4WD vs. AWD. What's the Difference? • Hacker News Board

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

#91

Post by mindslight » Sat, Aug 27, 2016, 7:43 PM UTC

dghughes wrote: post #6

>Torque goes to whichever of the four wheels has the least grip. That should be the tire with the most grip. If torque goes to the wheel where the tire on it has the least grip the tire still has no traction torque doesn't make grip appear out of nowhere. In four-wheel or all-wheel drive one wheel where a tire has no grip the other three wheel with tire that have grip move the vehicle isn't that the entire point of 4…

This article is careless with physical terms. What it means to say here is that rotation (or energy) goes to whichever of the four wheels has the least grip. The torque of all the wheels is basically equal [0]. The ones that are gripping are creating linear force, but not enough to move the car. The wheel that is spinning freely has its torque going into dynamic friction and rotational acceleration. When that wheel has accelerated to as fast as possible (which happens awfully quickly because a wheel isn't that massive), the engine's rev limiter kicks in and cuts the total torque down to 4x the dynamic friction.

[0] Assuming ideal naive differentials.

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

#92

Post by gambiting » Sat, Aug 27, 2016, 7:44 PM UTC

wil421 wrote: post #89

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 don't think you understand what I mean. When I say car it doesn't include SUVs/trucks. The difference used to be cars (unibody) vs SUVs/trucks (body on frame). A lot of SUVs are now unibody including my Grand Cherokee. There are many types of SUVs and trucks with 4wd in the US. I guess in Europe you only have Land Rovers or G-wagons. I don't think GLs or MLs are common in EU but you can get beefed up off road packa…

As I explained in my other comment - I forgot that in US, "car" and "suv" are two different things. You could have a Ford F150 or a Chevy Escalade here and it would still be a "car" , just like a car without roof is still a car but of a subtype "cabriolet".

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

#93

Post by jcbeard » Sat, Aug 27, 2016, 7:45 PM UTC

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…

This isn't always true either. 4WD can mean that you send equal amounts of power to each axle, but at that point there are things that can change the amount of power going to each wheel within the center differential casing. If you have a limited slip differential (why some 4WD vehicles can drive perfectly fine on pavement) it'll basically send a 50/50 split (lock the axles together when going straight) but allow the axles to turn at different rates when making turns (otherwise bad things would happen). Some cheaper 4WD vehicles (and many 2WD vehicles) have open differentials. Open differentials send the power to the path of least resistance (bad for traction, and also why if you're in gravel the loose wheel usually just spins while the other one doesn't). There's also a fully locking differential that literally locks the axles no matter what. These cannot be used on pavement for obvious reasons (well, think about turning a corner, the outside wheel will have to turn faster than the inside wheel). AWD vehicles have a center diff typically and also often employ limited slip differentials (but sometimes again, they have open and/or auto-locking diffs...ARB made air locking diffs at one point, not sure if they still do) at each axle as well which enables a mechanical distribution of power. Some newer vehicles have replaced the axle differential gears with anti-lock breaking systems that provide resistance to open diffs to regulate wheel spin (artificially add resistance to the spinning wheel).....sorry, before I became a computer geek I built motors and off-roaded in my 81 CJ-5, after that I drag raced for fun on the weekends at the local track. I had a limited slip in the mustang I raced and a fully locking one in my Jeep. Jeeps (and older 4WD) are interesting b/c many come with what are called locking hubs ( https://en.wikipedia.org/wiki/Locking_hubs ) which are not all that common today. Even if you do find them, they're usually auto-engaging vs. the ones you have to get out an manually turn to engage the front drive shaft.

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

#94

Post by Doctor_Fegg » Sat, Aug 27, 2016, 7:46 PM UTC

anexprogrammer wrote: post #27

tunap wrote: post #15

Earlier quoted context omitted.

Normally I would 2nd this, but it does not apply on this particular 4x4 road. It really sucks when opposing traffic appears as it's barely wide enough for 1 vehicle in most parts: The road gains 800 vertical feet (243.84 m) in 0.6 miles (0.9 km) at a 25% average grade, with steeper grades in sections. This is a paved public road but it is open only to 4 wheel drive vehicles. It is the steepest road of its length in t…

That doesn't seem that bad for normal traffic, conditions permitting - unless the steeper gradients are markedly so. I'm sure they have their reasons for 4wd only though - picking too many tourists out of the ditches or whatever. There's quite a few roads in the UK with 1:4 gradient, and one just down the road from me at 1:3 whose only restriction is no trucks and vans. Going downhill on the motorbike makes it feel l…

There are roads in Spain with 1:4 gradient, too. 200 of the world's best cyclists rode up some of them in today's stage of the Vuelta a Espana. I'm no slouch cycling up hills but my jaw dropped seeing today's footage...

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

#95

Post by giardini » Sat, Aug 27, 2016, 7:48 PM UTC

landryraccoon wrote: post #72

giardini wrote: post #68

4WD/AWD are almost never necessary in USA and Europe today, to the extent that, if you buy 4WD/AWD you're likely suboptimizing and even taking a risk (for your drivers who aren't familiar with the quirks of 4WD/AWD). As Saab has shown for decades, FWD will get you almost almost anywhere a 4WD/AWD will in bad weather. But sales of 4WD and AWD cars have been a boondoggle for the auto manufacturers. Cost, complexity and…

In California you are legally required to either have chains equipped or a 4WD vehicle with snow tires under R1 and R2 snow conditions: http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm It doesn't matter what the studies say, 4WD is legally recognized as safer than 2WD in California.

