4wd

   / 4wd #1  

Mark Page

Platinum Member
Joined
Sep 27, 2009
Messages
559
Location
Maryland
Tractor
Massey Ferguson 2615 48hp, 4wd, loader
Is 4 wheel drive really 4 wheel drive. I've owned 6 vehicles that were supposed to be 4wd. Never saw more than 3 wheels driving, usually 2 -1 front and 1 rear.
I'm wondering if tractors are the same way. I haven't had a chance to check out my new MF 2615 as we had no snow last winter. If I lock the diff in 4wd will all 4 wheels drive.
What should I expect?
 
   / 4wd #2  
Is 4 wheel drive really 4 wheel drive. I've owned 6 vehicles that were supposed to be 4wd. Never saw more than 3 wheels driving, usually 2 -1 front and 1 rear.
I'm wondering if tractors are the same way. I haven't had a chance to check out my new MF 2615 as we had no snow last winter. If I lock the diff in 4wd will all 4 wheels drive.
What should I expect?


If you will follow this thread all the way through, and watch the video then you will know more about differentials than the majority of people. and it will give you a good foundation for understanding the 4wd variation. Happy viewing

http://www.tractorbynet.com/forums/kubota-owning-operating/244523-tires-spinning.html



James K0UA
 
   / 4wd #3  
If you lock the diff you will have both rear spinning and 1 front. The differentials job is to differentiate power between the wheels. When you make a turn the outside wheel has to go faster than the inside wheel. The differential lock will lock both axles together so that both tires will spin. Under certain conditions a tractor or vehicle will spin both front or rear tires even though they do not have a locking diff. When they do this they are not putting the power down like a locking diff would. If you had true 4wd it would almost be impossible to make a turn. You should not have the diff on you tractor locked while you are making a turn because it puts extra stress on the axles.
 
   / 4wd #4  
I looked through the posted thread, but that still leaves me wondering how the mechanical four wheel drive works on my JD 3320.

I KNOW there is a differential between the left & right wheels of the rear axle.

I KNOW that when I press the Differential Lock pedal that locks the two rear wheels together - so they roll at the exact same speed. It says so in the manual - very clearly.

I assume that there is a differential between the left & right wheels of the front axle. Correct??? There has to be. If they are locked side-to-side it would make sharp turns in super difficult in any mode - and that doesn't happen. If the front wheels are free-wheeling, then MFWD couldn't work (I can't imagine power would be provided to only one wheel all the time).

When I engage the mechanical four wheel drive (MFWD) it puts power to the front axle - BUT I have two questions:

(1) Does the front differential - assuming it exists - ever lock? I don't think so.

(2) With MFWD engaged, is the front and rear axle locked together? I'm assuming they are - which is why the tractor doesn't like hard turns in MFWD and it tends to shred turf. If there was a differential between the front and rear axles - the tractor wouldn't have any trouble doing tight turns even in MFWD (which would be nice), BUT it would get stuck if even one wheel lost traction. So, my assumption has always been that the front & back axles are locked (no differential) in MFWD. Am I correct?

I know some cars have "limited slip differentials", but I suspect that my tractor doesn't have those - not side-to-side, nor back-to-front.

To summarize, I assume that there is a rear side-to-side simple differential that can lock by pressing the "differential lock pedal", that there is is a front side-to-side simple differential that never locks. I assume that in non-MFWD mode there is no power to the front axle - that the front axle is not at all connected to the rear axle, and in MFWD the front and rear axles are hard-locked. Am I correct?
 
   / 4wd #5  
To get "real" 4WD the front axle has to have limited slip and you have to lock the rear differential with the foot lever (on a Kubota).

The Kubota M7040, M8540. and M9040 have a limited slip front differential. Mine has engaged a few times in heavy conditions but there is no lever or way to permanently engage it.

When pulling over large trees I have left four very nice excavated holes in the ground.
 
   / 4wd #6  
I looked through the posted thread, but that still leaves me wondering how the mechanical four wheel drive works on my JD 3320.

I KNOW there is a differential between the left & right wheels of the rear axle.

I KNOW that when I press the Differential Lock pedal that locks the two rear wheels together - so they roll at the exact same speed. It says so in the manual - very clearly.

I assume that there is a differential between the left & right wheels of the front axle. Correct??? There has to be. If they are locked side-to-side it would make sharp turns in super difficult in any mode - and that doesn't happen. If the front wheels are free-wheeling, then MFWD couldn't work (I can't imagine power would be provided to only one wheel all the time).

When I engage the mechanical four wheel drive (MFWD) it puts power to the front axle - BUT I have two questions:

(1) Does the front differential - assuming it exists - ever lock? I don't think so.

(2) With MFWD engaged, is the front and rear axle locked together? I'm assuming they are - which is why the tractor doesn't like hard turns in MFWD and it tends to shred turf. If there was a differential between the front and rear axles - the tractor wouldn't have any trouble doing tight turns even in MFWD (which would be nice), BUT it would get stuck if even one wheel lost traction. So, my assumption has always been that the front & back axles are locked (no differential) in MFWD. Am I correct?

I know some cars have "limited slip differentials", but I suspect that my tractor doesn't have those - not side-to-side, nor back-to-front.

To summarize, I assume that there is a rear side-to-side simple differential that can lock by pressing the "differential lock pedal", that there is is a front side-to-side simple differential that never locks. I assume that in non-MFWD mode there is no power to the front axle - that the front axle is not at all connected to the rear axle, and in MFWD the front and rear axles are hard-locked. Am I correct?


There is a front differential as you assumed. It does not have any provisions to lock side to side. It is what is called an open differential. When you engage the MFWD you are powering a drive shaft that originates in the rear and goes to the front to power that differential and those wheels. Yes it is mechanically locked together rear to front. That is why you also have front wheel braking when in 4WD. There are no actual brakes on the front hubs, but when you engage the rear brakes your also are applying braking action to the front wheels thru the differential and the driveshaft. When you are in 2WD, there is no braking action and the driveshaft is not driven from the rear. (though it still turns from the front as the wheels turn on the ground.) You seem to handle a pretty good handle on everything. I should modify what I said about locking front differentials, and brakes for that matter.. On larger tractors these things are possible.. but I was refereing to the average CUT or SCUT most of us own.

James K0UA
 
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   / 4wd #7  
Thanks James, I'd always wondered about this.

About the brakes on my JD3320 ... I've never had the wheels off nor have I looked closely behind the rear wheels (kind of embarrassed to say). Are they just behind wheels - like a car? I did a search and see some pics of what looks like drum brakes.

(Sorry if this is hijacking this thread!!)
 
   / 4wd #8  
Thanks James, I'd always wondered about this.

About the brakes on my JD3320 ... I've never had the wheels off nor have I looked closely behind the rear wheels (kind of embarrassed to say). Are they just behind wheels - like a car? I did a search and see some pics of what looks like drum brakes.

(Sorry if this is hijacking this thread!!)
Their inside, and bathed in fluid... wet brakes. On most modern CUTs

James K0UA
 
   / 4wd #9  
It's a common misconception that 4WD is really 3WD. It's not.

With an open differential, the torque being delivered to the ground is determined by the wheel with the least traction, but both sides are ALWAYS receiving the same torque from the differential. So when one wheel is spinning and the other is not, they are putting the same power to the ground. In other words, it is 4WD. To get more pull you need a locking diff where the torque is always unequal, but the speed is always the same.

Open diff = equal torque and unequal speed for wheels on the same axle.
Locked diff = unequal torque and equal speed for wheels on the same axle.

If you want more power to the ground, with an open diff and one wheel spinning, apply the brake on the spinning wheel. Now you are applying as much torque to the non slipping tire as you are to the total of the brake and the ground on the slipping tire. But you are still applying equal torque, from the differential and through the drive axles, to each wheel.

Watch this video for a better explanation of an "open" differential:

[ame=http://www.youtube.com/watch?v=yYAw79386WI]Around The Corner (1937) How Differential Steering Works - YouTube[/ame]


A "locker" simply locks the two axles together as one and eliminates the differential. A "limited slip" applies friction between the two axles to resist them turning at different speeds, but doesn't lock them.

Each condition has it's advantages and disadvantages.
 
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   / 4wd #10  
It's a common misconception that 4WD is really 3WD. It's not.

{SNIP}

.

I dont think I could have explained it better myself:thumbsup:

Just because the wheel is spinning, doesnt mean it isnt driving.
If you see 3 wheels spinning, that fourth wheel is STILL driving, and having power applied. It just isnt spinning, and honestly, probabally doing the most work since it isnt spinning and has the most traction.

So...4wd really is 4wd, just not 4-wheel-spinning.
 
 
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