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AWD means that all the wheels receive power from the power train. Most AWD will stop providing power to a wheel if it loses traction (spins). This is because th

by jrs235 2y ago

AWD means that all the wheels receive power from the power train. Most AWD will stop providing power to a wheel if it loses traction (spins). This is because they have an unlocked differential. 4WD typically means all four wheels can receive power from the power train and can be locked so that that will not lose power if they spin.

falcolas 2y ago
> AWD means that all the wheels receive power from the power train. I used to think that, until my car (Dodge Charger) with AWD had its tail swing out on ice. Basically, AWD for that vehicle meant "Rear Wheel Drive until it slips," at which point the front wheels would receive power. Same with my Toyota Prius, though it was FWD until it slipped, then the rear wheels kicked in. Even my AWD EV with dedicated motors doesn't have them all active all the time. The Subaru is the only AWD vehicle I've owned which was full time 4 wheel drive.
- Klonoar 2y ago
  We typically refer to these as RWD-biased AWD, etc. BMW’s xDrive is a RWD biased AWD.
bryanlarsen 2y ago
Not all 4WD systems have locking differential's.
- alamortsubite 2y ago
  No, but "4WD" indicates they have a transfer case rather than a center differential.
nkurz 2y ago
Since the original citation was for a Subaru, it made me wonder how Subaru's X-Mode actually works. It says, among other things: X-MODE in Subaru vehicles is specifically engineered to push through tough conditions like snow and mud. By adjusting the throttle response and controlling the transmission, it ensures that power is distributed effectively to the wheels, reducing slip and enhancing grip. https://www.subaru.com/vehicle-info/articles/what-is-subaru-x-mode.html https://www.subaru.com/vehicle-info/articles/what-is-subaru-... Does anyone know what it actually does and how well it does it?
- bryanlarsen 2y ago
  https://en.wikipedia.org/wiki/Limited-slip_differential https://en.wikipedia.org/wiki/Limited-slip_differential is the general answer, I believe. Interesting reading whether it's the correct answer or not. The section labelled "electronic" is probably more applicable to the Subaru.
- rasz 2y ago
  On something like Forester snapped CV/Half Shaft Axle means car wont move.
- foobarbecue 2y ago
  It's so wild how consumer products don't disclose the basics of how they work to their customers any more.
  - darby_nine 2y ago
    Yes they do. User manuals just don't make for very good marketing.
    - eigen 2y ago
      >> consumer products don't disclose the basics of how they work to their customers > Yes they do. User manuals just don't make for very good marketing. user manuals tell you how to do something, not how it works. E.G. a motherboard user manual tells you how to enter BIOS Setup, not how secure boot works; where to set USB power output in system power states, not what the power states are and how they transition. https://www.asus.com/us/supportonly/a320m-c/helpdesk_manual/ https://www.asus.com/us/supportonly/a320m-c/helpdesk_manual/
      - shiroiushi 2y ago
        We need to go back to the 1960s/1970s, where Tektronix oscilloscopes and other such equipment came with user manuals that had detailed explanations of how the electronics worked. https://w140.com/tekwiki/wiki/Main_Page https://w140.com/tekwiki/wiki/Main_Page https://bama.edebris.com/manuals/tek https://bama.edebris.com/manuals/tek As an example, check out the user manual (not the repair manual, though that's available on this site too) for the popular Tektronix 465, and scroll down to the "Circuit Description" section: https://bama.edebris.com/download/tek/465/465_v6.pdf https://bama.edebris.com/download/tek/465/465_v6.pdf
kube-system 2y ago
If a wheel spins on an axle with an open differential, mechanically speaking, it receives all the power, being that it is the path of least resistance. That's a problem because the remaining wheels are the only useful ones for propelling the vehicle, but they stop receiving power. The reason power may be cut is a traction control system response to this slipping, not the mechanical drivetrain configuration itself. Depending on the vehicle, this may be done by cutting power at the engine, or by braking the individual wheel that is spinning, or both. A locking differential distributes the power evenly between both sides, all of the time. Limited-slip differentials do something in between the above two, depending on the type.
carabiner 2y ago
No, locking diffs are not standard with 4WD. Tacomas have 4WD without locking diffs. This guy on Quora claims that 99% of 4WD vehicles are open differential: https://www.quora.com/Do-all-4WDs-have-lockers/answer/Glenn-Slippy https://www.quora.com/Do-all-4WDs-have-lockers/answer/Glenn-...
- 10u152 2y ago
  You’re a bit confused here. A Subaru is AWD, and essentially has 3 diffs. Centre, front and rear. A Tacoma has 2 diffs. And a transfer case in the middle. A transfer case acts like a locked centre diff when engaged in 4wd, otherwise only sends power to the rear diff. When 4x4 people talk about locking diffs they mean a locking front and or rear diff. Something like a wrangler rubicon or a G-class would have a locking front and rear from factory which is rare. Transfer case and locking rear isn’t particularly unusual in an offroad vehicle.
londons_explore 2y ago
The funny thing is, I think (nearly) all cars, with minor changes to the ABS unit, could have the hardware necessary for brake based torque vectoring within the ABS unit. But, due to the ABS unit software being contracted out to a different company, it only is used for ABS and cannot act to make the car have a virtual diff lock, unless the car manufacturer wants to pay Bosch the extra $$$ for different software.
- fingerlocks 2y ago
  It’s not that simple. “Naive” torque vectoring will shatter CV axles if the drivetrain isn’t designed for it. Consider the scenario when three wheels are completely stuck in mud/sand and one is spinning. If the load is transferred from the free wheel and the others don’t gain traction, something will give.
  - londons_explore 2y ago
    Software would presumably be programmed with those limits to not break stuff. During normal operation, every gear in a differential holds the whole drive torque anyway. So really the only issue is sudden changes in wheel vectoring which could end up using engine/driveshaft momentum to smash gears. You'd just program the software to not do that.
fingerlocks 2y ago
Every AWD vehicle I’m familiar with has either a mechanical LSD (Ford, VW, Audi, early Subaru) or uses torque vectoring (late Subaru, Tesla). If most AWDs have open diffs, who makes them? Honda and Kia?
- xattt 2y ago
  Kia/Hyundai uses Magnadrive, which seems to be a clone of Haldex AWD systems.

