Front No Slip Differential ???!

[THEOFILOS2002]

Okay here is the situation:

I was driving home during heavy down pour a few days back. I switched to 4WD Hi for a little extra security. No problem. I got home and as usual I back my AV into the driveway and take a sharp right, cutting the wheel all the way so the AV rear just in front of the garage door. Half way through the 90 degree backwards turn my AV stopped moving freely. I gave it a littl extra gas and I felt the outside tire slip. I got out to see and sure enough the front right tire chewed up the grass just beyond the asphault. I tried moving back towards the driveway exit turning about 45 degrees and again the same thing occurred.

The questions that remains is: Does the AV have a positraction front differential? If so, if this slippage occurrs (or worse doesn't occurr) too many times is there a chance of damage?

Okay, I'll admit that this is my very first 4WD truck, and to some degree I am still learning the characteristics of the AV, 4WD vehicles, and trucks, in general, but this sort of design would seem to be a problem.

Can someone shed some light on my ignorance??

- THEOFILOS2002

 

[ShapeShifter]

4WD high or low should only be used in situations where the wheels can slip: snow, sand, other loose materials. If you try to use these 4WD modes an pavement, you will experiance torque lock up, which is exactly what you had described. This is caused by all four wheels trying to turn at different rates. It is not a lockup diff problem. I may be wrong, but I believe it originates in the transfer case, which is not really a differential like is used in the front/rear axles. It cannot cope with the front and rear axles/drive shafts turning at different speeds.

Yes, if you do this often, it will cause damage.

If you are in this situation again (slippery wet roads) you are better off being in the Auto 4WD position.

-- SS

 

[ShapeShifter]

More information is in this closely related topic: Steering in 4WD

-- SS

 

[simtechray]

The questions that remains is: ?Does the AV have a positraction front differential?

No, the avalanche does not have a posi front end.
The rear is available with an eaton posi, code G80.

 

[THEOFILOS2002]

Hey ShapeShifter,

Thanks very much. Auto 4WD for the heavy down pures it is.

For snow, mud, sand, gravel etc, I'll use 4WD HI/LO.

This now begs another question: Does Auto 4WD mode not engage or provide power to the front wheels until there is slippage in either front wheel? If so, why bother having 2WD at all?

Again, the question comes from never having owned a 4WD vehicle. To date I haven't heard of a generic manual/book that thoroughly explains 4WD HI, 4WD LO, Auto 4WD, Neutral, or Trailer Button (on shift column) for the AV or any other similar vehicle. Do you know of any such documentation? Yeah, I would not mind studying this this to death.

Thanks again!!

- THEOFILOS2002

 

[Chief]

OK, I hope I get this answer right on Auto Trac.

2-Hi: This should be used most of the time. Dry pavement, packed dirt road, trailering, even in light rain. If overall traction is good, this is all you need.

Auto-4WD: Also called Auto Trac. The hubs are locked but the differential is not engaged. If slippage is detected in the rear wheels then the differential engages sending power to the front and rear wheels. This is ideal on most paved situations where slippage might be an issue, heavy rain, snow, patches of ice, or say beach driving or driving on a mountain trail with loose rocks. I would say most Avalanche drivers will use 2-Hi and Auto-4WD 90% plus of the time.

4-Hi: The Avalanche has a PART TIME 4WD system. Continious operation as outlined above will cause damage and premature wear over time. 4-Hi should be used for times of poor traction or where a loss of traction could have catastophic results. Distance driving in standing water, deep snow, ice, mud, deep loose gravel, anywhere where the wheels will almost surely slip in the surface.

4-Lo: First let me say this 4-Lo in the Avalanche is AWESOME. Let me say this second, if you NEED to use 4-Lo, you're probably not where you should be. 4-Lo is good when you are in extreme conditions. Really deep soft sand, mud, snow, variable difficult terrain. Also because of the torque it provides as well as the traction it is ideal for pulling something a short distance when it is very steep, or hill climbing. You should never, ever, ever need to use 4-Lo if pavement of any kind is under your wheels.

Now with all this said you can fall into another trap. If you do mostly urban and suburban driving you may find that you're not using 4WD at all. Every four to six weeks you should make a point of engaging 4-Hi for a couple of miles of STRAIGHT road under 55 MPH. This keeps all the parts lubed, and assures the Auto Trac transfer case is operational.

Well - that's my story and I'm sticking to it!

 

[Crexis]

Well said, Chief!

Traditional "old school" part time transfer cases, when shifted into 4WD send positive torque to the front and back, regardless of whether or not it is needed. 4WD high and 4WD Lo on the Av are exactly that. (Lo just multiplies the gear ratio by 2.73:1 btw).

Now add in Auto-Trac. This nifty invention allows the front and rear drive shafts to turn at different rates without binding, sending torque where it's needed... much like a (posi) differential does left to right, so the auto trac transfer case does front to back.

The usage notes outlined above by the Chief pretty much say it all. 4WD Lo is indeed AWESOME. I've used it a handful to times to get the 5700 lbs beast up some pretty hairy rock steps and such. In delicate situations, better to have the control of more torque on tap at lower speed, it's safer and a little easier on the machine (although chances are whatever you're doing isn't too good for it anyway!! )

 

[SJProwler]

OK, I hope I get this answer right on Auto Trac.

2-Hi: ?This should be used most of the time. ?Dry pavement, packed dirt road, trailering, even in light rain. ?If overall traction is good, this is all you need.

Auto-4WD: ?Also called Auto Trac. ?The hubs are locked but the differential is not engaged. ?If slippage is detected in the rear wheels then the differential engages sending power to the front and rear wheels. ?This is ideal on most paved situations where slippage might be an issue, heavy rain, snow, patches of ice, or say beach driving or driving on a mountain trail with loose rocks. ?I would say most Avalanche drivers will use 2-Hi and Auto-4WD 90% plus of the time.

Good thing to point out Chief, I've known a few people that thought they should leave it in Auto and didn't understand that the front halfshafts were locked in in the Auto mode.

4-Lo: ?First let me say this 4-Lo in the Avalanche is AWESOME. ?Let me say this second, if you NEED to use 4-Lo, you're probably not where you should be. ?4-Lo is good when you are in extreme conditions.

Or anytime that you feel the need to drastically increase the cost of a tow bill.

Luckily I learned that in my Sierra before I got the Av

SJP

 

[MrBill]

Excellent subject. Great job explaining. Just one personal note:

I would say most Avalanche drivers will use 2-Hi and Auto-4WD 90% plus of the time.

Unless you live in Buffalo, NY Then it is:
50% 2 wheel
30% 4 HI
20% 4 Auto

The fun has started Its snowing.

 

[THEOFILOS2002]

Lots of good information in the replies to my initial questions. This is a keeper!

THANKS GUYS!!!

Now to find 3ft of snow at a 45 degree plus angle for my AV to exercise 4WD LO; with my 4WD experience a straight forward shot, no lefts or rigfhts.



I gotta see that feature in action.

- THEOFILOS2002