Steep Downhill in Snow/Ice - Advice

I read your post. That’s why I responded with a question. You think that having your truck in 4wd “keeps everything rolling” if you’re in neutral and sliding down an icy hill. I don’t know why you think being in 4wd has any impact if you’re not in gear. Why would that keep the front tires from locking up when braking.

Braking alone, mostly uses front brakes so those quickly break traction and slide so you no longer have traction or steering. That’s why people leave it in a low gear as well as 4wd - to let the engine braking method apply slowing resistance to all 4 wheels, (if not more focused on the rear wheels) so that front wheels aren’t doing all the work like in your situation if all you do is brake in neutral.

Your logic just doesn’t make sense to me. That’s why I asked you to clarify. Maybe I’m missing something.

 

*** Adding a note here - Sometimes engine compression braking (low range, 1st gear) isn't enough. I had a gear reduction t-case in one of the rigs I did a 180° and then a backwards slide. I literally could walk twice as fast or better as it idled along. Still no joy and no control on steep hill with ice. I used to call my driveway "the luge run". I had to be very careful in picking a line. Definitely no brakes (nothing more than a light drag)......

 

I'm sorry if my previous reply seemed short toward you. It wasn't my intent to be that way.

I think I see the confusion. You are correct that braking alone the front brakes do most of the braking. But being in 4WD mechanically connects the front and rear axles together, the transmission is just in neutral for that portion. If one wheel is turning while in 4WD they're all turning in most cases (see below). If the transfer case is in 4HI or 4LO even though the transmission is in neutral, if both of the front wheels are not turning then at least the rear driveshaft isn't turning. If there's an open differential in the rear axle, there is the potential of having one wheel turning forward on something with traction and one turning backward on the surface with less traction. Most likely though the rear will be stopped as well and at that point either the vehicle is sliding or stopped.

With open differentials there's the problem of for instance having plenty of traction on one tire on the axle and no traction on the other. Even when in gear there is a situation where open differentials would let the wheels with traction turn in the direction of travel and the wheels without traction turn in reverse while sliding. That can happen when in gear or not and slight braking will help here.

The concern Crow Horse has if I'm understanding him correctly is that braking in this situation will result in all 4 wheels sliding due to adding too much brake and breaking what little static friction is available. By letting the descent be controlled by engine braking you can use what little static friction is available and not inadvertently start sliding. I agree with this totally.

 

so if I put my truck in 4wd, then in neutral, jacked up the rear axle but left the front tires on the ground, the I couldn’t turn the rear wheels (though they’re off the ground and the truck’s in neutral, because the front and rear axle are locked together by the 4wd being engaged? This just doesn’t sound right to me. Maybe it is. I’m not an expert and I learn shit on here all the time. I just didn’t think being in 4wd would prevent your front tires from locking up just because your rear wheels are still turning (especially in neutral)

 

The transmission is upstream from the transfer case. When the transfer case is locked the front and rear axle have to turn at the same speed regardless of the transmission.

 

that is correct....engaging the 4x4 locks the center transfer case in these trucks at 50:50 front:rear

putting the transmission in neutral would just mean the engine can't push any of the tires. You're thinking of putting the transfer case in neutral, which we can't do in these trucks. Jeeps can do that. If you put the transfer case in neutral, the axels could rotate independently (but then you wouldn't be in 4x4 any more either)

 

copy that. Thanks. So in my Sequoia which has the lockable centre diff (otherwise 4 lo and hi is more like an awd system and doesn’t bind on dry roads) would this still be the case?

 

Tokyo drift

 

so for 4WD Auto there has to be a center differential that can allow the ratio of power to vary front:rear something other than 50:50....I don't know what it is for the Sequoia (or the 4Runners with that option) but I doubt it lets the split get as far as 0:100 or 100:0 (most center diffs are 70:30), so yes, I'd say you'd see the same thing in your Sequoia and the rear wheels wouldn't want to spin.

But if it could go 100:0 or 0:100 (all the power to the front or rear) then yeah, you could spin them.

