AWD in Sand

[Denali02]

Has anyone ever taken their AWD (not 4WD) GM truck off road in soft sand?  From what I read, AWD gets confused in soft sand and consequently goes nowhere.  Can anyone confirm or deny this or tell of other traction loss on soft terrain with AWD?  For example, gravel or mud?

[Stepside Z]

I cannot help you with your question but I do want to welcome you to the forum.

[TxDoc]

You basically have no lo range capability and a 38/62 torque split--no big deal since you are not running 44-inch mudders. You should be fine where GC is not a problem and you get high centered and have the correct tire for the terrain you will be in.  Do you have the G80 locker or does the C3 have a limited slip?

 

Some info from various sources:

"The beach was much the same story where we briefly drove the C3 right to the edge of the Pacific Ocean . Entering the beach we immediately executed a tight 180 degree turn with virtually no fish-tailing to speak of (no pun intended). Driving further down the beach we also tried slaloming the C3 around some piles of driftwood and seaweed. Again there was no oversteer on the sand beyond what would have been expected if we were on dry pavement with a two wheel drive truck"

 

 

"comes with a more sophisticated full-time all-wheel-drive system from supplier New Venture Gear that uses a planetary center differential set for a 38/62 front/rear torque split; a silicone viscous coupling unit progressively locks up if one axle or the other starts to slip. This is all contained in a cast-magnesium housing that saves 15 pounds compared to Autotrac. Aluminum front and rear prop shafts save even more weight and minimize vibration"

 

"C3 uses all-wheel drive (AWD) instead of four-wheel drive (4x4), the only pickup to do so. And AWD is standard. AWD drives all four wheels all the time and shifts power among them as traction dictates. Four-wheel drive, in contrast, drives two wheels (the back ones on a pickup) and transfers power to the other two only when needed. AWD typically is smoother than 4x4 and provides better dry-road stability.

 

Audi, BMW and Porsche have AWD systems. Using one on C3 makes it "international class," Kornas says.

 

AWD has no low-range gears, as conventional 4x4 does, nor does the driver have any control over how the power is being apportioned. There are no levers, buttons or switches.

 

No problem, Kornas says. "Not very many people are going to be in a situation where they need a low range," to slog through sand or climb over downed limbs. C3 "isn't intended to be an off-road vehicle." Nor do drivers often lock a 4x4 system into four-wheel drive instead of letting it work automatically, he says."

 

"Let's discuss the advantages of full time all wheel drive. GMC makes the claim that the Sierra C3 has the best on-road, wet or dry pavement handling performance ever engineered into a full size pickup. In theory its quite easy to agree with this statement.

 

Manually engaged four wheel drive trucks lack a component critical to full time all wheel drive trucks - a center differential. A differential is a set of gears that takes torque from the driveshaft and splits it evenly between the two output axles allowing them to rotate at different speeds. Without the center differential, when you engage a part time four wheel drive system, you wind up

 

locking the front and rear axles of the truck so that they rotate at exactly the same speed. In a straight line you won't notice too much of a difference between part time four wheel and full time all wheel drive but when you go to make a turn the laws of physics catch up with you. Because the distance the front wheels travel while turning is not the same distance as the non-turning rear wheels you wind up 'scrubbing' the tires on the road as they seek to even the difference in speeds between the front and rear wheels. You'll feel this as resistance in the steering wheel during a turn. This is also the reason the truck manufacturers recommend you don't engage part-time all wheel drive on dry pavement - you'll chew up your tires fast. On wet pavement the tire wear problem for part time four wheel drive trucks is significantly reduced because the slippery surface acts as a lubricant to reduce the amount of friction between tire and road as it scrubs off the difference speed.

 

In an all wheel drive truck, no matter how you turn the wheels or in what driving conditions, there is no scrubbing resistance because the center differential, acting with the front and rear differentials, continuously transfers torque from any slipping wheel to those with a firmer grip.

The Sierra C3's all wheel drive uses a viscous coupling to automatically lock the differentials at the first sign of traction loss. The viscous coupling is composed of a strong and lightweight magnesium housing that surrounds the

two axles with a silicone based fluid. The viscosity of the fluid varies from the consistency of karo syrup to the consistency of silly putty depending on the load it is placed under. As slippage occurs the disc plates within the differential shear this fluid so that faster rotating plates, those connected to slipping wheels, transfer power to slower rotating plates, the ones connected to wheels with better traction, which 'locks' the differential. Because of their fluidic nature viscous couplings continuously transfer

torque at all times and in a such a way that the driver never notices an adjustment in wheel traction.

