Selecting New Tires -- In a Nutshell

[Cr555]

I recently spent an inordinate amount time tying to figure out how to select new tires for my B7300. Here is what I learned in a nutshell -- I hope this helps someone else:

How to Select the Proper Replacement Tires for your MFWD Kubota -- So, your tires are worn out, dry rotted or damaged and you need to replace them. If you have a 4WD Kubota (or any other brand), you need to make sure that your front and rear tires are properly matched or you will risk damaging both your new tires and your tractor’s differential and related parts.

Here’s what you need to know. Kubota designed your tractor so that the front tires would “lead” the rear tires by about 3 – 7 percent (according to a Kubota factory technician I spoke with). You will find others on this board that recommend a 1 – 5 percent lead, but I am following the technician’s advice. Obviously, your rear tires are larger than the front tires. So, for every time the rear tires make one complete revolution, the front tires make approximately 1.3 – 1.6 complete revolutions depending on your tractor’s final drive ratio. You need to know this number. For my B7300, the final drive ratio is 1.475. For my B1750, the final drive ratio is 1.528. You need to know yours.

To determine whether your replacement tires are compatible with your tractor, you need to run one simple calculation:

(rolling circumference of your front tire / rolling circumference of your rear tire) x your final drive ratio = amount of “lead” or “lag” of your front tires. For my B7300, the rolling circumference for the factory turf tire is 61 inches. The rolling circumference of the factory rear tire (turf) is 86 inches. Therefore, the calculation looks like this:

(61 / 86) x 1.475 = 1.0462

Any result over “1” means that your front tires will lead the rear tires (good). Any result less than “1” means that your front tires will lag the rear tires (bad). In the illustration above, my front tires “lead” the rear tires by 4.62 percent, which is well within Kubota’s recommended range.

Understanding this simple calculation will give you tremendous power to select the proper tires for your tractor. For example, I am thinking about replacing the rear tires on my B7300 with the slightly larger rear tires on my B1750. Would this work? Well, I know that the Firestone Turf and Field 31x13.5-15 tires on my B1750 have a rolling circumference of 93 inches. If I installed them on my B7300 without changing the front tires the “magic” equation gives the following result:

(61 / 93) x 1.475 = 0.9674.

The result is less than “1,” which means the front tires are going to be lagging the rear tires – this is bad! But what if I also substituted larger front tires as well? I discovered that one of the optional front tires on the B2320 is a Titan Multi C/S 23x8.50-12 with a rolling circumference of 65 inches. Although this tire fits on a 12 inch rim (as opposed to the 10 inch rim that came from the factory on my B7300), I am fairly certain the new wheel/tire assembly will fit on my B7300 (adequate clearance, and the wheel is compatible). And, I believe the wheel/tire assembly is available at a reasonable price. So, would the rear tires from my B1750 and the front tires from a B2320 give me a proper match for my B7300? Let’s see:

(65 / 93) x 1.475 = 1.030.

The answer is “yes” – it would give me 3 percent front wheel “lead,” which is perfectly acceptable. So, with this simple equation, I can quickly calculate whether any combination of tires will work on my Kubota. Bottom line: 1) You must find out the final drive ratio for your tractor, and 2) you must know the rolling circumference of any tires you propose using as a replacement. Run the calculation. It will tell you whether you are on the right track.

 

[volp]

I think that Cr555 has made an excellent brief for selecting the right tyre size to your Kubota!

Anders

 

[L35]

I would go from R4 to turf when the time comes. Wouldn't I just be able to get the same sizes that are on there now? Or does each individual tire have the calculations listed with them and you would have to run the numbers?

 

[85Hokie]

Aw yes ....the old rolling circumference theory. I am sure your numbers are spot on, and as others have discussed before, the flat spot will change the RC as well as a load placed on the front or the rear. SO all of those numbers are great in a perfect STRAIGHT LINE world........however - since a 4WD tractor is designed to be on soil rather than a firm surface, something has got to slip at some point. IN a perfect straight line scenario(weight is same front and rear) - the front leads the rear, something on the front shooooulllld be spinning, the front having a smaller footprint than the rear - the rear should have a better grip - hear again assuming all things are perfect..........now 1% 5% it really doesnt matter what the %%% is .....S O M E T H I N G has to give. NOW.....depending on WHAT tire is sitting on what surface the right front might spin a hair faster than the left front - the differential in the front is taking up that little bit of slack, but the front to rear is still going to be "fighting" on another. If the machine is in a bind at 1% or 3% - there will be a bind being delivered to one tire on the front or the rear!! IN a turn the differential will be driving one wheel faster than the other but the diffy front and rear will still be "fighting" one another.

