Among modern hot hatch models, the Toyota GR Corolla remains the strangest of the bunch. While everyone uses four cylinders and a single driven axle (except the Subaru WRX), the GRC’s three-cylinder and its strengths are well known. But what’s really strange about the GRC is its unconventional all-wheel drive system. Here’s exactly how it works.
The main problem is that the GRC uses an all-wheel drive system that is identical in configuration but completely different from its contemporaries in terms of execution. Its layout is fairly standard: a transverse engine up front feeding a transaxle that leads to a transfer case that sends power to a rear differential. Externally, it’s closest to a Volkswagen Golf R, except the GRC’s transfer case doesn’t have a clutch pack. Technically, the GRC doesn’t even have a center differential, and that’s where it takes a real left turn.
The nerds among you will know that the GRC also has a variable torque all-wheel drive system, configurable to 60% front, 40% rear, 50/50 and 30% front, 70% in back. Conventionally, this torque distribution would be managed by a center differential with a clutch pack that manages the torque output to the secondary axle. For example, a Lancer Evo can go from 100% FWD to 50/50 depending on how its center differential locks. The two key points to remember about variable torque distribution are to figure out where the power is primarily going, and then move the torque elsewhere. In the case of the Evo and GRC, power goes to the front axle first, making it difficult to send more than 50 percent of power to the rear.
Older Subaru WRX STIs, which used a completely different style of all-wheel drive, had about a 40/60 split. Subaru’s all-wheel drive is mechanically locked, meaning the torque distribution is constant and determined by one thing: a difference in speed. With the STI, the rear was very slightly overdriven at the transfer gear that sends power to the rear of the car, while this slip was managed with the center differential. The GRC also uses a gear differential, but instead of a center differential it uses a clutch to absorb slip. This same clutch pack also determines the variable torque distribution.
Let’s get down to the meat and potatoes: The GRC’s rear axle is very slightly overdriven, just 0.7% at the transfer case. By managing clutch slip, the GRC can vary its torque distribution at will. When the clutch is open, the system spins more freely and sends more power forward. 50/50 is a little more locked in, while 30/70 becomes a real math problem.
Technically, the GRC does not send 70% of its power rearward. It sends 50%, but clutch slip determines how much goes back. But in 30/70, the advantage of rear axle gearing combined with a central locked clutch means the rear actually starts to subtract torque from the front axle. As the rear axle accelerates, the front axle slows down, sending its torque rearward. Are you still confused?
This is a technical innovation and workaround for the use of a transverse all-wheel drive system based on front-wheel drive. But it also makes the GRC completely unique and interesting to drive in that it doesn’t behave like it has 70% rear torque. In fact, Toyota itself says the 30/70 mode is largely intended for loose surfaces where the rear end can turn more quickly, rather than on pavement where most of that energy is absorbed by the clutch.
But if you’re a visual learner and that wall of text was just confusing, I highly recommend our video. Appreciate.
