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4 Wheel Drive Vs. All-Wheel Drive


Dasher

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Assuming that your vehicle is in 4WD High what is the difference in handling between that and all wheel drive? I was looking at the Caddy AWD STS and this subject came up and nobody has given me a very good answer. Naturally I turned to you guys for the real skinny.

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In most cases, four wheel drive, will split the power, 50/50 between the front and rear wheels. Most 4wd systems only operate with this parameter. All wheel drive, will vary how much power goes front to rear. Sometimes it will be 30% front / 70% rear, and then conditions can change (tire slip), and the power may change to 60% front / 40% rear. With both 4wd and awd, you will always have power to both front and rear wheels.

Don

"Modern warriors saddle iron horses of chrome."

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Some advanced systems can transfer up to 90% of all power to one wheel if it's the one with the most traction available. All wheel drive is much lighter and uses far less components (an added benefit is less to break!) If you are an On Road kinda guy AWD is better for the most part.

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AWD is usually full time, while 4WD is often "switchable". However, you only get fuel economy gains running in 2WD on a 4WD vehicle if you can also disconnect the hubs (if you have manual locking hubs). Most 4WD vehicles today have "auto" hubs, which are essentially just full-time locked. So you can run in 2WD, but the whole driveline still turns, so there's no real difference in economy between an AWD and modern 4WD drivetrain.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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How about fuel economy? Isn't AWD full time or can it be switched in and out like 4WD?

Jim

I remember back in about '75, when Audi's A5000 came out with their full time AWD, they were initially amazed to see that the AWD cars got better mileage than the identical 2WD cars. I think they rationalized that putting power to the other pair of wheels helped the tires lay down their tread better and reduced rolling resistance. The improvement was consistently 1-2 MPG.

Driving with power to all 4 wheels is supposed to improve handling, even on dry roads, but especially when it is wet

My 94 Bravada (now at 171,000 miles) has always gotten pretty good mileage and it is a a full time AWD car, with no low range transfer case, just a viscous coupling. I get about 15 in town and 20 on freeway.

Regards,

Jim in Phoenix

Jim in Phoenix

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The big difference is that the AWD has a third differential or viscous coupling in the transfer case. This helps eliminate the differences in tire wear and cornering. The result is that the vehicle can operate on dry roads without the induced drag and stress that a normal four wheel drive vehicle experiences.

Britt

Britt
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Keep in mind that even though an AWD car has a viscous coupling in the center differential, and even though your car may only be SENDING POWER to the front or the rear, the drivetrain at the other end is still turning, because the hubs are always locked. A RWD car has no drivetrain in the front end. Like a RWD Fleetwood for example. Now imagine an AWD SRX, and let's just pretend that on dry pavement, it's sending 100% of the power to the rear axle. The front drivetrain STILL turns, driven by the front tires as they roll down the road. Even though your engine may not be POWERING the front drivetrain, it's still turning, reducing mileage.

That's why 2WD pickups will always get better mileage than 4WD pickups, even if both run in 2WD. The only exception is if your 4WD truck has manual locking hubs, in which you can unlock the front wheels and keep the front drivetrain from turning. My '95 Nissan 4x4 truck was like that. With the hubs unlocked, in 2WD, I'd get about 20 mpg. With the hubs locked, even still in 2WD, my mileage would fall to around 15 mpg. I would also notice a severe increase in drag (and decrease in effective power). The difference wasn't whether my transfer case was in 2WD or 4WD, the difference was that I could engage--or disengage--the front hubs.

The main mileage reducer isn't the type of transfer case or center differential -- it's the ability (or not) to engage or disengage the drivetrain that's not being "used" at the time.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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All wheel drive is much lighter and uses far less components (an added benefit is less to break!)

Not sure I agree with this point... AWD systems delete the locking hubs at the front wheels but adds a "slippage" device to the transfer case... I've owned both 4x4 (F150) and an AWD (Toyota) and replacing Hubs is relatively cheap and easy... Transfer Cases not so...

Both systems need an extra drive shaft, drive axles and a differential.

There are two kinds of AWD systems:

True all wheel drive - or full time symmetric AWD has a conventional differential inside the transfer case - each of the wheels gets about 25% of the torque as long as traction is equal. However, the center diff cannot be mechanically locked.

