Why your SUV sucks in the mountains

[uglymoney]

A bunch of locked axles going down the road because you lock the diffs is likely to lead to more wheel slip because open diffs keep tires rolling the same speed as the road underneath them without interference from the other wheels. Uneven tire size or going around a corner will cause wheel slip which leads to some wheels having less traction than others. Juicing the throttle can result in multiple wheels spinning and the entire vehicle losing traction and directional control instantly rather than just 1 tire losing traction and the slower snowball effect that is much more controllable on a car with open diffs.

[bodywhomper]

I agree^^^. Subbies and audis have had various types of center diff lockers and LSDs. They used to have center lock switches. This is easy to look up.

This is a good discussion. Many vehicles have different systems. Some have an automatic method (mechanical or electronic) way to control wheel spin and some require user mechanical operations to control wheel spin. I believe many non-subbie/vw-based vehicles like Toyota's and trucks have a fully open differential and a mechanical center locker. It's useful to understand how they work.

Also that toyota video is oddly off. Jamal's explanation about torque is better.

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[GeezerSteve]

IME, Mazda AWD > Subie AWD

[plugboots]

IME, Mazda AWD > Subie AWD

Which of the 57 Subaru all-wheel-drive systems is it better than? All of them?

And of course they’re all symmetrical!!

[GeezerSteve]

Which of the 57 Subaru all-wheel-drive systems is it better than? All of them?

Ha, well, my n = 8 or so.

And of course they’re all symmetrical!!

worked for ABBA

[El Chupacabra]

^^^ Just pickin' nits, but dedicated rally cars use a limited slip center differential that is likely complex, expensive, and adjustable offering best of both worlds between an open diff and a locked diff. Today's road-going subies and audis have somethin similar. If you want lateral traction and stability, an open diff allows just that. If you want maximum forward traction, locked diffs are the best option. Limited slip differentials allow both.

That's oversimplifying it. Audi and Subaru use several different AWD systems. Some are pretty good. Some aren't anything special.

Locked differentials are not what you want on a road vehicle. Off road, selectable (ARB etc) or automatic (Detroit etc) can be very useful in difficult terrain. On snow, lockers are not desirable - the locked axle will just crab sideways downhill.

Limited slips vary a lot too. Torsen and gear driven (True Trac etc) can be quite good off road, esp in front axles that still need to steer and not overstress U joints or CVs. Clutch pack LSD can be set up more aggressive or fairly mild.

A LSD diff might help on snow, or might send you sideways into the ditch.

Center differentials can be open, locked, or somewhere in between (viscous coupling, Torsen, clutch pack).

IMHO on snow, on road, a computer controlled system that integrates yaw sensors along with wheel spin, and sends torque where useful, is ideal. These can be brake based or diff based, or on hybrids, even separate electric motor controls. Honda /Acura SH-AWD is a good example of what I'd prefer to have for a snow vehicle.

[summit]

Good discussion indeed. Since very few of us have a locking rear diff and we are discussing forward traction differences between LSD open and locking centers, I think now would be a good time to discuss BTM or Brake Throttle Modulation, AKA "left foot braking" AKA "one little trick to help you get unstuck a little bit maybe sometimes."

It is as simple as slightly apply the brakes and then gently applying the throttle. But it takes some practice.

This is a manual version of what your electronic traction control is doing, except the computer applies brakes to individual wheels where you are limited to applying it to all 4 wheels (or the rear two wheels if your e-brake is manual instead of electronic). Sometimes (if you are stuck) BTM can work better than the computer traction control. You may actually disable your traction control especially since in some vehicles (Toyota for example) it auto-throttles down when ATRAC detects traction loss (I don't remember what subies do). If your vehicle doesn't have a manual disable button (pre-2008 4Runners) for traction control, putting it in 4-Low usually does the trick, but having the brake engaged for BTM also usually disables ATRAC for the moment.

START at 53sec


This works on RWD with a hand brake as well.

4wheelers have been using BTM for ages, but I don't think most mountain drivers know it for getting unstuck in the snow. If someone who is good at this wants to take a better shot at 'splainin... please do!