 

For those out there who have doubts about the durability or superiority of full time all wheel drive versus part time four wheel drive consider the following. The Range Rover and Hummer H1 use permanently engaged all wheel drive systems and both vehicles are among the most capable off-roaders available, bar none. On road you need only look to the German luxury manufacturers which consider full time all wheel drive an essential safety component, such as Audi's Quattro system.

 

On most all wheel drive luxury cars you will normally find a 50/50 power split between front and rear power distribution but because this is a pickup truck with an unloaded bed, the default split on the C3 is set to 38/62.

 

You've read the above concepts and theories, but now that we have driven the Sierra C3 we can also practically state that it is indeed the most capable handling pickup truck on the planet.

 

We drove the Sierra C3 from Santa Barbara, California up the coast to Morro Bay. On this trip we tested the truck on highways, surface streets, hilly roads with lots of twisties and the beach.

 

Of course the first thing we tried to do was step on the gas and see if we could get the rear wheels loose - which turned out to be next to impossible unless you wanted to lose control of the truck entirely. The C3 sticks to the road under all conditions, period.

 

On the highway and surface streets you aren't even aware of the all wheel drive's handling improvements, nor should you be unless you are in a defensive driving position trying to avoid an accident.

 

Between the highway and the beach we drove the C3 on a hilly mountain pass for a few miles. The truck was quite stable and under the driver's control at all times. No way was the rear coming loose - even while taking the turns at what fellow passengers might have considered aggressive speeds. In a two wheel drive truck you would have seen the rear end come around you on some of the turns. It's amazing the confidence you feel from the all wheel drive as it translates to the steering - not too much oversteer, not too much understeer but just right.

 

The beach was much the same story where we briefly drove the C3 right to the edge of the Pacific Ocean . Entering the beach we immediately executed a tight 180 degree turn with virtually no fish-tailing to speak of (no pun intended). Driving further down the beach we also tried slaloming the C3 around some piles of driftwood and seaweed. Again there was no oversteer on the sand beyond what would have been expected if we were on dry pavement with a two wheel drive truck.

 

 

 

 

TYPES OF 4WD/AWD

 

Different systems have varying pros and cons. Adding confusion is the fact that automakers often give their systems names that make it hard to tell what type a particular vehicle has. All systems, however, are basically one of the following types.

 

Permanent 4WD or AWD. With this full-time system, found on SUVs, minivans, wagons, and a few pickups, the vehicle is constantly providing power to all four wheels, usually with power being shifted between the front and rear axles as needed. The advantages are that the vehicle always provides maximum traction in both dry and slippery conditions. As with all full-time systems, it requires no action from the driver.

 

For maximum traction, the center differential can usually be locked, which distributes power evenly to all four wheels. In some systems this is done automatically, while in others the driver must manually lock it. Most systems use a variable center differential to route power to where it's needed most. The BMW X5 and Mercedes-Benz M-Class use a four-wheel traction-control system to achieve a similar effect by minimizing wheel spin and sending power to the wheel with the most traction.

 

Automatic 4WD or AWD. Available on various SUVs, this type of full-time system lets the vehicle operate in 2WD (either front or rear, depending on the model) until the system judges that 4WD or AWD is needed. It then automatically routes power to all four wheels, varying the ratio between front and rear axles as necessary. In most systems, that is detected by a slipping wheel. More-sophisticated systems use software that switches the system to 4WD or AWD during specific driving conditions--before a wheel begins to slip. Automatic systems provide the key advantages of a permanent system, yet without having all drivetrain components constantly engaged.

 

Selectable 4WD. Found on some SUVs, this type of system allows the driver to select between several modes, including 2WD, a permanent or automatic 4WD mode, and, on some vehicles, a part-time 4WD mode designed to provide more traction.

 

Part-time 4WD. Found on SUVs and pickups, this basic type requires the driver to manually shift between 2WD and 4WD using either a lever or a switch. All current systems allow you to "shift on the fly" (or shift between modes while driving). Vehicles with part-time systems, though, aren't designed to be driven on dry pavement when in 4WD mode.