To prevent a complete loss of traction when one wheel or one axle would spin, a viscous coupling or a similar device like a Haldex coupling will try to "glue" both drive-shafts together to keep enough torque flowing to the axle with traction. Works kinda OK on slippery pavement when the vehicle has already sufficient momentum and the connecting device has to kick in very infrequently. Off-road or in other situations with slow speed and high demand for torque the glue box (viscous coupling or Haldex etc.) is overstressed and fails to deliver the needed torque. High torque transfers and continuous use make especially viscous couplings fail. Haldex units are much more reliable but cannot satisfy the constant high demand for torque at all wheels either.

Automatic asymmetric AWD - and in a way actually only a sophisticated 2WD system might not have a differential in the transfer case (Volvo, Honda, etc.) but some do (Jeep Grand Cherokee). Primary power goes only to one axle (makes spinning tires much more likely due to inefficient use of traction - as likely as in any other 2WD car). However, both drive shafts are joined by a viscous coupling or a similar device (see note) and as long as all 4 wheels turn at the same speeds the control unit remains inactive. Once the powered axle or one of the powered tires loses traction, the powered drive shaft rotates faster than the one that is just rotating along. The control unit reacts to the speed difference and kinda glues both drive shafts together. This way the previously unpowered shaft will get some of the torque and rescue the failing tires. Same story as in system one: Works kinda OK on slippery pavement when the vehicle has already sufficient momentum and the control unit has to kick in very infrequently. Fails miserably when need of high torque arises or when activated frequently. Cannot satisfy the constant high demand for torque at all wheels when off-road.

Hope this helps.

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Greg

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I guess "far less" components wasn't the phrase I should have used... but AWD is much lighter, and to an extent, simpler. I've seen Eagle Talons with almost 3 times their OEM rated HP doing 6000 RPM launches... and 99% of the time they break rear axles and other parts, but rarely hurt the AWD system itself. There are a ton of 400+ HP Syclones and Typhoons running around too. I am a rear wheel drive kinda guy when it comes to High HP vehicles (my latest purchase makes 547 HP at the rear wheels, but that's for a whole-nother web-site) but I would choose AWD everytime if it was staying "On road" the majority of the time.

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Interesting discussion.

I've never tried AWD but those that have them seem to like them. In any event it seem there are 2 differents types of AWD.

My Bronco (which i really like) had the auto hubs replaced about 3 years ago. i got tired of those plastic things breaking so I had a set of heavy duty WARN manual hubs put in. Yep ... it's nice to have 4WD on the fly but I can pull over and get out.

This past weekend because of the weather I drove home (200 miles) in 2 HI with the hubs locked and I did notice about a 2-3 MPG decrease in fuel economy.

One thing I've heard about auto 4WD hubs; if your sitting still and put the vehicle in 4WD it won't be in 4WD until the vehicle has moved a few feet.

On the other hand I've heard that with manual hubs, when they are unlocked the vehicle has to be backed a few feet to completly dis-engage it.

Jim

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Jim, yes, sometimes "auto" hubs really are "auto", and actually do engage and disengage. I think they're operated via engine vacuum often times. On most modern vehicles, though, the "hubs" are really full time...just like the FWD components on our FWD Cadillacs. There's no locking or unlocking.

I suppose that with manual hubs, if they got sticky, you might need to back up to get them to disengage, but I never had that problem on my Nissan. If I unlocked them, they were unlocked, and I could reach in and turn the half-shaft and it's spin freely in that wheel hub.

Driving your Bronco in 2HI with the hubs locked and seeing the mileage drop is exactly what you should be seeing -- and exactly the reason most AWD/4WD systems lower economy. Even if the system isn't directly driving those front wheels (or rear wheels, whichever), it's still indirectly driving them through the motion of the vehicle...unless you can manually disengage the hub.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Jim, yes, sometimes "auto" hubs really are "auto", and actually do engage and disengage. I think they're operated via engine vacuum often times. On most modern vehicles, though, the "hubs" are really full time...just like the FWD components on our FWD Cadillacs. There's no locking or unlocking.

I heard this from someone telling me how his 4WD (Dodge I believe) with auto locking hubs that was literally buried in snow over night was unable to move because the vehicle needed motion (I want to say he said about 15 feet) for the 4WD to engage. It sounded strange but I don't have any personal knowledge either way.

Jim

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A cousin of mine had an older Dodge Power Ram (mid 80s I guess) and it was a deal like that. I think to unlock the hubs, he had to drive in reverse 15 feet or something. It could have also required forward motion to engage them, I don't know. When I first got my Nissan truck, I didn't think much of the manual hubs, but after owning it for a while, I really liked them. I didn't pay a big penalty for having 4WD (there was still some added weight for that drivetrain), but it was always there if/when I really needed/wanted it.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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