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SUVs 4WD & AWD systems explained
1376 messages, Last post on Nov 23, 2009 at 9:11 AM
You are in the SUVs Forum. Your Hosts are steve_ & tidester
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Replying to: 2006SubLTZ (Jan 27, 2009 2:01 pm) This is likely an over-simplification, but All-Wheel Drive means that the system is either permanently engaged or engages automatically when the vehicle detects wheel spin on the primary drive axle (which would be the rear for a Suburban). The advantage of a permanently engaged system is better driving dynamics at all times. For a "part time AWD" system, which is the second one described above, the only real advantage is that the vehicle will automatically engage the secondary axle without requiring driver input. Depending on the needs of the driver, this may or may not be advantageous since the driver cannot provide input (keep the system in AWD even when wheel spin is not detected). |
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Replying to: 2006SubLTZ (Jan 27, 2009 2:01 pm) That's just wrong. ANY vehicle can slide depending on road conditions, the tires (as xwesx pointed out) and how the vehicle is driven. Even a bulldozer will slide given the right (though extreme) conditions.  tidester, host SUVs and Smart Shopper |
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i have a 95 ford explorer 4 wheel drive and have hit a snag in the brake system, there is a nut on the hub assembly bigger than most, it is about 2 1/4 inches, it is not a hex head nut, I can not find a tool to get it off anywhere. does anyone have info on this?
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Replying to: judahrisner (Feb 24, 2009 3:41 pm) I had some and they were slightly bigger, and rounded on the outer edges. You had to use a key adaptor to get it off. Did you ever have wheel locks? |
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Replying to: 2006SubLTZ (Jan 27, 2009 2:01 pm) The problem with 4WD simply is that it is not a true 4WD even though it is called so. When 4WD vehicles first came on the market that was the state of the technology and so they were called 4WD. 4WD system works as a real 4WD system only when you have condition that both front and rear wheels are slipping in same direction, i.e. if you are stuck or are pulling heavy load on slippery surface. The problem with the traditional 4WD system is that it is practically impossible to have condition that front and rear axles would want to turn at exactly the same speed. Depending on tire wear, tire inflation, vehicle load etc. the axles will want to rotate at slightly different speeds. If these axles are connected to 4WD which forces them turn the same speed it means that one of the axles as tire speed against road is faster than the other one. This causes very big forces between axles if vehicle is driven on dry pavement and can even damage the drive system. This is also the reason why all 4WD vehicles have a warning saying not to use 4WD on dry pavement. On slippery condition it is ok. Speed difference between front and read tires can also be hazardous on slippery road surfaces. This is simply because with 4WD system neither one of the axles has a good speed match on road surface so friction to road surface can actually be worse than 2WD under some conditions. AWD system was developed just to fix all of this. The only basic difference between 4WD and AWD systems is that there is a differential gear between front and rear axles. Differential allows each axle to turn at speed what ever the conditions requires, i.e. front and rear axles do not fight each other. I have seen some writing about automatically engaging 4WD and AWD systems. For 4WD system this is true as it makes sense. When ever the main axle slips the front axle engages to help but then drops off when good grip is sensed on both axles. For AWD I have never seen automatically engaging system and it does not make sense if it really exist. The differential between axles takes care of the axle speed matching for all conditions so there is no need to disengage. Differences between AWD systems from different car manufacturers are related to power split and slip control between axles and wheels. In short. If you want to have a good driving car in snow and other vice slippery conditions you definitely want to have an AWD system. Traditional 4WD can be even dangerous under some conditions. Are you absolutely sure your '06 Suburban has an AWD system? I thought that year it was available only on some Yukons. Could be wrong here though. Arrie
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Replying to: arrie (Mar 30, 2009 10:07 am) That assumes there exists a center diff at all. The Honda CR-V uses a rotary blade coupling but it doesn't really have a true center diff. The rear axle disengages instead. That's why it's only a part-time system.
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Replying to: ateixeira (Mar 30, 2009 11:37 am) AWD means there IS A CENTER DIFFERENTIAL, PERIOD. 4WD has transfer case which has two output shafts. One goes to front axle differential and the other to rear axle differential. When both output shafts are coupled to work it means that front and rear axles are turning at the same speed (if tires are designed to be same diameter as usually is the case) and this means that both front and rear tires should be exactly the same diameter under any driving conditions. Well, front and rear tires practically never are the same diameter and that is the problem. Different tire radius on road side of the axle means tire speed against road surface is different. This makes one of the axles pull the car and the other to slow it down. If Honda CR-V has a rotary blade coupling in the drive shaft going to rear axle it means it is a 4WD system with an automatically or manually controlled coupling that engages rear wheels when it is necessary. It is not an AWD system as what AWD is originally meant to be. Arrie
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Replying to: arrie (Mar 31, 2009 4:56 pm) Bob |
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Replying to: arrie (Mar 31, 2009 4:56 pm) 4WD has transfer case What about a hybrid with electric motors at each drive wheel? Now that's real four/all wheel drive, complete with lots of torque at any speed.. And you know they are coming.
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Replying to: steve_ (Mar 31, 2009 7:08 pm) separate electric motor on each wheel would be the best, just like it is already done using hydraulic motors in some earth moving heavy machines. That system would be a really good solution as electric motors waste very little power compared to mechanical gears that lose 1 - 2 % of input power at each gear mesh. This lost power of course is the reason why transmissions get so hot. Problem with electric motors is though that for slow speed motor diameter gets very big if it is connected directly to the wheel hub. This means that to keep electric motor physical size small enough a gear reducer has to be fitted in and the mechanical power loss is back. AWD name probably originally came from All time 4WD as differential between axles makes it a true 4WD but as that name was already in use something else had to me made to differentiate between the two. Arrie
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