4WD Front to Rear Tire Ratios

[rScotty]

Hello ALL!
SamofSweden started an interesting discussion in the thread "Kubota 7040 question" about Front to Rear tire sizes for 4wd tractors. It seems to me that the subject is one that affects all 4WD tractors, so I thought I'd start a new thread.
Here's where we were at this morning....feel free to jump right in.

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, rScotty here. In a earlier (12/11) post on the "Kubota 7040 question" thread I've wrote down some methods I've used 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 for some reason the ratios are hard to find and hardly ever 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. On one of my tractors (Yanmar 336D) I got curious enough to try different tires and found that at the low end of the ratio - that is when the ratios are nearly equal - there s 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.
Be interesting to see what others think.
rScotty

 

[LD1]

There is no way with a mechanical driven system to eliminate ALL tire slipping between front and rear. Even on trucks, with identical ratios and identical tires, you will bind and try to slip when turning. Cause the front axle is traveling in a larger arc than the rears. Over driving the fronts helps this, like with tractors, but doesnt do good when going straight.

Only way to eliminate it would be to have an open differential setup to split the front and rear halves of the tractor. Similar to the old 203 t-case in the full-time trucks of the late 70's era

 

[David Devuono]

Just put a string around the front tire and then the rear tire and then measure it. That will give you the ratio of front to rear tire. Alternately you could mark chaulk on the tires and see how many times the front tire rotates for one rear tire. That might not give you something like 3.73:1 but a pretty good ball park. I remember on a 4x4 counting driveshaft revolutions to make a tire go once around, not possible on the tractor rear.

 

[buickanddeere]

Put a string around a front and rear tire.
Drive 10 revolutions of the rear tires in 2WD . measure distance and number of front tire rotations.
Repeat with tractor in 4WD. Do the math on the difference.

 

[texasjohn]

rScotty, great first post...I learned things...including details of WHY I have very significantly more front tire wear than rear...more that justified by simple tire size differences.

 

[Piston]

I was following that thread a bit, and I think it was the one where sixdogs posted a link to some nice looking ag/forestry tires? I checked out that link and it got me thinking. I really liked those tires but figured with all the hassle of finding out my tire size ratios etc, I wouldn't even bother trying to figure out if they would work or not.

You've simplified it quite a bit, and really it doesn't seem that hard. I like the chalk-on-tire method that David mentioned.

Good discussion to post up Scotty

 

[rScotty]

Just put a string around the front tire and then the rear tire and then measure it. That will give you the ratio of front to rear tire.

David thanks for the thought and for thinking about the problem. I don't believe that method won't work here, although if the tire was measured up in the air it would come close to giving us the circumference if we took into account the diameter of the string.

But the string diameter - or even the string's stretch - isn't the problem. The problem is that the tire isn't a simple shape when it is moving. The weight of the tractor and the inflation of the tire come into play to make the tractors rotating center - the axle - closer to the ground than it is to the top of the tire. Add to that the bottom of the tire is being continually deformed into a flatter shape.

I'll admit those things don't seem like a large enough difference to matter, but it turns out to be enough of a difference to swamp the small 3 to 10 percent number we are looking for in order to match the ratios. This isn't a guess; I've tried it.

The string method comes closest to giving you the tire spec "circumference". But what you want is the tire spec described as "rolling or loaded circumference". Not all tire companys publish those tire specs but they all have them somewhere. It surprised me how much those two numbers differ. And also how much they differ due to the type and inflation of the tire.

I'll post a simpler method separately so this post doesn't get too long.
rScotty

 

[rScotty]

Heres the simplest accurate method of getting the tire measurements that I know of.Although it is still need to know the tractor's internal gear ratio if you have found it.

BTW, I sure agree with LD1 that turning arc matters...and with his comment that the only really right way to solve the 4wd ratio problem is by having a differential in the driveshaft connecting the fronts to the rears. That way we end up with an differential for each axle side-to-side, plus a differential in the drive shaft front to back. Three diffs in all.

For getting the rolling circumference then.....Put a wet splotch of paint on the outer surface of the front and rear tires - use different colors for the front and the rear tires. Immediately drive very slowly in a low gear over a long flat dry stretch of pavement keeping it going absolutely straight. Measure the distance between splotches and get an average. This will give an accurate real rolling circumference of the tires. If you can't find the numbers for your old tires in a tire book, that's the the easiest method I know of to get the numbers.

Then if you like the way the 4wd ratio you have has been working, you don't even need to find out the internal gearing ratio. You're almost done. Simply pick tires that the tire book says has a a rotating circumference that gives the same ratio that you just measured. That's the easy way and plenty good enough.

If you don't like the way the 4wd has been working we can work on what it is that is going wrong and what that means.
One kind of ratio mismatch causes the front wheels to be pushed around by the rears - and is especially noticible traveling in a straight line. The other type of mismatch causes excessive skidding on turns plus hanging up in 4wd. Both eat up tires.
Good luck,
rScotty

 

[art]

For the forty plus years we have been selling four wheel drive tractors with all things being considered including tire manufacturers and using ag tires we have found ideal to be between two and five percent lead.
It is harder then you might be thinking it is to really check lead if conditions vary! In soft ground it is not the right way to measure to the top of the lug but to the base of the lug! In soft ground the lug will penetrate to the tire body giving a different ratio all together vs on hard ground to include the lug height. This makes it far easier to do with turf tires and almost as easy as with industrial tires.
With mechanical engagement it is quite easy to balance with tires provided from manufacturer by air pressure and driving on a hard surface in and out of four wheel drive. You should be able to disengage with little or no effort.
Tires do vary in diameter from one manufacturer to another within the same size!

To go over five percent lead the potential excessive wear on fronts is quite noticeable and can cut front tire life to half with additional lead driveline components are also subject to premature failure. When I talk of premature failure with manufacturers knowing that approximately 75 hours a year on a compact is the normal you will not have issues during warrantee period!!!! It is only after 500 hours that the operation of unbalanced drivetrains become an issue.

 

[CobyRupert]

It is harder then you might be thinking it is to really check lead if conditions vary! In soft ground it is not the right way to measure to the top of the lug but to the base of the lug! In soft ground the lug will penetrate to the tire body giving a different ratio all together vs on hard ground to include the lug height.

:thumbsup:
Very true! .....but isn't the ratio only really a concern when on hard ground (top of lug measurement) where the linkage will fight/bind? (And why are you in 4wd then?) On soft ground you're going to get a whole different range of slip (between the tire and the ground in addition to between front and back axles) anyways, plus circumference is now at base of lug as you mention. Like walking in sand.