Determining Correct Tire Size

[Auto4]

Yes. In your calculations you are missing the difference between calculated circumference - which you are doing, and "rolling circumference or RC" - which I believe is the spec that Piloon is guiding you toward.

Your question has an easy answer, and I believe that you will find that answer in the formal definition of rolling circumference. RC is the measurement spec. that manufacturers use in their 4WD designs.
good luck,
rScotty

Thanks for the thoughtful reply...but as I understand it, the only difference between calculated circumference and rolling circumference is allowance for a 3%-4% deflection of the tire when it is in contact with the ground and is supporting weight. The fact remains that the rolling circumference difference between the front tires when they were new with full tread and the worn out condition that they are in right now I believe is still about the same rolling circumference difference as it would be by changing from a 7.2-16 tire to an 8.3-16 tire. So the more I am learning about the importance of matching four wheel drive front wheels with the transmission and rear rear wheels, the more concerned I become but I am suspecting that the difference I am dealing with does not matter. Thoughts anyone?

 

[Auto4]

Hi Kelvin....I just tried your simple test and it worked for me. I have been asking folks if I could switch 8.3-16 tires for 7.2-16 tires on the front of my four wheel drive Kubota with front end loader. I already made the switch earlier and they seemed to work fine but I began asking folks on this thread if it was ok just to confirm. After spending time calculating rolling circumference differences and consulting tire specs etc...I decided to try your simple roll test with the four wheel drive engaged. I couldn't budge my tractor by hand whereas it was really easy to move it before I switched the tires. So I now I know for sure I cannot make the substitution. Thanks for your great idea...it worked for me. Ron

 

[rScotty]

Thanks for the thoughtful reply...but as I understand it, the only difference between calculated circumference and rolling circumference is allowance for a 3%-4% deflection of the tire when it is in contact with the ground and is supporting weight. The fact remains that the rolling circumference difference between the front tires when they were new with full tread and the worn out condition that they are in right now I believe is still about the same rolling circumference difference as it would be by changing from a 7.2-16 tire to an 8.3-16 tire. So the more I am learning about the importance of matching four wheel drive front wheels with the transmission and rear rear wheels, the more concerned I become but I am suspecting that the difference I am dealing with does not matter. Thoughts anyone?

Yes, forming a hypothesis is part of the answer. But you seem to want to dig deeper. Science over opinion; yes, I applaud that. So.....we need to know much difference is significant? Is that 3 or 4% you mentioned significant? Hmmm....that reminds me, where did that 3 to 4% you are using come from? Years ago when I first looked into this problem I was surprised to find that the percentage of difference front to rear is an accepted range with a surprisingly short span.

So if we accept that a difference in ratio between the tires and the gears is required, and we know why the difference is always biased in favor of a higher ratio in front?....But which way does using calculated circumference versus rolling circumference bias the F/R ratio? Is it enough to move the ratio away from the normal span? Which way does it move things? How about tread type versus soil type? That has to make a difference, but in which direction?

BTW, I am aware of Kevin's test and disagree with the test he proposes. Even at the low end of the required difference in front/rear ratio I think you will find it difficult to impossible to hand push the tractor in 4WD.
rScotty

 

[Auto4]

Well I switched wheels again and with the 7.2-16 tires on the front, I could move the tractor by hand while it was in 4wd without much difficulty whereas with the 8.3-16 I could not budge it. So for me, that little test, along with some warnings from the good people in this thread, convinced me to stay with the smaller tires. Its a bummer for me because I could have swapped those almost new 8.3-16 tires for free as I am selling the tractor they are on...now I have to go buy tires instead. Oh well...glad I did not damage my tractor so thanks everyone for your help.

 

[Diddion]

Well I switched wheels again and with the 7.2-16 tires on the front, I could move the tractor by hand while it was in 4wd without much difficulty whereas with the 8.3-16 I could not budge it. So for me, that little test, along with some warnings from the good people in this thread, convinced me to stay with the smaller tires. Its a bummer for me because I could have swapped those almost new 8.3-16 tires for free as I am selling the tractor they are on...now I have to go buy tires instead. Oh well...glad I did not damage my tractor so thanks everyone for your help.

I have an answer to this question - suggested to me today, and absolutely obvious, when you think about it!

1 Jack the tractor up, so that it is on its side - one front and one back wheel off the ground.
2 Immobilise those two wheels which are on the ground - I hammered chocks in
3 Using a spirit level, put a vertical mark at the top of the raised front and back wheels. I also put another mark at the bottom, to give extra certainty. Have a willing helper look at the front wheel, whilst you look at the rear.
4 Place the tractor in first gear, low ratio. Using only a little throttle, start the engine, and allow it to tickover smoothly.
5 Count 10 revolutions of the rear wheel - you'll have to turn the engine off a little before the 10, to allow for some slowing down. If you miss it, just start the engine again and do one or two more revolutions.
6 At the same time, have your helper count the revolutions on the front.

You may have to estimate part revolutions - an exact quarter turn is 90 degrees. In my test on my Kubota B6000 I used a school protractor, and measured that the front wheel turned 13 full revolutions, plus just under a quarter - the protractor measured this as 80 degrees. I slightly under-rotated the rear, by 5 degrees short of the full 10 rotations. Remembering that there are 360 degrees in a complete rotation, the axle ratios are measured to be (13 x 360) + 80 divided by 10 x 360 - 5. = 4760/3595 = 1.324. If you do this yourself - on any tractor - the greater the number of revolutions, the more accurate will be the result.

