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4WD vs. AWD. What's the Difference?

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Re: 4WD vs. AWD. What's the Difference?

#21

Post by johngalt »

dghughes wrote: post #6

>Torque goes to whichever of the four wheels has the least grip. That should be the tire with the most grip. If torque goes to the wheel where the tire on it has the least grip the tire still has no traction torque doesn't make grip appear out of nowhere. In four-wheel or all-wheel drive one wheel where a tire has no grip the other three wheel with tire that have grip move the vehicle isn't that the entire point of 4…

Mechanically a drivetrain has no means of differentiating between a tire that is slipping vs a tire at that's on the outside of a turn. A 4wd car with three open differentials will absolutely send all of the engine power to one slipping wheel.

The author is describing the tricks used to get around that fact. Namely locking differentials or limited slip differentials. The end result is a fraction of the torque makes it to other Wheels depending on the design of the drivetrain. The author spent a lot of time on locking differentials when those are rare. Limited slip differentials are more common. Most awd cars will have a LSD in the center diff.

There are torsen diffs which do what you describe but they are rare.

Re: 4WD vs. AWD. What's the Difference?

#22

Post by brianwawok »

wil421 wrote: post #20
waterphone wrote: post #17

Earlier quoted context omitted.

> The downside is you shouldn't use it over 10mph or so. That's not strictly true, and varies between implementations/transfer case designs. In Jeep Wranglers, for example, which have both high and low gearing options in 4WD, the owner's manual will indicate that you can drive at any speed (on loose surfaces such as sand or dirt) in 4HI and 0–25 MPH in 4LO.

You right that's why I said 10 or so. Try taking a turn at 25 in 4LO. I'm not sure of the use case for 25 in LO. My Grand Cherokee can operate the same way but I doubt I would go into LO at those speeds. It also will switch to 4HI when it detects wheel slip. A friend of mine had a old truck that would randomly switch to 4LO on the highway. It's pretty scary driving 60 and having 4LO switch on.

In dirt taking the turn at 25 is fine. The inside wheel will just spin a little.

Re: 4WD vs. AWD. What's the Difference?

#23

Post by dsr_ »

ghaff wrote: post #13
blackguardx wrote: post #5

Earlier quoted context omitted.

Generally a sign saying you need 4wd in the US really means you need a lot of ground clearance. Your car probably would have high-centered, that is stuck resting on the middle of the car with the tires in the air. The annoyed ranger was probably just tired of pulling stuck tourists out.

The Waipio Valley road, as I understand it, is mostly paved but extremely steep and narrow/twisty. So the 4x4 requirement is probably more about being able to control speed without burning up brakes and dealing with often wet conditions. However, in the general case throughout most of the US West, "4WD roads" do indeed tend to be as much about having high clearance as about having 4WD per se. It's also just a way of…

And in particular, the Hawaiian rental car counters will tell you when you rent which roads your car is not allowed to go on.

Re: 4WD vs. AWD. What's the Difference?

#24

Post by gambiting »

wil421 wrote: post #11
bitL wrote: post #2

Great! Finally somebody explained this to me, I thought it was just a marketing term related to different patents of similar ideas. I was puzzled by this as I was stopped by a ranger coming out of Waipio Valley on Hawai'i, where there was a sign "4WD only" and my rental car had AWD. I thought they were the same, just different names, so I argued with the ranger that I do in fact have 4WD, which is in my case just cal…

AWD is meant for adverse conditions on a road and will probably help on a unpaved road. Think rain, ice, snow. It will keep you on the road. 4WD is meant for off road conditions. You get more torque so you'll be able to get over rocks and out of ditches. The downside is you shouldn't use it over 10mph or so. I can't think of any recent car that is 4WD. It doesn't make sense since you'll probably never be "off road".…

Any new Land Rover or Range Rover? They come with 2 locking differentials, with 3rd being an option that can be added extra. Same with Mercedes G-class - they come with 3 locks as standard. And I've no idea what you mean by "cars don't have proper ground clearance" - just buy a one that does?

Re: 4WD vs. AWD. What's the Difference?

#25

Post by kinofcain »

johngalt wrote: post #21
dghughes wrote: post #6

>Torque goes to whichever of the four wheels has the least grip. That should be the tire with the most grip. If torque goes to the wheel where the tire on it has the least grip the tire still has no traction torque doesn't make grip appear out of nowhere. In four-wheel or all-wheel drive one wheel where a tire has no grip the other three wheel with tire that have grip move the vehicle isn't that the entire point of 4…

Mechanically a drivetrain has no means of differentiating between a tire that is slipping vs a tire at that's on the outside of a turn. A 4wd car with three open differentials will absolutely send all of the engine power to one slipping wheel. The author is describing the tricks used to get around that fact. Namely locking differentials or limited slip differentials. The end result is a fraction of the torque makes i…

Even a torsen diff, or similar helical/gear type limited slip diffs are "torque multiplying", they need a load on one wheel in order to send the "multiplied" load to the wheel with traction. Modern stability/traction systems equipped with helical diffs will apply the brake to the "free" wheel in order to provide a load to the mechanical diff. You can also apply the brakes yourself if you have a torsen diff but no electronics.

