My LEs and other members. Question for all that drive different cars and own different brands as well. How much difference is there in different cars AWD systems? Is Audi’s Quattro that much better than the rest? If so where does Lexus AWD stand? Is the type of tire make a difference more than brand? Is there anyway Lexus can get better at AWD? Sorry lots of questions but with these lovely winter storms in the northern part of the US I just started wondering about this. Thanks LE!
It seems that the IS AWD system consists of a planetary geared center differential that has an electronically-controlled clutch pack, and a "silent chain" which transfers engine power to the front wheels. The default power distribution is 30 front, 70 rear which can vary all the way up to 50:50 depending on how tightly the clutch pack closes up. There doesn't appear to be a limited slip diff on the front or rear axles for the AWD model, I think there's some brake-based action going on to bring the car back in line, but I'm not sure.
It seems to be a simple, robust AWD system because it relies entirely on gearing to achieve that 30F/70R full time AWD, with the clutch pack only kicking in to vary the power distribution. It reminds me of the AWD system from the 2018+ Subaru WRX STI, but without a helical LSD on the front nor a Torsen on the rear (and no DCCD - just the various driver-selectable modes).
Other AWD systems I know of:
- VW uses Haldex, which relies entirely on a clutch pack to send power to the rear wheels (no gears). There might be other AWD systems that VW uses, but I can only think of Haldex at the moment (because I was looking into a Golf R at one point in time).
- Audi has a similar system to the IS' but they can add a sport differential to the rear, which is basically (as far as I know) two clutch packs to vary torque to either rear wheel. Audi has had a very good reputation for AWD systems, and for good reason. However they have been mucking around with a Haldex-like system they call "Quattro Ultra", which I hear is due to replace their original AWD system...even for cars with longitudinal engines.
- Ford's Focus RS has a Haldex-style AWD system but with two additional clutch packs (one per rear wheel), it's made by GKN.
- Acura's SH-AWD has a default torque split of 90F,10R and the rear wheels are over driven (ie. they are supposed to be spinning slightly faster than the front). There is no gearing involved here, just clutch packs (one per rear wheel), but SH-AWD was slightly different for older versions of the MDX in that there were gears and clutch packs working together for the rear axle.
- BMW, Kia, Hyundai and Genesis all use a unit made by Magna, which is branded xDrive in BMW and HTRAC for H/K/Genesis. I found this blurb from an old article. There is also a cheaper variant of xDrive that the X1s get, which is more like the above-mentioned Haldex and Quattro Ultra - this makes sense because I think the X1 is a FWD-based vehicle.
xDrive is the new, intelligent four-wheel drive concept with an active vehicle dynamic control system for the BMW X3 and BMW X5 model series as of model year 2004. This not only offers best possible traction in the event of difficult road surface conditions, but also simultaneously provides increased agility, vehicle dynamics and driving stability thanks to permanently variable torque distribution between the front and rear axle. The focus of this paper is on the transfer case of the BMW xDrive. In contrast to a planetary gear differential, in the xDrive transfer-case a rigid main shaft connects the rear axle, A wet multi-plate clutch is mounted on this shaft to continuously control the torque to the front axle. Important for the four-wheel drive function are the dynamics and the precision of the torque. To ensure both under varying operating conditions and over the life-time of the gearbox, the transfer-case control module continuously estimates the wear-off and mechanical tolerances as input parameters to the torque control of the clutch.
tl;dr The IS' AWD system seems to be very good because all 4 wheels are being powered full-time using a planetary gear system and that 'silent chain' I mentioned, so the clutch pack doesn't have to be constantly slipping just to send power to the front wheels.
