Any downsides to 4-wheel drive?

Factual Questions
1

A friend of mine is looking ot buy a new car and is committed to a 4-wheel drive car. Mind you, she will never, ever drive the thing off road…strictly paved road driving and mostly highway and city streets at that.

I’m not saying there is anything wrong with 4-wheel drive but I think that requirement of hers limits her choices unnecessarily and I don’t think she will encounter times when 4-wheel drive will be particularly useful to her very often. The odd snowstorm sufficient to halt most traffic where city crews can’t plow fast enough are fairly few and far between in Chicago. In the last 15 years living in Chicago I haven’t had a snowstorm stop me driving yet. There were two or three times where 4-wheel drive would have been clearly beneficial but I don’t think that is enough to ONLY look at 4-wheel drive cars around here.

As with most things there are usually tradeoffs you must get for your benefits and I see no reason why this should be any different.

Also, if anyone knows about Audi’s Quattro system of 4-wheel drive (or all-wheel drive?..is there a difference?) please speak to that as well. I’ve heard amazing things about Quattro and wonder if the truth lives up to the hype.

2

all wheel drive generally refers to a full time system that is more dynamic and spreads power to wheels as they slip.

4 wheel drive usually refers to static setups where x power is delivered to y wheel all the time

USUALLY.

If a full time all wheel drive, like Subarua and Audi (others make them too), then they are usually not an issue of snow traction or ice traction, but an issue of improving the balance and handling of the car in all weather conditions.

Draw backs? the car is USUALLY heavier with an AWD set up, and the repairs should it fail after warranty need to be considered. Drawbacks are minimal. Advantages to an Audi and Subaru set up are phenominal in all weather conditions, including rain and dry roads.

A truck style 4WD set up where the axles are locked in 4wd mode IS NOT for everyday driving. Having the axles locked on the most basic 4wd setups is a huge compromise in handling.

3

Well, for one thing I believe that all 4WD or AWD have lower gas mileage than comparable 2WD models and the added mechanical complications could mean high repair bills (if the system ever has a problem). How’s that for a start?

4

Warren Brown: Hello, Southington:
Yes, AWD mans different things at different companies. For example, some automakers try to pawn off AWD as the equivalent of four-wheel-drive, which it isn’t. The basic difference there is that 4WD means power simultaneously flows to all wheels, that there is a differential case and usually a four-wheel-low gear.
AWD essentially means that all four wheels are capable of receiving drive power, albeit not at the same time and in the same amounts. AWD means wheels share power on an as-needed basis, power shifting from slipping to gripping wheels, for example.
In that regard, the best AWD systems I’ve tested are in Subaru, Audi/Volkswagen and, yes, some Toyota vehicles.
Make sure that your dealer explains in detail the kind of system you’re getting. Hope this helps.
from Real Wheels, Washington Post
http://www.washingtonpost.com/wp-srv/liveonline/02/regular/business/r_business_brown101602.htm

5

Audi setup: let me say that AWD Audis were banned from some racing circuits because they handled too well versus non-AWD setups. And most races won by Audi in Quattro mode was on dry pavement.

Sure, your friend won’t be testing the handling limits, but AWD receives praise over criticism, almost universally.

6

The big difference (and it is a big difference) between “all wheel drive” and “four wheel drive” is the differentials. The differential is the part of the car that allows the wheels on the same axle to turn at different speeds. It’s purpose is to allow the outside wheel to rortate faster than the inside wheel during turns, thus avoiding undue wear, skipping, squeeling, and increasing handling quality. In a standard four wheel drive, if the left front tire comes off the ground as you go over a log, all the power to that axle will go to left tire and it will be spinning madly and uselessly. The right front tire is effectively robbed of all it’s power. You can get stuck this way. Additionally, in a four wheel drive you have to be very careful changing surfaces. When you travel from dirt to pavement, you can really screw up the drivetrain if you give it to much gas. When the front wheels start gripping the pavement, and the back wheels are on something slippery, undo stress can be put on the drivetrain if you givwe it to much power. In all wheel drive, both these drawbacks are eliminated. Don’t ask me to explain how, but the center differential knows when to transfer power to the rear or front differentials, and when not to.
The power (or more correctly, torque) of an all wheel drive “goes to the wheel that needs it”. That’s not necesarily true in a 4 wheel drive.

