geko29

Utterly false. Excluding the Mini/FWD-based cars, here is the most basic BMW xDrive system:



The actuator motor drives the yellow gear, which directly actuates a scissor clutch, which controls the F/R torque split. The default split is 40/60, but it can go all the way to 0/100 if desired. Fully electromechanical, no brakes involved. The M xDrive system replaces this with a 5-plate wet clutch that can allow for different levels of slippage between axles without affecting the targeted torque distribution. This is used in the "4WD Sport" mode to allow you to hang the rear end out while still retaining traction in the front. Also the front output shaft can be mechanically disconnected via a setting in the iDrive, for a true 2WD mode when you want it.

The "M lite" cars have an eLSD, while full M cars have a real mechanical LSD.

mmarshall

That's because Toyota is cheap, and still uses part-time 4WD systems on some of its trucks, lacking a center differential which would allow different tire-rotation speeds on dry corners.

It isn't just a dry-pavement issue with the part-time 4WD. On a partly-plowed road, or the right sunlight-angle conditions, in a partly-melted condition, the road surface under one's wheels can constantly vary from dry to wet to snowy to ice...it is changing and inconsistent. You don't always know what it will be at any given second.

JDR76

Not really a concern for me and I really don't worry about it. Most of the time in snow, I still just drive the truck in 2WD. A small patch of dry pavement here or there isn't going to kill it. When we get to the point in a snowfall where there are dry patches, I find 4WD isn't needed anyway. Just for the deep stuff. It's only going to bind on a long curve or sharp corner that's dry. It has proven itself extremely capable for me, so I wouldn't change it. One of the best vehicles I've owned, and our go-to in snowy weather. It's been a delight to own.

SW17LS

Incorrect. You need to do some research into these systems. No they are not torque vectoring, they can only distribute power front to rear but they are fully variable and can distribute 90% of power to the rear or front as needed. It can shift power side to side using the brakes. 4Matic + as found in the AMG cars can distribute 100% of the power to the rear. The Lexus and BMW systems are similar.

FrankReynoldsCPA

It's always those who don't actually own or drive trucks who insist they should be full-time 4wd.

peteharvey

FWD cheap and easy to drive for the novice driver.
Then, FWD-based AWD for a little more sophistication.
Then RWD.
Finally, RWD-based AWD.

mmarshall

Well, one reason we are seeing full-time 4WD and/or systems with center-differentials in more trucks today is that more buyers are demanding it.

See tihs link for more info:

https://pickuptrucktalk.com/2021/11/...hen-to-use-it/

FrankReynoldsCPA

Auto 4WD is not full-time 4WD. The system linked above is still using a transfer case rather than a center differential. It just has a clutch in it. And you can still run it in purely 2wd.

I prefer the traditional 2 speed mechanically locking transfer case that my XLT F-150 has in it. I don't need 4 auto, I can drive on the highway just fine without it. But the clutched transfer cases sacrifice some off-road capability because the clutch allows the front driveshaft to slip.

I would not want my truck to have either a clutched transfer case or a center differential. I only want the rear axle powered on the highway, and I need it to lock up when I'm off-roading.

signdetres

I thought this fit here well


Spoiler:
.
.
The RWD BMW won overall vs the FWD Audi in the snow, but the most important factor in snow is the tires, not which wheels move the car.

Stroock639

thanks for sharing that was interesting! i still can't believe i've gone all these years just assuming the A4, A6, and A8 were RWD architecture on the basis that they're executive german cars which are 'direct competitors' to the C, E, S classes and 3, 5, and 7 series respectively... but the audi looks and proportions make so much more sense now

but yes what that video ultimately shows is tires tyres still make the biggest difference

jwong77

Better to have some traction from 2 wheels than none from all 4 (cough cough, all season tires in snow)

LexFinally

A couple of factors are in play here. First, if you don't have to deal with inclement weather, RWD wins. But if you do, AWD wins.

Second, the question of whether FWD-based AWD can handle. Yes, RWD-based AWD generally wins, because its engine and transmission are farther rearward for better weight distribution. But PROPER torque vectoring can do a tremendous job of negating understeer. By "proper," I mean (electromechanically or whatever) actually propelling the rear wheels at different speeds. "Brake-based torque vectoring" isn't. It's just faking it by activating individual brakes; it can't push you forward.

Finally, there is a legitimate reason for on-demand AWD besides cheapness, and it's fuel eocnomy. Audi has changed over some of its models, and not just the cheapest ones, to on-demand for this reason. The electronic sensing of traction loss has gotten a lot faster, minimizing the squirrelly behavior that used to bedevil this setup.

Stroock639

i'd be surprised if weight distribution really played a role here... the biggest factor is that the power is sent to the rear wheels first so you're still able to "steer" with the throttle, whereas with FWD based systems (depending on how much power can be sent to the rear) hitting the gas will just cause you to push outward and not turn

LexFinally

Here's a link to the industry bible Ward's Auto World, stating as a fact that Audi imitated Honda/Acura's torque-vectoring "SH-AWD" system specifically to counteract the understeer caused by Audis' nose-heavy weight distribution. https://www.wardsauto.com/news-analy...r-differential

Stroock639

oh sorry i thought we were still talking about that snow test specifically lol... yes then i retract weight distribution plays a noticeable role