 

 

First of all, before we're "driven" to confusion by all of this terminology, let's lay a few ground rules. In most automotive circles (and for the purposes of this discussion) the following acronyms will hold true:

AWD = all-wheel drive

4WD = four-wheel drive

RWD = rear-wheel drive

FWD = front-wheel drive

 

Each of these drive systems has its own advantages and disadvantages which I'll cover in greater detail.

 

Since the majority of vehicles sold today are front-wheel drive, we'll start here. As the name suggests, FWD vehicles use only the front wheels to deliver power from the engine to the pavement. While relatively rare on the streets of America during the first half of the 20th century, FWD cars now account for approximately 70 percent of new car offerings. Why the recent switch from RWD to FWD in new cars? Well, since most vehicles carry their engines up front, it is a simpler task to get that engine power to the driven wheels when they are both on the same end of the car. Essentially, you turn the engine sideways and connect a short driveshaft between either side of the engine and the two front wheels. Less distance and fewer parts between the engine and the driven wheels means less horsepower loss through mechanical inefficiency. Additionally, fewer parts means reduced production costs and, ultimately, a lower sticker price. One final advantage of FWD is that it puts the engine weight directly over the driven wheels which can improve traction on slippery or snow-packed roads.

 

Despite its practical advantages, FWD has several disadvantages when related to vehicle performance. First of all, since vehicle weight shifts to the rear of a car during hard acceleration, FWD cars will always be fighting a losing battle when it comes to straight-line acceleration. Handling performance also suffers on FWD platforms since the option of using "throttle-induced oversteer" or sliding the rear end doesn't exist (unless you get really creative with the emergency brake).

 

In the end FWD is a very practical way to power a vehicle but its inherent design is contradictory to performance. Look at it this way: Your car came with four wheels, why put the stress of steering, braking, and acceleration on only two of them?

 

Take everything I just said about FWD cars and reverse it for RWD cars. Performance goes up because now the two rear wheels handle the duties of acceleration, leaving the front wheels to focus solely on directing the vehicle (plus the majority of braking). This means that when you punch it, vehicle weight transfers to the rear end, just where you want it during acceleration. You can also "steer" a RWD car with the gas pedal by applying power and sliding the rear end while in a corner. If you don't think this is an advantage, try driving a Mazda Miata and a Chevy Cavalier convertible through the same set of backroad twisties. Even though they're both top-down, four-cylinder compact cars, the experience will be very different with a clear victory in "fun factor" going to the Miata.

 

This doesn't necessarily make rear-wheel drive the "better" configuration since it has disadvantages too. As mentioned earlier, RWD cars require a driveshaft (and dreaded interior "hump") and a differential to get power from the engine to the rear wheels. These components add extra cost and weight to a vehicle while robbing horsepower and making RWD cars generally less efficient than FWD vehicles. This layout is also more challenging to drive in inclement weather where, without the aid of traction control, a RWD car can more easily end up on somebody's front lawn or stuck in a ditch.

 

I personally have an affinity for RWD because its biggest disadvantage is also its biggest advantage; it's less practical and, consequently, more fun.

 

Both of the previous drivetrain layouts are based on using only half of a vehicle's wheels to deliver power to the ground. But those other wheels are just sitting there, why not use them too? Actually, the concept of using all-wheel drive is not new. A Dutch manufacturer named Spyker had one of the first full-time 4WD vehicles on display at the Paris Motor Show in 1903.

 

While we're here let's go over what the supposed differences are between four-wheel drive and all-wheel drive. 4WD was basically the original term used to describe all four wheels being driven on early Jeeps and trucks. These systems were not very advanced in that they simply put power to all four wheels, usually after an interior shift lever was moved and/or the vehicle's hubs were manually "locked" by the operator. During this period 4WD was thought of as a purely off-road utility. Today 4WD still refers to vehicles that can opt for two- or four-wheel drive. These vehicles often have a 4WD "low" and "high" setting that can be switched using an interior lever. The "low" setting provides even greater torque for pulling or climbing in an off-road environment. The "high" is useful for slippery on-road situations like packed snow or ice. 4WD also uses a locking center differential to avoid unnecessary slippage between the right and left wheels while off-road.