I believe that the maximum lead permissible on this machine is 5% (and the minimum zero), which suggests that the smallest ratio should be 1.261.

To put it simply, I suggest that the ratio of the rolling circumferences (don't use the radii) of the tyres should be between 1.261 and 1.324, with a target of 1.30 to 1.31. Hope this helps!

ps. There is clearly the danger that your tractor might run off the supports. Take real care!! Make sure that your helper holds the throttle lever and can turn it off if problems develop, make sure that, if it does start to move, the tractor won't immediately smash into something or roll off a slope or ledge and, above all, make sure that you are out of the way if it does start to move or topple! If you have a long, flat area you can achieve the same measurements by riding forward a certain number of front or rear revolutions, and measuring the number of revolutions of the other wheel.

 

[rScotty]

Excellent!

Diddion, in my book that's an excellent way to determine the existing internal R/F gear ratio.

I did something similar with my old Yanmar YM336D when going from the OEM lugged tires to turfs back about a decade ago. That required a change of not only the tires but the wheels themselves....and so it was time to put on the thinking cap. I Jacked it up on one side as you suggest, and then mounted pointers to chalkmarks on both the front and rear tires. Rather than starting the tractor, I rotated the rear tire by hand, and kept track of the number of tire rotations until both chalk marks lined up again. From my notes, it says that when all the marks had lined up, the front had turned 61 complete revolutions while the rear was turned 36. Dividing 61/36 gives a F/R ratio of 1.694. Which means 1.694 is exactly the ratio of the internal machined gearing without any influence from the wheels or tires. That's cool...

But what does this ideal ratio of 1.694 mean in selecting tires? Well after an embarrassingly long time pondering the issue, it told me two things:
One is that if I select tires whose rolling circumference is very close to that 1.694 ratio the differential load on the 4wd mechancial parts is minimized. That's good....very good because obviously the farther we get from that perfectly matched ratio and the more serious the traction is then the more we load up those 4wd gears. If there is any mismatch at all, then the tires have to be allowed to slip or else something inside will break. With mismatched ratio the first thing we will notice is that it is hard to shift out of 4wd because of the "windup" on the gear shafts. We may even see a tire begin to slip when turning sharply. Both are warnings to back off before you hear expensive noises.

And the other thing it told me is that there could be a problem with ratios and steering - especially if the rear tires are covering more ground per drive shaft revolution than the front tires. In that case then the tractor's traction will tend to overpower it's steering. It's being pushed from the rear instead of pulled from the front. A mismatch where the rear tires are much larger than the fronts has the tractor rear wheels pushing the front wheels when in 4wd. That will slide both front tires excessively in soft soil making it hard to turn. BTW, some sliding of the front tires in 4wd is normal. More importantly, that type of mismatch - called an "Overdriven Ratio" makes steering squirrely at road speeds. Not much of a problem with a 12 mph tractor, but is absolutely something to avoid with a car or truck at highway speeds.

So since the perfect ratio is difficult to attain, and to avoid the problems with an overdriven ratio, most designers want to choose tires with a rolling circumference that will make the front tires tend to pull the tractor rather than being pushed by the tractor. They do that by selecting a front to rear circumference ratio that is 2% to 5% greater than the ratio of the gears themselves. The idea being that out where the tire meets the ground the front tire turns a little farther than ideal for each revolution of the gears....

The stock tire for that tractor is a Fireston 13.6x26 with a stated rolling circumfernce of 147" and the front is a Ohtsu 8x16 with a RC of 90.4". Note that now we are dividing Rear by Front, (WHY?) and that gives a R/F RC ratio of147/90.4 = 1.626 .... What's up with that? I was surprised enough to look into it a bit farther.

Being curious, the next step was to check how that internal gear ratio I had measured compared to my tractor's "real world" rolling circumference. After all, that's the only ratio that really matters. This tractor had low hours, normal wear, and still had the original lugged tires. There wasn't much wear on the rear tires but quite a bit on the front...maybe due to the loader and maybe due to the rolling circumference ratio. Anyway, this seemed the perfect chance to check how the factory did in selecting tires to match their internal ratio.... so I did two tests to check that. Both simply required marking the tread on the tire so that it would leave a obvious mark on the ground and then measuring distance between those marks. That would give a direct measurment of rolling circumference. We are only measuring the tire circumference now, not the internal gearing. In powdered snow I used a piece of tape spanning two lugs, and on dry pavement I used a blob of wet paint.

The first thing I found out was that I had to go in an absolutely straight line and even then the measurement was enough different (about 2%) from right tires to left tires that I had to mark all 4 tires and average the measured distance.
The second thing I found out is that I had to do this circumference test in 2wd. If I did it in 4wd, apparently the internal gearing caused the tires to slip an imperceptible amount.....just enough to make the measured distance keep coming out to exactly match the 1.694 ratio of the gears themselves. That made me suspicious so I switched to 2wd and repeated the test. This time I got an average of 1.618, which is a more believeable number because it was also quite close to the tire manufacturer's specs for the rolling circumference of the tires (1.626). Perhaps the difference was due to the extra wear on the front tires...or maybe not.

By now I was beginning to know quite a bit about the tires and wheels and gears as they came from the factory, ..... plus I was beginning to understand why most tractors acted as they did in 4wd. THere was still the problem of what it all meant when it came to choosing tires and wheels and ratios for the changeover from lugs to wide turf tires. But I was learning.
Enjoy! rScotty