7 ms·

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—

by japhyr 10y ago

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. This provides great traction, but a vehicle locked in 4WD cannot safely be operated on dry pavement because its front and rear axles are forced to rotate at the same speeds." I remember being scared when I bought my first 4wd vehicle, a '91 Toyota pickup. I knew I shouldn't have the wheels locked in 4wd on dry pavement, and I was worried about going from snowy roads to dry pavement. When I moved on to an Outback (AWD, not 4wd) I wondered why I didn't have to worry about transitioning between slippery roads and dry pavement. Now I understand. There's also a great video from 1937(!) that shows why we need differentials, and how they work. Here's a direct link to the video if you're going to skip the article: https://www.youtube.com/watch?v=yYAw79386WI https://www.youtube.com/watch?v=yYAw79386WI

neom 10y ago
woooow! Re: youtube video - Go to market in 1930s was actually how to go to market. When you think that Chevrolet really got going in 1920, 17 years later their marketing was super on point (here we are still watching it today, content marketing at it's best).
sunstone 10y ago
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 more sense.
- Mikeb85 10y ago
  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.
- andrewstuart2 10y ago
  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.
  - NickNameNick 10y ago
    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.
- tombrossman 10y ago
  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.
  - alanning 10y ago
    I loved my old Tercel wagon. Didn't use that low gear all that often but fun to have. CV joints were always my problem, ended up having to replace them quite often. One time in college I was turning too sharply while parking and the axle actually popped out of the right front. Loose axle would just spin in 2WD but switching to 4WD enabled me to make it to the nearby Mech E. building and get help. :)
    - icantdrive55 10y ago
      I gave a 89 Toyota that kinda took care of, with 240,000 miles. She still has 125 psi in the cylinders. Changed the oil when I felt like it. Changed the oil filter only sometimes. Never changed the air filter. Never repaired a tapping valve. In other words, didn't take care of her like other vechicles I owned. One day, I think, even 80's and 90's Toyotas will be collectors items. I know some of them are collectors vechicles now, but I feel the 80's to 90's vechicles will see a huge boost in interest.
      - madengr 10y ago
        I had an 87 pickup and 87 tercel. The engines will last forever, but rust is the killer on the truck. Currently have a 2005 Tacoma and it is rust free (plastic wheel wells) but even it has a recall for rusty suspension.
- gens 10y ago
  A well engineered differential [0]. [0] https://www.youtube.com/watch?v=JEiSTzK-A2A https://www.youtube.com/watch?v=JEiSTzK-A2A
- taneq 10y ago
  Everyone's focusing on the technology involved, but that's kind of pointless because AWD isn't a technical term. It's a term to distinguish an on-road car with four driven wheels from an all-terrain car, because when the first WRXes and such came out, people saw 4WD and thought that meant they could be used to go bush bashing.
  - zensavona 10y ago
    Take her for a bush bash m8
    - sctb 10y ago
      Please don't comment like this here. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
  - fungi 10y ago
    just got a 2nd hand impreza, can confirm more then a couple flogged cars in the market!
- avn2109 10y ago
  >> "...sends power away from the slipping wheel(s)..." Exactly this. The best ones are entirely mechanical, without any fancy computers or even viscous couplings. Audi sold (sells?) IMHO the most sophisticated AWD system on the market, which uses a system of crossed helical gears to continuously and instantaneously redistribute torque proportionally to wheel traction, up to a roughly 80-20 split. There are no computers and the whole thing is mechanical. The differential is a minor mechanical miracle sold under the trade name Torsen, short for "torque sensing." Despite Audi's German-engineering themed marketing, the differential was actually designed by an American. In the 80's, the station wagons even had a manually-actuated center locker, which was eventually replaced by a servomoter driven locker. I don't know what the new ones have.
  - joushou 10y ago
