Old Time 4 Wheel Drive Question

Back in the early days of 4WD vehicles, perhaps in the mid 1970s, a friend of mine had a Jeep Wagoneer. It had 4WD and there was a nob on the floor that was used to switch in and out of 4WD.

I don’t know how old the Jeep was, but I do remember my friend’s dad constantly reminding him never to use 4WD under normal driving conditions, i.e. while driving on dry pavement. This was in California so there was rarely a reason to use 4WD unless there was a downpour or we took it off road into the mud (without his dad’s permission), but I digress.

The admonishment always ended with something like “if you use 4WD when you shouldn’t you will destroy the transmission…” or something to that effect. I have since had a few 4WD vehicles, none of which were as old as that Jeep was, and can’t remember the owners manual ever warning me about the dangers of using 4WD on dry pavement. I currently drive a 2010 4WD truck and my wife drives a 2012 AWD SUV. We live in Montana so 4WD or AWD is a necessity in the winter.

So is it true that back in the day there was a real danger in using 4WD on dry pavement? I assume today’s cars and trucks with 4WD/AWD don’t have whatever limitation or problem the older designs had. Please enlighten me!

When you turn, the front wheels travel a different distance than the back. If the transfer case is locked, that puts a great deal of stress on the drive train and something has to give. Usually, a tire will slip on the pavement.

Same is true for some newer 4x4s as well. My 06 Pathfinder is that way.

I think Wranglers are still that way, also.

Wow. So it’s still verboten to use 4WD on dry pavement? I will have to look through the owners manual again because if it’s in there I must have missed it.

It seems a warning light should tell you if what you are doing is damaging the transfer case since it’s fairly easy to put a vehicle in 4WD and then to forget to take it out later…

True 4WD (not the full-time AWD from numerous makers or ‘Real Time 4WD’ variants from Honda – thanks, Honda, for confusing more people)…Anyway, true 4WD that you select and lock needs to run on loose surfaces only, because there is no designed ‘give’ in the drive-train when the wheels are locked.

Well, there is forcced ‘give’: ** It’s called incredibly rapid deterioration of the gears via grinding**. True 4WD isn’t even for use in the rain. When to use 4wd? We’re talking about surfaces where, when turning, the wheels will slip on the surface to keep their matching speeds at the cost of nothing mechanical. If a wheel can’t slip when locked in 4wd, it’s grinding away at something – and it’s making me cringe.

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The older systems had no differential between the front and rear drive trains. So as stated earlier, if you start making turns, something’s got to give.

Many of the newer four wheel drive systems have differentials between the front and rear drive trains to accommodate this issue. And of course it’s not an issue with the Full Time 4WD systems as they can handle the differences in travel between front and rear.

Yep - if you ever drive a true 4WD around a corner on dry pavement you will absolutely know it’s not good. It feels like trying to drive with your foot moderately hard on the brake at the same time. The strain that it puts on the drive train is palpable.

My last truck recommended driving a certain number of miles per month in 4WD to keep something lubricated - can’t remember if it was front differential or transfer case or something else. So when it was raining and I was on a relatively straight highway I would switch to 4WD to do that with hopefully minimal wear and tear on the drive train.

This isn’t true if you are using the term"grind" like most people do. Usually this refers to gears making contact wth the teeth not meshed.
My 1954 Willys jeep was run in 4wd on dry pavement. What happened was the tranny would not shift out of the gear it was in because the unreleaved torque on the system. I had to find a gravel spot and spin the tires to release the bind.

Yeah, Wranglers are still this way (at least my '98 is). I only use 4WD in snow or off road.

The older 4WD vehicles often had locking hubs for the reasons described above. Usually the locking hubs on the front wheels were sold as an aftermarket product (although dealers would offer them and install them before delivery of a new vehicle).

The locking hub looks like this:

http://scoutparts.com/products/photos/17884a_236187.jpg

If you were heading off road you would put the truck in 4WD, get out of the vehicle, and lock the hubs for full traction. Unlock them and the front wheels turned freely. Also, they served to unlock the wheels from the axle and driveshaft so that the vehicle rolled better on paved roads. So, they weren’t absolutely necessary. The truck would be OK in 2WD but locking hubs were usually the first addition a 4WD owner would make to his rig.

I really liked my '98: solid black with tan interior. I also had a 2001, but the '98 was sweet. Jeep takes a lot of heat for their 4WD system, which is basically unchanged for the last umpteen years. Don’t drive on pavement, don’t engage if driving at highway speeds, etc. But they can’t be beat for rock-climbing. You can almost feel the wheels reaching out and grabbing the next obstacle. :wink: Favorite video here.

It’s not really an “old time” 4x4 thing. There are still plenty of part-time 4wd vehicles being sold new and there have been AWD and “full time 4wd” vehicles going back to the 60’s. There are still a few part-time 4wd vehicles that only go into 4wd when the driver moves the lever or pushes a button like the Wrangler or FJ. But there are many more vehicles (including most full-size trucks these days, I believe) that essentially work the same mechanically, but automatically engage the 4wd when slippage is detected. You can also usually manually put the vehicle into 4wd, and you can still do damage to the drivetrain by running them in 4wd on dry pavement.

