Why no 4 wheel drive on roads?

[OzarkChris]

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 know I've been told this from way back when my Grandfather put me on a new tractor that had 4WD. I don't recall him every telling me exactly why, but it was pounded into my young mind way back then. As I thought more and more about it, it made sense that it would cause less stress on front end parts, maybe be easier on the tires, maybe easier steering, etc. but even after looking through this site and trying to research it via the interweb (maybe I was using the wrong search string, who knows), I can't find any "concrete" information as to why. I believe I gave this person a decent answer - and an answer he accepted, but I'll ask those of you that have been doing this since you were but a wee lad, if you have seen, researched, etc exactly why it is a good idea to turn off 4WD when on pavement. I would like to know so I can give concrete information to others that may ask something similar. BTW, the old timers 'round here just say "because its what we grew up learning" or "because, its the best thing for the tractor", etc

 

[sandman2234]

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

 

[RoostersGarage]

Not to mention that in turning, the outside tire turns faster than the inside tire, and all 4 tires are turning at different speeds. My wife learned this lesson the hard way one day when she pulled our neighbor out of the mud with my truck. She left it in 4wd when she hit the pavement and made a turn. I got a very panicked call from her thinking she broke something. Luckily it was just wheel hop, but she couldn't get it out of 4wd either. I had her finish the turn and straighten the wheels, then it came out.

 

[CausticUrbanCoast]

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.

 

[jeff9366]

In 4-WD the front wheels turn about 5% further than the equivalent units for the back tires.

This is primarily to aid steering in the dirt. Hence 4-WD tractor tires have tread.

2-WD front tires are grooved, not treaded.

4-WD also increases traction.

 

[OzarkChris]

Thank you all, I'm very appreciative that I now have something concrete to tell folks if the discussion comes up again.

 

[Diggin It]

Recent thread with a link to another thread with unfortunate results:

4 wheel drive all the time? or just when needed

I have a bobcat 2025 (kioti) with 55 hours so far; got it this spring. I really only need to engage the front axle when moving dirt or slogging through muddy trails. All my mowing and manure spreading are easily done in 2wd. Any reason to use the front axle every once and a while? I don't really...

www.tractorbynet.com

 

[ning]

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

Most of this difference is handled in the gearing.

Typically in 4wd trucks (as opposed to AWD vehicles which have an open center differential), the gearing is set up to have the front wheels turn about 3% faster than the rears because this produces more intuitive handling. Trucks usually have the same diameter wheels front and back, so they could be geared equally front and back in 4wd, but they're not. Similar with tractors, though, when turning the front and back axles will end up spinning at different rates regardless of gearing, so with a solid connection between front and back (ie typical transfer case, not an open center diff) tension will build up in the driveline which has to give somewhere.

If you're lucky, one or more tires will "hop" or skid to let go some of the tension.
If you're not lucky, you'll lose a gear tooth or two somewhere....

This is the same reason why sometimes when driving in 4wd, it's tough to switch back into 2wd - because there's a lot of tension in the driveline, and the gears are kinda stuck. This can happen even if you're not driving on pavement, though typically tire hop/skid is easier to happen here than on pavement.

 

[CausticUrbanCoast]

Most of this difference is handled in the gearing.

Typically in 4wd trucks (as opposed to AWD vehicles which have an open center differential), the gearing is set up to have the front wheels turn about 3% faster than the rears because this produces more intuitive handling. Trucks usually have the same diameter wheels front and back, so they could be geared equally front and back in 4wd, but they're not. Similar with tractors, though, when turning the front and back axles will end up spinning at different rates regardless of gearing, so with a solid connection between front and back (ie typical transfer case, not an open center diff) tension will build up in the driveline which has to give somewhere.

If you're lucky, one or more tires will "hop" or skid to let go some of the tension.
If you're not lucky, you'll lose a gear tooth or two somewhere....

This is the same reason why sometimes when driving in 4wd, it's tough to switch back into 2wd - because there's a lot of tension in the driveline, and the gears are kinda stuck. This can happen even if you're not driving on pavement, though typically tire hop/skid is easier to happen here than on pavement.

Until you get to the full sized tractors, the front axle is usually a locked differential when engaged.

Not sure about trying to compare it to a 4wd truck which would have an open, remote, or automatic differential locking system.

 

[Henro]

Until you get to the full sized tractors, the front axle is usually a locked differential when engaged.

Not sure about trying to compare it to a 4wd truck which would have an open, remote, or automatic differential locking system.

I do not think this is correct.

Correct me with specifics. I know both my sub compact and compact tractors do not have locked front differentials...