Kubota 7040 question.

[samofsweden]

Tire dealers--like Firestone--have this info. The Kubota dealer does as well.
You actually need the "ratio" so you can math out the tire size comparison.
Do not screw this up or you screw up tractor.

Best to use size it came with or prior approved tire option.

Hi sixdogs
Thx for helping, yes I know its VERY important.

 

[sixdogs]

Hi sixdogs
Thx for helping, yes I know its VERY important.

I can almost remember how to do the calculation off the ratio and the rolling circumference of the tires involved. I have it written down and that would refresh my memory if you get in bind--and have the ratio..

If you look at approved tire sizes for the 7040--or anything--and compare the rolling circumference to each other, you will find the front is 102% to 103% of the rear after the ratio is factored in. The front pulls the rear or you grind up the gears. That's why they say to stay out of 4WD on hard pavement or you'll wear out the front tires.

 

[ovrszd]

I also would think the tire dealer or manufacturer could answer your questions based on the current tires size/brand??

Curious to see how this works out. Please post your findings and end results.

 

[rScotty]

I can almost remember how to do the calculation off the ratio and the rolling circumference of the tires involved. I have it written down and that would refresh my memory if you get in bind--and have the ratio..

If you look at approved tire sizes for the 7040--or anything--and compare the rolling circumference to each other, you will find the front is 102% to 103% of the rear after the ratio is factored in. The front pulls the rear or you grind up the gears. That's why they say to stay out of 4WD on hard pavement or you'll wear out the front tires.

Hello sixdogs, in a earlier post on this thread (12/11) I've written down methods for figuring out the Front/Rear gear ratio and the rolling circumference tire ratio. Hopefully it will check what you have written down. Take a look and see ......

I've no idea why manufacturers make these F/R ratios such a secret. They are are at least as important as things like lubrication charts or internal motor part clearances - but seem to be little discussed. It would seem to me that the way the F/R ratio changes with Ag tires on the tractor versus Industrials or Turfs would be of great interest to the customer. It sure has a big impact on the way the tractor performs.

Continuing where we left off, generally the whole 4wd system will be dominated by the rear wheels because they have the greatest traction. So that relative to the rears, you then want the front wheels to always be rotating at either the exact same ratio as your rears or "pulling" slightly (higher ratio) so that you always have steering control. Having the ratio exactly the same front to rear would be workable and just fine....except for times when the relative traction, or tire wear, or inflation, would combine to make the rears overpower the fronts - which would result in loss of steering. So in order to make sure that you always have steering, the front tires are usually given a slightly higher ratio. The closer to 1:1 the better, but it must never be less. So as you say, the result is usually more on the lines of 1.02:1 all the way up to 1.1:1. That's a 2 to 10 percent higher ratio - just as we've all heard.

The trouble is that 2 to 10 percent is a rather large spread. At the low end is some loss of steering control and skidding when turning on sloppy surfaces . On the high end there is a huge amount of tire wear and stress on drivetrain components. And all of us have seen what happens to a nice finished grassy lawn if we drive over it and turn around in 4WD as opposed to 2WD. Folks who use belly mower tractors for lawn mowing are very aware of this.

It gets worse. In practice, nobody can really nail down the right amount of extra "overdriven front ratio" to design for because it is so dependent on inflation, wear, and surface traction. IMHO, Variable Ratio coupling is the real answer...that's what AWD cars use. But Variable Ratio is expensive and most of Variable Ratio systems reduce ultimate traction. So for a simple ol' tractor the best the manufacturer can do is change the Front to Rear gearing either in the transfer case or differentials enough to make sure the final ratio always favors the fronts enough to keep it pulling from the front end and therefore the steering working right. Common practice is to do this by giving the front end a 3 to 10% higher ratio than the rears. And then to overbuild the U-joints and transfer case pieces enough so that they can skid a tire as needed without breaking any transmission components. BTW, that is another reason why AWD vehicles have fluid F/R coupling....and so should tractors....but that's another story. It's also why a tractor tranny can be hard to shift out of 4WD sometimes.

At present, most tractor manufacturers seem to just tend to build the drivetrain parts oversize (hopefully) and let the machine just go ahead and skid a tire when turning on a hard surface like asphalt. Trading tire wear for mechanical wear.

