All-wheel drive and four-wheel drive explained


#1

Originally published at: https://boingboing.net/2018/11/23/the-mysteries-of-all-wheel-dri.html


#2

Probably worth pointing to this https://boingboing.net/2017/02/08/how-a-cars-differential-work.html (edited not link to the wrong link https://www.youtube.com/watch?v=Zlg7S-qVeHw) which explains how differentials work. Now that film talks about how differentials are important because the left and right wheels rotate at different speeds when the car is turning. That is also happens to a lesser extant between the front and rear wheels. Which is WHY you might want a differential rather than a transfer case between the front and rear axles. The downside of a differential is that it is possible for one output to spin while the other is static. So once the left wheels (Or possibly the front wheels for an AWD) start spinning, no power is going to the non-spinning wheels which does little to help you out of a ditch.

And don’t forget, none of this makes your car magically more controllable while stearing or stopping. All that it really does in the snow is make your car a bit less likely to spin the wheels when you step on the gas.


#3

Usually differentials have a limited slip function (mechanical or electronic managed) which prevents that one (semi) axle spin freely with the other stuck


#4

I wouldn’t say “normally”, not by a long shot. Outside trucks, Subarus, and some off-road vehicles limited slip and/or locking differentials are not the norm. They require an additional clutch pack (be it remote controlled or not) and significantly more expense. It is much more common to use the ABS to prevent slip through throttle control since that requires no additional hardware.

Don’t forget every vehicle has a differential, and very few 2WD cars have limited slip.

And if we’re talking only 4WD vehicles, again things like lower model 4WD Silverados and F150s and even Jeeps only have a dumb one: 4Hi = unlocked, 4Low = locked. And I’ll bet my left gonad that’s the majority of 4WD/AWD vehicles on American roads.


#5

This video seems to lean a bit to the side that simpler is better.
I would say that isn’t so.
I would say that simplest is better though.


#6

I have tried and tried and tried to get certain people to understand that 4WD/AWD is not magically safer in winter.

  • Yes, you might be able to get unstuck from a snowdrift.
  • It won’t help you stop.
  • You really need winter tires for that.
  • Most “all season” tires are a poor compromise.

The reason winter tires work is a combination of rubber that stays softer in the cold and grooves that channel water/slush and provide grip on snow/ice. The downside is they wear out quickly in warmer weather. Summer tires get stiff in the cold and will slide easily.

Expensive solution: Upgraded my particular individual to a spiffy high-spec SUV with ridonkulous 21” wheels for which only summer tires are made, thus forced to buy a second set of (smaller) wheels to fit winter tires. Done.


#7

That video is Sho’nuff not about differentials at all.


#8

I was just thinking - vehicles that use computers to tell different electrical motors how much torque to use (such as multi-motor Teslas), could easily electronically lock the wheels together. I don’t know whether any do this, but it really would go beyond these definitions.


#9

Oops. Fixed. Wern’t they going to do a remake? That movie is far better than it has any right to be.


#10

It’s a video of a guy talking in front of several cars and trucks for 6 minutes.
I expected that at some point he would use some of those cars and trucks to demonstrate the real world differences under actual use.

But no.


#11

Getting winter tires is probably the most helpful thing you can do to improve your driving safety at winter. And if you’re commonly facing icy roads, instead of just packed snow or slush, studded tires are where it’s at. (Dunno if they’re available / legal in all parts of US; here in Finland, studded winter tires are extremely common and often are a literal lifesaver.)


#12

4-wheel/all-wheel drive: just means you’re further from help when you need it… :grinning:


#13

No need to electronically lock wheels together if you have electronic traction control. Locking differentials is just a mechanical way to get to a crude form of traction control.


#14

Doesn’t that mean you need to re-calibrate your speedometer/odometer each time you change wheels because of the different circumferences?


#15

Right - but computerized control can make sure that the wheels move at the exact same speed - or any other speed ratio without having any wheels spinning, loosing traction. Each wheel can have the best torque and speed.


#16

Sigh. Video capability is so wasted on so many modern users. It’s so cheap now, you can video anything.

I have to admit, I was kind of hoping for some “Nova” style animation showing the various mechanisms and how they are similar/differ.

It adds zero, frankly, to be in front of the various vehicles, telling us they are 4x4 or all-wheel. They could be run by a flux capacitor for all I can see of their works.

And in the cold! He could have delivered the lecture in a warm classroom; even a blackboard with a few line drawings of differentials and transfer cases, and it would have been more clarifying.


#17

If the video doesn’t add value by showing something or playing audio that gives insight, it would be better off as text I can read quickly. :-/


#18

I get very annoyed when the video states “all you need to know” or “here is THIS explained” and the person starts saying things like “I’m not really sure” or “It may be THIS, but I highly doubt it”.

Yeah. cool. That means you don’t know or couldn’t have been bothered to do some googling and research before making your video. Fuck off then.


#19

Taller rubber solves that. Increase the sidewall height so the outside circumference matches. Most glovebox manuals give alternate sizes that will work for exactlyvthis purpose.

So now we have 295/35R21 summer and 265/55R19 winter.


#20

What in the hell are you driving with tire sizes like that?