Why no 4 wheel drive on roads?

[DieselBound]

apologize for the obvious, to relieve my '92 4x4 toyota pu switching back to 2wd, i come to a full stop, reverse & it will usually do the trick
on the tractor, either by reversing, or by slightly lifting the ft end w/FEL will disengage a stuck 4wd. thinking my limited slip ft end also helps w/the 4wd binding. love the higher end M series...

My Polaris also will get locked into 4wd such that I have to reverse and then switch to 2wd to release 4wd. Seems like physics is in play with everything

 

[ljjhouser]

My first experience driving a 4wd vehicle was in 1976. We had an IH Scout. Had to get out and lock the front hubs for 4WD. The instructions back then were to avoid driving in 4WD on dry pavement. I'm no engineer, but I don't think it is just a tractor thing.

In 1995 (seems like yesterday) I bought a Ford Bronco full size ( I think that was the last year for them). It had "self locking hubs on the front. I did not like them, you could never tell for sure if they were locked or not. Seemed that they wouldn't lock until they would slip. Too late them, stuck. I changed them to Mile Marker manual locking hubs. Liked that much better.

 

[ning]

In 1995 (seems like yesterday) I bought a Ford Bronco full size ( I think that was the last year for them). It had "self locking hubs on the front. I did not like them, you could never tell for sure if they were locked or not. Seemed that they wouldn't lock until they would slip. Too late them, stuck. I changed them to Mile Marker manual locking hubs. Liked that much better.

The only unreliable thing about my '97 F350 was that the autolock hubs (the left one, actually, for whatever reason) failed twice in 170k miles.
Now I've got the good old fashioned manual hubs too. Lock them if there's any hint I'll want 4wd, maybe I'll remember to unlock them by summer...

 

[ruffdog]

It's called transmission wind-up and there can be a tremendous amount of torque induced into the driveline.

 

[CausticUrbanCoast]

Odd segway to diesel-electric drivetrain.

It is unlikely to happen, that is why hydraulic final drives exist. Too expensive for most compact tractors though, at that price point people buy a skidsteer or telehandler instead.

 

[Farmer495]

Tractor: Front wheels lead (turn faster) so you are binding up the drivetrain and the front tires either slip or something lets go in the driveline.

Loader tractors used on concrete floors scraping alleys in barns that run in 4wd all the time tend to wear the front tires bald in a hurry.

You will feel the bind if you leave tractor in 4x4 on dry pavement.

The binding is considerably noticeable at faster speeds. Have an operator's manual for one tractor that clearly states not the use 4wd in higher gears.

I do a lot of road travel, hundreds of km a year, all in 2wd. Icy or snowy roads I will use 4wd if I need the traction.

Pickup Truck: Front wheels normally turn same speed as rears, you can drive as fast as you want, shift in and out of 4wd on the fly on most newer trucks. Try to turn a corner on dry pavement and you will feel it....

 

[k0ua]

Tractor: Front wheels lead (turn faster) so you are binding up the drivetrain and the front tires either slip or something lets go in the driveline.

Loader tractors used on concrete floors scraping alleys in barns that run in 4wd all the time tend to wear the front tires bald in a hurry.

You will feel the bind if you leave tractor in 4x4 on dry pavement.

The binding is considerably noticeable at faster speeds. Have an operator's manual for one tractor that clearly states not the use 4wd in higher gears.

I do a lot of road travel, hundreds of km a year, all in 2wd. Icy or snowy roads I will use 4wd if I need the traction.

Pickup Truck: Front wheels normally turn same speed as rears, you can drive as fast as you want, shift in and out of 4wd on the fly on most newer trucks. Try to turn a corner on dry pavement and you will feel it....

Yes, unless you have a vehicle with a viscous coupling between front and rear driveline. Example Chevrolet Silverado of mine has an "Auto" setting on the 4WD selector. The idea is you can drive on a road that is dry is some places and maybe a little icy in others. You need 4WD one second and don't the next. It works pretty well, and prevents torque windup, but does throw torque toward the front driveline when needed. Of course it "eats" some fuel mileage. So you don't want to run in it all the time as 2WD will give better gas mileage. But our CUT tractors have nothing like that.

