[ARCHIVED THREAD] - differences between 4WD and AWD? (Page 1 of 2)
Posted: 2/9/2005 3:59:59 PM EDT
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I did my homework to try to understand the differences in between. I am not an auto expo. I'd like to hear your opinions and experience about them. thanks. |
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This is a bigger subject than most people give it credit for. Think 1980's. 4WD tipically ment you had a vehicle that was RWD, but you could engage the front wheels to make them have pwerfrom the engine distributed to them. Now 4WD usuallymeans thesame thing, but it can get confused and blurred with AWD since peopel keepon wanting new fangled shit to break on their vehicle. AWD 'usually' means that all wheels have some amount of power from the engine distributed to them all the time. But as the world turns we have drivetrains that automatically kick into 4WDwhen needed and out when not needed. So it that 4WD or AWD? That is your choice I suppose. Now with all of that aside one main difference that is usually present is some type of front to rear power distributer. As in front gets say 75% of the power and rear gets 25%. Nowdays it is even adjustable electronically. With standard 'old school' 4WD this is not the case. the front and rear wheels are locked to gether basically at the same speed and power level. This is why you can't take a 4WD vehicle on pavement because when you turn your tires follow 4 different paths and when your front and rear wheels are speed locked you will stress the shit out of your drivetrain. With AWD this isn't so much of a problem because of the slippage that allows for the front and rear wheels to get different amounts of power. Think of it as slipping the clutch between the front and read wheels around a corner. hope some of that made sense. Now for what is better. You should have to ask. A good solid set of front and rear axels with lockers, and a nice beefy transfercase. This is of course the only TRUE 4WD. You must read up on LSDs and lockers and open diffs. and spools and such to understand the true part. ETA: I am not that bad of typer, I need a new keyboard. I will claim all the mispellings though. |
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4WD is usually part time and has a multiple speed transfer case. AWD is always on and as such, must have open differentials between the front and rear axles. Of important note is the Audi quattro IV system in use on the A4 to A8 series. It uses the ABS unit to prevent wheelspin on any corner and has a Torsen center differnetial. The Torsen differential is NOT a friction type limited slip but a TORqueSENsing all gear type. It does not diminish the fuel economy of the car because there is no friction in normal driving. The Audi quattro IV system is far advanced and is the best AWD, hands down. For 4WD, Jeep is still the best, unless you count the Daimler Unimog. |
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All wheel drive means that all four wheels can drive the vehicle if need be without any input from the driver. 4 wheel drive is a generic term that is placed on any vehicle that is meant to be driven (primary) by only one axle (two wheels). With todays high end SUV 4 wheel drives the point is moot. My opinion? A 4 wheeler is a vehicle you need to get out of to lock the front axles manualy! 
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I don't believe that is correct. If the rear wheels have a locker, they will both move. In general an AWD vehicle is ususally FWD until slip is noticed up front, then the gear boxes will transfer power to the rear. A 4wd vehicle is usually a rear wheel drive vehicle. When 4wd is needed, you usually have to do something to engage the front wheels. Either push a button, shift the transfer case, and/or lock in the front hubs. It's important to know, that usually, 4wd and AWD both only turn one wheel on each axle, unless a limited slip differential, or a locker is used. Some 4wd transmissions will do this automatically, like the bmw x5 or the quadratrac2 on a jeep. This allows power to be sent to any individual wheel (the one with the most traction) |
close AWD have a center diff like in the rear but instead of splitting power between rear wheels it splits it front to rear and sends power to the wheel with least resistance. 4WD has a transfer case and will send equal amount of power to the front and rear clear as mud |
depending on the brand vehicle and type of system used frnt or rear wheels may be the primary drive axle. AWD will allow limited slip then engage a % of power distribution to frnt and rear. 4WD will lock in the frnt axle rather than use a slip sensor to engage. This is even more clouded with the new auto 4WD systems which operate the same as AWD. Some 4WD come with locking diffreentials in the rear too. They sense slip and lock the rear for better traction. AWD it selects the % of drive power to each axle , 4WD you pick locked in or out. I believe 4WD is better for off road , as you pick when to lock the axle. 
