4 wheel drive question...ratios

DENNIS MIN · Dec 28, 2016

All,

Please forgive my ignorance, but if I had a 4 wheel drive tractor/truck, and there was a little slip, would I be better off having the front wheels pulling or being pushed, as far as driveline wrap? This assumes no differential betwixt front and rear axles.

I am of the thinking the front wheels probably should pull a whisker more than the aft can provide. I am calculating the tire size as well as the ratios for getting a good balance.

I am thinking of gravel surfaces, not tar roads.

As a side, I don't know exactly where to put a comma when I type these sort of things.

D.

rrman61 · Dec 28, 2016

I know fwa tractors have a 3% gain on the front to be able to steer right

504 · Dec 28, 2016

4X4 have different ratios front to rear. With all of the different types of part time, full time, and automatic all wheel drive,it depends what system it uses.

moresmoke · Dec 28, 2016

Mfwd tractors run faster on the front to pull around a corner. A 4wd truck that runs on hard surface at times needs to be the same speed front and back.

An articulated vehicle would also be the same on both ends.

tim s · Dec 28, 2016

I just went through all this a couple years ago building my 60 4X4,,I found that the front ends run from 103% to 107% of the rears mine is 105%,,,on most 4X4 pickups if they have 411 rears they will have 410 fronts

DENNIS MIN · Dec 28, 2016

Tim S,

I have not seen any JD tractor with that sort of drive.

It's either extraordinarily rare, or you have made a great adaptation. Thanks for the reply regarding ratios.

I will do a bit more math prior to any wheel diameter and ring/pinion combination.

D.

mj · Dec 28, 2016

(quoted from post at 18:16:33 12/28/16)..........................A 4wd truck that runs on hard surface at times needs to be the same speed front and back.
....................

I've never seen that, what brand? 8)

showcrop · Dec 28, 2016

(quoted from post at 18:28:16 12/28/16)

(quoted from post at 18:16:33 12/28/16)..........................A 4wd truck that runs on hard surface at times needs to be the same speed front and back.
....................

Click to expand...

I've never seen that, what brand? 8)

I don't think that it goes by brand, but by type. If it is the type that you need to be able to steer the front would need to turn a little faster.

Goose · Dec 28, 2016

Don't know about tractors, but as I recall '73 to '80 half ton Chevy pickups with full time 4wd had 3:08 in front and 3:07 in the rear. That would have made the rear slightly taller than the front.

When I questioned it during the era, I was told it was simply the way the gears had to be designed to keep the front differential from being overly large. The full time transfer case with it's own differential compensated for it, anyway.

Makes sense, though, that the front diff be slightly taller than the rear for steering control.

the tractor vet · Dec 28, 2016

Not no more as this all changed in about 80-83 and now they run the same gear in the ft as in the rear . I know that Ford started this in 78 on the F250's with the Dana 60 ft and rear axle and trust me the bad road handling went out the window . Before they ran a 4.09 in the ft and a 4.10 in the rear and the ft pulled just a little more and made all the difference on bad roads . My 95 Dodge one ton has a 4.10 in the ft and a 4.10 in the back m y buddy's Dodge has a 3.54 ft and a 3.54 rear . They did this to eliminate drive line wind up as the older trucks would at time hard to almost impossible to get out of four wheel drive if ran on a hard surface . The one way to unwind one was to back up for a short distance .

RodinNS · Dec 29, 2016

I've never looked into the ratios on trucks, but I would expect there is a small amount of lead on the front axle. There needs to be to steer in slick conditions. Otherwise the rear will push and the front will plow...
With MFWD tractors the specified lead is between 1 and 5%. Less and they won't steer... and more and stuff starts cracking up. Equal wheel machines that are articulated... as far as I know have the same ratios front/rear.