 

[ptsg]

Yes, unless you have a vehicle with a viscous coupling between front and rear driveline. Example Chevrolet Silverado of mine has an "Auto" setting on the 4WD selector. The idea is you can drive on a road that is dry is some places and maybe a little icy in others. You need 4WD one second and don't the next. It works pretty well, and prevents torque windup, but does throw torque toward the front driveline when needed. Of course it "eats" some fuel mileage. So you don't want to run in it all the time as 2WD will give better gas mileage. But our CUT tractors have nothing like that.

Or a vehicle with AWD capability, just like Mitsubishi have been doing since the 90's with their Super Select 2 transfer case on the Mitsubishi Pajero/Montero and the L200 Pickup trucks.

Basically the transfer case has 4 positions: First - 2WD, Second - 4x4 with open center diff, Third - 4x4 with locked center diff and Forth- 4x4 Low Range with locked center diff. Some also had a Neutral position used for a mechanical front winch.

The 4x4 with open center diff can be engaged on the go as needed and it will disengage the front diff automatically in 2WD.

 

[LouNY]

Or you can get a transfercase like the RAM 1500's do and I wouldn't be surprised if many of the other vehicles with an auto mode and a loc mode do.
In the RAM half ton in the fancier trims the transfer case has Auto 4wd, Hi 2wd, Hi4wd and Lo4wd, the earlier models actually called it Auto 4wd and Hi lock 4wd and Lo Lock 4wd which was an out right lie.
Any of the RAM's with an Auto 4wd mode have a wet clutch setup in the transfer case going to the front axle. The vehicle computer decides how much of a voltage signal to send to the clutch depending on throttle position wheel speeds and steering angle as well as other inputs.
And it does this in both the Auto mode and the so called Lock mode. The fifth generations (new style 19's and up) do a much better job of "locking" then the earlier ones did.
My 15 was terrible, my wifes last 2 vehicles had a better awd system then my 4wd so called Lock, a Chevy Equinox and a little Jeep Compass.
I ended up with an aftermarket wiring harness that would feed the clutch full 12V+ with the flip of a switch which helped that truck a lot.
You could put that truck in 4 ho or 4lo lock and gently accellerating on ice just spin the rear tires the front would just grab a tiny bit every once in awhile. And it even had a limited slip differential in it.
The "19" that I have now actually bucks a bit in 2wd in a corner not much but a bit. When I got this "19" that was a check I did before I bought it as I had heard that it was a different and "improved" transfer case. I was thinking of a Chevy half ton till I learned that low range was an "option".
I still much prefer a solid mechanical transfer case.

 

[Farmer495]

Yes, unless you have a vehicle with a viscous coupling between front and rear driveline. Example Chevrolet Silverado of mine has an "Auto" setting on the 4WD selector. The idea is you can drive on a road that is dry is some places and maybe a little icy in others. You need 4WD one second and don't the next. It works pretty well, and prevents torque windup, but does throw torque toward the front driveline when needed. Of course it "eats" some fuel mileage. So you don't want to run in it all the time as 2WD will give better gas mileage. But our CUT tractors have nothing like that.

Yes, drove a few with that mode. My last truck with the transfer case on the floor was fun driving in the urban areas after snow storm, side streets not plowed, main streets plowed, moved that lever quite a bit going between dry pavement and snowpack, every turn seemed to be onto dry pavement!

 

[Farmer495]

I still much prefer a solid mechanical transfer case.

Yes. Less to go wrong, bonus with mechanical hubs. Just a pain to preplan needing 4x4. I've had to replace the hub actuators in my current vacuum shift front axle.... Never had to touch the manual hubs.

 

[ning]

Yes. Less to go wrong, bonus with mechanical hubs. Just a pain to preplan needing 4x4. I've had to replace the hub actuators in my current vacuum shift front axle.... Never had to touch the manual hubs.

Preplanning needing 4x4: "Oh it may snow soon - lock hubs" 5 months later: "I guess we're done with snow - unlock hubs"

 

[Farmer495]

Preplanning needing 4x4: "Oh it may snow soon - lock hubs" 5 months later: "I guess we're done with snow - unlock hubs"

That about sums it up perfectly. Also the "I really don't want to stop right now and I think I can make it through here in 2wd" which ends up with you locking the hubs after you get stuck.

