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

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

#191

Post by markdown »

beachy wrote: post #121
anexprogrammer wrote: post #27

Earlier quoted context omitted.

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…

Here in New Zealand we lay claim to the steepest street in the world at 1:2.86 gradient https://en.wikipedia.org/wiki/Baldwin_Street . There's an annual charity event that involves the rolling of over 30,000 Jaffas (spherical confectionery-coated chocolate confectionery) down the hill.

How have I not heard of this event? Sounds similar to (and less dangerous than) cheese rolling.

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

#192

Post by markdown »

sunstone wrote: post #33
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 m…

In the famous Saudi videos (there are hundreds of them), the wheels that lose traction completely stop. See https://www.youtube.com/watch?v=MQm5BnhTBEQ

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

#193

Post by mtreis86 »

This article is good but goes overboard in explanation. AWD and 4wd by all accounts are just buzz words for having front and rear differentials. In actual use, a 4x4 (a term I mean to refer to both 4x4s and awds as its just a buzz word) has three differentials. A center diff splits power from the transmission out to the other diffs, which split power to the wheels. There are three general types of diffs, whether centered or front or back:

Open (just gears, sends power the the least resistant path)

Locked (a spindle connects the input directly to the output and all three spin proportionally)

Posi (multiple ways of actuating this, can be viscous clutch, electric or air powered actuation, or mecanical geared torsion style - regardless the point is to keep power at all wheels, some with variable speeds allowing for turns - others act like a spindle)

Most "awd" cars have a open gear in all three diffs. My jeep has a spindle in the front and center and an air locker in the rear (not the stock design). My old BMW 325ix has a viscous clutch in the center and in the back, and open in the front. My dad's audi has open gears all around and uses the brakes to actuate were the power is delivered. At the end of the day tho, its three differentials.

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

#194

Post by joushou »

This definition is broken.

You have 4WD if you have 4 driven wheels (and unless you count the spare, that's all my wheels). Then, you can have a multitude of differential configurations on the 3 differentials (open, lsd, locker, diff-less viscous clutch, diff-less locker).

A Jeep WK uses a locking differential center, and electric engaging LSD's front and rear. By that article, the car is AWD on road, and sorta 4WD when off. The Jeep WJ is either controlled by a viscous clutch (Quadra-Drive), or has manually selectable 2WD, 4WD open ("AWD") and 4WD locked ("4WD") modes. A 98 Ford Explorer pulses an electric clutch to go between 2WD and 4WD locked. These cars all behave very differently both on and off road, and they don't fit well in the authors categories.

If you're buying or owning a car that drives all wheels, the only useful information is the diffs. Labels like "4x4", "4WD", "AWD", "Quadra-Trac", and "Quattro" mean nothing.

Most new 4WD's use open diffs with electrically controlled lockers or LSD's, as that gives the best combined on/offroad experience. Permanent LSD's/viscous clutches (Jeep WJ, for example) is a bit more responsive in surprise low-traction conditions, but it eats your tires when you make tight u-turns, as the turn will progressively engage a complete diff lock. Center lock without diff works like a regular 2WD until you lock, but remember that if you have ANY type of locker on ANY axle (not LSD), then engaging in high traction conditions might snap your axels in a very loud and explosive manner, even if you think you're driving straight.

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

#195

Post by madengr »

icantdrive55 wrote: post #159
alanning wrote: post #97

Earlier quoted context omitted.

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. :)

I gave a 89 Toyota that kinda took care of, with 240,000 miles. She still has 125 psi in the cylinders. Changed the oil when I felt like it. Changed the oil filter only sometimes. Never changed the air filter. Never repaired a tapping valve. In other words, didn't take care of her like other vechicles I owned. One day, I think, even 80's and 90's Toyotas will be collectors items. I know some of them are collectors ve…

I had an 87 pickup and 87 tercel. The engines will last forever, but rust is the killer on the truck. Currently have a 2005 Tacoma and it is rust free (plastic wheel wells) but even it has a recall for rusty suspension.

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

#196

Post by chiph »

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 Honda VTM-4 system is quite good in it's intended operating environment - which is operation during winter weather. It uses electromagnetic clutches in the rear differential housing to direct torque to the rear wheels with left/right independence. This gives you the effect of a locking differential, up to the friction limit of the clutch material. There's no center differential in the system, and the front differential is open (but Honda includes a TCS, which is cheap because once you have the ABS sensors & pump, it's just software).

Even during good weather on dry roads, the system has benefits by engaging the clutches when accelerating from a stop, so all four wheels are driven when you need to get going. The clutches are disengaged after about 20 mph so you get the fuel economy of a FWD vehicle.

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

#197

Post by joushou »

avn2109 wrote: post #182
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…

>> "...sends power away from the slipping wheel(s)..." Exactly this. The best ones are entirely mechanical, without any fancy computers or even viscous couplings. Audi sold (sells?) IMHO the most sophisticated AWD system on the market, which uses a system of crossed helical gears to continuously and instantaneously redistribute torque proportionally to wheel traction, up to a roughly 80-20 split. There are no compute…

Audi Quattro is great, although IIRC, it also eats a bit tires on tight turns like anything but an open diff would. I also like the viscous clutch on the jeep, although it definitely is less on-road friendly (A tight 90 degree turn is enough to hear the tires, 180 and you can almost feel the car get lower). My worst system was my 98 explorer - it engages the clutch completely when front axle falls behind, waits a second, disengages to check if axle is still behind, if so, reengages... It's jumpy as hell when turning, and sucked no matter what. You could mod it for pure 2WD operation by switch, but stock was awful.

However, I don't think you should underestimate electric systems. I mean, I hate car software with a passion, but when they work, they actually have really good characteristics. Things like LSD's where the pump is electrically enabled give quite smooth transitions, and can simulate the response of both on and offroad mechanic systems. Response time is also nearly instant. If car computers weren't bloody black boxes (or at least leave only important things to the black box!), these systems often end up being much simpler than their purely mechanic black voodoo counterparts.

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

#199

Post by mcguire »

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…

You know what really sucks? Discovering that you have food poisoning while you are out and about, and pulling off to the side of the road to throw up. Then realizing you pulled a little too far off onto wet grass and mud, and need the kind property owner's help to push your truck back onto the road.

Or going to the SO's cousin's for a family thing, parking in the yard like everyone else, getting rained on, and needing their tractor to pull you out.

Now, I'm fine with my 2WD truck. But if I had to pull those kind of antics more than once a year, I would seriously consider 4WD.

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

#200

Post by sctb »

zensavona wrote: post #178
taneq wrote: post #155

Earlier quoted context omitted.

Everyone's focusing on the technology involved, but that's kind of pointless because AWD isn't a technical term. It's a term to distinguish an on-road car with four driven wheels from an all-terrain car, because when the first WRXes and such came out, people saw 4WD and thought that meant they could be used to go bush bashing.

Take her for a bush bash m8

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