fidelity101 said:
Berck said:
In reply to Keith Tanner :
The Focus RS is similar, but a little more fancy using a computer controlled dual clutch pack in the rear and a fixed power takeoff unit instead of a center diff. Modern clutch materials are impressive, but of course these are wet clutches which makes dealing with constant slip easier. I suspect that worry about those clutch materials at high temps is why the Focus RS only lets me drive about 20 minutes on the track before it decides the rear drive unit is too hot, opens the clutches and turns it into a 350hp FWD machine which is funny, but not particularly fun.
this is why I hate the focus RS. huge let down - the Toyota should triumph gloriously in this area I hope.
The "GR AWD" system uses the same clutch coupling system as the Focus RS, but one clutch before the rear diff instead of a clutch after the diff at each axle (which allowed the RS to torque vector). So in a way it is less complex and less capable than a "dual clutch" AWD system. Notably, both systems overdrive/overspeed the rear differential.
In that sense it is most definitely possible to overheat the clutch packs, just like the RS, and it also would default to a FWD drivetrain with AWD limp mode. Most modern AWD Systems with a clutch pack use a temperature model for the clutches (sensor for the sump oil temp), and puts the system into limp or protection mode at a certain temp. Even RWD based AWD systems do this. How often it overheats and in what conditions depends on how the system was designed, the temp model is calibrated, and driving conditions. More wheelspeed differential will drive higher clutch temps, so low-mu gravel/snow will increase clutch temps in general, this might not bode well for the GR AWD system in rally use. Team O'Neil ran into these issues with trying to campaign the RS in stage rally.
From Yaris AWD System explanation:
"The system’s ECU controls the drive torque distribution to suit the driving situation, according to data gathered on vehicle performance such as speed, acceleration, braking and steering. Electronic control of the multi-plate clutch uses information from the wheel speed and G sensors and the clutch’s temperature sensor." - Not common to have a clutch temp sensor built into the clutche coupling themselves, so this may be marketing jargon and there might be a temperature model with a oil temp sensor.
https://www.autoevolution.com/news/toyota-gr-yaris-awd-system-can-overheat-this-is-what-happens-when-it-does-166717.html
The Evo X had a similar system as the RS (and Honda's SHAWD) with two clutch packs at the rear diff, but there aren't many reports from overheating the rear diff on those.
Compared to the RS: as mentioned it is a FWD trans with PTU, as opposed to an AWD transmission with center differential. Rear diff is also oversped/overdriven. The
Yaris GR AWD has a single clutch pack before the rear differential, which is oversped from the front (so the clutch is constantly slipping with AWD engaged, except in the 30/70 split where the clutch packs are locked).
https://www.gr-yaris.co.uk/forum/threads/gr-yaris-awd-system-explained.916/
https://www.youtube.com/watch?v=uetY5CSckPg