[jamal]

That trick works really well with a torsen, because those use gears to send a multiplication of torque of the wheel with the least traction to the other, and also they do not rely on a speed difference like a clutch or viscous diff.

With an open diff, braking doesn't do you that much good, as the wheel with grip has to overcome the braking force which is the same on both sides so the net torque applied doesn't change.

With a torsen, the multiplication factor might be 4:1. That means if your wheel with the least traction can apply, say, 10 lb-ft, the other wheel with grip could get up to 40. There problem with that type of diff is that 0 x 4 is 0, so with very little traction, or a wheel in the air, it doesn't work. But if you brake, the side with the most traction gets more torque and can overcome the brakes to move you forward.

IMO a helical diff is the best purely mechanical option if all the wheels are going to stay on the ground and have a reasonable amount of grip. Audi thinks so too and uses them in a lot of cars.

Clutches i think are better when there's less grip, or bigger differences in traction, or when wheels come off the ground. Being able to control then electronically is even better because they work more smoothly in some situations (i.e open when you're trying to park), and can be locked pre-emptively instead of needing to wait for a speed difference.

Subaru uses electric clutches in automatic transmissions and the sti. The sti even gives you a little dial to adjust it yourself plus different presets for auto mode that give more or less lockup across the board. The vtd autos and cvts have a very similar center diff but without the dial.

Honda's sh-awd seems neat, although i'm not completely clear on how it works. I think each rear axle gets it's own clutch pack so that power can be sent to just one wheel or the other. And there's no actual differential, just a solid shaft turned by the ring gear (also called a spool).

As far as rally cars go, i think they tend toward more lockup, and fancy e-diffs are usually not allowed. The popular rally center diff for subarus is the group n sti part that is just a viscous center with a very strong/stiff coupling that acts almost like it's locked.

[El Chupacabra]

Good points. True, none of the crossover SUVs have locking rear diffs. Some of the off-road oriented truck based ones, and some trucks, do from the factory - which is a great option for offroaders, but probably best to not use then on road, ever.

Unless it's donut time in the snowy parking lot...

[Jong Lafitte]

Back when I had an 88 4Runner with auto hubs, if the truck was in 4wd, the center diff was locked. There were times where I’d drive hundreds of highway miles in mixed road conditions in 4wd (as required by CHP) without problems.

I don't believe this vehicle employed a center differential, just an old fashioned t-case, power to the front is either "on" or "off" and is operator selected.

I have to disagree. One of the comments in the video covers it extensively:

1. Locked center offers less directional control on any surface in order to gain surety of traction in complex uneven terrain (ie the example with the front axle high with a wheel fully off the ground. The second example is RWD truck vs truck in 4WD (locked center).
2. That's why rally racers use AWD (FT-4WD). You see subies and audis... if a locked center was better, they'd use it.
3. The logical progression to validate this thought is adding yet another locker (rear locker) think about what that does, now go back to the center lock quandary.

Do agree that locked center diff sacrifices directional control due to increased wheel slip from driveline wind up (understeer), nonetheless, for a while, years, the regs called for WRC cars to not employ a CD at all. Back to CD in 2017. May have more to do with what the regulating body is trying to accomplish with rule changes vs. what is the "best." Remember they are race cars that can be set up to compensate much more easily that a passenger car built for (relative) comfort and longevity.

As far as rally cars go, i think they tend toward more lockup, and fancy e-diffs are usually not allowed. The popular rally center diff for subarus is the group n sti part that is just a viscous center with a very strong/stiff coupling that acts almost like it's locked.

Some were locked without using a CD at all for a while, here's a discussion:
http://wrcbehindthestages.blogspot.c...otorsport.html
The new version is high tech and once again uses [at least a version of what could be called] a CD:
https://www.roadandtrack.com/motorsp...ial-explained/

Good points. True, none of the crossover SUVs have locking rear diffs. Some of the off-road oriented truck based ones, and some trucks, do from the factory - which is a great option for offroaders, but probably best to not use then on road, ever.

Unless it's donut time in the snowy parking lot...

