Why no 4 wheel drive on roads?

[CausticUrbanCoast]

Interesting. All of my engines will go at different rpms depending on where I have the throttle set.

Hahaha yeah, not so clear on my part.

Engine only runs at one rpm speed at any given moment. Engine only transfers one rpm speed to the transmission.

The gears sending shaft rpm front and back are close in wheel size compensation ratio, but not perfect. Even hydraulic final drives have the same out of sync issue (getting them rebuilt at the same time or purchased matched somewhat helps with this).

For two wheels of different sizes traveling one rotation, he is correct, a smaller wheel rotates slower for both to complete one rotation at the same time.

However, to travel as a connected chassis the same distance, the smaller wheel will have to rotate more times than the larger wheel. The smaller wheel will have a larger rpm rate.

Tractors, to compensate for different wheel sizes have different gear ratios going to the front and back to take advantage of only having one engine input speed.

My understanding is that it is easier to error on having the front slightly faster than the rear in the gear ratio. On soft ground this allows the gears to push themselves back in sync on the following edge of the tooth.

If one were to short the ratio for error and wear, the gears would always be trying to drive the gear teeth into the leading tooth. This causes more wear, accidental teeth breaking if the there is a driveline shock, not to mention if it shorts too far and locks up the gears on a slip or mismatch.

The old tight vs right gear/belt issue.

 

[Kolas]

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

I'm pushing a large snow thrower in 4-wheel drive with chains on the rear agricultural tires. I'm guessing the front wheels are not getting enough traction to worry about dragging.

 

[ROUSTABOUT]

On 4 wd trucks and tractors I stay in 2 wd until I see a bad spot. Put in 4, go through, switch back to 2. On hard surface it will wear stuff out completely. Plus break something. I have fixed it all. It will wear ring and pinion usually first. Ask anybody that maintains reverse chute concrete trucks 6x6. Always front axle worn gears, broke axles. Why don't people read tractor instructions? All wheel drive in trucks and cars have a differential in the transfer cases to alleviate this problem. Subaru has them with a posi differential in them. Full time Ford dodge Chevy had 203 transfer cases that had a diff on t case. You had to get an elimination diff kit to put lock out hubs and use 2wd. I still have a brand new kit. I am very stingy with my 4x4. But, I have never broke or worn out anything either. Even my 55 Willys pickup, never broke a thing. I have a 74 Ford 4x4, never broke a thing. Been stuck so bad had to crawl out a window.

 

[ROUSTABOUT]

When you turn it puts your front and rear at different ratios, stuff gets in a bind. Something is wearing out during bind time.

 

[sandman2234]

When you turn it puts your front and rear at different ratios, stuff gets in a bind. Something is wearing out during bind time.

Isn't the binding during a turn negated by the spider gears in a differential?
David from jax

 

[lman]

Isn't the binding during a turn negated by the spider gears in a differential?
David from jax

There is no differential between the front and rear axle.

 

[W MORRIS]

Hey Guys,

I'm new to TBN, just signed up yesterday. Signed up looking for insight into a problem I'm having with my LS XR4040C. But I'll tackle that later. Started reading this line of questioning as to why you do not run 4-WD on a hard surface. It's pretty simple to most of us but I gather from comments of some that there is not a clear understanding of a simple (old school) 4-WD tractor or a 4-WD truck. I'm a State Certified Mechanical Contractor in the State of Florida. (think a/c, refrigeration, etc.) Not an auto, truck or tractor mechanic by any means. So with that out of the way let put in my 2-cents to this discussion. Although I did not read every comment made, I didn't hear anyone effectively explain how the differential actually works. Again this is for the ones that don't understand. Understanding this will help understand the main question at hand. My description will use an "unlocked" differential and I will use an example that is not technically accurate but simple, to keep it simple to follow.

