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...

 

[ning]

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

It's a similar issue.

If the front axle is a locked differential when engaged, you still get driveline tension and wheel hop; in this case, the tension will be between the front and rear diffs (because the front axle will still be turning at a different speed than the rear) as well as to the right and left of the front diff (between the front diff and each of the wheels) because once again the left and right front wheels will be turning at different rates.

The same thing happens in the rear when you lock the rear differential and try to turn.

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

I stated "4wd truck" in particular because 4wd systems are generally considered to have a solid connection between front and back (ie, transfer case) as opposed to a center differential, which may or may not be lockable (generally considered to be "AWD" and specifically called out in my posting).

 

[CausticUrbanCoast]

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...

You are right. Now that I think on it, the front axle is a type of bevel pinion drive.

 

[Torvy]

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.

 

[Jstpssng]

My Colorado has a knob to run the electric to engage 4WD. I'm constantly hitting it with my knee when getting in or out of the truck. I know within 50 feet that it's engaged, because the truck starts bucking and binding.

 

[k0ua]

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...

No, they don't BUT there isn't a center viscous coupling Between the front and rear differential either. So the driveline windup that can occur on solid surfaces can't slip or cannot be relieve by a center coupling differential. Your CUTS are solidly locked Between the front and rear. Sure there is differential action in your front differential as well as your rear differential BUT none between the differentials.

 

[mcfarmall]

Do you have a 4WD pickup or SUV? If so, put it in 4 High and drive it that way for 90 days and report your conclusions back to us.

 

[sandman2234]

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...

Neither are mine!
David from jax

 

[Agvg]

It just a bad design of the drive train, tractors with full time 4WD do eksist but might be a little on the expensive side

The Fendt 1000 Vario | Model year 2021 | Tractors

Award-winning masterpiece. 500 HP. The new Fendt 1000 Vario. Typically Fendt. You can find all the highlights here.

www.fendt.com

This drivetrain drives front and rear totally independent so it can do very cool things and are very effective, steeples s from zero to 60km/h.

 

[RoostersGarage]

I wouldn't say it's a bad design, I'd say it's a less expensive design and since tractors spend most of their life on soft surfaces there's no reason to have the added expense of full time 4wd. Plus, it's very expensive to transfer a lot of torque through a full time 4wd or AWD system and the mechanical lockup tractors have is more efficient.

 

[Agvg]

I have spent a few hours to day in a Toyota Hilux and has very few good things to say about 4WD whitout some form of constant 4WD, it's simple and there stops all advantages.

A 4WD as Fendt makes them will be very effectiv, when you can distribute torque between front and rear and use the front axel to activ help steering will also be very nice.