    Audi Quattro is great, although IIRC, it also eats a bit tires on tight turns like anything but an open diff would. I also like the viscous clutch on the jeep, although it definitely is less on-road friendly (A tight 90 degree turn is enough to hear the tires, 180 and you can almost feel the car get lower). My worst system was my 98 explorer - it engages the clutch completely when front axle falls behind, waits a second, disengages to check if axle is still behind, if so, reengages... It's jumpy as hell when turning, and sucked no matter what. You could mod it for pure 2WD operation by switch, but stock was awful. However, I don't think you should underestimate electric systems. I mean, I hate car software with a passion, but when they work, they actually have really good characteristics. Things like LSD's where the pump is electrically enabled give quite smooth transitions, and can simulate the response of both on and offroad mechanic systems. Response time is also nearly instant. If car computers weren't bloody black boxes (or at least leave only important things to the black box!), these systems often end up being much simpler than their purely mechanic black voodoo counterparts.
- markdown 10y ago
  In the famous Saudi videos (there are hundreds of them), the wheels that lose traction completely stop. See https://www.youtube.com/watch?v=MQm5BnhTBEQ https://www.youtube.com/watch?v=MQm5BnhTBEQ
jsmthrowaway 10y ago
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 dry corner if you can disengage in motion. If not, yeah, bigger concern. Front axle locker is similar, but with that you feel how difficult it is to steer as soon as you lock it. (In my Wrangler if I'm finding that I need the fronts we are in some serious shit anyway.)
- dsfyu404ed 10y ago
  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...
  - jsmthrowaway 10y ago
    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.
jcbeard 10y ago
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 axles to turn at different rates when making turns (otherwise bad things would happen). Some cheaper 4WD vehicles (and many 2WD vehicles) have open differentials. Open differentials send the power to the path of least resistance (bad for traction, and also why if you're in gravel the loose wheel usually just spins while the other one doesn't). There's also a fully locking differential that literally locks the axles no matter what. These cannot be used on pavement for obvious reasons (well, think about turning a corner, the outside wheel will have to turn faster than the inside wheel). AWD vehicles have a center diff typically and also often employ limited slip differentials (but sometimes again, they have open and/or auto-locking diffs...ARB made air locking diffs at one point, not sure if they still do) at each axle as well which enables a mechanical distribution of power. Some newer vehicles have replaced the axle differential gears with anti-lock breaking systems that provide resistance to open diffs to regulate wheel spin (artificially add resistance to the spinning wheel).....sorry, before I became a computer geek I built motors and off-roaded in my 81 CJ-5, after that I drag raced for fun on the weekends at the local track. I had a limited slip in the mustang I raced and a fully locking one in my Jeep. Jeeps (and older 4WD) are interesting b/c many come with what are called locking hubs ( https://en.wikipedia.org/wiki/Locking_hubs https://en.wikipedia.org/wiki/Locking_hubs ) which are not all that common today. Even if you do find them, they're usually auto-engaging vs. the ones you have to get out an manually turn to engage the front drive shaft.
- rascul 10y ago
  Very hard to read because you bunched everything into one huge paragraph but if I read it right then you are absolutely correct.
  - jcbeard 10y ago
    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.
jlarocco 10y ago
Wow, that video does a really great job building up the explanation. I already knew how a differential worked (thanks to Legos), but I had never thought about it that way before.
ptaipale 10y ago
Nice video (well, I'd say motion picture). Reminds me of the Master Hands: https://www.youtube.com/watch?v=8bT6txm4RpA https://www.youtube.com/watch?v=8bT6txm4RpA
sandworm101 10y ago
I would add that in addition to locking between the front and rear axles, many 4x4s allow for the wheels to be locked across single axles. Right and left tires then turn at the same rate. This is horrific on dry pavement as one wheel will drag in every corner, but an enormous advantage in rough terrain. Some of these can be locked manually/physically, but more often these days they are controlled electronically. Of course this defeats any traction control systems, normally leading to a warning light telling you that you are on your own in that regard. Antilocks also often disabled in this mode.
sevenless 10y ago
I learned about limited slip differentials from Marisa Tomei's great courtroom scene in My Cousin Vinny: https://www.youtube.com/watch?v=CFdJza0AbeA https://www.youtube.com/watch?v=CFdJza0AbeA