As far as drawbacks, the only one I can think of is fuel efficiency, which tends to be worse in four wheel or all wheel drive cars than it is in standard two wheel drive. However, I don’t think gas mileage for Audi’s is too bad, so that may not be that much of a concern.

7

quote from Beeblebrox: "In a standard four wheel drive, if the left front tire comes off the ground as you go over a log, all the power to that axle will go to left tire "

Not true. The power will go to the axle, split between the wheels. The left front tire will spin, but only if the right front tire can spin. In standard 4wd, the diffs are locked and if you see a wheel spinning, then a wheel is spinning somewhere else…and that tire could have grip…or it could not.

Actually, a commom criticism of many AWD systems is that there is no option to LOCK the differential(s), and in extreme traction situations, 4wd is superior to AWD. Enthusiasts who are into off-roading know that 4wd w/ a locked diff is superior than AWD, if the AWD set up won’t let you lock the diffs.

A least one AWD system lets you, but it’s rare that an AWD system lets you lock the diff. I believe Hummers do the AWD thing AND let you lock the diff. for extreme conditions.

8

There aren’t any hard-and-fast rules about these names, but here are the general ways the names are used:

4-wheel drive: There’s a lever to turn the system on and off; this is a part-time system. As far as going off road or getting un-stuck from the snow, it’s the best type of system. But if you switch it on while driving on clean, dry pavement, the car handles badly, and the drivetrain components take on LOTS of stress----and you don’t use 4-wheel drive at highway speeds. You’d just turn on 4-wheel drive to get up a steep driveway or an unplowed side street.

All-wheel-drive: The driver has little or no control over the system; the car “decides” when and why to put power to all four wheels. This is still helpful in the snow, but it can also make the car handle better in any weather, as Philster mentions.

What are the downsides?

Poorer fuel economy—the systems not only add weight to the car, but the engine has to expend more energy rotating all these extra parts.

Maintenance/repairs: There’s obviously more stuff to break. Also, some 4-wheel drive cars call for changing the front differential fluid every so often.

Ride quality: Unless the car’s a very expensive one, compromises have to be made: on some trucks, to get 4-wheel drive, you have to trade the sophisitcated independent front suspention for a more old-fashioned solide axle.

Purchase expense: I suppose you already know this, but the system is obviously going to increase the price of the car. This may or many not come back to you at resale time; check the book value of some used cars of the same type.

I don’t mean this to insult your friend, but if she’s getting stuck in the snow with her current car, she might just need some education. I love it when I stop to “help” people get their car unstuck…they’re usually expecting me to push their car, or shovel snow out or whatever…then instead I just get inside and drive the car right out of the same spot they couldn’t.

Make sure she knows about all the various concepts of getting a car unstuck: Rocking the car back and forth, turning the steering wheel versus keeping it straight, rear-wheel drive versus front-wheel drive, and so on. A couple hours’ worth of tutoring, along with a bag of kitty litter and a folding shovel, can go a long way.

9

No, no, no. The power takes the “easiest path” which is the wheel not touching the ground. I’ve found a cite to back me up;

See? The left tires spin uselessly, while the right tires don’t budge, which is exactly what I said

10

This is just annecdotal, and I don’t know much about cars, or if this would even be considered a drawback for everyone. In high school I would go to a parking lot with a friend and do donuts when it snowed. He claimed that it wasn’t as easy to donuts with the 4-wheel drive on. He would have 4-wheel drive on when on the streets, but when doing donuts he turned it off.

11

That cite talks about everything BEFORE the diff is locked. The heart and soul of 4wd in extreme conditions is locking the diff. The cite continues on to explain how things change when the diff is locked.