Yeah, makes me wonder how a Detroit locker would do. Lock once in November and then dance around until the snow melts in April and you round a turn on the dry? If I had the need, I'd lean heavily towards operator actuated as opposed to the automatic lock up/unlock, but, yeah, those are for true off road use, would be great in the unplowed or the ditch as pointed out before.

[jamal]

GM puts an auto locking rear in a lot of their trucks. I'll notice it working sometimes even when it's dry but for the most part it's unobtrusive and effective.

Actual lsds are probably getting more rare though. Like why pay for torsens when you can just use traction control. And when you can brake an individual wheel, you do get a benefit and can change the torque bias to the ground with an open diff. Downside is that can wear out and overheat brakes, and it's applying the brakes and maybe cutting power. Sometimes you need, or want a little wheel spin.

They're still out there though- honda and audi for example do make some pretty fancy diffs still.

[El Chupacabra]

I put a Detroit locker in the rear of my XJ Cherokee, with a manual transmission. That was entertaining on snow... So long as it was in 4WD, I could keep it under control. Far from ideal for a ski vehicle.

[ill-advised strategy]

4wheel drive doesn't equal 4 wheel stop.

Actually, in my old pickup trucks with kind of disproportionate braking and no newer tech, having the hubs and case engaged keeps from just locking the fronts while the rears turn...or vice versa. Braking on ice is a lot better with the case locked on that kind of old style setup.

[auvgeek]

This is a manual version of what your electronic traction control is doing, except the computer applies brakes to individual wheels where you are limited to applying it to all 4 wheels (or the rear two wheels if your e-brake is manual instead of electronic). Sometimes (if you are stuck) BTM can work better than the computer traction control. You may actually disable your traction control especially since in some vehicles (Toyota for example) it auto-throttles down when ATRAC detects traction loss (I don't remember what subies do). If your vehicle doesn't have a manual disable button (pre-2008 4Runners) for traction control, putting it in 4-Low usually does the trick, but having the brake engaged for BTM also usually disables ATRAC for the moment.

Subie (at least the 2015+ OB and 2014+ Forester) lets you manually disable -- for those unfamiliar, there's a button on the left side of the steering wheel that looks like a car fishtailing, you have to press and hold for 3 second until the same symbol pops up on the instrument display. Crucial to turn it off if stuck (or even better, before you get stuck) because the computer WILL cut power to all wheels when none has enough traction ... and then you'll lose momentum and become stuck, if you weren't already. Best I've found is traction control off and BTM. Sometimes X-Mode on can help because it supposedly engages a lower (electronic) gear ratio, so it's kinda like a crummy version of 4-lo. But it also increases the sensitivity of the VDC and applies very liberal use of BTM, so I don't like to keep it on very long, if I bother with it at all. I guess it's helpful for the masses who don't understand "left foot braking."

I'm sure jamal or someone else could give you a better run down of X-Mode. I'm honestly not sure how exactly it works when traction control is turned off.

[GPP33]

In the 4wd community there are folks who claim Detroit lockers are the best option in slick condition because it’s predictable. In other words you will always be on the verge of loosing control. No thanks.

[markcjr]

Didnt't read the whole thread but figured I would throw in Nokian hakkapeliitta 8. Got those this year and damn, it's like ice skates carving tracks down the street

Had studless Nokian and blizzaks in the past, like these the best so far. Mostly because on siped roads on I-80 going down to Reno, these don't shimmy whereas the studless ones shimmied like crazy. I thought I had a broken axle/ rear sway bar the first time I drove on that section of Highway.

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[bodywhomper]

Didnt't read the whole thread but figured I would throw in Nokian hakkapeliitta 8. Got those this year and damn, it's like ice skates carving tracks down the street

Had studless Nokian and blizzaks in the past, like these the best so far. Mostly because on siped roads on I-80 going down to Reno, these don't shimmy whereas the studless ones shimmied like crazy. I thought I had a broken axle/ rear sway bar the first time I drove on that section of Highway.

i bet the hakkaluuggie 8 are great. I always thought that a shimmie was due to tire/wheel balance problems, which can be corrected. i've had several blizzaks versions on 3 different vehicles and never had shimmie issues at highway speed.

