If you have time and you want to read thru this, I will outline some "scenarios"
Lets start with a normal 2 Wheel drive vehicle. This could be a car, or truck or even a 2 Wheel drive tractor.
Lets say you have one tire on a solid surface like concrete or dry asphalt. Now the other tire is sitting on glare ice with a little water on top, or some really slick mud. What happens when you attempt to start the vehicle in motion? Why doesn't it move forward, and why does the tire sitting on the slick surface just spin"? Why doesn't the tire sitting on the perfectly dry surface with good traction move the vehicle forward?
Here is why. Imagine if you will the drive shaft coming from the transmission has a gear called the pinion on the end, that meshes with the ring gear that is hard bolted to the differential frame. Also on this differential frame is either 2 or often 4 spyder gears. These spyder gears are attached to the differential frame by "jackshafts" or "stub shafts".
These spyder gears mesh with the two bevel gears that are on the ends of the axle shafts that go out to the wheels thru the axle tube. As the ring gear rotates as it is driven by the pinion and therefore the differential frame rotates the jackshafts rotate, BUT the spyder gears
DO NOT rotate on their shafts, they just rotate and push against both of the bevel gears (remember that are on the ends of both of the axle shafts) and in pushing against them equally, both of the axle shafts turn and of course the wheels and tires turn and the vehicle moves. Whew! that was a mouthful.
This all happens above if and only if the traction applied to the tires is equal. In other words going down a straight dry road. The spyder gears
DO NOT rotate on their shafts. Those nasty little spyder gears don't even have bearings, just bushings because they don't rotate a bunch.
When you go around a corner, the traction is unequal. and the spyder gears rotate on their shafts which causes the outside tire to go faster than the inside tire.
When you have one tire on a slick surface, the spyder gears rotate real fast on their shafts, so fast that if you persist in spinning the tire on the slick surface, the spyder gears will eventually overheat their bushings on their shafts from the high speed rotation. By the way your spinning tire is going twice as fast as a speedometer would indicate. So if it says 120 mph, your tire is frying at 240 mph. And your spyder gears will eventually turn blue. (don't ask much more about this)
So your "2 wheel drive" vehicle became a 1 wheel drive vehicle with the one wheel/tire on the slick surface doing the rotation. But since is is just spinning, you might as well have a 0 wheel drive vehicle. Pretty neat huh? 2 equals 1 and 1 equals none!
Now lets multiply our drive system by 2. So now we have a 4 wheel drive vehicle So they say...
Now place one of the front wheels/tires on the slick and one of the rear wheels/tires on the slick, and what happens? Same as before the 2 wheels with the least traction spin and the other two on the dry surface don't move. So now 4 equals 2, and 2 equals no forward motion either. Pretty cool huh?
So what can we do to get this tractor moving off of the slick stuff? We can one of two different things. We can apply the rear differential locking mechanism which will solidly lock the two REAR axles together and now we have 3 moving tires. AND guess what ONE of them is on a solid surface and the tractor will move forward. OR we could apply a braking force using our split brakes (not all tractors have split brakes but many do) to the rear wheel that is spinning. When we apply this braking force to the rear spinning wheel, this slows down the axle shaft and gives those spyder gears something solid to push against and stops their rotation on their jackshaft, which in turn allows force to be applied to that other axle shaft that is attached to the wheel/tire on the dry surface and it begins to move. So you wind up with a 3 wheel drive system. The only time you have all 4 wheels pulling equally is when the traction surface under each tire is equal.
If you are in a car or truck and you are stuck you either have some sort of many different locking mechanisms built into the rear end or the newer "traction control" which does independent braking when a wheel sensor indicates wheel spin OR if you have none of this, you get out and push or call a tow truck.
So remember 4 equals 2, 2 equals 1, and 1 is none. Happy tractoring.