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Marketing.. that is the only difference. If you have four wheels and they are all powered then you have 4wd, if all your wheels are powered then you have awd. If all your wheels add up to 4 then guess what.. yup, you got it. When passenger vehicles started arriving on the scene in mass the marketing folk didn't want to label them as 4wd, sounded too truck like and their target market did not want to ride around in a truck. The solution is to simply come up with a new name. That is how awd was born. Anyone remember the old subarus and AMC eagles? they were originally advertised as being 4wd, now they are advertised as awd. Some types of 4wd 1. part time 4wd, full time rwd. (think pick up trucks or jeeps) 2. part time 4wd, full time fwd. (think subaru and most of the recent "awd" vehicles) 3. Full time 4wd, all wheels/all the time Axle type 1. Open differential 2. limited slip differential 3. Locking differential All these axle type can be used on the rear of a vehicle. for the front axle it is best to go with an open or possibly a limited slip, lockers on the front are NOT recommended and should only be used by experienced drivers under special conditions. An exception to this might be something like an air locker on a front axle where it can be disengaged whenever not needed. There are different types of transfer cases also, part time, full time and torque biasing but I am not up to going into that subject right now. Last word on this subject, do not believe what the car salesman tells you, he read his spiel out of the book the marketing folk gave him. I would bet he has little or no idea what the hell is going on in the drivetrain. ETA: as for which axle drives, how torque is distributed, locking and unlocking axle I ask you to remember the following terms.... Quadra-trac (reffered to as 4wd) and every one of those old 4wd trucks with two transfer case levers with these patterns 2wd N 4wd and.... LO LO Loc N HI HI Loc (Is that right? It has been a long long time) |
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Types Of Four-Wheel Drive Systems In general, 4WD refers to a vehicle's drive train system that can send power to all four wheels, but the four wheels are not necessarily under power all at the same time. Here's an explanation of the different types of four-wheel drive systems: 1. Full-time 4WD (or AWD) means the vehicle is constantly providing power to all four wheels, usually with power being shifted between the front and rear axles as needed. This provides maximum traction in both dry and slippery driving conditions and requires no action from the driver to activate it. IMPORTANT: Full-time 4WD (or AWD) doesn't provide as much mobility off-road as part-time 4WD does, because the system is designed such that it allows a set of wheels (front or rear) to spin if they don't have traction. 2. Part-time 4WD refers to a vehicle with selectable 4x4 or 4x2, requiring the driver to manually shift between 2WD and 4WD using either a lever or a switch. With part-time 4WD, you can "shift on the fly" (switch between 2WD and 4WD while driving). Part-time 4WD gives you better traction on slippery surfaces because the front and rear sets of wheels are locked together. Thus, this is the optimum choice for most off-road conditions. IMPORTANT: Vehicles with part-time 4WD systems should not be driven on dry, smooth road surfaces when in 4WD mode, or you will soon be spending a lot of money on repairs. 3. (or AWD) is a full-time system that lets the vehicle operate in 2WD (either front or rear) 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. Usually a slipping wheel activates the system, however, some of the more sophisticated systems use software that switches the system to 4WD or AWD during specific driving conditions -- BEFORE a wheel begins to slip. 4. Automatic 4WD (or AWD) vehicles are not recommended for serious off-road driving because all four wheels are powered at all times, which is not wise under certain off-road conditions. |
True, but some more info as I understand it: - 4WD as in jeeps, pickups is aka 4WD on demand, e.g. you have to shift into 4WD - AWD is like 4WD but you don't have to shift on demand, the 'puter's do it - 4WD is really 2WD, only one front and one rear wheel gets all the power at any given time (the one's that are slipping) - Except if you have a locking differential in the back and it's 3 wheel drive, the power goes to both rear wheels and 1 front wheel (the one that's slipping) - AWD have a differential in the front, jeep/truck 4WD's don't, j/t 4WD is harmful to your jeep/truck on non slippery surfaces for the front wheels since the front doesn't have a differential |
