Why no 4 wheel drive on roads?

[CausticUrbanCoast]

smaller wheels rotate slower...so your wrong.

Okay

 

[SmallChange]

Funny to see this just now. I just filled one of those Bagster folding dumpsters with rotting fence panels and construction scrap, and approached it from the non-road side and picked it up by the handles with my pallet forks so I could tie the handles towards one another and be sure I satisfied the requirement that the lifting straps can meet in the middle. While it was hanging from the forks I decided I'd left it too close to the road, so I tried to back up in 2WD, but my rears slipped -- they're filled with 500 lbs, but I was not wearing my 800 lb ballast box. I put it in 4WD, which worked fine.

But then I forgot, and later (without the Bagster) made a tight turn onto my paved driveway. I startled myself with a vigorous case of wheel hop. So, there's a good reason. The tractor looks funny with wheel hop, and the startled driver looks even funnier.

 

[Farmer495]

The ft axel is designed to run slightly faster than the rear to allow the driver to shift to 4wd while moving at speed. If both axels were geared the same, you could not engage 4wd smooth. Awd vehicles have a limited slip between ft & rear end. On vehicles that have lock up function on awd they still have to be unlocked to drive on hard surfaces.

Tractor front axle runs faster to allow the tractor to steer better, if it didn't lead(run faster) the front end would slide and be pushed by the rear axle when turning on soft/slick surfaces.

Since gearing is "%" faster, the difference is greater the faster you drive, your 4wd engagement with be harsh or grind if mechanical (versus clutch pack) Try shifting mechanical box tractor into 4x4 in first gear versus road gear.

Kubota M-110 etc had "bi speed" front axle, it ran one ratio normal, but would upshift (automatic powershift) the front axle to a faster speed when you turned sharp. I drove one years ago when new, the 110 HP tractor would almost pivot on one rear wheel without using brakes.

Pickup truck front axle runs the same speed as rear...easy engagement at highway speed, even on dry pavement if you are going straight.
Pickup truck also doesn't like to turn as easily in snow or mud, especially towing a trailer....BTDT.

 

[Farmer495]

smaller wheels rotate slower...so your wrong.

To go the same travelled distance (when rolling) smaller wheels will turn more often then larger ones since they have less circumference.

If the driven axle shaft is the same rpm, a small wheel will rotate slower, but that would mean on a tractor the rear axle is pushing skidding/locked up front tires. The front tire shaft RPM has to be increased to make them travel the same, or slightly farther (oversped) distance. So yes, the front wheels end up rotating faster.

 

[ning]

smaller wheels rotate slower...so your wrong.

In case this isn't a troll despite appearances:
1: smaller wheels rotate more times per distance traveled than do larger wheels (ie, they rotate faster)
2: you can choose the final drive (differential) gearing to closely match the front and rear driveshaft speeds even when there's substantial wheel size difference.

The front wheels of my tractor are about ½ the diameter of the rear, yet the gearing is such that the front and rear driveshafts spin at almost the same speed, as evidenced that the tractor doesn't grind to a halt or wheel hop as soon as it hits pavement in 4wd.

 

[JWR]

smaller wheels rotate slower...so your wrong.

No, ed1978, that is not the case. As several posts have pointed out, the front wheel gearing on all 4WD tractors is designed to cause the front wheels to pull ahead slightly more inches per unit time than the rear. That is what Caustic... meant saying that the smaller (front) wheels "rotate at a faster rate than the rear ones." OK, maybe his description was worded imperfectly but he is correct. Assuming std size tires and wheels it has nothing in the world to do with the size of the tire. It is the intentional gearing driving those tires. That design is on purpose to cause the front wheels to pull ahead just slightly.

This entire topic is covered, if I may say so, IMHO as I see people say, by my post #59.

 

[coast40]

My 69 Bronco ended up with approximately 3.54 rear gears and 3.50 front gears. When I got it the ratios were about opposite; it was a bucking Bronco. For more confusion, Google Ackermann steering... https://en.wikipedia.org/wiki/Ackermann_steering_geometry

 

[4570Man]

No, ed1978, that is not the case. As several posts have pointed out, the front wheel gearing on all 4WD tractors is designed to cause the front wheels to pull ahead slightly more inches per unit time than the rear. That is what Caustic... meant saying that the smaller (front) wheels "rotate at a faster rate than the rear ones." OK, maybe his description was worded imperfectly but he is correct. Assuming std size tires and wheels it has nothing in the world to do with the size of the tire. It is the intentional gearing driving those tires. That design is on purpose to cause the front wheels to pull ahead just slightly.

This entire topic is covered, if I may say so, IMHO as I see people say, by my post #59.

He’s wrong on every level. The smaller wheels turn more RPMs to drive the same speed and the front wheels are geared to go faster as in distance traveled than the rears.

 

[CausticUrbanCoast]

No, ed1978, that is not the case. As several posts have pointed out, the front wheel gearing on all 4WD tractors is designed to cause the front wheels to pull ahead slightly more inches per unit time than the rear. That is what Caustic... meant saying that the smaller (front) wheels "rotate at a faster rate than the rear ones." OK, maybe his description was worded imperfectly but he is correct. Assuming std size tires and wheels it has nothing in the world to do with the size of the tire. It is the intentional gearing driving those tires. That design is on purpose to cause the front wheels to pull ahead just slightly.

This entire topic is covered, if I may say so, IMHO as I see people say, by my post #59.

He’s wrong on every level. The smaller wheels turn more RPMs to drive the same speed and the front wheels are geared to go faster as in distance traveled than the rears.

Not sure about every level, my engine only goes at one rpm.

 

[K5lwq]

Not sure about every level, my engine only goes at one rpm.

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

 

[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.