Why no front differential gear?

Will · Jun 5, 2004

If you ever leave your tractor in 4WD and then make a turn on a hard surface, you'll find that you lose a layer of rubber from your front tires. If you make a sharp turn on grass, you'll remove a layer of turf instead.

It can't cost that much to put a differential gear on the front of a tractor, so why isn't this an option? Maybe some tractors have this? Maybe there are some disadvantages that I can't think of? What do you think?

BGL990 · Jun 5, 2004

I believe there IS always a differential between the two front tires as there is between the two back tires, but there is generally NOT a diff. between front and back. Because of the slightly different paths taken by each end when turning (and a variety of other things like driveline lead/lag, differences in tire size due to inaccurate air pressures, etc, etc.), the front and rear drive paths want to turn slightly different speeds. They can't, so something has to skid.

Brad.

To20Chris · Jun 5, 2004

BGL990 is right. Without a center diff, it forces an even torque split front to rear. If there were a center diff, then it would need some kind of "limited slip" device, or if one end lost traction, then the other end would get no power. Just like on a car with full-time 4WD and open diffs - it is barely better than 2WD, at least at the point of slipping.

All needless complication on a tractor - if you're on pavement and doing a lot of tight turns, put it in 2WD & you'll have no problems

RobertN · Jun 5, 2004

Ya, what he said /forums/images/graemlins/cool.gif

My Kubota does it, My Dodge 4x4 pickup does it, my two Ford 4x4 pickups did it, and all the Jeeps I grew up around did it.

That's why they say not to use 4x4 on hard surfaces unless necessary. The fron traks different than the back in a turn. The drive line is tied together in the transfer case.

All of the "All Wheel Drive" cars have diff's in the transfercase to get around this. It would cost A LOT to do this in a tractor; it's cheaper to just shift to 2wd...

ZJ_HR · Jun 5, 2004

Differential between front and rear axle haven't advantage to standard tractor setup. Just disadvantages. Why?

- More teethwheels more noise and less efficient tranny.
- Worse traction capabilities, because of different load distribution to front/rear end, depending of implements. For example: with front loader and full bucket, tractor will spin rear wheels when front wheels will stop, or with plow will be opposite.
For good traction you will need lock differential between front and rear axle - so why differential then.
- No problem with tractor stability in curves when 4 wd is on, because of low road speed.
- Cheaper to make automatic 4wd (like on NH TNS) than central diff - auto disengaging when steering angle is over 20-30 deg.
- When you use full pulling power, for best traction you need 5-10% of sleep, so difference is sleepage of front and rear wheels are not important.
- Usually you don't need 4wd often on hard surfaces or lawn.

BTW, wide front wheels on my tractor (360 mm) make traces on lawn or concrete during sharp steer, as you described, even in 2wd, and unloaded tractor. /forums/images/graemlins/blush.gif

There are some advantages of this setup, like on PU-s or Jeeps, but they are minor on standard tractor.

MikePA · Jun 6, 2004

<font color="blue">Maybe there are some disadvantages that I can't think of? What do you think?
</font>
Increased costs to manufacturer and therefore buy, more parts to break, another differential to service, all to address a problem that can be solved by taking the tractor out of 4wd in these situations.

RickB · Jun 6, 2004

ZJ has posted an excellent response to this question. Don't miss it.

Will · Jun 6, 2004

Thanks for the information. I used my loader to pick the front wheels off the ground, and sure enough, if I rotate one wheel forward, the other goes backwards. I'm surprised that all that skidding around on hard surfaces is because the front and back wheels are coupled. All of the explanations for why it should be done this way make sense to me.

_RaT_ · Jun 7, 2004

Heres another experiment. If you really want to find out just what it would be like to have no differential, lock up the rear differential lock and try turning. The front/rear tracking 4WD difference will seem rather minor by comparisson. Some tractors even allow you to lock the front differential.

ZJ_HR · Jun 7, 2004

Will, I understood your question wrong. You asked does front axle has differential - yes it does, as you concluded after lifting front end. I tried to explain why there is no differential between front and rear axle (like on Land Rover, for example).
Tracks on pavement, when 4wd is engaged, are because of different rolling radius of front and rear wheels, as somebody mentioned before.
I didn't mention that my bigger tractor has both mechanical (100%, not limited slip front) differential locks. Diff locks can be engaged simultaneously by pressind pedal on right pedestal.
When both diff locks are engaged, and 4wd on, all 4 wheels have similar circumference (rolling) speed, and is forbidden to steer on hard surfaces (I tried this on pavement, and saw traces of all 4 wheels) /forums/images/graemlins/blush.gif
Diff locks survived this test /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif