AWD vs 4WD....

[The Stigenator]

My friend who live in Truckee, CA (sierra mountains) and commutes with I-80 all the time has a Lexus RX 350 (2007) and her husband has a Chevy Tahoe 4WD.

They both have snow tires, her husband insisted she have a set of snow tires but also bought her chains just in case.

Apparently CHP would not let her go on without putting on chains on top of her snow tires and AWD but if she drives her husbands Tahoe, they will let it go through without chains cause its got snow tires and 4WD.

The way I remember it is this. 4WD = all 4 wheels turning at the same time.
AWD = power shifts from one wheel when it slips but its on 4WD mode in slippery conditions.

I wonder if its because she drives a Lexus..

Anyway tonight she had to have someone put on a chain. She lives alone (husbands out of the country for the moment) and right now she is not going to take off the chains cause she does not know how, but she might get a neighbour to do it.

Anyway what are your thoughts?? I didnt think she needed chains on a car with snow tires and AWD.

 

[SonicIce]

I think people interchange the naming and they're really the same thing. Theres many differant systems out there. But when people say 4WD they sometimes mean that the vehicle has locked diffs.

 

[Zenmervolt]

They are not the same thing.

4WD properly involves locking front and rear differentials and a transfer case that has a low range option.
AWD can use several methods, but it does not lock all wheels together the way that true 4WD does and will not offer the same level of traction in severe situations. However it requires less driver intervention and is overall "better" in 90% of all on-road situations because of not requiring driver action.

ZV

 

[The Stigenator]

Well her owners manual says "automatic AWD"..
her husbands tahoe has a switch to use the 4WD.

Even so the issue is why is CHP making her put on chains when she has AWD and Snow tires on her Lexus RX350
when she can drive the same road in similar conditions with her husbands Chevy Tahoe 4WD with snow tires and not be required to put on the chains. She does not drive her husbands car cause she finds it too big and unwieldy.

 

[caspur]

In a town call Truckee, one would naturally assume only truck-based SUVs are allowed through.

All joking aside, the trooper is not concerned so much with locking diffs, awd/4wd, etc...in deep snow, his concern is ground clearance. Interestingly, a heavier vehicle like the Tahoe would actually do worse in the snow compared to a lighter RX. (w/the same tire footprint).

In addition, locking differentials are mainly for offroading, and contribute very little to snow performance. The reason being that on snow, there is no need to transfer large amounts of torque between wheels (which is needed in hill climbs for example.) A well designed AWD system with even an open center diff, LSD and/or a traction control setup is just as capable on flat snowy surfaces without the weight penalty of a dedicated 4wd system.

 

[The Stigenator]

that still does not answer why CHP requires her to use chains on her vehicle and not her husbands..

it does not make sense..

 

[caspur]

To put it plainly then, in the troopers mind, he feels the Lexus is less capable in those conditions than the Tahoe.

So your initial assumption may be right, it could be a brand bias.

Or it could be in his experience, he's had to pull out more stranded Toyota branded vehicles than GM vehicles. Which leads him to doubt the capability of the Lexus AWD system.

 

[dug777]

I would describe three open diffs as AWD, and two or less open diffs in a vehicle that drives all four wheels as 4WD (with open meaning open or limited slip)

4WD wipes the floor with AWD in any real off-road situation, but I've had some fun with my folks Prado seeing how far you can go in AWD before you shift down

 

[iamwiz82]

Originally posted by: dug777
I would describe three open diffs as AWD, and two or less open diffs in a vehicle that drives all four wheels as 4WD (with open meaning open or limited slip)

That's pretty much the accepted definition now, as long as you mention a low gear, as well.

 

[Demon-Xanth]

AWD: A viscous coupling attaches the front and rear axles
4WD: a transfer case or center differential does that.

Basically the same difference as a limited slip differential and a locker.

 

[Dman877]

Originally posted by: caspur

All joking aside, the trooper is not concerned so much with locking diffs, awd/4wd, etc...in deep snow, his concern is ground clearance. Interestingly, a heavier vehicle like the Tahoe would actually do worse in the snow compared to a lighter RX. (w/the same tire footprint).

Actually, heavier vehicles are better in the snow. They are able to push through it easier so their tires contact pavement.

 

[nismotigerwvu]

I know this is a little bit out of the context here, but but about the STI and EVO's of the world....not all AWD systems pack a set of open difs...i believe the EVO has 3 lsd's now

 

[Vic]

AWD and 4WD are mechanically different. However, for the legal purposes being discussed by the OP, they are the same thing, and CHP is wrong. The Lexus with AWD and snow tires does not have to chain up, and the CHP officer's opinion to the contrary is irrelevant. text

 

[ayabe]

I thought chains were illegal now pretty much everywhere because of the damage they cause to the pavement. I was in Iowa last week and I know you can't use chains there.

 

[Demon-Xanth]

In California chains are required over the passes during certain times of the year. During those times, contacting pavement at all isn't reliable.

 

[dainthomas]

Originally posted by: ayabe
I thought chains were illegal now pretty much everywhere because of the damage they cause to the pavement. I was in Iowa last week and I know you can't use chains there.

