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Comment by jrs235 | Hacker News Reader

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.

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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.

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-...

Does anyone know what it actually does and how well it does it?

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.

On something like Forester snapped CV/Half Shaft Axle means car wont move.

It's so wild how consumer products don't disclose the basics of how they work to their customers any more.

Yes they do. User manuals just don't make for very good marketing.

>> 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/

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://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

I'm sympathetic to the reason why: Most people don't read. It's wasted work to write elaborate manuals.

Also, many people are busy. Very busy. Even if someone wanted to read, he's way too tired to immerse in new knowledge. Source: Me. I simply have neither the time nor the energy to bother with learning anymore unless there's a very good reason to.

cars are a comparatively rare purchases, that costs a ton of money, and that has life-ending safety risks. Read the book.

If you don't read the microwave's instructions and don't understand how 50% defrost power works chances are it's not going to permanently cripple you, or lead to $10k's in liability and repairs.

>cars are a comparatively rare purchases, that costs a ton of money, and that has life-ending safety risks. Read the book.

No. Ain't nobody got the time.

Operating a car safely doesn't require reading through a 2 inch thick tome every time you buy a car.

I thought I did when I was still a sweet summer child, reading every manual within reach and being told "How do you know that????" by every adult around me. That felt good.

I'm an adult now. Ain't nobody got the time. I know nothing; as in the third stage, not the first stage. I will ask the nearest kid "How do you know that????" and save myself the time and energy if I must learn.

The very first car you buy? Okay, sure, read through the tome since you have nothing to build on. But second car onwards? Ain't nobody got the time.

What? You're trying to tell me I don't know what the fuck Subaru X-Mode is? You're bold to assume I would even use Subaru X-Mode. That button isn't getting pressed for the life of the car ownership; users by and large do not change defaults and hate having dials and buttons to mess with, and I am slowly but surely becoming one of them.

Ain't nobody got the time.

But at least I don't suffer from a bad case of acne anymore.

You sure do seem like a busy person with no time for frivolous pursuits

My hours are mostly spent eating (cooking), working (making a living), maintaining (mostly laundry and cleaning), and sleeping (because I'm exhausted).

Precious few hours are left for recreation.

As such, no, I sincerely do not have the time to go read tomes I have no need or interest in. As an adult with responsibilities I literally can't afford the time to be irresponsible. I certainly wish I could, the sheer amount of time I had as a kid was amazing.

I was teasing. Your post was long.

I mean, they'll take the time to write flowery marking BS like "our transmission delivers power intelligently right where it's needed for the smoothest ride, optimizing your comfort and blah blah blah use dirt mode for pushing your adventure to the extreme and sport mode when it's time to shine blah blah" but they won't say "The system can apply the brakes to an individual wheel if the tire loses traction."

And then they'll sell several different models which all have equivalent but different marking BS and no way to tell what the actual difference between the products is. Give me a damn table of specifications.

> Often our instinct is to apply more power to the accelerator when we are stuck in ice, snow, or mud. Unfortunately, this can deliver too much torque at once, resulting in the throttle opening up too quickly and creating rapid wheelspin with no traction. With X-MODE activated, the engine will deliver torque gradually

Translation: If you punch the throttle and it's slippery, the throttle is going to ignore you just a little bit.

> Subaru vehicles improve on the already outstanding AWD system by increasing the front/rear coupling force, splitting power more evenly between the front and rear sets of tires.

Translation: limited slip center differential. It'll mechanically send power more-or-less evenly to the front and the back of the car.

> With X-MODE in use, the Vehicle Dynamics Control (VDC) system provides enhanced Limited-Slip Differential (LSD) control. This means that braking is applied much faster and only to the wheels that need it most because they are slipping or spinning without traction.

Translation: For slippage from left-to-right, the traction control system will brake the wheel that is spinning, thus encouraging the power to be used at the wheel that is gripping the road, rather than being wasted spinning a tire.

If it’s anything like my Hyundai, it uses brakes to slow loose wheels.

It works amazingly well for snow (which is all I’ve tried it in).

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.

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.

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.

> 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.

We typically refer to these as RWD-biased AWD, etc.

BMW’s xDrive is a RWD biased AWD.

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-...

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.

Not all 4WD systems have locking differential's.

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

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?

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

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