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Okay here is anecdotal experience. I had a set of treads on my rear tire at 15/32 while the fronts where at 9/32. Viscous coupled AWD with traction control via
by o-__-o 6y ago
Okay here is anecdotal experience. I had a set of treads on my rear tire at 15/32 while the fronts where at 9/32. Viscous coupled AWD with traction control via four-wheel braking. I first noticed something was up when the car would launch rather quickly. I added an odb-2 reader and saw my AWD clutch was engaging a lot. Thought this was by design due to my heavy foot. Then the clunking started.
The AWD would rather stay engaged which puts strain on the transmission to shift harder. Over time this showed up as increased transmission temperatures. My transmission is sensitive to heat but has a poor design which makes any heat a very big deal. At about the same time I was researching new wheels and determined I could fit 9” wide tires in the rear vs 8.5” in the front. A staggered wheel set would give me more rear wheel power bias, but the closest I could get in tire size was still almost .5% larger in diameter.
In all my research it turns out you need tires of the same size within 3/16 wear of each other to avoid non-full time AWD transmissions from doing dumb things like my car does. You see, the AWD clutch simply engages a power transfer unit and then depends on ABS to control power sent by the differential to individual wheels. That system needs to recalibrate itself regularly and it does this when the clutch disengages. When it doesn’t, all sorts of warnings are tripped (in my car I went down 6 floors of a spiraling parking garage exit at 15-20mph and at the 2nd floor a “service stabili-trak” icon appeared on my dashboard)
In cars with a well designed AWD system this will not happen.. but most cars are not designed to engage in AWD for prolonged periods of time and are really there as a beefed up version of traction control. For instance my Torsen Quattro AWD Audi had full time AWD and never had this problem nor would a Nissan GT-R. But for that fancy Lexus or Hyundai AWD, you may want to make sure you stick to the recommended tire rotation schedule
To end my story, I was at the service center and they mentioned I should get my transmission checked out by the bigger repair bay because of the clunking. I suggested it may be the wear of the tires and they agreed to see if that solved the problem. New set of tires and the clunking, extra AWD engagement, and transmission temps went away.
- ComputerGuru 6y agoInteresting story. And yes, I was specifically referring to full-time AWD with or without a rear bias, like that in the Audi Quattro (who I personally think does an excellent job). I actually don’t consider a car that doesn’t have full-time AWD to be AWD at all, isn’t that considered 4WD?
- o-__-o 6y agoI understand AWD to be exactly that, all wheels providing driving traction. We have advanced long from the Audi 200 days to where part-time AWD is as simple as $700 in parts (for my car). 4WD (4x4) is full time (typically selectable but cannot be changed while in motion). 4WD has a greater gear ratio to really put power to the wheels that have grip, so while all 4WD cars can be considered AWD, AWD cannot be considered 4WD. I’m sure some jeep owner can give a better rundown on why 4WD is named the way it is..
- macintux 6y ago> typically selectable but cannot be changed while in motion Not true for a long time now. Manually locking hubs required egress while stopped but at some point 4wd tech progressed.
- frankchn 6y agoYeah, the most advanced systems can switch between 4WD and RWD while the drivetrain is under considerable stress too. For instance, the F90 M5 can engage the front wheels on command while in the middle of a power slide and let the front wheels pull the car out of the slide.
- o-__-o 6y agoM5 is not 4WD. Just an electronically controlled clutch like my car.
- ComputerGuru 6y agoI don’t think 4WD/4x4 is a strict subset of AWD or the other way around. One is typically manually selected, the other is typically always on at a certain ratio but (at least in modern variants) dynamically adjusted (or at least theoretically adjustable) either on a front/back or even on a per-wheel basis based on dynamic real-time feedback. TBH the line has blurred considerably in recent years and what once would have been 4x4 (manually engaged fixed-ratio four wheel mode) is being marketed as AWD. I’m sitting in a rental GMC Terrain (very crappy car, with or without AWD) and it must be manually switched from 2WD to 4WD mode but it’s an electronic button (rather than gear selection lever) that can be changed at speed, and can also be used in 4WD mode at highway speeds without redlining. This is considerably different from body-on-frame SUVs/pickups with “true” 4x4 mode that have regular 2WD but can be changed into 4L or 4H to get different gearing ratios inside the transfer case for four wheel drive.
- Dylan16807 6y ago> In all my research it turns out you need tires of the same size within 3/16 wear of each other to avoid non-full time AWD transmissions from doing dumb things like my car does. 9/32 + 3/16 = 15/32 so I guess not