by jrs235 2y ago

falcolas 2y ago

> AWD means that all the wheels receive power from the power train. I used to think that, until my car (Dodge Charger) with AWD had its tail swing out on ice. Basically, AWD for that vehicle meant "Rear Wheel Drive until it slips," at which point the front wheels would receive power. Same with my Toyota Prius, though it was FWD until it slipped, then the rear wheels kicked in. Even my AWD EV with dedicated motors doesn't have them all active all the time. The Subaru is the only AWD vehicle I've owned which was full time 4 wheel drive.

Klonoar 2y ago
We typically refer to these as RWD-biased AWD, etc. BMW’s xDrive is a RWD biased AWD.

bryanlarsen 2y ago

Not all 4WD systems have locking differential's.

alamortsubite 2y ago
No, but "4WD" indicates they have a transfer case rather than a center differential.

nkurz 2y ago

Since the original citation was for a Subaru, it made me wonder how Subaru's X-Mode actually works. It says, among other things: X-MODE in Subaru vehicles is specifically engineered to push through tough conditions like snow and mud. By adjusting the throttle response and controlling the transmission, it ensures that power is distributed effectively to the wheels, reducing slip and enhancing grip. https://www.subaru.com/vehicle-info/articles/what-is-subaru-x-mode.html https://www.subaru.com/vehicle-info/articles/what-is-subaru-... Does anyone know what it actually does and how well it does it?

