By Jim Kerr
Putting the power of a high performance engine to the ground has always been a challenge. At the drag strip, the fastest cars will use a “spool” which is a machined part that connects the axles on both driven tires together. Power is split equally between the two tires.
This method of getting the power to the ground doesn’t work on road racetracks or the street, however. When a car turns a corner, the tire on the outside of the corner has to travel further than the tire on the inside of the corner. To allow this, vehicles are built with differentials, a set of gears that allow one wheel to turn faster than the other does so the vehicle can go around corners smoothly.
Unfortunately, a differential can also send the power to one tire only, and if that tire is on ice, it will spin and the vehicle won’t move. Most vehicles have differentials like this, as they are the most economical to manufacture. High performance vehicles need some help with traction and a common way to do this is for the manufacturer to install a limited-slip differential.
Limited-slip differentials have gears inside that allow the wheels to turn at different speeds just like a simple differential, but there are also clutch mechanisms. When one axle starts to spin faster than the other axle, the clutches engage to slow the spinning axle and transfer power to the other axle. This splits the power to both drive tires.
A limited slip differential has disadvantages too. In order for the clutches to engage, they need to be preloaded with them slightly engaged. When you turn a corner, the clutches have to slip so they do wear some. This preload also can cause the vehicle to lose traction on ice covered corners. The tires don’t have enough grip on the ice to make the clutches slip, so the tires slip instead. If the surface is very slick, the drive wheels can slide sideways!
Limited slip differentials have been almost exclusively used on rear-wheel drive vehicles or in the transfer cases of several all-wheel drive vehicles. Front-wheel drive cars haven’t used them because the drag on the clutches could make it more difficult to steer the vehicle. A Torsen differential has been offered on a few high performance vehicles because this design will allow the wheels to turn at different speeds without any clutch drag, but will drive both wheels when power is applied. To do this, the differential uses several gears driving through a complex arrangement – efficient but very costly to manufacture.
Electronics are playing an important part in maximizing traction in modern vehicles without the use of limited slip differentials, and not just high performance ones either. Traction control may reduce a wheel from spinning but it really doesn’t help performance. Acura’s Super Handling All Wheel Drive (SH-AWD) is one of the first examples of adding traction through electronics. The system uses several inputs including steering and yaw sensors to monitor vehicle direction. When the stability control computer senses the vehicle straying from the intended path, it will electrically activate clutches in the rear axle that speeds up the rotation of one of the rear tires. Putting more power into one rear wheel “steers” the car back in the correct direction without any reduction in performance.
Ford’s 2012 Focus is the newest vehicle to include torque vectoring. This system uses lateral accelerometers, yaw sensors, steering angle input and wheel speed sensors as inputs to the vehicle stability control computer. Monitoring the vehicle a 100 times a second, the system detects when the front inside tire starts to slip on the road surface. By applying the one wheel brake incrementally, the spinning wheel is slowed slightly and torque is transferred to the other drive wheel, maximizing the car’s grip on the road. It acts like a limited slip differential without the disadvantages. As for performance, the torque vectoring is imperceptible but it definitely makes the car perform in a spirited manner on a twisty road. On slippery roads, the system can also help maintain directional control.
Diehard performance enthusiasts still love limited slip differentials, but the new electronic systems are actually much better, both in performance and driving feel.