by japhyr 10y ago

neom 10y ago

woooow! Re: youtube video - Go to market in 1930s was actually how to go to market. When you think that Chevrolet really got going in 1920, 17 years later their marketing was super on point (here we are still watching it today, content marketing at it's best).

sunstone 10y ago

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 more sense.

Mikeb85 10y ago
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.
andrewstuart2 10y ago
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.
- NickNameNick 10y ago
  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.
tombrossman 10y ago
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.
- alanning 10y ago
  I loved my old Tercel wagon. Didn't use that low gear all that often but fun to have. CV joints were always my problem, ended up having to replace them quite often. One time in college I was turning too sharply while parking and the axle actually popped out of the right front. Loose axle would just spin in 2WD but switching to 4WD enabled me to make it to the nearby Mech E. building and get help. :)
  - icantdrive55 10y ago
    I gave a 89 Toyota that kinda took care of, with 240,000 miles. She still has 125 psi in the cylinders. Changed the oil when I felt like it. Changed the oil filter only sometimes. Never changed the air filter. Never repaired a tapping valve. In other words, didn't take care of her like other vechicles I owned. One day, I think, even 80's and 90's Toyotas will be collectors items. I know some of them are collectors vechicles now, but I feel the 80's to 90's vechicles will see a huge boost in interest.
    - madengr 10y ago
      I had an 87 pickup and 87 tercel. The engines will last forever, but rust is the killer on the truck. Currently have a 2005 Tacoma and it is rust free (plastic wheel wells) but even it has a recall for rusty suspension.
gens 10y ago
A well engineered differential [0]. [0] https://www.youtube.com/watch?v=JEiSTzK-A2A https://www.youtube.com/watch?v=JEiSTzK-A2A
taneq 10y ago
Everyone's focusing on the technology involved, but that's kind of pointless because AWD isn't a technical term. It's a term to distinguish an on-road car with four driven wheels from an all-terrain car, because when the first WRXes and such came out, people saw 4WD and thought that meant they could be used to go bush bashing.
- zensavona 10y ago
  Take her for a bush bash m8
  - sctb 10y ago
    Please don't comment like this here. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
- fungi 10y ago
  just got a 2nd hand impreza, can confirm more then a couple flogged cars in the market!
avn2109 10y ago
>> "...sends power away from the slipping wheel(s)..." Exactly this. The best ones are entirely mechanical, without any fancy computers or even viscous couplings. Audi sold (sells?) IMHO the most sophisticated AWD system on the market, which uses a system of crossed helical gears to continuously and instantaneously redistribute torque proportionally to wheel traction, up to a roughly 80-20 split. There are no computers and the whole thing is mechanical. The differential is a minor mechanical miracle sold under the trade name Torsen, short for "torque sensing." Despite Audi's German-engineering themed marketing, the differential was actually designed by an American. In the 80's, the station wagons even had a manually-actuated center locker, which was eventually replaced by a servomoter driven locker. I don't know what the new ones have.
- joushou 10y ago
  Audi Quattro is great, although IIRC, it also eats a bit tires on tight turns like anything but an open diff would. I also like the viscous clutch on the jeep, although it definitely is less on-road friendly (A tight 90 degree turn is enough to hear the tires, 180 and you can almost feel the car get lower). My worst system was my 98 explorer - it engages the clutch completely when front axle falls behind, waits a second, disengages to check if axle is still behind, if so, reengages... It's jumpy as hell when turning, and sucked no matter what. You could mod it for pure 2WD operation by switch, but stock was awful. However, I don't think you should underestimate electric systems. I mean, I hate car software with a passion, but when they work, they actually have really good characteristics. Things like LSD's where the pump is electrically enabled give quite smooth transitions, and can simulate the response of both on and offroad mechanic systems. Response time is also nearly instant. If car computers weren't bloody black boxes (or at least leave only important things to the black box!), these systems often end up being much simpler than their purely mechanic black voodoo counterparts.
markdown 10y ago
In the famous Saudi videos (there are hundreds of them), the wheels that lose traction completely stop. See https://www.youtube.com/watch?v=MQm5BnhTBEQ https://www.youtube.com/watch?v=MQm5BnhTBEQ

jsmthrowaway 10y ago

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 dry corner if you can disengage in motion. If not, yeah, bigger concern. Front axle locker is similar, but with that you feel how difficult it is to steer as soon as you lock it. (In my Wrangler if I'm finding that I need the fronts we are in some serious shit anyway.)

dsfyu404ed 10y ago
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...
- jsmthrowaway 10y ago
  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.

jcbeard 10y ago

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 axles to turn at different rates when making turns (otherwise bad things would happen). Some cheaper 4WD vehicles (and many 2WD vehicles) have open differentials. Open differentials send the power to the path of least resistance (bad for traction, and also why if you're in gravel the loose wheel usually just spins while the other one doesn't). There's also a fully locking differential that literally locks the axles no matter what. These cannot be used on pavement for obvious reasons (well, think about turning a corner, the outside wheel will have to turn faster than the inside wheel). AWD vehicles have a center diff typically and also often employ limited slip differentials (but sometimes again, they have open and/or auto-locking diffs...ARB made air locking diffs at one point, not sure if they still do) at each axle as well which enables a mechanical distribution of power. Some newer vehicles have replaced the axle differential gears with anti-lock breaking systems that provide resistance to open diffs to regulate wheel spin (artificially add resistance to the spinning wheel).....sorry, before I became a computer geek I built motors and off-roaded in my 81 CJ-5, after that I drag raced for fun on the weekends at the local track. I had a limited slip in the mustang I raced and a fully locking one in my Jeep. Jeeps (and older 4WD) are interesting b/c many come with what are called locking hubs ( https://en.wikipedia.org/wiki/Locking_hubs https://en.wikipedia.org/wiki/Locking_hubs ) which are not all that common today. Even if you do find them, they're usually auto-engaging vs. the ones you have to get out an manually turn to engage the front drive shaft.

rascul 10y ago
Very hard to read because you bunched everything into one huge paragraph but if I read it right then you are absolutely correct.
- jcbeard 10y ago
  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.

jlarocco 10y ago

Wow, that video does a really great job building up the explanation. I already knew how a differential worked (thanks to Legos), but I had never thought about it that way before.

ptaipale 10y ago

Nice video (well, I'd say motion picture). Reminds me of the Master Hands: https://www.youtube.com/watch?v=8bT6txm4RpA https://www.youtube.com/watch?v=8bT6txm4RpA

sandworm101 10y ago

I would add that in addition to locking between the front and rear axles, many 4x4s allow for the wheels to be locked across single axles. Right and left tires then turn at the same rate. This is horrific on dry pavement as one wheel will drag in every corner, but an enormous advantage in rough terrain. Some of these can be locked manually/physically, but more often these days they are controlled electronically. Of course this defeats any traction control systems, normally leading to a warning light telling you that you are on your own in that regard. Antilocks also often disabled in this mode.

sevenless 10y ago

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