How the GR Yaris and GR Corolla AWD system works
The GR Yaris and new GR Corolla have been setting enthusiasts' underpants on fire with excitement for a while now with a return of fun all-wheel-drive performance cars from Toyota. The Japanese giant has a long history with rad all-paw cars, like the Celica GT-Four and Caldina, but the GR Yaris employs a few new tricks.
The GR Yaris and Corolla use a transversely-mounted (ie: sideways) engine with a front diff that is either an open unit or Torsen limited-slip unit depending on the spec of the car. The rear diff is also either open or a Torsen LSD depending on the spec of the car.
Where things get a lil' funky is joining these two units.
In the rear of the drivetrain, ahead of the rear diff, is an electronically-controlled electro-magnetic multi-plate clutch pack. This isn't a centre diff, like what is found in Subarus, but something much more clever for splitting the torque between front and rear.
The electronically-controlled clutch pack (officially known as the Intelligent Torque Controlled Coupling, or ITCC) is key to the AWD system's performance, as it changes the torque split going to the front or rear depending on which of three driving modes has been selected by the dial in the cabin.
This is similar to the Haldex system used in modern Volkswagens, but Toyota claim the in-house-developed ITCC is smaller and lighter than Haldex. This is important given the huge packaging constraints that Toyota faced when adapting AWD into the Yaris platform.
In Normal driving mode the centre diff uses a 60:40 front/rear torque split, while Track mode shifts this to a 50:50 torque split, and Sport mode flips to a 30:70 torque split to lend rear-drive-style handling characteristics.
To make the AWD system work, even in 50:50 mode, the rear differential is geared 1% faster than the front, as this will allow a greater percentage of the drive force to be transmitted to the drive wheels, even when the front wheels aren't slipping.
Normal mode (60:40 split) sees the clutch plates partially compressed, permitting an amount of drive to the rear. While the four tyres turn at the same speeds, the sets of clutch plates turn at different speeds thanks to the different gear ratios between front and rear diffs, allowing the torque split to vary.
As the amount of rear-drive that is required increases (in Sport and Track modes), the compression increases on the clutch packs. The 12 pairs of clutch plates, seen below, are covered in DLC (Diamond-Like Coating) to make them super-hard-wearing. This should see a longer life for the hard-wearing plates compared to an un-coated finish.
All up a very clever system that Marty has proven is enough to chop even Volkswagen's finest!