I dunno, but MY AWD vehicle drives all four wheels at the same time. edit: upon further investigation, my audi works the same as MB's 4matic and SUbaru's AWD ; that is, they are driving all four wheels. This system is called "permanent AWD". Other systems, called "Full-time AWD" only send 5-10% of the power to the rear, and revert to FWD when coasting. |
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Everyone has a different theory on this subject. Here is mine... A 4WD has a transfer case with a selectable low range. It can have part-time 4WD, full-time 4WD, or 2WD. I have seen "LO, LO Loc, N, HI, HI Loc" on an old Dodge transfer case shifter. This is a truck with a full-time and part-time transfer case. LO and HI are full-time settings which do not 'lock' the front drivetrain to the rear, meaning it allows for different speeds to negotiate turns easier. LO Loc and HI Loc will 'lock' the drivetrain for maximum traction. It is always in 4WD, selectable between full time (unlocked) and part-time (locked). My Dodge has 4LO, N, 2HI, and 4LO. If it is in 4WD, the driveshafts are locked together, meaning it is part-time. I cannot drive it on dry pavement in 4WD. Here is a pic of me out four wheeling in my Dodge...  |
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The Subaru WRX STI isall wheel drive with adjustable adjustable front/rear power distribution from what I hear. So how can you say what you said? I don't own one, but I thought that is how they worked. Correct me if I am wrong. |
I disagree. There are 4WD trucks which have 'full-time' transfer cases. These allow for different speeds between the front and rear axles, which permits turning on dry pavement. Some Jeep Grand Cherokee's have a 4WD full-time capable transfer case. |
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I have always had 4WD trucks and I built AWD racecars for about 10 years. My wife has a AWD SUV that fully selectable into AWD, 4WD and 2WD- I still have to read the technical manual to figure out what type of system is in a type of car. Its quite confusing as what your actually getting when you purchase a 4wd/awd/all-time 4wd vehicle. I have seen the above mention SAE document and wish that the marketing teams knew it existed. There are several types of AWD: viscous coupled AWD systems and electronic lock up systems that use the wheel speed sensors to measure wheelspin- these are the most popular types. The true 4WD systems are explained in the above posts pretty well. The AWD systems in small/medium size SUVs are intended for poor road conditions and to prevent getting stuck in shallow mud or wet grass- not for tackling the Rubicon like a proper 4WD system you find in light to medium trucks and Jeeps. AWD does work well and has a place- the system tends to apply the power where need it (if you have limited slip in the diffs) and only as much as you need to get you out of trouble. The AWD systems can be primary front drive or primary rear, it depends on the vehicle type. In the case of the primary front systems the AWD tends to perform better than a true 4WD truck on flat, slippery surfaces (like a snowy or icy road) as the front drive system will straighten out the vehicle and unlock the rear without the yaw effect that is encountered with 4WD. The front drive systems are inherently light duty applications as there is limited space to locate the transmission and transfer case next to the engine so you wont find it on larger vehicles (and the negative effects with towing as well- another reason larger trucks will never adopt it). You will also find that 4WD system advertise "automatic 4WD"- this could mean auto locking hubs (you dont have to get out- like in the 70's and early 80's) or it could mean that it is also using some type of wheel spin detection (electronic or mechanical again) to lock the transfer case in. The short and simple is that there is a lot of systems out there now that blur the two types so try to identify your need in a 4 wheel driven vehicle (slick roads or true off road) and read about the system in any vehicle you are considering to see what the car maker is selling it as. 4WD truck guys might want to argue this point (and I drive a 4WD 1500 Ram everyday) but if you are plodding along on a snow and ice covered road and a Subaru or Mitsu passes you doing fine at 15 mph faster- assume two things: they might be stupid and are going to stick it in the ditch up ahead OR the little jap car has a AWD system that is better suited for the slick roadway than your "mud toy". If its the latter and you speed up because the jap car is doing just fine you might be the one calling your friends to come get you out of the ditch. 
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Blah, Blah, Blah... |
What does determin mean? 
