Honda’s Staggering Active Torque Transfer System

From the video explaining the Prelude Type SH’s Active Torque Transfer System (ATTS), it’s clear Honda’s research and development engineers are glued to the tube when Indy car racing is on. During those ESPN broadcasts, pit expert and former Indy car driver Jon Beekhuis often explains "stagger" -- the oval-track technique of employing a taller right rear tire -- by laying a paper cup on its side, with its bottom to the left, and rolling it across a table; he points out that it turns left on its own. Honda’s ATTS video contains an identical depiction. ATTS is the latest and most technically advanced (Honda says it’s "a world’s first") weapon in the war to quell front-wheel drive’s inherent power-on understeer. (If you didn’t know, "Understeer" is when the front tires lose traction before the rears, and the car slides, nose-first, wide of its intended arc.)

With stagger, the tread of an Indy car’s taller right rear travels faster than that of its shorter left rear to help negotiate an oval’s left-only turns. ATTS duplicates stagger’s effect with computer-controlled, clutch-activated gear sets that divert power originally destined for the inside tire and employ it to speed up the outside tire.

The system, which is wholly unlike a limited-slip differential, is positioned between the differential and the left halfshaft. The Prelude’s computer monitors steering-wheel angle, wheel-speed, lateral-acceleration, and, for all we know, ESPN2 to determine that you’re, say, blasting out of a left-hand freeway transition ramp, near the edge of tire adhesion, foot to the floor. Without ATTS, this is where the car would start to push toward the guard rail, and you’d have to get off the gas.

With an 80-millisecond response time, the computer tells a linear solenoid to progressively direct hydraulic pressure from the ATTS’ dedicated pump to incrementally engage the appropriate clutch (similar to those found in automatic transmissions). The planetary gearset then steals torque intended for the left (or inside) tire and diverts it back through the differential, where it’s used to speed up the right tire. In extreme cornering, the ATTS shifts up to 80 percent of engine torque to the outside tire, which is spun up to 15 percent faster than the inside. The process works much like a kayaker paddles hard forward on the right side to make the canoe turn left.

A side benefit of ATTS, though not directly attributable to the system, is a reduction in drop-throttle oversteer: Because there’s less understeer with ATTS, you’ll have less steering angle dialed in, so if you have to get off the gas in a corner, the car is less likely to tuck its nose toward the inside and spin out.

In normal operation, the new Prelude transmission challenges the best of today’s wondrous fleet of computer-controlled, seamless-shifting automatics. Also, thanks to its own dedicated 16-bit electronic brain and eight shift maps (Porsche’s Tiptronic transmission has but five), it thinks for itself to, for instance, downshift early on inclines.

Switch to "Sequential Sport Shift" mode, and the driver is required to do the thinking. With a touch of the Indy car -- like sequential shifter, the Honda automatic snaps off sharp manual-transmission-like upshifts -- one-third faster than a conventional automatic -- and downshifts at your command. And only at your command. Like a manual and unlike the Tiptronic, the Honda won’t upshift until you push the shift lever. Stay away from the lever and keep your foot in it, and the engine will bounce off its rev limiter. Yet the transmission’s digital brain won’t allow downshifts at inappropriately high engine speeds.

Opposite of shifter karts and Indy cars, the Prelude’s lever is pushed forward to change up, back to downshift. The reason: Honda felt most people are used to selecting lower gears on an automatic by pulling back, higher gears by pushing forward.