claims Harty.
Once rear-drive was handled, front-drive became the next hurdle. One that Harty and crew weren't sure they wanted to tackle. "Our feeling was that, whenever you do anything on the front of a front-drive car, the steering torque corruption will worsen," he says. The customer, who was "very keen" that Prodrive adapt ATD to his front-drive chassis, insisted they try it anyway. "Providing that you engage the differential at the right times," says Harty, "you get a superb interaction between the front axle and the steering system." It was a characteristic of the ATD system that he says previously hadn't been used to its full potential.
When the back of the car threatens to swap places with the front, engaging the differential on a front-drive vehicle allows the system to "turn up the volume" on the steering through the interaction of the driveline forces and steering geometry. "It massively increases the message that the back end of the car is moving," says Harty, "to the extent that–if you let go of the steering wheel–the back of the car will very elegantly steer itself out of an incipient spin."