Determining Correct Tire Size

[Kelvin]

To all,
Rather than go back to an older thread, I'll start a new one to finish up this topic.

When correct 4wd tractor front-rear tire size pairings are unknown or unavailable, I have come up with a simple test to determine if questionable substitute tire sizes are acceptable AFTER they are installed on the tractor. While I have an engineering degree, my statement is far from scientific or educated so please feel free to debate this issue.

Kelvin's Questionable Test: To determine if your tractor's 4wd gearing is compatible with the installed wheels & tires, do the following test with the tractor engine shut off and transmission in neutral (this is easiest on smaller tractors):

(1) Put the tractor in 2wd mode and see how easy the tractor is to push straight forward or backward.
(2) Put the tractor in 4wd mode and then see how easy the tractor is to push straight forward or backward.
(3) If there is resistance in 4wd mode that was not encountered in 2wd mode then the tires size may be a problem.

In my specific case, the Kubota B6100/B7100 tractors call for 20.5x8.00-10 front turf tires which are no longer available (there are golf car and trailer tires available in that size, but they (1) don't have the correct tread and (2) supposedly are not the same size as a tractor turf tire bearing the same numbers).

With my AG tires, my improvised test was successful. With turf tires mounted (correct rear tires with 20.0x8.00-10) the test failed (I had a lot of difficulty pushing the tractor in 4wd).

While comparing the B7100 to the B7100's replacement (B7300) I noticed that all of the recommended Ag and Turf tire sizes were the same except for the front turf tires; instead of 20.5x8.00-10 the B7300 recommended a 21.0x8.00x10 which is a tire that is available. I bought a pair, mounted them and performed the above test. The tractor rolled equally easy in 2wd and 4wd mode.

So is the above mentioned simple test valid? If so, whomever reads this may want to pass the tire size information to anyone with "turf tire-equipped 4wd B6100s or B7100s" that the 21.0x8.00-10 is an acceptable substitute front turf tire.

 

[MChalkley]

Your test sounds like it should work well enough on that size equipment to tell you if you've got a go/no-go situation.

Suggestion: call Kubota and ask them what the mechanical ratio between the front and rear axles are. With that number, you can calculate the rolling circumference (RC) of the front tires by multiplying it by the actual RC of the front tire. The result should be 0-3% larger than the RC of the rear tires. Then, they only thing you'll need to worry about is whether they'll physically fit or not.

Mark

 

[rbargeron]

This may be old news to many readers but the reson manufacturers of 4wd pickups say not to engage it on dry pavement is that there is some difference in front-rear tractive speed by DESIGN. Same difference is true in 4wd tractors. Mark's comment about 0-3% is certainly true in theory - maybe in practice there is more like 5%.

If the tire sizes are correct, you may not be able to push it easily in 4wd. If you can, there may not be enough difference to get the full 4wd effect in loose going.

Some simple evidence I've noticed: 4wd tractors on dry pavement are in "synch" when turning sharply. Reason is the front tires have a slightly longer travel length (along the arc) than the rear tires do. You can verify this by putting the 4wd lever in and out while moving - it's easy to disengage when turning (no gear load) but it's hard when going straight.

 

[WVBill]

Kelvin:
I need to replace the Ag tires on my B6100D. I would like to put R4's or Bar-turf tires on is as I need the traction but want a "wider" tire.

I read your post and understand that 21.0X8.00-10 are correct for a front turf tire. What size/type tire do you have on the rear? Also, what manufacturer.

I'm guessing that if I look up the specs for these tires and find out what their rolling circumference is that I can calculate the front-rear ratio and then go find a set of R4s with the same ratio.

Thanks.

Bill

 

[Kelvin]

Bill,

First I'll re-visit the numbers for the turfs and then go into the Bar-turfs and R-4s discussion.

Front Turf Specs: From the owner's manual the front turf tire specs for both the B6100D and the B7100D are 20.5x8.00-10. As mentioned before this tire is no longer produced in a farm turf and I have found the 21.0x8.00-10 to be an acceptable substitute on the B7100.

Rear Turf Specs: As for the rear turfs, the B6100D calls for a 27x8.5-15 whereas my B7100D calls for a 29x12.00-15.

Turf Availability & Pricing: Firestone makes all three (21x8.00-10, 27x8.5-15 and the 29x12.00-15) in a "Turf & Field" model turf tractor tire. Go to the Firestone site http://www.firestoneag.com/tiredata/tiredatabook.asp and enter 15 in the search box at the bottom (for 15" tires). A picture and tires specs will come up (no prices). I believe that the retail for the smaller front tire was around $88 and the rear tire was around $150...each. I bought my front tires discounted from a dealer whom I purchase a lot of stuff from.

Bar Turf: On the tractors that I've seen with Bar-Turf tires, they usually have Bar-Turfs on the rear and standard turfs on the front. Therefore, I would guess that the tire sizes for your B6100D would be the same as standard turf tires.

R-4 Tires: I don't know what the tire size would be required. The Firestone site listed above gave the dimensions of the turf tires (width, height, etc) from which you should be able to come up with at least one set of satisfactory R-4 tire specifications (whether they produce that set is another story). Comparing the ratio of actual heights of (1) your current AG tires and (2) the turf tire specs from the Firestone site should allow you to come up with a pretty close approximate ratio required for the R4 tires. I forgot to look for R4 tires while on the Firestone site but will look again later.

For my tractor, I noted that the front turfs were approximately 1" shorter than the advertised height number (the 20.5 was a little over 19" and the 21.0 was around 20" tall) before the tires were mounted on the rim. I haven't checked them after mounting.

Hope this at least points you in the right direction. I'll look around and if I come up with something else I'll pass it along.
Kelvin

 

[hayden]

The turf Bar tire do use regular turf tires on the front, but they are a different size since the turf-bar rear tire is a smaller diameter that a turn rear tire.

 

[Auto4]

Hey guys I have a Kubota L2350 four wheel drive tractor with front end loader that has 7.2-16 Ag tires on the front. Do you think I would have any problems if I switched them out with 8.3-16 Ag tires? I think it should be no problem but thought I should ask you guys to be sure. You seem to know a lot more about this topic than I do. thanks, Ron

 

[PILOON]

Hey guys I have a Kubota L2350 four wheel drive tractor with front end loader that has 7.2-16 Ag tires on the front. Do you think I would have any problems if I switched them out with 8.3-16 Ag tires? I think it should be no problem but thought I should ask you guys to be sure. You seem to know a lot more about this topic than I do. thanks, Ron

If the circumference (rolling distance) is the same you are good to go.
Go online as most tire manufacturers list the specs.

 

[Auto4]

If the circumference (rolling distance) is the same you are good to go.
Go online as most tire manufacturers list the specs.

Hmm now you've gotten me thinking. The circumference of the 7.2-16 tires is 92.63 (diameter of 29.5) and the circumference of the 8.3-16 tires is 98.91 (diameter of 31.5) so the rolling distance is 6.28 inches more but does it really matter? I mean the front tires on my tractor are worn as smooth as a new baby's behind so the diameter must be at least 2 to 4 inches shorter than when it was new. Assuming the tires lost just one inch of tread all the way around since they were new (I am sure it is more actually), the tires would have a diameter of 27.5 vs 29.5 when new which would mean that they have a circumference of 86.36 now vs 92.63 when new. The difference in the rolling distance then would be 6.28 inches, the same amount with the tire change I am considering. Am I missing something?

 

[rScotty]

Hmm now you've gotten me thinking. The circumference of the 7.2-16 tires is ......Am I missing something?

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

 

[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