 

AWD generally operates all the time with no "low" or "high" gearing options. While 4WD is basically an SUV term, associated with off-road use, AWD is a car/wagon/minivan term that means traction improvement for performance or bad weather reasons. Some vehicles, like the Mercury Mountaineer or the Lexus RX300, are tougher to classify because they are SUVs in appearance but use AWD instead of 4WD.

 

The "all" part of all-wheel drive is a bit misleading since the majority of AWD vehicles use the front wheels primarily and only direct power to the rear wheels when a sensor detects front-wheel slip. Other cars, like the Porsche 911 Turbo, use AWD for performance purposes like getting from zero-to-sixty in 3.7 seconds. Even with sufficient horsepower, a Porsche with only two-wheel drive would be traction-limited to high 4- or low 5-second zero-to-sixty times. AWD also helps keep Subaru wagons and Chrysler minivans from sliding on wet or snowy roads by redirecting power from "the wheels that slip to the wheels that grip."

 

So, it seems like 4WD/AWD is the best way to go, right? Well, not exactly. Both of these systems add substantial weight, complexity and cost to a vehicle. They also reduce gas mileage because of increased drag on the drivetrain. When you consider that traction control is becoming more and more common on front- and rear-wheel drive cars, the supposed advantages of 4WD/AWD start to wane.

 

By now it should be obvious that there is no "best" drivetrain layout; just different designs with different characteristics. Hopefully, after reading this column, you'll have a better idea which one is right for your circumstances and not get suckered into buying an all-wheel drive Subaru for your daily commuter in Phoenix or a BMW M3 for exploring the Yukon.

[Denali02]

Thanks for your reply.  I don't know if the Denali has a G80 or Locker, probably neither.  I'll have to research this.  The truck does have AWS (Quadrasteer).  Primarily, I need traction on slippery flat road like surfaces, not rough off-road terrain.  However, I'm headed to Cape Hatteras, NC where the sand can be really soft.  I know most AWDs get lost in the sand.  I am looking for some testimonials or criticism on the Sierra Denali in the sand.

[MarkD]

Let it never be said that TXdoc gives an incomplete reply!!!  

 

Man! do you cut and past or just type that much!

 

Impressive dedication to a thorough answer!

[Stepside Z]

 

Sorry man but you said it all.

[GnatGoSplat]

The C3 had the rear locker, but I think due to quadrasteer, the locker is not available in the Denali.  I could be wrong though, check your glovebox sticker for the "G80" code.  If it's there, you have the locker.

[TxDoc]

I just ordered a 1500 with Quadrasteer (not a Denali), I wanted the locker and they were going to submit it and see if it would go through.  On the car pricing site automatically selected locking differential as required and another said limited slip was required. So, my dealer's computer did not say it wasn't. Hopefully, I will get the G80.

[TxDoc]

btw--the G80 is an Eaton and the limited slip with the Quadrasteer trucks (I was told) is a Dana.

 

From Carquotes.com and CarsDirect.com

 

LOCKING DIFFERENTIAL (N/A w/NYS 4-Wheel Steering) -- Code: G80

 

LIMITED SLIP DIFFERENTIAL (REQ: NYS 4-Wheel Steering) -- Code: G86

 

4-WHEEL STEERING -inc: pickup box w/flared fenders, roof marker/fender flare lamps, limited slip differential, 71" rear track width, 145 amp alternator, HD disc brakes, ride control suspension, HD tra -- Code: NYS

 

4-Wheel Steering Package :

 

• Engine: Vortec 5300 SFI V8

8-cylinder engine with 5.3-liter displacement, sequential fuel injection .

• Quadrasteer .

• Autotrac Electronic Transfer Case .

• Roof Marker Lamps .

• Heavy-Duty Rear Limited Slip Differential .

• 145 Amp Alternator .

• Manual Selectable Ride-Control Suspension Package .

• Heavy Duty Trailering Equipment .

• Heavy Duty Air-to-Oil External Transmission Oil Cooler .

• High Capacity Air Cleaner .

• Gross Vehicle Weight Rating: 6,600 lbs (2993 kg) .

• Includes 4WS mode selector switch on the instrument panel, redesigned pickup bed with flared fenders and marker lamps, 71 in. rear track width and heavy-duty power disc brakes..

[Denali02]

I checked my glovebox VIN sticker.  G80 is on the label.  I guess from what TxDOC reports I have the Eaton locker. Great!