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4WD vs. AWD. What's the Difference?

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Re: 4WD vs. AWD. What's the Difference?

#31

Post by gambiting »

petepete wrote: post #28
gambiting wrote: post #24

Earlier quoted context omitted.

Any new Land Rover or Range Rover? They come with 2 locking differentials, with 3rd being an option that can be added extra. Same with Mercedes G-class - they come with 3 locks as standard. And I've no idea what you mean by "cars don't have proper ground clearance" - just buy a one that does?

I think he means regular 'cars' rather than 4×4s (or SUVs).

Sorry, I forgot that in US "cars" and "SUVs" are two different things. Where I am from there is no such distinction, a Land Rover is still a "car".

Re: 4WD vs. AWD. What's the Difference?

#32

Post by s917 »

gambiting wrote: post #24
wil421 wrote: post #11

Earlier quoted context omitted.

AWD is meant for adverse conditions on a road and will probably help on a unpaved road. Think rain, ice, snow. It will keep you on the road. 4WD is meant for off road conditions. You get more torque so you'll be able to get over rocks and out of ditches. The downside is you shouldn't use it over 10mph or so. I can't think of any recent car that is 4WD. It doesn't make sense since you'll probably never be "off road".…

Any new Land Rover or Range Rover? They come with 2 locking differentials, with 3rd being an option that can be added extra. Same with Mercedes G-class - they come with 3 locks as standard. And I've no idea what you mean by "cars don't have proper ground clearance" - just buy a one that does?

>Any new Land Rover or Range Rover? They come with 2 locking differentials, with 3rd being an option that can be added extra.

Source? Center locking diffs are more or less standard, rear lockers are options in LR3/4/post 2004 Disco's, Sports and Range Rovers (although standard in some, I think the Supercharged and possibly the SVRs, but I'd need to check) Front lockers are not factory options and I don't ever recall seeing after market lockers for these models advertised.

Defenders, Discos upto 2004, Range Rover classics (and probably P38s) and all series vehicles have the standard after market options for front and rear lockers.

Re: 4WD vs. AWD. What's the Difference?

#33

Post by sunstone »

japhyr wrote: post #3

Here are the key takeaways for me: "Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels." "4WD works by locking the front and rear axles together, splitting torque 50:50 between them. T…

And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. "

So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes more sense.

Re: 4WD vs. AWD. What's the Difference?

#34

Post by rconti »

dsr_ wrote: post #23
ghaff wrote: post #13

Earlier quoted context omitted.

The Waipio Valley road, as I understand it, is mostly paved but extremely steep and narrow/twisty. So the 4x4 requirement is probably more about being able to control speed without burning up brakes and dealing with often wet conditions. However, in the general case throughout most of the US West, "4WD roads" do indeed tend to be as much about having high clearance as about having 4WD per se. It's also just a way of…

And in particular, the Hawaiian rental car counters will tell you when you rent which roads your car is not allowed to go on.

I've always wondered why this matters. I've got car insurance, I've got additional rental coverage on my credit card, and there are plenty of other vendors from whom to rent cars if you want to blacklist me.

Re: 4WD vs. AWD. What's the Difference?

#35

Post by dsfyu404ed »

ebbv wrote: post #19

One thing that this article only touches on briefly is that more and more AWD systems today are just open differentials plus the TCS instead of using more expensive and heavier limited slip differentials. These modern systems are lighter and cheaper but they have disadvantages; not least of which is that using the brakes to force power to move around wastes torque, and it's only going to be as good as the sensors. If…

The brake based system is great if the traction control is programmed for "get me up this snowbank" and not "prevent me from being an idiot"

There was an episode of Dirt Every Day where Fred and co. took a Jeep Renegade(?), or whichever the small one is, off-roading there were some great scenes of the traction control in action. Basically by spinning the dial to rocks the traction control knows to use ABS to approximate how one would work the brakes with a torsen.

Re: 4WD vs. AWD. What's the Difference?

#36

Post by rconti »

ghaff wrote: post #13
blackguardx wrote: post #5

Earlier quoted context omitted.