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And tons of older AMC Jeeps that had Quadratrac ... CJ's and Full Size (Cherokee and Wagoneer) ... The "Quadratrac" system was full time four wheel drive. It has, for lack of a better description, a differential in the transfer case that slips for normal street driving to allow the front and rear axles to turn at slightly different speeds(while making turns, etc) but can be locked manually from the interior for situations when more traction is needed. |
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No, there are subtle differences. The Subaru uses a viscous center differential that requires a finite speed difference in the front and rear axle speeds in order to differentiate torque. The Audi quattro IV system divides torque based on traction. The difference is NO WHEEL SPIN happens on any one axle. And the automatic traction control system (which works off the antilock braking system, only in reverse) allows only ONE wheel to spin. The quattro can only spin ONE wheel and that is only for ~20 milliseconds. Not enough time to notice. You really need to drive something like the S4/S6/RS6 or S8 to feel it. The other models don't have enouugh torque to spin a wheel unless on snow or ice. Rainy slick streets are not a reason to moderate acceleration. |
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Also, Full time 4 wheel drives are geared lower compared to the all wheel drive systems. Full time 4wd are usually set on 4 high on pavement. The Humvee/Hummer H1 has the following: 4HL, 4H, N, 4L. Also, unique to the Humvee/H1 are the Geared Hubs which provides a 2-1 gear reduction for more torque. The brakes are relocated inboard right beside the front/rear differential.   |
Yes I know that quattro and subaru awd and 4matic are all subtly different, my point was they were the same w.r.t. the default power level at each wheel. |
FWIW there are all-mechanical AWD systems. I dont know if you were discounting this or not based on your verbiage |
If you could afford three Torsen differnetials, you could make a wonderful, all mechanical non-friction AWD system. |
Your name fits. Never said that 4WD was inferior or that an AWD car had the magical ability to stop and better than anything else on the road- just that AWD system can get power to the ground in some situations better than a 4WD system. Sorry if hurts your truck's ego. If you would like some other examples of AWD come out to Pikes Peak in CO in July. I'd be happy to show you some of my work.  Your the guy in the ditch. |
Correct, take the Subaru for instance URL •All-Wheel Drive (AWD) •Suretrac limited-slip front differential •Mechanical limited-slip rear differential Another example example The new all-wheel-drive system adds a front differential and a transfer case to the standard rear-wheel-drive configuration. All-wheel drive continuously divides the power between the front and rear axles at a 38- percent to 62-percent ratio. In addition to the front differential and transfer case, the all-wheel- drive system adds a unique short- and long-arm front suspension to accommodate the front drive axle and related components. This unique suspension uses a forged upper control arm and single-piece case-iron lower control arm, efficiently packaged around the front-axle drive system. The steering gear effort and response and the suspension characteristics also are tuned to the handling characteristics of all-wheel-drive vehicles. You must be confusing the transfer case with a front differential |
You're misreading the info about the Subaru. They are not describing what an AWD system is, they are describing what changes were made by Subaru to change their stock RWD car into an AWD car. Adding a transfer case and a front diff is what converts a 2WD pickup into a 4WD pickup. Going back to your earlier comment that AWD has a front diff and 4WD doesn't, if there wasn't a diff in the front of a 4WD then how would the drive shaft get connected to the axles? |
I think you're cofused. 4WD vehicles have a transfer case and a front differential. |
This is wrong too. With an open differential, both tires on the axle ALWAYS receive the same amount of torque. |
I guess it all depends what type of differential we have then. I had in mind that part time 4WD don't have the type of diffrential that allows the front wheels to move at different rates as when cornering vs AWD that have a type of differential that allows the front wheels to move at different rates. |
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A couple of points. ALL driving axles have differentials, it allows the wheels to turn at different speeds to accomodate the different radius (and distance) the inside versus the outside tire take while turning. An open differential will send the power to through the path of least resistance. I.E. the spinning tire. Limited slip allows X amount of torque to be tranferred regardless of resistance at the individual wheel. Locking differentials will mechanically lock the axles halves together. I.E. Either they both spin or not. Transfer cases are similiar. Some simply allow connecting/disconnecting front and rear driveshafts. When engaged they are mechanically locked together. When disengaged there is no connection/transfer of power at all. Some transfer cases have a differential in them, this allows power to be sent to both front and rear axles at all times but still allows for the difference in rotation when turning such vehicle. Usually the transfer cases have the option to lock out the differential action at which time it will behave as the previously described transfer case. Subaru's are a strange bird. Technically they DO NOT have a front differential, they have a transaxle. Of course there is a differential unit inside the transaxle but for those who love to quibble over minutae, well, it is technically not a differential. What Subaru did was to split power out of the transaxle and send it via a driveshaft to a rear differential. When in two wheel drive the Subarus just power the transaxle, putting it in 4wd simply engages a gear which sends power to the rear of the vehicle. Operation of 4wd/awd is all basically the same, how it gets done can be quite varied. Manually is the old school way, just pull a lever (or two, maybe engage the hubs also) and away you go. Modern vehicles have computers that do all the work for you. If the computer senses wheel slip it will divert power to the slower/non-moving wheels. This can be done electronically by activating various servos and soleniods. Of course there are mechanical systems too, when a torque sensing device (Think along the lines of a centrifugal governor) an imbalance it activates the appropriate device and power is transferred to the non-moving/slipping wheel. I know I have blathered on here for quite a bit and I doubt anyone has read all of what I have typed but as I thought back on all the replies here I would have to say this about 4wd vs. awd The difference is strictly a marketing ploy. The term 4WD is usually used for the more serious working vehicles and is usually a much stroger system, AWD is usually much lighter duty and is found in passenger vehicles marketed to people who don't want to have a vehicle that "drives like a truck". Ok, I am off my soap box now, please continue to discuss. 