AFAICT you're required to have the chains available, so they're usually in the trunk, unless you're in a situation where they're required. Of course if you buy chains you should immediately put them on your vehicle, if only to ensure that later, when you need them, they will fit. Once you've done that, then you can put them in the trunk.

I disagree with your interpretation of the law: dot.ca.gov simply state what they require and say nothing about which is superior. And note that 4WD/FWD vehicles _also_ must carry chains in the R2 & R3 areas (from the URL you posted http://www.dot.ca.gov/hq/roadinfo/chcontrl.htm):

"(NOTE: Four wheel/all wheel drive vehicles must carry traction devices in chain control areas.)"

Unfortunately for an AWD/4WD vehicle, following this provision costs twice as much as for a FWD car and will take twice the time to install (not an insignificant factor in freezing rain) and, should you fail to install on both front and rear properly, or should they suffer mechanical failure, may put one in a bad situation.

Sometimes it's better to do less.

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

#96

Post by s917 » Sat, Aug 27, 2016, 7:51 PM UTC

gambiting wrote: post #54

s917 wrote: post #32

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. 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…

I had a 2005 Discovery LR3 V8(US Spec, imported to EU) and I could swear it had 3 diffs - at least the 4x4 Info screen would always show 3 diffs working independently when I was driving it offroad. Of course I might be remembering it incorrectly, but a quick search on google shows "genuine Land Rover Discovey 3 front differential" part on ebay, so I guess it must exist? http://www.ebay.co.uk/itm/like/182246825443?lpi…

They have differentials on the front, but they're not "locking" differentials, they're "open" differentials, no locking capability at all.

Center diffs are for the most part (in Land Rovers) locking differentials, though some Discovery IIs had open center diffs and things like Freelanders use a viscous coupling in place of a mechanical diff lock.

Newer models have an electronically controlled locking diff in the gearbox (center) and an optional electronically controlled locking differential at the rear.

But front diffs on modern Land Rovers are open.

Edit: Here is an image showing the locking diffs on an LR3/Discovery 3: http://www.landroversonly.com/forums/attachments/f41/37257d1...

Note the center diff is unlocked (or open) and the rear is locked, there is no option to lock the front, which will always be open.

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

#97

Post by alanning » Sat, Aug 27, 2016, 7:52 PM UTC

tombrossman wrote: post #48

sunstone wrote: post #33

Earlier quoted context omitted.

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 m…

That's right. A poorly-engineered 4WD system means you are stuck on a hill because one wheel is on slippery wet leaves. AWD should mean you can get up that hill with three wheels on slippery wet leaves, and one with grip. That's the simplest explanation. I had an old Toyota Tercel 4WD station wagon and it was very difficult to steer sharply at low speed with 4WD engaged because of the lack of a differential. It would…

I loved my old Tercel wagon. Didn't use that low gear all that often but fun to have. CV joints were always my problem, ended up having to replace them quite often.

One time in college I was turning too sharply while parking and the axle actually popped out of the right front. Loose axle would just spin in 2WD but switching to 4WD enabled me to make it to the nearby Mech E. building and get help. :)

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

#98

Post by jlarocco » Sat, Aug 27, 2016, 7:52 PM UTC

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…

Wow, that video does a really great job building up the explanation. I already knew how a differential worked (thanks to Legos), but I had never thought about it that way before.

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

#99

Post by brbsix » Sat, Aug 27, 2016, 7:53 PM UTC

mindslight wrote: post #79

giardini wrote: post #68

4WD/AWD are almost never necessary in USA and Europe today, to the extent that, if you buy 4WD/AWD you're likely suboptimizing and even taking a risk (for your drivers who aren't familiar with the quirks of 4WD/AWD). As Saab has shown for decades, FWD will get you almost almost anywhere a 4WD/AWD will in bad weather. But sales of 4WD and AWD cars have been a boondoggle for the auto manufacturers. Cost, complexity and…

FTA: > That’s why AWD ... helps you safely navigate both inclement weather This is certainly how AWD/4WD etc is marketed to consumers, but it's worth remembering that every car has four wheel stop . Better acceleration really just helps you hit that snow bank first . (Also this article loved to misuse the word "torque" when they really meant rotation or energy. Sigh.)

It's not just "better acceleration", it's more consistent and predictable acceleration. I find it makes driving backroads on a dark stormy night quite a pleasant experience.

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

#100

Post by justinph » Sat, Aug 27, 2016, 7:53 PM UTC

In real life, in most situations, ground clearance, good tires, traction control and ABS make more difference than driving all four wheels, most of the time. Personally, I'm more concerned with stopping than starting, and that's where ABS and good tires matter and what kind of transmission you have doesn't.

This is a sales pitch video, but it is interesting to illustrate the differences in AWD approaches, with different vehicles on rollers: https://www.youtube.com/watch?v=9cuZYTQLfA0

I spent a lot of time looking at this several years ago when my family was planning on taking a vacation to the Outer Banks of North Carolina where you have to drive on the sand. We ended up renting a Ford Explorer that had a "sand mode" in it's electronically controlled awd setup. Worked great.