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

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

#131

Post by qyv »

dsr_ wrote: post #23
ghaff wrote: post #13

Earlier quoted context omitted.

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.

I have heard this before as well, but having rented cars in Maui many times I can't say that I have ever been told not to drive on certain roads.

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

#133

Post by justinph »

analog31 wrote: post #108
justinph wrote: post #100

In real life, in most situations, ground clearance, good tires, traction control and ABS make more difference than driving all four wheels, most of the time. Personally, I'm more concerned with stopping than starting, and that's where ABS and good tires matter and what kind of transmission you have doesn't. This is a sales pitch video, but it is interesting to illustrate the differences in AWD approaches, with differ…

Something I've learned is that snow tires make a big difference during the winter, and I've read that they make a bigger difference than AWD.

Absolutely. Have owned a Prius and a Subaru. The Prius with good snow tires was phenomenal in the snow, better than the Subaru with all-seasons.

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

#134

Post by slr555 »

All this talk of traction and moving forward is great but one of the most important things to remember about 4WD or AWD especially in snow and ice is that is doesn't enhance your ability to brake. It is easy to feel fearless being able to accelerate on a slippery road but a bummer to discover you can't stop (yes, I get that engine braking plays a role...).

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

#135

Post by paganel »

analog31 wrote: post #108
justinph wrote: post #100

In real life, in most situations, ground clearance, good tires, traction control and ABS make more difference than driving all four wheels, most of the time. Personally, I'm more concerned with stopping than starting, and that's where ABS and good tires matter and what kind of transmission you have doesn't. This is a sales pitch video, but it is interesting to illustrate the differences in AWD approaches, with differ…

Something I've learned is that snow tires make a big difference during the winter, and I've read that they make a bigger difference than AWD.

Have been a driver for only 2 years but I can confirm that. I live in Eastern Europe where we tend to have at least one or two snow storms per winter, and, indeed, having winter tires and ABS makes a ton of difference, even on a 1.4-liter FWD vehicle like mine.

And, even more important, you have to keep a level head when you see that there's snow on the road and not try to do anything crazy, like making sudden left or right wheel movements. Also try to keep a safe distance from the vehicle in front of you, and, before you venture on a trip outside of the city, just check the weather forecasts: if there's a "red code" storm announced you'd better postpone your journey, only a tank can get you out from something like this: https://www.youtube.com/watch?v=Frf2lV77fl0

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

#136

Post by mcpherrinm »

rsync wrote: post #113
Animats wrote: post #38

There are lots of variations on this theme. There are off-road differentials which limit the speed difference between the output shafts to a fixed maximum ratio, usually in the 2:1 to 4:1 range. It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels…

"It's possible to do some or all of this in software. Tesla has an ordinary open differential but will apply the brake on an overspeeding wheel. Tesla's all wheel drive system has a separate motor for the front and rear wheels. Power distribution between the two during acceleration is mostly equal, but once speed stabilizes, the rear powertrain takes most of the load. Off-road slip handling doesn't seem to be well do…

One more thing to think about, on this line of questioning, is efficiency in "normal" driving situations.

If you're driving in a straight line, with good traction on all wheels, will a single larger motor be more efficient than two smaller ones on each wheel? For many people, I think that's 99% of their driving, so it makes sense to optimize. Especially on electric vehicles, where increasing battery capacity is much harder than just adding an extra gas can.

Maybe in that case the complexity of AWD/4WD/Traction control makes sense in that case.

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

#137

Post by irahul »

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…

https://www.youtube.com/watch?v=2xGp5anlXSA

The video posted in another comment doesn't look that bad.

I am not sure about the grade, but one of the Bangalore's weekend gateway has some steep sections(a low average grade but some sections are steep).

https://www.youtube.com/watch?v=C0_MyaPzM88

That footage is from a rally but every weekend, fwd cars with 800/1000cc engines and a peak torque of about 70 Nm do it just fine. Most of the cars have 2-5 occupants decreasing the already low torque/kg, and almost the whole climb is done in a bumper to bumper traffic(no momentum for the climb).

https://dc-cdn.s3-ap-southeast-1.amazonaws.com/dc-Cover-nj6b...

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

#138

Post by NickNameNick »

andrewstuart2 wrote: post #47
sunstone wrote: post #33

Earlier quoted context omitted.

And in the next paragraph "To counteract this [sending of power to the wheel of least resistance] , the better AWD cars are fitted with a center differential that contains a clutch or viscous drive unit. This splits torque front-to-rear, directing it away from the spinning wheel. " So, a well engineered AWD system actually sends power away from the slipping wheel(s) and towards the wheels with traction, which makes m…

Well, not quite; the viscous drive unit really just ensures that every path has some resistance floor, and thus that not all torque goes to one wheel. At one point I remember there was a workaround mentioned by the Hummer engineers (I believe) to apply the brake simultaneously with the gas if you ran into the "one wheel spinning in the air" problem. This is a simply a manual application of the same physical solution…

The trick with the hummers was slightly different.

Hummers have torsen diffs, which have a pair of worm gears where a conventional open diff would have single star gear. As a result, the torsen diff sends some fixed multiple (changes with model) of the torque used by the easier to spin side, to the harder to spin side.

Of course if one wheel is spinning in air, the easy side uses effectively 0 torque, and 0 times anything is still 0, hence the brakes.

Applying the brakes increases the torque requirement of all wheels by a fixed amount, but the hard to spin wheels get More than that amount of extra torque.

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

#139

Post by rascul »

jcbeard wrote: post #93
japhyr wrote: post #3

Here are the key takeaways for me: "Thanks to that differential between your axles, an AWD car will send your engine’s power down the path of least resistance—the wheel with the least grip. Where a two-wheel drive car can only choose between two wheels, an AWD system looks for that least resistance across all four wheels." "4WD works by locking the front and rear axles together, splitting torque 50:50 between them. T…

This isn't always true either. 4WD can mean that you send equal amounts of power to each axle, but at that point there are things that can change the amount of power going to each wheel within the center differential casing. If you have a limited slip differential (why some 4WD vehicles can drive perfectly fine on pavement) it'll basically send a 50/50 split (lock the axles together when going straight) but allow the…

Very hard to read because you bunched everything into one huge paragraph but if I read it right then you are absolutely correct.

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

#140

Post by jcbeard »

rascul wrote: post #139
jcbeard wrote: post #93

Earlier quoted context omitted.

This isn't always true either. 4WD can mean that you send equal amounts of power to each axle, but at that point there are things that can change the amount of power going to each wheel within the center differential casing. If you have a limited slip differential (why some 4WD vehicles can drive perfectly fine on pavement) it'll basically send a 50/50 split (lock the axles together when going straight) but allow the…

Very hard to read because you bunched everything into one huge paragraph but if I read it right then you are absolutely correct.

hah, I'll edit after a bit :) . Wrote while re-enacting the battle of hoth in lego form with my son...hard to do paragraphs on iphone.
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