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

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

#41

Post by samwillis »

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 Jaguar C-X75 [1] concept car uses exactly this idea, it uses a tiny and very efficient jet engine that is charging the batteries. Very neat idea.

1: https://en.wikipedia.org/wiki/Jaguar_C-X75

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

#42

Post by kinofcain »

rconti wrote: post #37
kinofcain wrote: post #18

There's a lot of blurring and different labeling of 4WD systems based on the market segment. Generally it's more helpful to think of 4WD/AWD as fitting into three categories: 1: A system with a transfer case that predominantly drives one axle (almost always the rear, and almost always labeled "4WD"). This is your typical pickup truck 4wd system. When it is engaged, the two axles are locked together and may as well be…

Right. I've got a Golf R with Haldex; it allows 30+mpg under cruising conditions but can "lock 100%" in the sense that the fronts and rears are locked together. It's created a lot of forum wars because some people read the marketing materials saying "100% of torque to the rear wheels" or "max 50/50 split" and don't understand the difference. If it's fully locked, on dry pavement, of course you're getting 50/50. If th…

Great car, and yeah, it's even more fun when you explain that the Haldex systems often pull off either the left or right half shaft...

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

#43

Post by dsfyu404ed »

jsmthrowaway wrote: post #39
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…

Locking the center differential in a 4WD vehicle is fine on pavement as long as you don't turn, so forgetting for a small straight of pavement in the middle of the trail is no big deal. Some trails are half dirt road, half long-forgotten pavement, so constantly reaching down can get tiring if one follows that rule verbatim. Snow transitions are similar; your truck would have been fine and I'd just reach down for a dr…

If locking the transfer case and doing U turns on pavement is going to find a weak link in your drive-train then you're not doing something right...

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

#45

Post by jsmthrowaway »

dsfyu404ed wrote: post #43
jsmthrowaway wrote: post #39

Earlier quoted context omitted.

Locking the center differential in a 4WD vehicle is fine on pavement as long as you don't turn, so forgetting for a small straight of pavement in the middle of the trail is no big deal. Some trails are half dirt road, half long-forgotten pavement, so constantly reaching down can get tiring if one follows that rule verbatim. Snow transitions are similar; your truck would have been fine and I'd just reach down for a dr…

If locking the transfer case and doing U turns on pavement is going to find a weak link in your drive-train then you're not doing something right...

Well, yeah. Mine will bark pretty loudly if you do that but it did survive the one time it happened. Said experience is also why nobody but me sits left seat.

I'd have to find my paperwork but I think such maneuvers are cause to void drivetrain warranty. Chaining the tires is, too, for unrelated reasons.

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

#46

Post by Mikeb85 »

sunstone wrote: post #33
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…

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 both tactics work in different scenarios. When power goes to the slipping wheels, it helps keep the car drive strait. For example, in the rain or light snow. If one wheel slips, then suddenly power goes to the wheel with traction, you'll veer off-course.

When it goes to the wheels with traction, it helps get the car un-stuck. This is usually what you want when you're doing serious off-roading, stuck in snow, etc... Also, in this scenario you're usually going very slowly/not moving, so the sudden change of direction won't be a problem.

Modern AWD does a bit of both - it'll send power to wheels with traction, but will also compensate by sending power to a wheel on the other side of the car. For example, if there's no traction on your left front tire, it'll send power to your right front tire and left rear tire, and reduce power on your right rear tire to keep you going strait.

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

#47

Post by andrewstuart2 »

sunstone wrote: post #33
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…

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 of creating at least some resistance everywhere.

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

#48

Post by tombrossman »

sunstone wrote: post #33
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…

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…

That's right. A poorly-engineered 4WD system means you are stuck on a hill because one wheel is on slippery wet leaves. AWD should mean you can get up that hill with three wheels on slippery wet leaves, and one with grip. That's the simplest explanation.

I had an old Toyota Tercel 4WD station wagon and it was very difficult to steer sharply at low speed with 4WD engaged because of the lack of a differential. It would lurch and I had to press harder of the accelerator to get straightened out and pop it back into 2WD. It was my first car so I didn't know any better. Fun fact: In 4WD, you got an 'extra low' gear below 1st and that car could very nearly climb trees. The gear ratio was insane. No ground clearance but I used to scare the crap out of friends by going up very steep inclines in extra low gear with 4WD engaged. They would have to park their pickup trucks at the bottom of the hill and ride with me, and this caused them some embarrassment. Good times.

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

#49

Post by ynniv »

Of course, this only matters when you are driving four wheels with one engine. Now that we're going electric you can have one engine per wheel, or one front engine and one back engine and use ABS across single axles. It's kind of amazing how much progress we have made with simple internal combustion engines.
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