 

AWD has a center diff with a spider gear to allow differential front-rear action as well as side-to-side. 4 runner limited has a locking Torsen style limited slip which is probably the same as the sequioa. It’s 50/50 unless there’s slip, in which case it can move some torque forward or backward.

 

Everyone is discussing ways to controllably go slower down the hill. I say go faster.

It’s impossible to slide down the hill if you go faster than the truck would naturally slide down the hill.

 

It's still a dicey situation. Often times when there's a significant snow or has been, keeping tires on the "unmolested" ditch side can be treacherous where the ditch isn't clearly defined and you can dump into the ditch.

Regarding icy driveways, going up is usually easier to negotiate if you pick a good line and have the correct speed. Going down, when you get a serious case of gravity and have zero traction is where it can be uncontrolled.....

 

(1) I have a long, steep gravel/dirt driveway with curves on top and bottom...20% grade.
(2) If it's iced anywhere, I don't go down it....,
(3) In the event of an emergency, I put on lugged chains rear, cable chains front (they are the only recommended thing for Taco fronts.) and 4WDL

 

In this scenario (4wd) the front and rear driveshafts must turn the same speed. If the front driveshaft cannot turn (tires on ground) then the rear driveshaft cannot turn. So you cannot rotate the driveshaft by the rear tires. With the open diff when you spin one of the rear tires the opposing one will spin opposite through the differential. If you and a buddy each try to spin a rear tire forward together you will find nothing will turn.

 

The simple explanation is that in 4wd in order for a single wheel to lock up and begin sliding, all wheels must lock up. This is due to the front and rear driveshafts being locked together and the axle differentials. So if you apply excessive braking torque to a single wheel, instead if it locking up, the torque is redistributed via the drivetrain to one, two, or all of the other wheels. The redistribution is a fairly involved vehicle dynamics problem. So no wheels will lock up and slide until the sum total of the braking torque applied to each individual wheel exceeds the sum total of the grip of all four tires.

In 2wd a front wheel may lock up alone if excessive braking torque is applied to that wheel. This creates a cascading effect as the loss of grip at the one wheel demands more contribution from the remaining three which likely breaches their capability in low traction situations inducing more wheels to lock up.

Not intuitive but it's interesting how it works and it's a bit counterintuitive for folks when you start applying torque at the wheel back through the drivetrain and not distributing torque from the engine to the wheels. Granted, most can't get torque distribution from the engine to the wheels right anyway.

 

it definitely makes sense, and it’s been explained well. I get applying brakes to turn LSD into immediate lock and all that stuff. I’ve come to understand the nuances of pros and cons of 4 hi/lo and center locked/unlocked and all that stuff. I did the yellow wire mod when I had the Tacoma and I love the 4wd system and options in the sequoia (with the torsen center diff) I’ve just never considered an influence of 4wd while in neutral. I use 4wd in low and slow gears to get the benefit of engine braking on all corners and never thought about it’s antilock braking effects in neutral. TW FTW.

 

Nice looking Jeep

 

When we get snow in the south it usually turns to ice eventually, sometimes we just get sleet and icy roads. I’ve done 360s in a 4 x 4 just as easy as in a 2WD. Best solution is not to drive on ice. Beyond that I liked the sand on the ice solution someone posted. In a 2WD I’ve done well on ice going slow, never braking and never turning haha. Seriously using your brakes at all on ice downhill is a losing proposition, I go in lower gear and steer gently and slowly. Worst is to have someone in front of you hitting their brakes downhill even when it’s straight back up a hill - they slide and you have to brake. I treat it like a sled on such hills, let it gain speed to get up the hill.

 

I also have a steep section of driveway that never sees direct sun and with a turn at the bottom. Luckily not much ice in my part of TN. But, one year when we did get some ice i ripped off the fm radio antenna sliding backwards down the hill and off through a magnolia tree. Didnt quite have enough momentum to go all the way up.

I found that scattering dry-sorb or kitty litter granules helps just enough to bite the ice. Makes sense why previous owner of the house left a 50lb bag of it in the garage.