I don't believe this vehicle employed a center differential, just an old fashioned t-case, power to the front is either "on" or "off" and is operator selected.

yes. that's correct. it was a transfer case. thanks for the correction. from a driver's perception, it essentially operates the same as a center diff lock, right? rotational speed is the same between the rear and the front axles, one does not want to make turns on dry pavement because of the equal rotational speeds, open diff on each axle, torque is even between the left and right wheel of each axle, and you're stuck if one wheel on each axle has zero traction (unless you have some other kinda traction control issue on an axle). is that true?

i found an interesting thread on the mud forum with a lot of input from some driving coach at a steamboat ice thing.

[markcjr]

i bet the hakkaluuggie 8 are great. I always thought that a shimmie was due to tire/wheel balance problems, which can be corrected. i've had several blizzaks versions on 3 different vehicles and never had shimmie issues at highway speed.


yes. that's correct. it was a transfer case. thanks for the correction. from a driver's perception, it essentially operates the same as a center diff lock, right? rotational speed is the same between the rear and the front axles, one does not want to make turns on dry pavement because of the equal rotational speeds, open diff on each axle, torque is even between the left and right wheel of each axle, and you're stuck if one wheel on each axle has zero traction (unless you have some other kinda traction control issue on an axle). is that true?

i found an interesting thread on the mud forum with a lot of input from some driving coach at a steamboat ice thing.

The shimmy was just on the siped part of the road made to drain water witch Tahoe to Reno has a fairly large section. So it was like the sipes of the really soft blizzaks against the many grooves/ sipes on the highway going downhill. Otherwise blizzaks were fine.

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[bodywhomper]

The shimmy was just on the siped part of the road made to drain water witch Tahoe to Reno has a fairly large section. So it was like the sipes of the really soft blizzaks against the many grooves/ sipes on the highway going downhill. Otherwise blizzaks were fine.

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Got it!

[Jong Lafitte]

i bet the hakkaluuggie 8 are great. I always thought that a shimmie was due to tire/wheel balance problems, which can be corrected. i've had several blizzaks versions on 3 different vehicles and never had shimmie issues at highway speed.


yes. that's correct. it was a transfer case. thanks for the correction. from a driver's perception, it essentially operates the same as a center diff lock, right? rotational speed is the same between the rear and the front axles, one does not want to make turns on dry pavement because of the equal rotational speeds, open diff on each axle, torque is even between the left and right wheel of each axle, and you're stuck if one wheel on each axle has zero traction (unless you have some other kinda traction control issue on an axle). is that true?

i found an interesting thread on the mud forum with a lot of input from some driving coach at a steamboat ice thing.

Yeah, exactly, it’s the same as if there were a diff but locked all the time and yes, one front and one rear with zero traction and your spinning those two tires with zero traction. With full traction say on tarmac, yeah, the driveline will “wind” up and you’ll feel it binding, due to the same rotation speed being employed but far reduced ability to slip to make up for it as it would in snow or such.

I had this old Jeep and trashed the rear driveshaft. I dropped the pulled it off and drove in “4x4” (fronts only) for a week til I could afford a new rear drive shaft.




I shoulda never put those aux lights on there.


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[LiveLarger]

Given how car-crazy the US is, I find it interesting how uneducated they are about proper tires. Running winter tires is pretty much kindergarten knowledge in scandinavia.

[charles martel]

yeah but he is british and they can't drive ...

Tell that to Lewis Hamilton.


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[GPP33]

Given how car-crazy the US is, I find it interesting how uneducated they are about proper tires. Running winter tires is pretty much kindergarten knowledge in scandinavia.

Given how car crazy we are I find it interesting how little people know about cars or how to operate them.

[El Chupacabra]

Given how car-crazy the US is, I find it interesting how uneducated they are about proper tires. Running winter tires is pretty much kindergarten knowledge in scandinavia.

Climate is pretty different in the majority of the US than in Scandinavia.

[fastfred]

Given how car-crazy the US is, I find it interesting how uneducated they are about proper tires. Running winter tires is pretty much kindergarten knowledge in scandinavia.

I bet 9 out of 10 people can't change their own oil