4WD and loader work

   / 4WD and loader work #101  
".... But turns left or right do generate differing distance traveled by the two front tires so there ARE both kinds of slippage. It does have to do with turning versus driving straight as well as the designed-in other reason. And if you happen to be on high traction surfaces like pavement you will feel the jerkiness of tires slipping on the pavement. Not all windup or slip is deliberate, there is also some of it inherent and unavoidable."

That's an interesting way to look at it, and would be correct if the two front wheels were connected to each other by a solid axle. But the front wheels are NOT connected by a solid axle. One front tire doesn't know or react to what the other is doing. Let me help with what is happening there....

Inside the middle of the front axle housing there is a heavily built mechanical bevel and pinion device called a "differential" which always allows one front tire to travel a different distance and independently from the other tire. That way the different distances won't cause either tire to slip or skip regardless of the surface.

The rear wheels have the same set up. There is a mechanical differential in the middle of the axle which connects the two rear rear tires.

Both differentials do the same thing - they prevent "wheel hop" due to one side traveling a different distance or on different traction surfaces.

You can prove this to yourself by using the loader to lift the front wheels off the ground. Hold one wheel from turning, and the other wheel can still be easily turned by hand. The mechanical differential in the center of the axle allows each half axle to rotate independently of the other.

So there is no way no windup can occur on the front or rear axle or between one front tire and the other. The mechanical differential prevents the windup there and also on the rear axle

So where is the windup anyway?? And why Full Time 4wd versus Part time 4wd?

In all Full Time 4wd vehicles - like a lot of modern SUV - there are the same two differentials on the front and rear axles as explained above - but in addition there is also a third mechanical differential in the front driveshaft which carries power to the front axle. We are now looking at that front driveshaft which is located under the center of the tractor and connects the front of the transmission to the back of the front axle. That extra differential in the front drinveshaft in all wheel drive vehicles is what prevents windup in their front driveshaft and allows the vehicle to be a Full or All Time 4wd.

So in Full Time 4wd vehicles there is no way no windup can occur on the front or rear axle or between one front tire and the other. The mechanical differential prevents the windup there.

But our compact and utility and small Ag tractors are Part Time 4wd and they don't have that extra third (high dollar) differential in that front driveshaft. They rely on gearing within the transmission to match the different circumferences of the front and rear tires. That gearing is deliberately made with the ratio slightly "off" of perfect one to one F/R to allow the front driveshaft to turn a few percent faster than the rear driveshaft and always maintains positive steering.

The key takeaway is that without a third differential such as Full Time 4wds have, when we put our 4wd tractors in 4wd mode they rely on tire slippage to prevent windup in the front driveshaft and the parts that connect it.

Whew.. What a long dissertation. Maybe it will help a bit. A good question is, "If Part Time 4wd vehicles without the extra differential always have some windup when in 4wd mode, why is this only a problem with the front and not the rear?"
The answer is simply that the forces that cause windup do also happen at the back end, but the rear driveshaft is short and so heavily built that it is almost immune. It would be possible to build the front driveshaft that heavily as well - but most are not.

Hope this helps,