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Yet another car thread - 4WD vs. AWD

T

tontod

Diamond Member
I was wondering no cards have 4WD on them? Is it because the 4WD system is too complex or too big to be put on cars? Or is it some other reason?
 
How many wheels does your car have, 6?

What's the difference between ALL WHEEL DRIVE (AWD) and 4WD if your car has 4 wheels?

Just wondering
 


<< I was wondering no cards have 4WD on them? >>



I hear the GF4 has 4WD and the R300 will have it as well with a supercharger. 😛
 
Well, pickup trucks and SUV's (talking about the ones with 4 wheels) usually have 4WD instead of AWD on them. Now, a car like a Subaru will never have a 4WD system. I was just wondering what the reasoning behind this was.
 


<< Well, pickup trucks and SUV's (talking about the ones with 4 wheels) usually have 4WD instead of AWD on them. Now, a car like a Subaru will never have a 4WD system. I was just wondering what the reasoning behind this was. >>


So, seriously, what's the difference? 4WD the one that you can switch back to 2WD, whereas AWD is always on?
 


<< So, seriously, what's the difference? 4WD the one that you can switch back to 2WD, whereas AWD is always on? >>


I believe so.


<< Well, pickup trucks and SUV's (talking about the ones with 4 wheels) usually have 4WD instead of AWD on them. Now, a car like a Subaru will never have a 4WD system. I was just wondering what the reasoning behind this was.
>>


Subaru advertises the AWD as being "more safe" generally than 2WD; hence they have their cars being AWD all the time, as opposed to 4WD, which is only AWD when you need it. (Boy, that was a mouthful). I'm not sure of the technical reasons for doing this, but I'd imagine that there must be some as well...

Rob
 
RS Memphis: Oh, I misunderstood you. Yeah, AWD is always on, while 4WD usually you have to switch it to that mode from 2WD.
 


<< RS Memphis: Oh, I misunderstood you. Yeah, AWD is always on, while 4WD usually you have to switch it to that mode from 2WD. >>


Shoulda thought so.

In that case, 4WD is much more expensive, probably the reason why manufacturers don't use it as often anymore.
 


<< Is it because the 4WD system is too complex or too big to be put on cars? >>

Actually most AWD systems are more complex because of how the distribute power. Most 4WD systems, when engaged, have only one setting for how the power is distributed where as AWD systems can change the power output to each wheel based on traction conditions.
 
AWD is more sophisticated and is meant for low-traction conditions, but can be used fine on dry pavement as well, due to the ability to change the power ratios to the wheels, and that it isn't locking.

4WD usually involves locking, and doesn't actively manage power distribution, and hence is not suitable for dry pavement. when the drive train is locked and you have good traction taking a corner will result in the drive train binding as the outer wheel tracks a longer path than the inner one. 4WD low is great for off roading in mud or on rocks where each wheel might not be in contact with the surface.
 


<< AWD is more sophisticated and is meant for low-traction conditions, but can be used fine on dry pavement as well, due to the ability to change the power ratios to the wheels, and that it isn't locking.

4WD usually involves locking, and doesn't actively manage power distribution, and hence is not suitable for dry pavement. when the drive train is locked and you have good traction taking a corner will result in the drive train binding as the outer wheel tracks a longer path than the inner one. 4WD low is great for off roading in mud or on rocks where each wheel might not be in contact with the surface. >>



Good answer, that's exactly how I would have explained it.
 
AWD = usually has a viscious center differential or electronic diff. If either front or rear wheels start slipping, the center diff locks (never 100% torque transfer though) and transfer torque to the wheels that aren't slipping.
4WD = simply locking the center diff so there's always equal torque distrobution.
 
There have been cars with selectable, 2wd or awd. The amc eagle, older subarus I think had it, and I am pretty sure there was a mazda with it.
 