A clutch-based diff or some other type of locking diff would otherwise be required to solve the "wheel in the air" problem without applying an artificial load to the free wheel.

Re: 4WD vs. AWD. What's the Difference?

#26

Post by mschuster91 »

I wonder why car manufacturers still use direct force transmission with hugely complex (and in case of ignoring the instructions, outright dangerous) like 4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on locomotives.

A computer could instantly shift torque with both of the hybrid solutions, you wouldn't need a clutch or a gearbox any more, not to mention cheap all-wheel-drive including the trailer in trucks. And a fuel-electric drivetrain could easily plug in a battery or supercaps for braking energy recuperation, and the fuel engine could always run at peak efficiency/exhaust gas condition...

Re: 4WD vs. AWD. What's the Difference?

#27

Post by anexprogrammer »

tunap wrote: post #15
blackguardx wrote: post #5

Earlier quoted context omitted.

Generally a sign saying you need 4wd in the US really means you need a lot of ground clearance. Your car probably would have high-centered, that is stuck resting on the middle of the car with the tires in the air. The annoyed ranger was probably just tired of pulling stuck tourists out.

Normally I would 2nd this, but it does not apply on this particular 4x4 road. It really sucks when opposing traffic appears as it's barely wide enough for 1 vehicle in most parts: The road gains 800 vertical feet (243.84 m) in 0.6 miles (0.9 km) at a 25% average grade, with steeper grades in sections. This is a paved public road but it is open only to 4 wheel drive vehicles. It is the steepest road of its length in t…

That doesn't seem that bad for normal traffic, conditions permitting - unless the steeper gradients are markedly so. I'm sure they have their reasons for 4wd only though - picking too many tourists out of the ditches or whatever.

There's quite a few roads in the UK with 1:4 gradient, and one just down the road from me at 1:3 whose only restriction is no trucks and vans. Going downhill on the motorbike makes it feel like you're falling off! :)

https://en.wikipedia.org/wiki/Hardknott_Pass

Re: 4WD vs. AWD. What's the Difference?

#28

Post by petepete »

gambiting wrote: post #24
wil421 wrote: post #11

Earlier quoted context omitted.

AWD is meant for adverse conditions on a road and will probably help on a unpaved road. Think rain, ice, snow. It will keep you on the road. 4WD is meant for off road conditions. You get more torque so you'll be able to get over rocks and out of ditches. The downside is you shouldn't use it over 10mph or so. I can't think of any recent car that is 4WD. It doesn't make sense since you'll probably never be "off road".…

Any new Land Rover or Range Rover? They come with 2 locking differentials, with 3rd being an option that can be added extra. Same with Mercedes G-class - they come with 3 locks as standard. And I've no idea what you mean by "cars don't have proper ground clearance" - just buy a one that does?

I think he means regular 'cars' rather than 4×4s (or SUVs).

Re: 4WD vs. AWD. What's the Difference?

#29

Post by UnoriginalGuy »

spacerace wrote: post #14

Jesus what a SHIT article It gets so many things wrong I don't even know where to start. Why is this garbage spam on Hacker News?

I always find it suspect when people claim that something is so bad that they cannot find a single concrete thing to criticise it on.

I'd suggest you start with the thing you believe they got "most wrong" and go from there. Otherwise this comment seems to lack substance.

Re: 4WD vs. AWD. What's the Difference?

#30

Post by kinofcain »

mschuster91 wrote: post #26

I wonder why car manufacturers still use direct force transmission with hugely complex (and in case of ignoring the instructions, outright dangerous) like 4WD/AWD instead of diesel-electric or diesel-hydraulic systems like on locomotives. A computer could instantly shift torque with both of the hybrid solutions, you wouldn't need a clutch or a gearbox any more, not to mention cheap all-wheel-drive including the trail…

The diesel-electric/hydraulic systems are used in situations where the gearing ratio would have to be absurdly high, they're not used because they're efficient or because they have good torque vectoring capabilities. The electric motor can produce nearly 100% of torque at zero RPM, which helps locomotives get several million pounds of train moving. In your car, truck, or even semi, first gear plus some clutch/torque converter slippage is plenty of mechanical advantage to get going, and once you're underway, a lockup torque converter or a fully engaged clutch is more efficient than the combination of an electric generator and a motor.

There's also the matter of power distribution, if you have four independent electric motors, you can only ever send 25% of available torque to one wheel, if you have front, rear, and center lockers, you can send 100% of torque to one wheel.

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