In part time “traditional” 4WD, the axles are mechanically locked together so that torque is split 50/50 to each one axles. From there it is split 50/50 to each wheel via a differential, so each wheel gets 25%. But torque only works with resistance. If one wheel slips, it’s torque is suddenly very low. Since the other wheel gets the same amount, it suddenly loses torque too. It’s the same principal as the car talk in My Cousin Vinny. One wheel spins like crazy in the mud while one sits on the pavement and doesn’t budge. They both have the same torque but torque is being produced in response to resistance and that one wheel isn’t getting any resistance, so no torque is being produced and you’re stuck. The big advantage of 4WD is that you have an extra axle that will hopefully still have some traction and get you out of the mess.

A locking differential is an option on some 4WDs. It mechanically locks both sides of the axle together so that both wheels will have the same rotational speed regardless of resistance. This is what the axles themselves do. They are mechanically locked together so if one axle is spinning, the other will still get torque regardless of the resistance on the spinning one. Without it, one spinning front wheel could cause the front axle to become useless which would cause the rear axle to become useless. But on a part time 4WD, the axles are locked together so neither one can make the other become useless.

This is why the 4WD will “wind up” in a bind and buck and break things on pavement. Both axles get exactly the same amount of power, but when turning they need different amounts of power. Offroad, the wheels slip a little to relieve the stress. Onroad, the wheels don’t slip so something has to give somewhere else.

To be able to drive on the road, AWD requires a center differential which allows the axles to operate at different speeds. Now the axles are no longer mechanically locked, so loss of resistance on one wheel can cause the front axle to become useless which can cause the rear axle to become useless as well. Loss of traction on a single wheel will literally leave the vehicle stuck.

This is countered by traction control systems which can be complicated and expensive, but works pretty well. The end result is that, on the road, it is better than 2WD in slippery conditions, but off the road, 4WD is mechanically simpler and more reliable and has the advantage over AWD.

I remember reading somewhere, and I don’t know if it’s true, that if you get caught in a situation where you have a wheel spinning that you can apply the brake to slow down the spinning wheel in an attempt to deliver some of that torque back to the stuck wheel.

The thing is, the car doesn’t know you’re doing any damage. As has been said, all the wheels are turning at the same rate and the car doesn’t realize that they’re trying to turn at different rates. It’s usually pretty noticeable when you try to make a tight turn (like in parking lot) and the rear end starts skipping around. Fishtailing at 4 miles an hour on dry asphalt is an odd feeling.

Every year in fall we get our plow ready for the winter (a Ford Bronco with a blade) we always make sure to test out the 4WD on the gravel part of our parking lot so that the rear end can skip without too much resistance when we make turns.

If I’m reading you correctly, there was an option that was offered on rear wheel drive cars called “limited slip differential”. There was the problem in snow and ice (and performance cars, think of a dragster where one tire has lost traction) where the wheel with the least traction would just spin. You were stuck. If one rear wheel had no resistance and the other wheel was on firm ground the car would be immobile. With a limited slip differential it was somehow mechanically balanced out so that if the difference in rotation became great enough, power would be sent to the wheel that wasn’t spinning. I’m not sure of the mechanics but it was an option on some cars. “Limited slip” was incorporated into some 4WD vehicles but I can’t comment on what extent and how effective and safer it was.

Yes and yes. I’m no expert on limited slip, but the basics are that the LSD has a mechanism like a clutch linking the two sides of the axle shaft and the clutch grabs tighter when more power is applied and tries to keep the two wheels rotation speeds from becoming too much different. Many 4WDs have LSD and it does help a lot, but not as much as a locking differential which mechanically hardlinks the wheels together. LSD limits the difference between a wheel with traction and one without, and lockers eliminate it. LSD is great though, and it, or something like it, is pretty much a necessity on AWDs. Of course I’m talking cars here, not drugs. Tripping on acid is probably not so good either on or off road.

Applying the brake when stuck does sometimes work and is often called “the poor man’s limited slip.” AWDs often use the ABS system to apply brakes to wheels that lose traction.

I was taught to put “two clicks” on the e-brake if I ever got stuck in mud or snow. It’s not as good as having lockers, but it does do wonders for getting out of a snowed-in parking spot or climbing up a muddy hill.

That would make sense (assuming a rear wheel is spinning) since it would only lock up the rear wheels. At least that way if you do start to get free you’re front wheels aren’t going to be holding you back.

I think I originally heard the trick here mentioned in reference to army vehicles when they have a wheel off the ground.

Check this out

http://www.rubicon-trail.com/4WD101/driveline-bind.html

Then read this

http://www.4x4abc.com/4WD101/def_turnpart.html

This site shows what is going on in a way that made me understand it, all of my cars have 4matic so it doesnt really matter but I do have a off road vehicle so I wanted to learn about using 4 high on the highway in the rain. Good thing that i didn’t do that.