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To most marketing dept's there is not difference. In true 4wd, all 4 wheels turn at least as fast as the ring gears, none of the 4 will slip at all. One wheel might be allowed to turn faster than the others, like the outside wheel in a turn, but none can turn slower unless something breaks. This is the best "off-road" system out there. Everything else you ever hear about is an attempt to use this system on dry pavment without having disasterous effects. All Wheel Drive typically means that any one of the 4 wheels might or might not be "pulling" at a given time. The way the system is designed to operate can get quite complicated. |
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Most AWD systems are viscous couplings, sort of like the torque convertor in an automatic transmission. These are "fire and forget" type of 4x systems, torque is distributed to other tires without having to do or activate anything. It’s pretty good if you are not 4x4 savy, no thinking involved. But they really only engage when the main tire is spinning. It's not the best traction system when the road gets really sloppy since the main wheel has to really spin a lot for the other side to engage. Fast rotation = low traction = wheel spin. I've seen many AWD 4x4's stranded on sloppy ice or stuck on a bump spinning one of their wheels with the other wheels barely moving. There is nothing quite like actual gear to gear 4 wheel drive when conditions require constant high torque motion from all or most of the wheels to get you through. Sloppy snow, deep mud, sand etc. On a recent trip to Big Bear I was involved in a huge traffic jam. There was something like 170 cars and trucks stranded on highway 330 for 4 days. I got out with my Jeep's Select-Trak. It was caused by too many people going up with 2wd cars and AWD cute-utes slipping and sliding all over the ice. They were stuck in every angle and direction across two to three lanes. Conditions weren't really that bad, I've been in worse, but the road was really sloppy with a fairly thick layer of broken ice on it combined with a mixture of snow and rain coming down. I had to worm my way around several Honda CRV's, Rav4's and Ford Escape's to get through. I can't believe how low some of those CRV's are, the last one that I drove around was high centered on an ice burm in the middle of the lane it was like 5" tall at the most. His passengers tried to push him through but he just sat there with primarily one wheel spinning fast. I had to climb up on to the snow the snow plow burm just to clear his front fender, and that was at least 14" tall. AWD’s have had problems getting into my cabin’s driveway too; I’ve always been able to plow right up without clearing snow even with giant ice clumps in the way. If you run full lockers although, you have to be extra cautious on ice, lockers tend to increase sideways sliding when engaged. |
| It seems most "AWD" vehicles are based on a car/ lower chassis than most 4x4 trucks. To me give me dana with lockers and a solid front axle. Do "AWD" owners know what a high lift is? I mean "AWD" is cool for some people, I just like the the big 4x4 mud slinging, big block, gas guzzling. truck. |
No, Open differential = will send all the power to whichever wheel is turning easiest. Ever seen a car stuck with 1 drive tire sitting on an area of good traction while the opposite tire is spinning in muck? That's an open differential. Limited Slip differential = This one will send power to both tires, until a certain amount of speed/force is exceeded. Usually done with spring clutches or other mechanical devises. More expensive than an open differential. May be more likely to fishtail if turning under power, compared to an open differential. Much better traction under most conditins than an open differential Locker = acts like a solid axle. Better traction than open or LSD, but DOES NOT respond well to turning. As I understood it, open, LSD, or locked, differentials are not a factor in whether a vehicle is AWD or 4WD. It used to be many domestic 4WD trucks, came with LSD's in the rear axle. Foreign came mostly with open differentials. 4WD is a heavier duty system that sends appx the same amount of power to the front and rear axles. 4WD always has hi-lo speeds for the transfer case. AWD, some systems are full time, some are part time. AWD sends more power to 1 axle. So it could be a 75/25 split or a 62/38 split. One axle is generally much lighter construction than the other, and has enough power to basically keeps itself unstuck, or allow "power turning". Also some AWD systems only send power to the secondary axle when the primary axle begins slipping excessively. |
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With an open differential, the torque in the axles are always the same. The torque pushes at both wheels equally at the same time. If torque is like voltage, then an open differential is like two paralell resistors. An open differential has no resistance to current when either resistor ----> zero ohms. The current--torque x rpm--goes wild. A locker behaves like voltage across two resistors in series. Both resistors have to ----> zero ohms for the current to run wild. It's that simple. |
I'm pretty sure four-wheel-drive means two powered axles and a two- (multiple) speed transfer case. I'm sure open or locked diffs make no difference. |