All tractors designers know this, and then again - most all tractors are placed on a "field" a "backyard" a "farm" that allows the bind to be placed on something that will give.

NOW doing all those calc's - the impressive thing is, being able to find something quickly to get a final @ 1:1- nicely done!........no MATTER what the tire type.....hell you could have ags on the front and industrial tires on the rear.....as long as you stayed on the 1-5% margin you would be "ok" the variables will drive the calculator crazy!

now somewhere I remember someone ( Vic?) explaining that the BX25 is reallllly 3 wheel drive.....that the rear really doesnt use both wheels UNTIL the diffy lock is used......

very nice with the math btw!

one needs to remember - if you are on asphalt or concrete .....that four wheel drive lever best not be in the engaged position!!! Unless you like paying for $$$ stuff inside the machine!~

why do we place any vehicle in 4WD???? cause something was slipping in the first place? or .....we dont want to slip?!!! YET something must slip to gain traction! ok ....this is crazy !

 

[tiredguy]

Actually what you came up with is an extremely complicated formula and
I'm actually surprised that a Kubota tech would have told you what he did
or maybe it was simply a misunderstanding on his part, who knows.
Here's what you do to determine whether your replacement tires are compatible with your tractor, you need to run one simple calculation:

(rolling circumference of your front tire / rolling circumference of your rear tire) x your final drive ratio = amount of “lead” or “lag” of your front tires. For my B7300, the rolling circumference for the factory turf tire is 61 inches. The rolling circumference of the factory rear tire (turf) is 86 inches. Therefore, the calculation looks like this:

(61 / 86) x 1.475 = 1.0462

Now here's the simplified and correct formula to determine what you need to know in order to keep the proper ratio:
Divide the front RC by the rear RC 61 divided by 86= .07093 which we'll round up to .71 which is the magic number you need to keep the difference at no matter what sizes you end up using it must be within 2% of that number and you won't have any problems with the lead/lag front to rear.
It's really that simple and no matter what size or brand of tractor it is if the correct tires are on it to begin with and you get the manufacturers rolling circumference specifications on the tires do the math to find out what the ratio is on a particular setup.
If you can't find the specifications then you can measure the travel yourself but it really should only be figured on new tires.
Al

 

[volp]

I have got a slight problem with the calculations.
My B6001 has original size tyres 7 x 14 back and 5.00 x 12 front.
Using Firestones table the diameter is 27.2 and 20.8. The circumference would be (x pi) 85.4 and 65.3

(65.3 / 85.4) x 1.475 = 1,13 that means that the front wheels goes 13% more.

Have I got something wrong?

Anders

 

[Cr555]

Anders,

Yes, you have made a misstep. You are using the "outer diameter" of your tires to calculate the rolling circumference (i.e., you are relying upon the formula, pi x d). One would think from high school geometry that is all you need to do. However, a tire is not a simple beast. When it is loaded (from the tractor's weight), the sidewalls of the tire deflect, causing the actual rolling circumference of the tire to decrease somewhat. See the attached illustration -- note the small, but significant, deflection labled as "f". Illustration 2 also demonstrates the difference between the "static" and "loaded" circumference of a tire.

Anyway, the "magic formula" requires that you use the loaded "rolling circumference" of the tire. In addition, you are using the final drive ratio for MY tractor (a B7300). Your B6001 may very well have a different final drive ratio. I had to contact Kubtoa (Corporate) to obtain the final drive ratio for my B7300 and B1750. Fortunately, Firestone publishes the rolling circumference of the tires that originally came with my tractors. However, when I considered using Carlisle replacement tires, I had to contact Carlisle technical support for the "rolling circumference" of the tires I wanted to use. They sent me a detailed tire "tech sheet."

I don't claim to be a tire expert, but after investigating this issue thoroughly, I do believe that you really do need to know 1) the final drive ratio of your tractor and 2) the rolling circumference of your front and rear tires to accurately determine whether your tires are compatible with your tractor. You can utilize the methodology proposed by tiredguy (who I certainly respect -- he's in the tire business after all), but I believe his methodology could lead you astray in certain circumstances. For example, my B1750 came from the factory with EITHER 29x12.50-15 Firestone Turf and Field tires on the rear, or 31x13.50-15 Firestone Turf and Field tires on the rear. The 29's have a rolling circumference of 86 inches. The 31's have a rolling circumference of 93 inches. The factory front tires are 21x8.00-10 Firestone Turf and Field Tires, which have a rolling circumference of 61 inches. And, and the final drive ratio for my B1750 is 1.528.

