If he said that about AWD he’d have gotten it backwards.
Traditional AWD will let all your engine’s power flow to the freespinning wheel, while the other wheels under tension will lose power.
This ia so that you can make turns at high speeds without side slip and ruining your tires and so on.
AWD will lose torque on contacting wheels if there’s a free spinning wheel.
Now he did explain that in some newer AWD vehicles there’s a sensor that will divert power to the contacting wheels or even lock the differential if it senses a free spinning wheel. But that’s not a normal part of AWD.
Maybe I’m just used to Subaru and it’s phenomenal AWD but it seems utterly useless to give preference to the spinning wheel(s). I guess this is why many other automakers’ AWD seem more gimmicky than anything. I know on my WRX the drivetrain can divert up to 100% power to one wheel if it has traction and AFAIK all AWD Subarus work this way.
Giving preference to the spinning wheel is a by-product of the slipping differential. At highway speeds if you have a locked differential, it puts a lot of twist on the axles when you make a turn, because the inner wheels travel a shorter distance than the outer wheels. The slipping differential lets the outer wheels “free spin” at a faster rate than the inner wheels so that the axle doesn’t have to twist.
As I understand it, with Subarus, they work like a normal AWD under high traction conditions, but when the drive train senses much faster free spinning than expected under high traction conditions it locks the differential.
4wd can prevent some accidents with rwd vehicles like trucks. If the rear drive wheels break loose and spin on a rwd at highway speed it can send you off the road. I know because it happened to me. That’s less likely if power is going to all wheels. That particular situation is about traction, not stopping.
This video, via Road and Track explains 4 types of subaru awd. It says that all of them have a mechanism for sending power toward the less slippy axle (either a viscous differential or a computer controlled clutch differential). They then detect a slipping wheel and apply breaks to send power toward the other wheel.
Fully locked 4WD power is going to go to the wheels that are gripping in proportion to their grip (they all spin at the same speed, and whatever grip there is is the resistance to the engine, so equal and opposite force: all the power is going into the grip, wheels that don’t grip are providing no resistance to the engine).
The viscous differential is pretty cool, I have read about this before and was reminded.
It uses a non-newtonian fluid (like ooblek, but obviously not that one ). Two plates are spinning. As long as they are spinning at about the same speed the fluid is… fluid… and its a slippy differential. If the two plates are spinning at different speeds the fluid becomes much more viscous locking the differential (or mostly locking it I suppose).
I’d imagine that viscosity switch just wouldn’t work at really low speeds though. Inching over some giant boulder or whatever.
But it sounds like that is now being replaced by computer controlled clutch mechanisms or by fancy gear based mechanisms in some cars.
Really at any speed, if the rear wheels lose traction and start to spin, they no longer provide directional stability. Static friction between two bodies at rest with each other is greater than sliding friction. When the wheel is rolling in a way that is synchronized with surface that it is rolling on, you have to overcome that static friction for it to slip sideways. So the surface can exert enough sideways force on the wheel to turn the vehicle sideways or to keep it going straight.
But if the rear wheels are spinning faster than the car is moving, the friction between the tire and the surface is MUCH lower. And it is no longer much easier for the tire to roll along straight than for it to slide sideways. Which is why RWD vehicles tend to fishtail and skid in the snow. I remember some very white knuckle times driving my first car up hills in the snow. (1977 Mercury Capri II)
Possibly because it was supposed to be a sports car, but only came with a 4-speed automatic as Subaru didn’t have a manual that could deal with the torque from the EG33 engine (which at the time was the largest they had ever made).
Definitely. Studded winter tires were pretty much a requirement when I lived in Alaska, due to more ice than snow most winters. Moved to Denver and just used regular winter tires since there was way more snow than ice. That was with AWD. I live in a much milder climate now but with occasional snow and now a RWD car, I still use winter tires for safety.
I would consider the main difference between RWD and AWD to be the center diff. In either setup there is no guarantee that you have an LSD or locking diff in the front or rear. Thanks to ABS and stability control being standard on all new cars wheel slip can be limited by the ABS system even without a mechanical differential solution (obviously up to a point). There isn’t going to be a best configuration because each type of environment iis different. A setup with locked front and rear diff and a standard transfer case would give you power at all 4 wheels all the time. But that’s going to be hard on tires and drive poor on pavement. Subaru’s AWD setup with the addition of an LSD diff would probably be the best mix between off road capability and drivability.
). Two plates are spinning. As long as they are spinning at about the same speed the fluid is… fluid… and its a slippy differential. If the two plates are spinning at different speeds the fluid becomes much more viscous locking the differential (or mostly locking it I suppose).
