Car Talk

January 26, 2004 1:26 PM

Car talk: Why is it, when either of our two cars are in 4WD, that the wheels feel so wobbly when making a tight turn? It's unnerving, feels like the wheels are going to fall off. It goes away when the case is switched back to 2WD.

BTW, the tires are inflated properly. Everything feels fine in situations other than making a tight turn (at any speed).

posted by stupidsexyFlanders to Travel & Transportation (8 answers total)

I'm not very knowledgeable about part-time 4WD systems, but it seems to me that this could be a consequence of locking differentials. Going around a turn, the inside and outside wheels have to turn at different rates of speed (the outside arc is wider, and thus the outside wheel has to rotate more than the inside wheel does for its shorter distance). This is normally no problem, as the drivetrain allows the wheels to move independently of each other.

When you engage the 4WD system, however, the differentials lock, so that equal power is sent to all wheels all the time, and if one wheel spins, the others are mechanically forced to move with it. This makes traction excellent in slow snowy situations, or when climbing rocks, or whatever, but is also awkward for actual driving. Because everything is locked together, wheels cannot rotate independently on a turn and get dragged for the ride.

Full time 4WD systems, and AWD cars have more sophisticated systems that allow the car to be driven normally, but if I remember right, most cars that allow you to drive in 2WD should not be driven normally in 4WD. Usually you're only supposed to use it when necessary, for the reasons above, and not above slow speeds.

Disclaimer: The above is spewed from memory and conjecture, and may be completely incorrect, both as to 4WD mechanics and as to the cause of your problem.
posted by tirade at 1:43 PM on January 26, 2004

If you truly have 4WD which you shift into and out of, what you're experiencing is the nature of the beast: The front and rear tires on your vehicle are turning at the same time and same speed. The effect of this is when you turn the steering wheel on dry pavement your rear wheels are actually moving faster than the front, therefore the wobble. 4WD systems are more common on trucks and older jeeps. The manufacturers usually warn against engaging 4wd on dry pavement.

On the other hand, if you're vehicle is AWD (all wheel drive), all 4 wheels are only powered when the system detects slippage. On dry pavement only 2 wheels are powered. This system is found on Subarus and later model SUVs.

On preview, what Tirade said.
posted by SteveInMaine at 1:52 PM on January 26, 2004

When making a turn, all four wheels trace different arcs with different arclengths. The differential ensures that the tires rotate at different speeds, corresponding to the different arclengths.

Suppose you are using front wheel drive. The left front wheel may want to go at 98 rpm while the left right wants to go at 102 rpm. Let's simplify and say that the drive shaft will then turn at the average of 100rpm.

Now you switch into 4WD mode. The left front wheel wants to go 98 rpm, the left right is at 102 rpm, the left rear is at 95 rpm, and the right rear is at 98. Now there's a problem, because the front wheels need a drive shaft turning at 100 rpm, but the rear wheels need the shaft turning at 96.5 rpm. The car can't do both (unless it's a *full-time* 4WD vehicle), so one set of wheels ends up turning at the wrong rate.

If you're on snow or dirt, you probably won't notice a big problem since the tires can skid over the surface relatively easily. On pavement you have a lot more friction, so the car will vibrate/wobble/jump around.

It's probably not real good for the drivetrain to be using 4WD on dry pavement.
posted by Galvatron at 1:54 PM on January 26, 2004

To reinforce what tirade's saying: You should not use locking differential four-wheel drive on dry pavement. One of your wheels pretty much has to just "scrub". Or, what everybody else said.
posted by LionIndex at 1:54 PM on January 26, 2004

You've got several different answers here. Galvatron's is correct. Full-time 4WD is also often called all-wheel drive, the only difference is that full-time 4WD is usually used for trucks, and AWD for cars.

Part-time 4WD has a simple clutch (transfer case) that splits the power front/rear. AWD has a center differential to split the power, so the front wheels and back wheels can turn at different rates.
posted by adamrice at 6:53 PM on January 26, 2004

An oldie but still relevant is Eliot Lim's All-wheel drive FAQ, which has commentary about the differences between AWD, 4WD, etc.
posted by gen at 10:40 PM on January 26, 2004

Hang on... what everyone has said here implies that 4*4s don't have limited slip diffs in them. Why the hell not?
posted by twine42 at 4:24 AM on January 27, 2004

twine42, it might just be old technology that's slowly being phased out. I mean, to shift my own relatively old 4x4 into 4WD, I have to get out of the car and lock the hubs on my front wheels. Then I can get back in and mess with the gearshift.
posted by LionIndex at 7:27 AM on January 27, 2004