Imagine a differential that has a 1-1 ratio for this example. 1 revolution of the drive shaft will result in 1 revolution out of each axel in a straight line. So for the following example lets call that 1+1=2 The power from the drive shaft spins the Pinion gear which in turn drives the Ring gear. The Ring gear then sends the power through the Spider gears before it gets to the axles. Its the Spider gears that allow for different wheel speed while turning. Simple example, in a turn left the the left rear (inside) tire will make .75 turns and the right (outside) tire will make 1.25 turns. .75 + 1.25 = 2 In a tight turn left the inside may do .50 and the outside will do 1.50 turns. My point in this is to explain that the Spider gears is how an (unlocked) differential does not bind up in a turn. Now think of the Front and Rear drive as right and left. There is no Spider gears in between them to compensate for different rotations of the front vs the rear axle while traveling straight or turning. It is a solid connection with no slippage. The only slippage that happens and is by design is for the different axles to skid, slide, be pushed or drug when on a soft surface. In a turn the front axle has to travel further requiring more revolutions than the rear axle. In a tight turn this can be about twice as much or so. Something has to give when the same revolutions is sent to the front as the rear. Others spoke about the front axle being faster than the rear (on simple old school drive lines) and that is true to help compensate for that. If they were the same ratio the rear axle would try to push and slide the front axle and your turn would not be as tight, nor as comfortable operationally. Now imagine all this slipping, sliding, pushing and dragging of the tires on a hard or paved surface and you should be able to understand how the hole drive line will become bound up. Again I'm no expert but just thankful for the things my Daddy exposed me to as a little kid. Then as a young guy messing with my old 1959 F-100 4 x 4, and so on, so on. I know, elementary for most folks on this site. But for the ones with less understanding, hope this helped.

 

[PILOON]

OK, here is one statement that can't be argued.
The smaller fronts will wear out much faster than the large rears!

(*and at very rapid rate if in 4 or all wheel when driven on asphalt)

 

[lman]

Hey Guys,

I'm new to TBN, just signed up yesterday. Signed up looking for insight into a problem I'm having with my LS XR4040C. But I'll tackle that later. Started reading this line of questioning as to why you do not run 4-WD on a hard surface. It's pretty simple to most of us but I gather from comments of some that there is not a clear understanding of a simple (old school) 4-WD tractor or a 4-WD truck. I'm a State Certified Mechanical Contractor in the State of Florida. (think a/c, refrigeration, etc.) Not an auto, truck or tractor mechanic by any means. So with that out of the way let put in my 2-cents to this discussion. Although I did not read every comment made, I didn't hear anyone effectively explain how the differential actually works. Again this is for the ones that don't understand. Understanding this will help understand the main question at hand. My description will use an "unlocked" differential and I will use an example that is not technically accurate but simple, to keep it simple to follow.

Imagine a differential that has a 1-1 ratio for this example. 1 revolution of the drive shaft will result in 1 revolution out of each axel in a straight line. So for the following example lets call that 1+1=2 The power from the drive shaft spins the Pinion gear which in turn drives the Ring gear. The Ring gear then sends the power through the Spider gears before it gets to the axles. Its the Spider gears that allow for different wheel speed while turning. Simple example, in a turn left the the left rear (inside) tire will make .75 turns and the right (outside) tire will make 1.25 turns. .75 + 1.25 = 2 In a tight turn left the inside may do .50 and the outside will do 1.50 turns. My point in this is to explain that the Spider gears is how an (unlocked) differential does not bind up in a turn. Now think of the Front and Rear drive as right and left. There is no Spider gears in between them to compensate for different rotations of the front vs the rear axle while traveling straight or turning. It is a solid connection with no slippage. The only slippage that happens and is by design is for the different axles to skid, slide, be pushed or drug when on a soft surface. In a turn the front axle has to travel further requiring more revolutions than the rear axle. In a tight turn this can be about twice as much or so. Something has to give when the same revolutions is sent to the front as the rear. Others spoke about the front axle being faster than the rear (on simple old school drive lines) and that is true to help compensate for that. If they were the same ratio the rear axle would try to push and slide the front axle and your turn would not be as tight, nor as comfortable operationally. Now imagine all this slipping, sliding, pushing and dragging of the tires on a hard or paved surface and you should be able to understand how the hole drive line will become bound up. Again I'm no expert but just thankful for the things my Daddy exposed me to as a little kid. Then as a young guy messing with my old 1959 F-100 4 x 4, and so on, so on. I know, elementary for most folks on this site. But for the ones with less understanding, hope this helped.

That's what I said.

 

[ROUSTABOUT]

Isn't the binding during a turn negated by the spider gears in a differential?
David from jax

No it makes it a different ratio. Even army trucks have an overrun clutch in transfer cases. Old WW2 stuff didn't but they break.

Anything full time 4x4 has differential on each axle and one in transfer case because of this. If it is not full time you need to be careful with the part time.

Fixed one in early morning for a brick layer in town. The engagement wasnt working. Brought it back before lunch with a broke front axle. On a hard surface in 4x4 with a loaded bucket of mason sand. Pop it was over. He was hot about it at first until he read some instructions.