I think most people could handle the lofty peaks of Iowa without chains.

They're often required in Oregon.

 

[thomsbrain]

My understanding is there shouldn't be a distinction in these cases. The Caltrans website does use the term "4-wheel-drive" and not "all-wheel-drive," but I have never seen the CHP force chains on AWD vehicles in the Sierras. You see Subarus driving around without them all the time. However, there are conditions where the CHP will require ALL vehicles to chain up, and it is possible she has been hitting the road during these times, but when it gets that bad, the CHP usually chooses to close the road instead. She may also not be adequately communicating that her Lexus has both AWD and snow tires, especially when so many RX's are equipped with neither.

Regardless, there is no excuse for not knowing how to put on or take off a set of chains if you LIVE in Truckee.

 

[Kelvrick]

Originally posted by: thomsbrain
My understanding is there shouldn't be a distinction in these cases. The Caltrans website does use the term "4-wheel-drive" and not "all-wheel-drive," but I have never seen the CHP force chains on AWD vehicles in the Sierras. You see Subarus driving around without them all the time. However, there are conditions where the CHP will require ALL vehicles to chain up, and it is possible she has been hitting the road during these times, but when it gets that bad, the CHP usually chooses to close the road instead. She may also not be adequately communicating that her Lexus has both AWD and snow tires, especially when so many RX's are equipped with neither.

Regardless, there is no excuse for not knowing how to put on or take off a set of chains if you LIVE in Truckee.

She shouldn't even need to communicate that she has snow tires. M + S rated tires are lumped in together with snow tires (I know there is a difference). AWD + M+S tires = go on ahead. I've never seen them stop awd/4wd cars instead of just closing the road. Maybe bad california drivers are the cause for this?

They usually see the stars on my subaru badge and wave me past.

This is of course only from my experiences driving up to tahoe/reno from sacramento.

EDIT: Iono about her lexus, but I won't put chains on my subaru, it says right in the manual it'll hurt the car. If I have to, I'll use cables on all four wheels.

 

[ayabe]

Originally posted by: dainthomas

Originally posted by: ayabe
I thought chains were illegal now pretty much everywhere because of the damage they cause to the pavement. I was in Iowa last week and I know you can't use chains there.

I think most people could handle the lofty peaks of Iowa without chains.

They're often required in Oregon.

Hey man, I'm born and raised in FL, I was expecting to see everyone driving around the snow with chains on in the -13 degree weather.

Live and learn

 

[Savij]

****I'm very likely wrong and don't mind being corrected****

I found this site very useful:

http://www.4x4abc.com/4WD101/difference_4WD_awd.html

Basically, my understanding of it is that 4WD is basically the same as a symmetric AWD system and only subaru and volvo (or was it audi) have that. Everyone else has asymmetric AWD on their cars which isn't (edit:wasn't?) as good and that's why caltrans officers make the distinction.

 

[Throckmorton]

They are justified IMO, because the Lexus is powering the front wheels until they slip. The Tahoe is splitting power 50/50 front and rear with the center diff. BTW an open center diff should be safer than locked. You wouldn't want to lock unless you're offroad.

 

[Kelvrick]

Originally posted by: Savij
****I'm very likely wrong and don't mind being corrected****

I found this site very useful:

http://www.4x4abc.com/4WD101/difference_4WD_awd.html

Basically, my understanding of it is that 4WD is basically the same as a symmetric AWD system and only subaru and volvo (or was it audi) have that. Everyone else has asymmetric AWD on their cars which isn't (edit:wasn't?) as good and that's why caltrans officers make the distinction.

Originally posted by: Throckmorton
****I'm very likely wrong and don't mind being corrected****

They are justified IMO, because the Lexus is powering the front wheels until they slip. The Tahoe is splitting power 50/50 front and rear with the center diff. BTW an open center diff should be safer than locked. You wouldn't want to lock unless you're offroad.

I get it now. That makes more sense since the car is basically a fwd before slippage.

EDIT: I :heart: my subbie

 

[Vic]

They're both wrong. CA law in this regard does not differentiate between full-time and part-time 4-wheel/all-wheel drive systems. The Tahoe, for example, is rear wheel drive until the driver engages the 4-wheel drive system.

The CHP officer with the Lexus in the OP was wrong, plain and simple. I've had a similar experience with a CHP in my Subaru, trying to force me to chain up even though I had AWD and snow tires.

 

[dug777]

Originally posted by: Savij
****I'm very likely wrong and don't mind being corrected****

I found this site very useful:

http://www.4x4abc.com/4WD101/difference_4WD_awd.html

Basically, my understanding of it is that 4WD is basically the same as a symmetric AWD system and only subaru and volvo (or was it audi) have that. Everyone else has asymmetric AWD on their cars which isn't (edit:wasn't?) as good and that's why caltrans officers make the distinction.

Anything that has three diffs provides AWD, which means the Toyota Landcruisers and Prados have full time AWD (with rear LSD)...

 

[IronWing]

Since the problem seems to be that your friend has trouble getting the chains on and off by herself she might look into RUD chains which are supposed to be easier to mount. I haven't tried them myself.

http://www.bb4wa.com/articles/Chains.htm