Why no 4 wheel drive on roads?

[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.