# Four wheel drive vs. all wheel drive. **URL:** \ **Category:** In My Humble Opinion\ **Created:** [January 3, 2016, 9:15pm UTC](https://boards.straightdope.com/t/four-wheel-drive-vs-all-wheel-drive/742102 "2016-01-03T21:15:38Z")\ **Posts on this page:** 5\ **Page:** 4
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [January 8, 2016, 7:58pm UTC](https://boards.straightdope.com/t/four-wheel-drive-vs-all-wheel-drive/742102/61 "2016-01-08T19:58:58Z")
> [@scabpicker](#): > > Hmm, I’m not sure about that. [Subaru](http://media.subaru.com/newsrelease.do?&id=580&mid=123) claims the CVT and the viscous diff can both deliver 100% of the torque to either end of the car. Well, the only place I see them claiming that is that one press release, so I’m guessing whatever underling wrote it simply made a mistake there. > [@swampspruce](#): > > I have to ask because it really isn’t clear to me; shifting torque between the front and rear has absolutely nothing to do with going around corners, per se so what is the argument here? > > If an AWD system has a centre diff (and if that centre diff is a [viscous coupled version](https://www.youtube.com/watch?v=w2bRb17jJ1U) it almost acts like a clutch, varying the viscosity between the plates to balance torque), it will transfer as needed when the front and rear diffs are moving at different speeds. > The cornering issue is the difference in speed between the inner and outer wheels, which is addressed by either having an open differential (usually geared) or having some sort of limited slip system. > To my understanding of it, the main difference between AWD and 4WD is a matter of what is now called part time systems where the center, rear and/or front diffs can be locked to provide an even split of torque on all 4 wheels and the “slip” is at the tires interface with the ground (which is why my Cherokee didn’t like going around corners in 4 low on dry pavement) vice a “full-time” system that introduces that slip in the diffs themselves via viscous coupling or clutches. > > Please enlighten this simple Subaru driving person. > FWIW, I have the 4EAT Legacy Wagon and [it does not transmit 100% front or back like the STI or WRX did/does](http://www.awdwiki.com/en/subaru/) due to mechanical center diff vice the VC. See, I think that is kind of why **Throatwarbler Mangrove** and I keep going past each other here. With a center differential, the change in axle rotation speed to go around a corner is necessarily accompanied with a shift in where the torque is going. With the electronic clutch system, it’s not, or at least not as directly so. **Throatwarbler** seems to think this is an important enough distinction that the two systems are utterly incomparable, but I think that to a typical bad-weather driver it doesn’t really matter. Both systems can send power to both axles while accommodating turns on dry pavement; the only two things the driver really cares about is whether the system can shunt enough power to the rear to move the car if the fronts are spinning (and in virtually all situations 50% is going to be fine), and how much power is sent to the rear under _normal_ driving conditions, which affects how the car handles and how prone the front wheels are to losing traction in the first place. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [January 9, 2016, 7:03am UTC](https://boards.straightdope.com/t/four-wheel-drive-vs-all-wheel-drive/742102/62 "2016-01-09T07:03:53Z")
> [@GreasyJack](#): > > Well, the only place I see them claiming that is that one press release, so I’m guessing whatever underling wrote it simply made a mistake there. Ya know, that’s just about the weakest evidence you could provide. Why wouldn’t one set of clutches be able to slip 100% while the other one locks? I’ve been thinking about why Subaru opted to not use a conventional LSD to distribute power from the CVT. It’s because it’d be a waste to put one in there. LSD’s react to a torque differential mechanically, and are pretty crude. Since the CVT is an electronic transmission, it seems like a no-brainer to eliminate the weight and have the clutches being electronically controlled. Also, since the output and input shafts are parallel, and connected via a chain, there doesn’t seem to be a loss from changing the direction of power. I’m not a fan of CVTs in general, but that’s a pretty nifty transmission. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [January 9, 2016, 2:33pm UTC](https://boards.straightdope.com/t/four-wheel-drive-vs-all-wheel-drive/742102/63 "2016-01-09T14:33:26Z")
> [@Stranger\_On\_A\_Train](#): > > In that context, a front mounted winch, where the operator can have eyes on the object being winched, makes more sense. Winches don’t need to be operated from the drivers seat. Few are. There is a wired control that plugs in to the winch that has the controls. Anyway, you want to be out of the vehicle so that you can closely watch the progress (or lack of). [Warn winch controls](http://www.amazon.com/s/?ie=UTF8&keywords=warn+winch+control&tag=mh0b-20&index=aps&hvadid=3480761113&hvqmt=e&hvbmt=be&hvdev=c&ref=pd_sl_7hftft1z88_e) The top one looks like mine. ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [January 9, 2016, 10:28pm UTC](https://boards.straightdope.com/t/four-wheel-drive-vs-all-wheel-drive/742102/64 "2016-01-09T22:28:05Z")
> [@scabpicker](#): > > Ya know, that’s just about the weakest evidence you could provide. Why wouldn’t one set of clutches be able to slip 100% while the other one locks? Because it doesn’t have a clutch for the front axle. The most it can do is lock the two axles together, sending 50% to the rear, but under no circumstance can it send more power to the rear than the front. (Notice that on those videos where they put three wheels of the car on rollers and have it drive off, they one wheel with traction is always on the front. Not that the car wouldn’t get off the rollers if it were a rear wheel, but in order to send power to that one rear wheel, it’d also have to send power to the front and so the front wheels would also be spinning.) It might be that the press release writer meant that the system can electronically shift 100% of the available power from axle to axle or something like that. Or maybe they were just mixed up. > [@scabpicker](#): > > I’ve been thinking about why Subaru opted to not use a conventional LSD to distribute power from the CVT. It’s because it’d be a waste to put one in there. LSD’s react to a torque differential mechanically, and are pretty crude. Since the CVT is an electronic transmission, it seems like a no-brainer to eliminate the weight and have the clutches being electronically controlled. Also, since the output and input shafts are parallel, and connected via a chain, there doesn’t seem to be a loss from changing the direction of power. I’m not a fan of CVTs in general, but that’s a pretty nifty transmission. Subaru has been touting the ability of the system to control power split electronically with the whole “proactive AWD” thing where it monitors the same sensors as the VSC system and tries to predict the need to shift power ahead of time. However, I’m pretty sure the major reason to use it instead of a viscous coupling is that it’s cheap. The manual still uses the VC because the clutch system would need a separate hydraulic system, but on the automatic it can use the same one as the transmission. ---
**Author:** ![scabpicker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scabpicker/32/8268_2.png) [@scabpicker](https://boards.straightdope.com/u/scabpicker)\ **Post date:** [January 10, 2016, 12:11am UTC](https://boards.straightdope.com/t/four-wheel-drive-vs-all-wheel-drive/742102/65 "2016-01-10T00:11:01Z")
> [@GreasyJack](#): > > Because it doesn’t have a clutch for the front axle. Ahh, hadn’t noticed that. I figured it was one set for the front, one for the rear. [Previous page](https://boards.straightdope.com/t/four-wheel-drive-vs-all-wheel-drive/742102.md?page=3)