Rod

ohiojim · Dec 29, 2016

fronts need to turn faster or you couldn't steer around corners

MarkB_MI · Dec 29, 2016

You get the best steering control if there is NO torque on the front wheels. The more torque to the tires doing the steering, the more likely they are to break traction. That's why, if you're turning a front wheel drive or 4x4 on ice, it's a good idea to pop the transmission into neutral as you make the turn.

In the particular situation you mention, it doesn't really matter whether the front axle is "pushing" or "pulling". Either way, if a tire breaks traction you will lose steering control on that tire. And if both front tires brake traction you lose ALL steering. Which is why most vehicles do not have limited slip differentials on the front axle.

Mike M · Dec 29, 2016

I remember the literature on the JD 750 compact mentioned the front ran slightly faster.
Kubota also at one time had a bi speed turn where when turning the front end shifted gears to a faster speed to help you turn better. It did work too !

Part Time Pete · Dec 29, 2016

On my 86 F-350, the front is 3.56 and the rear is 3.55
Pete

4wdtom · Dec 29, 2016

I don't know anything about what it should be but I will say that in a corner the front wheels travel a longer distance than the rears. The sharper the corner the bigger the difference.Look at sharp corner tracks in snow or mud. The difference would be almost nothing at speed on the road.

wilson ind · Dec 29, 2016

PLEASE EXPLAIN TALLER AS I HAVE HEARD IT Before . sorry cap,lock was on

mj · Dec 29, 2016

(quoted from post at 08:42:04 12/29/16) On my 86 F-350, the front is 3.56 and the rear is 3.55
Pete

You sure it's not the other way 'round? 8)

T_Texas_Tony · Dec 29, 2016

Tractor Vet, I need to talk to you: I'm having PTO Hell and really could use some input. Please send me an email or PM me and let me know the best way to get in touch with me. Many thanks in advance.

Texasmark1 · Dec 29, 2016

Let me jump in here as I am buying rear tires for a 4wd and think I am correct. Will take numbers posted by TIM S below with his Deere:

He says: 411 rears and 410 fronts. Assume engine is turning 5000 rpm. 5000/411 = 12.165 rpms. The front is 5000/410 = 12.195.

12.195/12.165 = 0.00247 or 0.2 ?% faster than the rear (aka just slightly). I agree that it has to run faster to be able to pull around a corner. Otherwise it is going to be pushed sideways (like turning a 2wd.....obviously by a much smaller amount but pushed none the less) by the rears.

On the different radii of the circles, I don't think it has a thing to do with it. The drive is from the engine to the differentials regardless of the wheel positions. The wheels will have to follow the gearing, unlike a 2wd where the front wheels are free to follow the friction of the surface.

On taller one would think higher ratio and the 411 would be taller than the 410.

This is all speculation on my part so I am open for contributing comments as I too am interested in the correct answer.

495man · Dec 30, 2016

Modern pick ups use same ratio. Lets you drive it at road speeds in 4x4. The same ratio however does not let it turn easy on hard surfaces. You can feel a noticeable bind in the steering and drivetrain.

FWA tractors have some lead (faster front axle) Drive one on a packy but loose surface and if you watch closely you can see the front wheels slip slightly in the tread. This is why FWA tractors wear the front tires down faster than the rears, especially ones run on concrete, i.e. barn duty. Turns are easy, tractor pulls right around.

Try to drive a FWA down the road with front axle engaged and you can feel/hear the binding in the drivetrain.

495man · Dec 30, 2016

Modern pick ups use same ratio. Lets you drive it at road speeds in 4x4. The same ratio however does not let it turn easy on hard surfaces. You can feel a noticeable bind in the steering and drivetrain.

FWA tractors have some lead (faster front axle) Drive one on a packy but loose surface and if you watch closely you can see the front wheels slip slightly in the tread. This is why FWA tractors wear the front tires down faster than the rears, especially ones run on concrete, i.e. barn duty. Turns are easy, tractor pulls right around.

Try to drive a FWA down the road with front axle engaged and you can feel/hear the binding in the drivetrain.

4 wheel drive question...ratios

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