<< AWD = usually has a viscious center differential or electronic diff. If either front or rear wheels start slipping, the center diff locks (never 100% torque transfer though) and transfer torque to the wheels that aren't slipping.
4WD = simply locking the center diff so there's always equal torque distrobution >>



It seems to me that those 2 thing are essentially same (like some company's calling their soft-roaders to SUV's and other's SAV's or whatever). IMO, car makes doesn't follow those rules when putting "AWD" or "4WD" stickes on their vehicles. There's one more technology that sometimes is called "4WD": part-time 4WD with no centre diff.
 


<< It seems to me that those 2 thing are essentially same (like some company's calling their soft-roaders to SUV's and other's SAV's or whatever). IMO, car makes doesn't follow those rules when putting "AWD" or "4WD" stickes on their vehicles. There's one more technology that sometimes is called "4WD": part-time 4WD with no centre diff. >>


They are almost the same thing, with the exception that awd can transfer torque while 4wd does not.

And I've never heard of a center diffless 4wd system. How would that work?

EDIT: I looked it up, it's called part time 4wd.
 


<< They are almost the same thing, with the exception that awd can transfer torque while 4wd does not.

And I've never heard of a center diffless 4wd system. How would that work? >>



Just like all-time/full-time 4WD with central diff locked 😉 2+2 system really, you can engage 2nd axle on heavy road conditions but you use your car as usual 2WD vehicle on pavemen. Probably oldest and most used 4WD system. More about all those technologys can be read from here.
 
Dont both systems use more gas than their 2WD or 4x2 counterparts?

I can see the advantages of each system, but, is it really necessary for urban dwellers to have such systems? I know it helps resale value. That urban jungle we call So. Cal, whew.....

 


<< So AWD is "power given from the wheels that slip, to the wheels that grip"? 🙂 >>


Kind of, I have a Subaru SVX and from what I gather, it's 80% front and 20% rear. Then when I get on slippery terrain it goes to 50/50 and then distributes power to that which has the most grip after that.

Here is a pic of it all muddy after I did some offroading in it, and let me tell you...I couldn't get it stuck.
 
AWD: All wheel drive is always on. The power fromt he engine goes to basicly a differential that will split the power from the front axle to the rear axle while still providing "slip" so that the front and rear tires can turn at different rates, and still be driveable. AWD vehicles aer usually light duty, and do not have to deal with the torque transfer of a 4 wheel drive. AWD power systems usually put 100% of the torque to the rear tires until tire slipping is sensed, then power begins to be transfered to the front. ( some, such as subaru work the exact opposite way, power goes tot he front, then when slipping is sensed, it transferes to the rear ) AWD only have one transfer case speed, high. Meaning the transfer case can not reduce the gearing, it just transferes the torque.

4WD: 4 wheel drive is switchable. If a vehicle is true 4 wheel drive, the ability to go to 2 wheel drive ( rear only ) exists, and is made to be operated that way. Torque goes from the engine, then is split 50-50 from front to rear. No sensing exists, it always splits torque 50-50. The splitting is done by a very large chain ( usually ) and the chain has no "give" in it for slippage. Without that slippage, you can get some wierd handling around cornders and at high speeds because the only way for the tires to spin at different rates is for one to "scrub" on the pavement. That is what causes most tire wear. also, in most 4wd vehics if you make a sharp turn, you will tend to "plow" or keep going straight. 4WD vehics also have a "low" range, where the transfer case reduces the gearing, to get more torque, but slows down top speed. (in low, I am at redline in 4th at about 40 mph ) but the power increase is huge. Also, 4WD is manual selection. The driver must manualy pull a lever to put the vehic in 4WD or low, while a AWD the car does it for you. 4WD parts are also 4-5x as big as AWD, as AWD is just meant to move the car, 4WD is meant for desert womping, mud running, or pulling AWD cars out of ditches.

<edit>
And yes, 4WD and AWD cars use more gas than the same vehic with 2WD, as parasitic losses are incurred by the transfer case, and the other axle parts.
</edit>
 
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Evadman - That is a very good description of the way those terms are usually used today.
I don't think that anyone really has a solid definition of AWD or 4WD.
 
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