Generally a sign saying you need 4wd in the US really means you need a lot of ground clearance. Your car probably would have high-centered, that is stuck resting on the middle of the car with the tires in the air. The annoyed ranger was probably just tired of pulling stuck tourists out.

The Waipio Valley road, as I understand it, is mostly paved but extremely steep and narrow/twisty. So the 4x4 requirement is probably more about being able to control speed without burning up brakes and dealing with often wet conditions. However, in the general case throughout most of the US West, "4WD roads" do indeed tend to be as much about having high clearance as about having 4WD per se. It's also just a way of…

In Death Valley they often specify 4x4 high clearance. Of course, you don't REALLY need 4wd or even 2wd; you can tackle it on a motorcycle :)

Re: 4WD vs. AWD. What's the Difference?

#37

Post by rconti »

kinofcain wrote: post #18

There's a lot of blurring and different labeling of 4WD systems based on the market segment. Generally it's more helpful to think of 4WD/AWD as fitting into three categories: 1: A system with a transfer case that predominantly drives one axle (almost always the rear, and almost always labeled "4WD"). This is your typical pickup truck 4wd system. When it is engaged, the two axles are locked together and may as well be…

Right. I've got a Golf R with Haldex; it allows 30+mpg under cruising conditions but can "lock 100%" in the sense that the fronts and rears are locked together. It's created a lot of forum wars because some people read the marketing materials saying "100% of torque to the rear wheels" or "max 50/50 split" and don't understand the difference. If it's fully locked, on dry pavement, of course you're getting 50/50. If the front wheels are on a magic 0 friction surface, of course, all of your torque is going to the rears because the fronts have nothing to grip on, but they're all spinning at the same speed (theoretically).

Re: 4WD vs. AWD. What's the Difference?

#38

Post by Animats »

There are lots of variations on this theme. There are off-road differentials which limit the speed difference between the output shafts to a fixed maximum ratio, usually in the 2:1 to 4:1 range.

It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels. Power distribution between the two during acceleration is mostly equal, but once speed stabilizes, the rear powertrain takes most of the load. Off-road slip handling doesn't seem to be well documented, but Tesla cars aren't intended for off-roading.

The most advanced systems are seen on locomotives. Modern heavy locomotives (GE Evolution series) use three-phase AC synchronous motors driven by IGBT inverters from power from a Diesel generator. Each axle has its own motor. All axles are normally locked together electronically by the control software, so no wheel can slip ahead of the others. Locomotives can be cabled together so that the axles on multiple locomotives synchronize. Now that's all-wheel drive. This is a huge win when starting a heavy freight train. Locomotive axles that are slipping provide little pulling power and damage the tires and rails.

Re: 4WD vs. AWD. What's the Difference?

#39

Post by jsmthrowaway »

japhyr wrote: post #3

Here are the key takeaways for me: "Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels." "4WD works by locking the front and rear axles together, splitting torque 50:50 between them. T…

Locking the center differential in a 4WD vehicle is fine on pavement as long as you don't turn, so forgetting for a small straight of pavement in the middle of the trail is no big deal. Some trails are half dirt road, half long-forgotten pavement, so constantly reaching down can get tiring if one follows that rule verbatim. Snow transitions are similar; your truck would have been fine and I'd just reach down for a dry corner if you can disengage in motion. If not, yeah, bigger concern.

Front axle locker is similar, but with that you feel how difficult it is to steer as soon as you lock it. (In my Wrangler if I'm finding that I need the fronts we are in some serious shit anyway.)

Re: 4WD vs. AWD. What's the Difference?

#40

Post by dsfyu404ed »

The performance of either system is completely dominated by the traction aid at the axles. The article is otherwise good because most AWD vs 4WD stuff completely igores the axles (because it's a heck of a lot to explain if you really want to compare the various systems) but kinda misses the main point.

Open, bad traction control (cuts throttle, applies a little brake), good traction control (all brake no throttle), the many varieties of LSDs, automatic lockers, manual lockers, Lincoln lockers and spools all have a huge affect on traction.

Lincoln lock the diffs of any AWD crossover, screw with some wiring so it's full time and you'll have something that can out wheel a $50k Jeep but trades off a tiny bit of grocery getting capability.

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