I'm surprised this ratio thing isn't more discussed. It sure has a large effect on how we use our tractors.
I think I'll post this here and also use the message to start another thread on Front to Rear Tire Ratios.
rScotty

 

[sixdogs]

RScotty--I red your earlier post and that's a lot of work and can be subject to operator error. The easiest way is to use of the tire combos that Kubota already suggests. Piece of cake, that way.

Next easiest is to find out the front axle "ratio" as it pertains to computing the tire thing. As mentioned you can call a Firestone ag dealer and they have a book with that info in it. If I couldn't find it that way I would call Kubota in California and get it from an engineer.

Plan "C" would be to do the math on all the tire combos that Kubota offers and pick a ratio on a new tire combo that is within the range of the numbers you came up with on the approved option tire list. Nothing wrong with back engineering.

All these are pretty easy choices. Anyway, that's my take on it.

 

[rScotty]

RScotty--I red your earlier post and that's a lot of work and can be subject to operator error. The easiest way is to use of the tire combos that Kubota already suggests. Piece of cake, that way.

Next easiest is to find out the front axle "ratio" as it pertains to computing the tire thing. As mentioned you can call a Firestone ag dealer and they have a book with that info in it. If I couldn't find it that way I would call Kubota in California and get it from an engineer.

Plan "C" would be to do the math on all the tire combos that Kubota offers and pick a ratio on a new tire combo that is within the range of the numbers you came up with on the approved option tire list. Nothing wrong with back engineering.

All these are pretty easy choices. Anyway, that's my take on it.

Thanks for the feedback. Yes, I agree that knowing what works is sure a lot easier than figuring out why something works. But figuring out why something works in the first place sure does open up a whole lot more choices.

I don't see figuring out the front to rear drive ratio as being very difficult. One trick that helps me is to use a notebook rather than rely on my memory for things like measurements and counting tire revolutions.
In fact, I'm so simple minded that when counting wheel revolutions to calculate the front to rear gear ratio I put down a pencil mark every time the front or rear tire makes a complete revolution. Then when the marks on the tires line up with my coathanger pointers exactly as they were when I started I just count the front and rear pencil marks, divide the front tire number by the rear tire number, and that's the Front to Rear gear ratio right there.
After that it's just a matter of going to any of the tire manufacturer's specs and choosing tires whose rotating circumference give the same ratio.
I think that's worth the doing....and frankly I'd trust the numbers that you or I came up with on our own tractors at least as much as the numbers I'd get by asking my dealer. Your dealer might be different, but the guys at our local dealership are mostly oriented towards sales and accessory parts than to mechancial things.

Besides.....isn't it more fun to figure things out?
Enjoy! rScotty

 

[sixdogs]

Thanks for the feedback. Yes, I agree that knowing what works is sure a lot easier than figuring out why something works. But figuring out why something works in the first place sure does open up a whole lot more choices.

I don't see figuring out the front to rear drive ratio as being very difficult. One trick that helps me is to use a notebook rather than rely on my memory for things like measurements and counting tire revolutions.
In fact, I'm so simple minded that when counting wheel revolutions to calculate the front to rear gear ratio I put down a pencil mark every time the front or rear tire makes a complete revolution. Then when the marks on the tires line up with my coathanger pointers exactly as they were when I started I just count the front and rear pencil marks, divide the front tire number by the rear tire number, and that's the Front to Rear gear ratio right there.
After that it's just a matter of going to any of the tire manufacturer's specs and choosing tires whose rotating circumference give the same ratio.
I think that's worth the doing....and frankly I'd trust the numbers that you or I came up with on our own tractors at least as much as the numbers I'd get by asking my dealer. Your dealer might be different, but the guys at our local dealership are mostly oriented towards sales and accessory parts than to mechancial things.

Besides.....isn't it more fun to figure things out?
Enjoy! rScotty

Yup...for the thrill of it. That's why I did it. On the rolling circumference, just look it up in the maker's tire book and forget counting.

On the tire ratio, it is the relationship of how the front and then how the rear relates to the axle ratio and that tells how they relate to each other. Just front to back is not all that's going on. As well, I searched many tire combos and ended up using the original Kubota options. They pretty much put every usable combo as a tire option so even though I have that rebel streak, they already did the heavy lifting and the tire sizes were there in plain view. Yawn.

But I'm not done. My next goal is to find a small B type Kubota and figure out a really tall tire combo that spaces the tractor up to maybe 24' of crop clearance and turns it into a garden side dress fertilizer unit for sweet corn or whatever. Also a parade vehicle and conversation piece.