12

I am considering making my next car an Audi A6 Quattro. I have looked into the technology, (i.e. read the third party reviews and what technical data I could get my hands on), to sum up:
[list=1]
[li]AWD Rocks[/li][li]Audi’s Quattro AWD Rocks harder.[/li][li]The Germans know a few things about bad weather.[/li][li]They also know a few things about craftsmanship.[/li][/list=1]

AWD is the system by which, under normal driving conditions, the least number of tires necessary actually drive the vehicle.
Audi’s Quattro Site
Animation of how it works.

Even more impressive than Audi’s AWD system is that used by Porsche on the new Turbo 911, and the Mega-system on the Original Hummer which can transfer ALL engine power to any wheel. Which means that it can climb it’s maximum slope with three wheels on roller bearings. In fact they have a video of this on their website.

13

Beeble:

With the diff locked, all four wheels must travel at the same speed, meaning two lefties can slip and turn on ice while the two righties sit there motionless. This very same feature makes turning on dry pavement hard on the car/truck components and terrible to drive with locked diffs.

.

14

correcting my typo: With the diff locked, all four wheels must travel at the same speed, meaning two lefties CAN’T can slip and turn on ice while the two righties sit there motionless.

locked diff = all wheels rotate together, come hell or high water. Either that or the drive train snaps.

15

Yes, yes. But you still have a front differential that is not locked. which will send all the power of the front axle to the left, thus cutting your available torque by half, which may not be enough to get you over the hypothetical log. My original quote was:

I never said anything about the rear wheels.

Additionally, since only the center differential is locked, your statement, “The power will go to the axle, split between the wheels. The left front tire will spin, but only if the right front tire can spin” is simply untrue. The right front is on the same unlocked diffential as the left front.

There are a few specialty “hill climbers” which will allow you to lock all three differentials, but you are not going to find one of those stock.

16

Whoops, my mistake. Diffential locks on the axles are a good deal more common than I thought, and come standard on many 4WD vehicles. It’s not a good idea to drive around with all of them locked, however, as all four wheels need to rotate at different speeds. The only time to lock 'em all is when you have to get through something really hairy.

So I guess Phil and I are both right, depending on which differentials are locked.

17

A HUGE problem with full-time 4WD/AWD…whose axles cannot be disengaged in neutral…is towing: you MUST have the vehicle towed on a flat-bed; rolling the vehicle one one axle with the other axle’s wheels locked in the tow-sling will damage the center diff in the transfer case.

18

Can I make a slight hijack? My father has an 86 Isuzu Trooper II with selectable 4WD. Now, the system isn’t so old that you have to go out and lock/unlock the hubs manually; you can stop, shift it into either 4WD-High or 4WD-Low and keep going, and then stop, shift out, and then go backwards a few feet to unlock the hubs. On a system like this, is it a bad thing to drive in 4WD at high speed, compared to a full-time 4WD?

This, and the fact that most people with 4WD don’t understand that it doesn’t work on ice, came to my mind as the possible downsides.

19

Don’t forget the added weight means increased stopping distances.

I thought the differential (if it’s not locked) sent the power to the wheel with the least resistance. That’s why one powered wheel in sand can stop a regular car- it has the least resistance so it gets all the power.

PC

20

I think the disadvantages have been well summarized in this thread. Compared to the same vehicle with 2WD, there’ll be greater initial expense, insurance expense, maintenance, weight and gas consumption. For this you get improved traction in slippery conditions and, with a sophisticated system like Audi’s, better performance during hard handling in all conditions.

We hear that something like 30% of all vehicles sold with selectable 4WD are never placed in 4WD during the vehicle’s entire life. Perhaps the owners derived some status or peace-of-mind benefits, but the price was high.

I’ve lived in New England for a long time, and have found that 4WD is really needed only on unpaved roads. I’ve never been stuck with 2WD, and have passed uncounted numbers of 4WD vehicles that were.

It’s notorious in New England that in bad weather 4WD vehicles have problems at a rate higher than their market share. The reason isn’t that they are inferior, but rather that driving skill and judgment are still important, and too many 4WD owners buy their vehicles to compensate for shortages in these areas.

The OP’s friend sounds like someone for whom a little driving education would be far more useful than 4WD.