bryanlarsen 2y ago
https://en.wikipedia.org/wiki/Limited-slip_differential https://en.wikipedia.org/wiki/Limited-slip_differential is the general answer, I believe. Interesting reading whether it's the correct answer or not. The section labelled "electronic" is probably more applicable to the Subaru.
rasz 2y ago
On something like Forester snapped CV/Half Shaft Axle means car wont move.
foobarbecue 2y ago
It's so wild how consumer products don't disclose the basics of how they work to their customers any more.
- darby_nine 2y ago
  Yes they do. User manuals just don't make for very good marketing.
  - eigen 2y ago
    >> consumer products don't disclose the basics of how they work to their customers > Yes they do. User manuals just don't make for very good marketing. user manuals tell you how to do something, not how it works. E.G. a motherboard user manual tells you how to enter BIOS Setup, not how secure boot works; where to set USB power output in system power states, not what the power states are and how they transition. https://www.asus.com/us/supportonly/a320m-c/helpdesk_manual/ https://www.asus.com/us/supportonly/a320m-c/helpdesk_manual/
    - shiroiushi 2y ago
      We need to go back to the 1960s/1970s, where Tektronix oscilloscopes and other such equipment came with user manuals that had detailed explanations of how the electronics worked. https://w140.com/tekwiki/wiki/Main_Page https://w140.com/tekwiki/wiki/Main_Page https://bama.edebris.com/manuals/tek https://bama.edebris.com/manuals/tek As an example, check out the user manual (not the repair manual, though that's available on this site too) for the popular Tektronix 465, and scroll down to the "Circuit Description" section: https://bama.edebris.com/download/tek/465/465_v6.pdf https://bama.edebris.com/download/tek/465/465_v6.pdf

kube-system 2y ago

If a wheel spins on an axle with an open differential, mechanically speaking, it receives all the power, being that it is the path of least resistance. That's a problem because the remaining wheels are the only useful ones for propelling the vehicle, but they stop receiving power. The reason power may be cut is a traction control system response to this slipping, not the mechanical drivetrain configuration itself. Depending on the vehicle, this may be done by cutting power at the engine, or by braking the individual wheel that is spinning, or both. A locking differential distributes the power evenly between both sides, all of the time. Limited-slip differentials do something in between the above two, depending on the type.

carabiner 2y ago

No, locking diffs are not standard with 4WD. Tacomas have 4WD without locking diffs. This guy on Quora claims that 99% of 4WD vehicles are open differential: https://www.quora.com/Do-all-4WDs-have-lockers/answer/Glenn-Slippy https://www.quora.com/Do-all-4WDs-have-lockers/answer/Glenn-...

10u152 2y ago
You’re a bit confused here. A Subaru is AWD, and essentially has 3 diffs. Centre, front and rear. A Tacoma has 2 diffs. And a transfer case in the middle. A transfer case acts like a locked centre diff when engaged in 4wd, otherwise only sends power to the rear diff. When 4x4 people talk about locking diffs they mean a locking front and or rear diff. Something like a wrangler rubicon or a G-class would have a locking front and rear from factory which is rare. Transfer case and locking rear isn’t particularly unusual in an offroad vehicle.

londons_explore 2y ago

The funny thing is, I think (nearly) all cars, with minor changes to the ABS unit, could have the hardware necessary for brake based torque vectoring within the ABS unit. But, due to the ABS unit software being contracted out to a different company, it only is used for ABS and cannot act to make the car have a virtual diff lock, unless the car manufacturer wants to pay Bosch the extra $$$ for different software.

fingerlocks 2y ago
It’s not that simple. “Naive” torque vectoring will shatter CV axles if the drivetrain isn’t designed for it. Consider the scenario when three wheels are completely stuck in mud/sand and one is spinning. If the load is transferred from the free wheel and the others don’t gain traction, something will give.
- londons_explore 2y ago
  Software would presumably be programmed with those limits to not break stuff. During normal operation, every gear in a differential holds the whole drive torque anyway. So really the only issue is sudden changes in wheel vectoring which could end up using engine/driveshaft momentum to smash gears. You'd just program the software to not do that.

fingerlocks 2y ago

Every AWD vehicle I’m familiar with has either a mechanical LSD (Ford, VW, Audi, early Subaru) or uses torque vectoring (late Subaru, Tesla). If most AWDs have open diffs, who makes them? Honda and Kia?

xattt 2y ago
Kia/Hyundai uses Magnadrive, which seems to be a clone of Haldex AWD systems.