Therefore, the "magic calculation" provides the following results:

1) for the 29 inch rear tire option:

(61/86) x 1.528 = 1.083 (this tells me this tire combination results in 8.3 percent front wheel "lead" -- this falls slightly outside the Kubota recommended preference for 3 - 7 percent front wheel lead. Apparently Kubota decided this was "close enough.")

2) for the 31 inch rear tire option:

(61/93) x 1.528 = 1.002 (this tells me this tire combination results in zero percent front wheel "lead" -- again, this falls slightly outside the Kubota recommended preference for 3-7 percent front wheel lead. And, again Kubota must have decided this was "close enough.")

The point is, Kubota sometimes selects factory tires that are at the margins of the preferred or recommended range. Perhaps they do this because they are trying to use the same tires across a variety of models. Anyway, if you use the factory original tires as a starting point for selecting your new tires, you could already be slightly outside the ideal lead/lag ratio. The "magic formula" insures that you are selecting a tire that falls squarely within the proper ratio.

 

[85Hokie]

Anders,

Yes, you have made a misstep. You are using the "outer diameter" of your tires to calculate the rolling circumference (i.e., you are relying upon the formula, pi x d). One would think from high school geometry that is all you need to do. However, a tire is not a simple beast. When it is loaded (from the tractor's weight), the sidewalls of the tire deflect, causing the actual rolling circumference of the tire to decrease somewhat. See the attached illustration -- note the small, but significant, deflection labled as "f". Illustration 2 also demonstrates the difference between the "static" and "loaded" circumference of a tire.

Anyway, the "magic formula" requires that you use the loaded "rolling circumference" of the tire. In addition, you are using the final drive ratio for MY tractor (a B7300). Your B6001 may very well have a different final drive ratio. I had to contact Kubtoa (Corporate) to obtain the final drive ratio for my B7300 and B1750. Fortunately, Firestone publishes the rolling circumference of the tires that originally came with my tractors. However, when I considered using Carlisle replacement tires, I had to contact Carlisle technical support for the "rolling circumference" of the tires I wanted to use. They sent me a detailed tire "tech sheet."

I don't claim to be a tire expert, but after investigating this issue thoroughly, I do believe that you really do need to know 1) the final drive ratio of your tractor and 2) the rolling circumference of your front and rear tires to accurately determine whether your tires are compatible with your tractor. You can utilize the methodology proposed by tiredguy (who I certainly respect -- he's in the tire business after all), but I believe his methodology could lead you astray in certain circumstances. For example, my B1750 came from the factory with EITHER 29x12.50-15 Firestone Turf and Field tires on the rear, or 31x13.50-15 Firestone Turf and Field tires on the rear. The 29's have a rolling circumference of 86 inches. The 31's have a rolling circumference of 93 inches. The factory front tires are 21x8.00-10 Firestone Turf and Field Tires, which have a rolling circumference of 61 inches. And, and the final drive ratio for my B1750 is 1.528.

Therefore, the "magic calculation" provides the following results:

1) for the 29 inch rear tire option:

(61/86) x 1.528 = 1.083 (this tells me this tire combination results in 8.3 percent front wheel "lead" -- this falls slightly outside the Kubota recommended preference for 3 - 7 percent front wheel lead. Apparently Kubota decided this was "close enough.")

2) for the 31 inch rear tire option:

(61/93) x 1.528 = 1.002 (this tells me this tire combination results in zero percent front wheel "lead" -- again, this falls slightly outside the Kubota recommended preference for 3-7 percent front wheel lead. And, again Kubota must have decided this was "close enough.")

The point is, Kubota sometimes selects factory tires that are at the margins of the preferred or recommended range. Perhaps they do this because they are trying to use the same tires across a variety of models. Anyway, if you use the factory original tires as a starting point for selecting your new tires, you could already be slightly outside the ideal lead/lag ratio. The "magic formula" insures that you are selecting a tire that falls squarely within the proper ratio.