 

[LouNY]

My first Scout didn't have changeable hubs.
It had two sets of caps for the front axle,
one set had splines which matched the axle splines.
You slide the drive hub on lined up the bolts and bolted the hub on.
If desired in the summer you could remove the splined hubs and
install the other hubs that were smooth bored and wouldn't engage the axle splines.
If you had a lot of highway driving planned.
I had them for a couple of years before I found out what they were for

 

[CoyPatton]

The front wheels pull faster than the rears, and on pavement that stress and good traction will wear out a lot of the components. Tires dragging, gears stressed and axles binding.
David from jax

Totally agree with this when tires are different sized front to rear.
However that in my opinion is only part of the reason! Just as with your 4x4 highway vehicle, you should not run 4x4 on dry hard surfaces!
Some passenger vehicles take this control away from you by being computer controlled, but those Full time all wheel drive vehicles do jot always put power to all 4 wheels, but is controlled by the computer— when programmed conditions occur, then and only is power engaged.

 

[sandman2234]

Totally agree with this when tires are different sized front to rear.
However that in my opinion is only part of the reason! Just as with your 4x4 highway vehicle, you should not run 4x4 on dry hard surfaces!
Some passenger vehicles take this control away from you by being computer controlled, but those Full time all wheel drive vehicles do jot always put power to all 4 wheels, but is controlled by the computer— when programmed conditions occur, then and only is power engaged.

Seldom see a tracor with exactly the same size tires in front as in the rear. Manufacturers change gears in axle to account for this and a little extra so that fronts are always out-pulling the rears. Otherwise it would require rears to push fronts which would decrease efficiency. The difference between the actual ratio creates a binding on the driveline if not allowed to slip (on soft surfaces).
David from jax

 

[Bradleyt55]

Less about driving on pavement and more about going fast in 4WD. 4WD is meant to go slow. I was always told to only use 4WD when you need it. ie rough terrain.

 

[fleecer]

The ft axel is designed to run slightly faster than the rear to allow the driver to shift to 4wd while moving at speed. If both axels were geared the same, you could not engage 4wd smooth. Awd vehicles have a limited slip between ft & rear end. On vehicles that have lock up function on awd they still have to be unlocked to drive on hard surfaces.

 

[JWR]

This might open a real hornets nest, but Saturday am I was asked a question from one of our "Big City Weekend Folks" that I really had to stop and think about. The question was: "Sombody told me to turn off 4 wheel drive (on my tractor) when I'm on the road (Pavement), Why?"

I would not say a hornet's nest but there sure is a lot of going off on tangents here. Harmless but distracting. Buried in a few of the other posts is the meat of the answer to your very basic question. Here is WHY:

1) Tractor 4WD is designed, built and intended only for off-road use. Key to that is that there is no 3rd differential to facilitate the front drive running at a different rotation rate than the rear. ALL "4wd" cars and trucks use the third differential in a transfer case to accommodate the front and rear running at slightly different speeds.. You tractor doesn't have one.
2) The implications of 1) are that if you run in 4WD on pavement the front and rear tires have a disagreement on exact rotational speed. In a perfect straight line they may disagree by only a few %. Around curved maneuvers, much more so. Something has to give so you get hopping/jerking on the pavement because the tires must slip going slightly different speeds. The end result is higher wear on tires, on the gearing, on the entire drivetrain. Therefore the extremely well-founded and logical directive is "Don't do it."

It should be footnoted that on soil and gravel and other surfaces (where tractors are designed and intended to be used) slippage to accommodate differing speeds of the front and rear drives, is radically easier and normal than on dry pavement, occurs all the time and no one notices. Even on normal off-road soil, etc. you are reducing wear and tear on the drive train slightly by being selective about when you turn on your 4WD. There is a recent "other thread" that discusses this in some detail and I am too lazy to hunt it. Suffice it to say each operator has unique and individual good reasons for being in or out of 4WD such as steepness, load being pulled, surface slipperiness, etc. Safety/braking is a big one since your brakes use all 4 wheels only when in 4WD.

 

[ed1978]

Most tractors have smaller wheels on the front than the back. The smaller wheels rotate at a faster rate than the rear wheels.

There is a small amount of play in your gears, as the rotation rates are slightly different, they will quickly reach their maximum and start to bind against each other.

Attached is an image of a front axle shaft where I suspect the previous owner did exactly what you are asking about. At least it was only a shaft and coupler that was damaged instead of gear teeth in the transmission or transaxle.

View attachment 717488

smaller wheels rotate slower...so your wrong.