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Old 09-06-2017, 03:28 PM   #1
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Poor man's four-wheel drive

Looking at potential tow vehicles, I noticed that the GMC Yukon comes with an "automatically locking rear end."

Salesman artfully termed this "the poor man's four-wheel drive."

Anyone have experience with this?

Curious to know if it works as promised.

I'm looking for something that could get me out of a grassy campground on a wet, soggy morning.

Would prefer not to go the 4WD route, but not ruling it out.
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Old 09-06-2017, 03:46 PM   #2
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Sounds like "posi-traction", limited slip rear differential. If you don't want to go 4WD then that's your only other option. My TV has full time 4WD and I really like it, no more rear wheel slipping on wet pavement when I go around the corner. When I'm pulling the boat out of the water, no slipping on the ramp. In my year 4Runner they made a v6 too. The gas mileage was only 1 MPG more, even running in just 2 WD. For me, for a TV, I wanted a V8 and 4WD. I'm 13 years down the road and haven't changed my mind yet.
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Old 09-06-2017, 05:46 PM   #3
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Hi

Limited slip rear ends have been around for a long time. Electronic "auto locking" is an improvement over that. The gotcha is the smarts that makes it lock up. That may or many not always do what you want it to do. A manual locking rear end is a better (cheap) option if you can get it.

Bob
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Old 09-06-2017, 06:11 PM   #4
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The GM unit has an open differential with an automatic lock up when it senses 1 wheel slipping. This give you 2 wheel drive without the side effect of slipping around corners .

It worked seamlessly in my Avalanche .
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Old 09-06-2017, 07:57 PM   #5
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I've had it in my last 2 chevy pickups. Works great for snow!

Basically, it's a clutchless automatic limited slip differential. It has a set of gears which lock both sides together when one spins (loss of traction).

This has nothing to do with 4 wheel drive. It does work great WITH 4 wheel drive; gives you rock-solid traction.
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Old 09-06-2017, 09:37 PM   #6
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Limited slip differentials can be great to have but they can be a handful as well.

Up here in the snowy white north on ice or snow having one wheel spin while the other stays stationary will at least keep you on the road. Having a wheel start to spin, the differential locking up, and then both start to spin can put you sideways faster than you can realize.

It's always at the start of an incline as you put power down. It once happened to me at 50 mph on black ice. Nothing tightens your spincter gland faster than going sideways at 50 mph.

At least with a manual locker; you control it; plus it remains locked, as opposed to an automatic locker, that will disconnect once it senses traction for however short a time it occurs.

Cheers
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Old 09-07-2017, 04:13 AM   #7
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Interesting.

I'm in the Deep South USA, so snow is not a big concern.

Doesn't snow where I live, but that means if I get caught in a storm while on a camping trip in lovely Canada ... wish me luck.
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Old 09-07-2017, 08:19 AM   #8
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"It worked seamlessly in my Avalanche"

Could you elaborate?

I'd like to know how it works in practice. I understand the theory.

Thanks.
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Old 09-07-2017, 09:44 AM   #9
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Quote:
Originally Posted by Isuzusweet View Post
It's always at the start of an incline as you put power down. It once happened to me at 50 mph on black ice. Nothing tightens your spincter gland faster than going sideways at 50 mph.

Cheers
Tony
In just my car one time, I did a 180 on black ice across 4 lanes of traffic on a divided highway (paved islands). I was just driving along, 45 on a straight / level road, on patchy snow, no foot on the brake, came up to a shaded area of highway that was all black ice. There was full oncoming traffic. By the time I crossed the median I was looking out the passenger window to see where I was going. There was an opening in oncoming traffic and I slid right through it, now going backwards and across into the opposite emergency lane before stopping. I had to sit there for a minute to regain my composure, but I was thankful for my very good luck that morning.
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Old 09-07-2017, 09:53 AM   #10
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If you don't have 4WD or limited slip, here's a little trick I learned that worked very well on my old '72 Duster and '89 Toyota pickup, pulling a boat up a dirt/grass ramp. (both were rear wheel drive, emergency brake on the rear axle.)

If one wheel is spinning and the other isn't you need to equalize the pressure. Keep the wheel spinning but start putting on the emergency brake. This will put equal pressure on both rear wheels (eventually, as you keep putting on the brake) and you'll start to move forward. When you do, release the brake.

This works best with a hand brake but I've done it with center and left side hand brakes and a foot emergency brake. I haven't tried it with FWD, but if the emergency brake is on that axle I would think it would.
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Old 09-07-2017, 10:27 AM   #11
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RichW46 - that same trick works on my '41 Ford 9N tractor in the snow - I just stand on the brake for the tire that is spinning and it settles the spin down and both tires pull. 4WD is by far the best for pulling in steep, slick, and / or loose surfaces but to be honest we've only used 4WD one time towing our trailer and that was in a sudden snow storm that was not forecast but I was dumb to be towing in it regardless ...
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Old 09-07-2017, 10:42 AM   #12
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Posi-Trac was the GM name for a limited slip differential.

It was recommended by the Big Four automakers starting in the middle 1960s. Definitely a help in slippery situations. When you bought that big block Dodge Monaco, it was one of the boxes you checked off.

A 2WD pickup with same does most of what 4WD can do but without the substantial penalties

A "locking" differential is a somewhat different critter. More effective, but much more limited as to surfaces and speeds.

A 4WD pickup needs locking differentials in both axles to avoid being a one Drive wheel per axle vehicle. An expensive aftermarket upgrade most times.

4WD is no panacea.

And these aren't off-road trailers.
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Old 09-07-2017, 10:47 AM   #13
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No way would I buy a 2WD when 4WD is available. Yeah, I know 2WD cost less, weighs less, uses less gas and so on..... The truck I use to tow my AS is 2WD. We have towed at least 7 trailers with it and have never gotten stuck. However, there have been some tense moments. Twice as of recent. Once was on a very steep gravel and washboard public road. If I would have had to stop, I don't think I could have gotten going again. The second was last week. I set up in a large field that had been getting rain. I eased in, but knew that one spin of a wheel probably would have buried it to the axle.
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Old 09-07-2017, 11:11 AM   #14
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The GM locking diff is great. Its different than a posi. When a locking diff senses 1 wheel spinning it will lock the diff. All this happens under 20mph as it will unlock above 20mph.
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