Originally Posted by smithers
I had the understanding that 4 low is not the right mode for driving in ice or going up icy inclines. More of a tool for deep snow or sand.
covered most of this. I don't use 4lo in general I-70 or pass driving because I have lower differential gears to compensate for larger tires (5.29:1) and an inline 6 engine that has a ton of compression braking, so first gear is effectively very "low" and I'm not usually lower than 2nd.
In any case, gearing is the mechanical drivetrain basis for traction, because it is the master function for multiplying and applying torque. Being in 4lo to go up an icy hill allows far greater throttle control (I actually have a hand throttle that is very useful for starting on a steep grade with no rearward roll), and that is what keeps tires from slipping. The descent control is incredible - in the vid I posted I took my foot off the brake and don't exceed 1 mph down a 14% grade. If you aren't on the brakes, they can't lock.
I'm not necessarily a Luddite on this stuff - it's more a recognition that all of the electronic stuff attempts to mimic what was formerly mechanical traction. It does some things exceptionally well, like modulating differential torque distribution via the brakes at higher speeds, and some things exceptionally poorly, like putting on the brakes at all four corners at slow speeds when all the tires have low traction. But there is no electronic substitute for gearing. They try to create descent control by deactivating engine cylinders, etc., but gearing is a mechanical application of torque (and in compression braking a resistance to acclerating) that has no substitute.
I am in 4lo here for sure - this was in November after coming back to my neighborhood after a ski day. It is almost never this icy here, but we had a the ridiculous warm November and then first heavy snow on warm pavement. I was just out to get a beer with the mrs and then check on conditions. I stopped midway up that 14% grade (not pictured below), which had been the site of many accidents, and then tested to see what it took to restart on that kind of ice on that steep of a grade. It was definitely low range and all three diffs locked.