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Comment by ComputerGuru | Hacker News Reader

Interesting 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?

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I 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..

In the US, 4WD (4x4) means when the system is engaged, the center diff is locked. Sometimes there'll be some combo of front and rear lockers or LSD units, but not always. When engaged, the center diff is locked, so these systems are typically only engaged off-road or in snow.

And AWD is any system that drives all 4 wheels permanently. In fancy systems, the ratio sent to front vs rear might vary from 50/50 to 0/100, but it's always engaged to some extent. These systems are safe for regular on-road use.

Pick-up trucks and truck-based SUVs usually have 4x4 systems (with a few exceptions like the Land Cruiser and Lexus GX which are both AWD).

Cute utes and cross-overs, plus the odd-ball sedans and wagons (Subaru, VW 4Motion, Audi) are usually AWD.

Edit - as noted in a sibling comment, the line has definitely blurred with fancy electronically controlled and viscous couplings, but generally "truck" = 4x4 and cross-over/wagon = AWD.

I 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.

> 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.

Yeah, 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.

M5 is not 4WD. Just an electronically controlled clutch like my car.

there are different forms of awd. Some "awd" is mechanical with even 50:50 or a 40:60 distribution of power like the quattro system that have a center differential. These systems still work great with large differences in traction: if you put snow tires on the front and regular tires on the rear, you can bust some sweet drifts and drive right out no problem. I wouldn't try bad tires on the front though. Other systems use electronic sensors and have a front bias giving 80+% to front wheels while giving rear varying amounts when the front loses traction. It reacts much slower and there is probably a bit of over/under correction from the control system if the tires don't have even traction. I could see this leading to a quickly cycling clutch that would over heat your system.

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