4WD vs. AWD. What's the Difference? • Hacker News Board

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

#161

Post by kls » Sun, Aug 28, 2016, 1:24 AM UTC

tiglionabbit wrote: post #103

tombrossman wrote: post #48

Earlier quoted context omitted.

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…

> 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 think you just said the opposite of the guys above you while saying you agree. 4WD should not have a problem on slippery terrain.

It actually has nothing to do with 4WD or AWD, the article is poorly written and confuses several terms. At the surface 4WD is manually selected and AWD is 4WD all the time. Thats it they are the same thing, it's just 4WD means you opt to put the vehicle in 4WD.

The term the are talking about when traction is applied to the wheel with the most binding to the ground and away from the one that has the least traction is called a Limited Slip Differential and by virtue of the slipping of the low traction wheel it will spool the high traction wheel and bind it transferring torque to it. This can also be done by what is called a locker, either electronic or manual but manual lockers can be dangerous at high speed if you are not used to their manor. You generally only see manual lockers and welded differentials in offroad trucks. Limited Slip differentials and electronic lockers are available in both 4WD and AWD vehicles.

The term they are talking about to apply power to both the front and rear wheels is not a differential but rather a transfer case and it is not always at a 50:50 ration man times they run 60:40 or 70:30. Both 4WD and AWD have a transfer case, 4WD's allow you to select whether they are engaged or not.

The term they are talking about when they talk about adjusting power front to back is called a Traction Control Unit and is handled by a Body Control Module that may or may not also communicate with the Engine Control Unit or the Transmission Control Unit to defuel or detorque the powertrain, as well as adjust torque via electronic lockers in the differential to the wheels that need it and away from the ones that do not. As well as engage or disengage front and back axles via the transfer case. Many newer AWD systems have this but it is also available in most newer 4WD vehicles.

TLDR is 4WD and AWD can be virtually identical depending on the subsystems but the article confuses a lot of those subsystems as being unique to one or the other. Though the upper end of 4wd's have more "hardcore" options not generally found or offered in AWD vehicles.

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

#162

Post by sevenless » Sun, Aug 28, 2016, 1:28 AM UTC

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…

I learned about limited slip differentials from Marisa Tomei's great courtroom scene in My Cousin Vinny: https://www.youtube.com/watch?v=CFdJza0AbeA

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

#163

Post by lsllc » Sun, Aug 28, 2016, 1:34 AM UTC

Alright, time to bust a myth. As an AWD owner, every time I have a tire issue, they tell me I need to replace all 4 even if the others are good ... because AWD. "Won't someone think of the tires ... the tires need to to be the same size so they can rotate at the same rate."

Clearly, with AWD the car is designed to allow wheels to rotate at different rates (hence the diffs). Otherwise you would't be able to make a turn. Once you dispel that myth, they then tell you to replace the pair ...

Why do dealerships, repair shops and tire places keep insisting on replacing all 4? ... good for tire sales I guess.

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

#164

Post by esaym » Sun, Aug 28, 2016, 1:35 AM UTC

magicbuzz wrote: post #129

samwillis wrote: post #41

Earlier quoted context omitted.

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

Jet engines are not "efficient". The SFC is worse than piston engines and in fact, the smaller the turbine the worse the efficiency: https://en.wikipedia.org/wiki/Brake_specific_fuel_consumptio...

I've never heard that jet engines are less efficient. From what I remember piston engines are 33% thermally efficient, diesels 36% and turbine engines are upwards of 60%+. Of course there are many types of jet engines and ways to get power off of them.

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

#165

Post by mariusc23 » Sun, Aug 28, 2016, 1:38 AM UTC

lsllc wrote: post #163

Alright, time to bust a myth. As an AWD owner, every time I have a tire issue, they tell me I need to replace all 4 even if the others are good ... because AWD. "Won't someone think of the tires ... the tires need to to be the same size so they can rotate at the same rate." Clearly, with AWD the car is designed to allow wheels to rotate at different rates (hence the diffs). Otherwise you would't be able to make a tur…

The reasoning I've heard is that your differential will get worn out.

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

#166

Post by lee » Sun, Aug 28, 2016, 2:04 AM UTC

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.

It really depends on the amount of snow and type of snow. Where I live in Canada, we get enough snow that AWD > snow tires. Given enough snow, or conditions, even snow tires will get stuck. Having power on all 4 wheels really does give superior traction in those situations.

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

#167

Post by analog31 » Sun, Aug 28, 2016, 2:09 AM UTC

lee wrote: post #166

analog31 wrote: post #108

Earlier quoted context omitted.

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.

It really depends on the amount of snow and type of snow. Where I live in Canada, we get enough snow that AWD > snow tires. Given enough snow, or conditions, even snow tires will get stuck. Having power on all 4 wheels really does give superior traction in those situations.

Definitely a good point. For reference, I should have mentioned that I'm in the middle of Wisconsin. We actually had very little snow over the past two years, and I'm hoping for more this winter.

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

#168

Post by kalleboo » Sun, Aug 28, 2016, 2:11 AM UTC

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.

There's a good reason snow tires are legally mandated in Sweden during the winter. They work.

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

#169

Post by userbinator » Sun, Aug 28, 2016, 2:15 AM UTC

No mention of the "Detroit Locker", IMHO one of the more clever automatic-locking differential designs --- it uses a pair of dog clutches which are normally engaged, one for each side, and the wheel that spins faster is allowed to. It's self-contained and requires no additional controls, just like a regular differential. I think the only reasons why they didn't become more popular are due to their cost and the noise they make --- a very noticeable clunking on tight turns as the outer wheel tries to go faster and unlocks, then locks again as the vehicle straightens out.

http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differen...

Having driven a "4WD" with all three differentials of this type, I now wonder if it would be classified as 4WD or AWD.

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

#170

Post by morganvachon » Sun, Aug 28, 2016, 2:22 AM UTC

jcbeard wrote: post #149

morganvachon wrote: post #143

Earlier quoted context omitted.

I once had a Jeep Grand Cherokee that had a "full time 4WD" system. Basically you could choose between 4WD low (traditional 4WD with front and rear diffs locked to each other, in a low gearing for massive torque), 4WD high (traditional 4WD as above, in normal gearing), 2WD (front diff unlocked and spinning freely as if it were a regular 2WD car) and full time 4WD (front wheels would automatically lock together only i…

sounds like maybe the locking clutch pads were malfunctioning (pretty sure they used clutch packs vs. gear type). the hopping is never fun.

From what I remember when I had to replace the transfer case a year after I bought it (it was a 1996 that I bought in 2006 and the previous owner abused it), it was more or less a "traditional" Jeep 4WD setup but depended on the transfer case to keep the front and rear axles independent until slippage was detected. I think it was pretty much fully mechanical, except for the auto locking front hubs. I believe Jeep's marketing term for it was Select Trac or something like that.

I think the issue was with the auto locking hubs; when it malfunctioned it would lock the front hubs together and in turn the transfer case would lock the front and rear diffs together, causing the wheel hop. This defect probably contributed to the death of the original transfer case, now that I think about it.