Difference between AWD and full time 4x4

[H3 Hummer]

I have some extra wheels and wanted to get spare studded snow tires (for when I go skiing) but I don't want 4 I just want two for the front. The guy at the tire store said, "you should never have just two of the same tire on a all wheel drive vehicle." Is he just trying to get me to buy 4 new tires or is this true? http://www.elcova.com/infopop/emoticons/icon_confused.gif
I know there is a difference between AWD and 4x4 but I really don't know what.

Thanks! http://www.elcova.com/infopop/emoticons/icon_biggrin.gif

[Mike E]

I'm guessing you can cause a problem in the differentials of you have different size tires on the front versus the rear. Someone else may be able to go into more detail but I wouldn't do it. OR i would make sure the outside diameter of the front tires matches the rear so your rotation is the same.

[f5fstop]

Per SAE, and what s used to determine the differences in the auto industry....
AWD: A drive system that applies power to all four wheels, according to a percentage of slip at a wheel.
If the main drive wheels are at the rear on an AWD vehicle, there will be 5-20% of the power to the front wheels, with the remaining power to the main drive wheels in the rear. If slippage is detected at one of the front wheels, more power is applied up to a maximum of usually 50%. The control to supply more power can be mechanical (a set of clutches similar to a limited slip rear end diff), or a speed sensor sends the signal to the controller, that pulses a signal to an actuator to supply more power to the front wheels. The pulsing of the actuator (pulse width modulation) allows the computer to vary the power; for example, a 50% pulse width means 50% more power, 100% pulse width means 100% more power, equaling the 50% to the front wheels.
(Note: There were some AWD systems that supplied no power to the front drive wheels, until slippage was detected at the rear primary drive wheels. When slippage was detected, the computer activated the front diff via an electronic actuator. Not sure if there are any like that anymore...could be.)

4X4: Means the front and rear axles are supplying a set amount of power at all times, regardless of slip. You have partial 4-wheel drive, where the system has to be manually selected for 4-wheel drive (lever or a push button), and you also have automatic 4-wheel drive, where a computer sees slippage and it goes into 4-wheel drive supplying 50/50 power to front/rear.
This auto 4-wheel system is different from an AWD system in that power is only supplied to the front axle when slippage is detected, whereas, in an AWD system, there is always some power to the front wheels.
The H3 is a full-time 4x4 system, but has only 40% power to the front/60% power to the rear. When you push the 4-wheel drive HI LOCK or LO LOCK buttons, you lock the transfer case into 50/50 power to front/rear wheels. (4-Lock LO also changes the gearing in the T/case so you have a lower gear range.)
All above examples were for vehicles with primary rear drive. Vehicles with primary front drive, work the same, but in most cases, the rear drive only receives about 10% power until slippage is detected.

In addition, power to the non-primary drive wheels differs with different systems used, so that is the reason for the large range I supplied.

The guy in the shop is correct. The grip of all wheels should be similar. We know, due to wear, they can't be exact, but they should be close.
Adding a high-grip tire with studs to the rear will cause handling problems with a 4-wheel drive system when engaged in 4-wheel drive, so with a Hummer I personally would not recommend it. If only off-roading it may be ok, but possible highway use could cause problems.

[H3 Hummer]

Thanks so much for the info. It's hard to purchase 4 new wheels and tires for about 5 trips a year. At least I know the truth now and won't worry about just being ripped off.

Thanks again. http://www.elcova.com/infopop/emoticons/icon_wink.gif