I wonder, lets say on a backhoe model - if you lifted the rear tire off the ground and took a piece of fabric - or seatbelt and wrapped the middle of the tire and took the "actual" outside diameter, measured it , and then allowed the tire to sit on the ground "loaded" and using the same piece of fabric, would the squatting tire have a "smaller" measurement - and if so, what % difference would be.

this would be done on a very hard surface so the fabric would mimic the tire. And the PSI would need to be right too.

Wonder what the % difference is and does this change if you use ag or ind or turf? Will an Ag tire squat more than a turf? They are NOT all going to be the same - wonder what the "true" RC is when all other things are equal - and then what the changes when someone has a #1000 in a bucket or ballast box with #800 of concrete - both are going to squat the tires and the terrain will change that too, going down hill will load the front tires more than the back and vice-versa. But the true RC and loaded RC should be a little different sitting still

 

[volp]

Can anyone on this forum give the final drive ratio for different Kubota models?
Especially B6001 and B6100.
Also the "rolling circumference" for my standard tyres

Anders

 

[Cr555]

85Hokie, you have made good points. I suspect it is impossible to select any set of tires that will provide the ideal lead/lag ratio under all conceivable operating conditions. Perhaps the best we can do is select tires that are appropriate for the most typical operating conditions. At a minimum, it seems we should all have a set of tires that are approprate when the tires are mounted and supporting the full weight of the tractor. I believe this is what the "magic formula" helps us do. When you have 1,000 pounds of gravel in your front bucket, or 800 pounds in a ballast box, you are subjecting your tires and tractor to additional stress that the formula, admittedly, does not address. As for myself, I consider operating in 4WD to be inherently stressful (for the tires and affected drive train components). Accordingly, I intend to use 4WD only when it is necessary. It's like having nitro in a performance car -- its nice to have it when you need a performance boost, but it comes at a price (in terms of additional wear and tear).

 

[85Hokie]

85Hokie, you have made good points. I suspect it is impossible to select any set of tires that will provide the ideal lead/lag ratio under all conceivable operating conditions. Perhaps the best we can do is select tires that are appropriate for the most typical operating conditions. At a minimum, it seems we should all have a set of tires that are approprate when the tires are mounted and supporting the full weight of the tractor. I believe this is what the "magic formula" helps us do. When you have 1,000 pounds of gravel in your front bucket, or 800 pounds in a ballast box, you are subjecting your tires and tractor to additional stress that the formula, admittedly, does not address. As for myself, I consider operating in 4WD to be inherently stressful (for the tires and affected drive train components). Accordingly, I intend to use 4WD only when it is necessary. It's like having nitro in a performance car -- its nice to have it when you need a performance boost, but it comes at a price (in terms of additional wear and tear).

Amen brother - you said a mouthful! and well said !!!! I agree 105%

 

[tiredguy]

HOLD THE BUS!
Therefore, the "magic calculation" provides the following results:

1) for the 29 inch rear tire option:

(61/86) x 1.528 = 1.083 (this tells me this tire combination results in 8.3 percent front wheel "lead" -- this falls slightly outside the Kubota recommended preference for 3 - 7 percent front wheel lead. Apparently Kubota decided this was "close enough.")

2) for the 31 inch rear tire option:

(61/93) x 1.528 = 1.002 (this tells me this tire combination results in zero percent front wheel "lead" -- again, this falls slightly outside the Kubota recommended preference for 3-7 percent front wheel lead. And, again Kubota must have decided this was "close enough.")

The above figures DO NOT WORK on the same tractor without changing gears. On the first calculation your ratio on the tractor using the 21x800-10's with RC's of 61 & 86 equaling .71 If you use the 31" tire instead of the 29" tire dividing 61 by 93 = .66 ( rounded up ) if you take 71 minus 66 that equals 5. If you divide that 5 by the corrected targeted ration of 71
the answer is 7 which means there's a 7% difference in the ratios which is
WAY off the 2% difference that's allowable.

Mind you know these figures are to determine ONE particular tractor. If you have Two different tractors of that both have the same front tires and
a different size on the rear, you can take it to the bank that those two tractors have different gear ratios. It's the only way it will work without
breaking something gear wise.

If I missed something in what I read I apologize if I misunderstood something but I'm extra careful when it comes to figuring these out as it
causes extremely expensive damage and I'm always trying to save myself and others hard earned money. Also, if I make a mistake at work telling someone the wrong answer and selling them the wrong sized tires, then I'm the bad guy and am responsible and liable and I'd have to pay for the
damages.
Al

 

[Cr555]

Tiredguy,

Per my factory manual, my B1750 came with either 29 inch rear tires having an RC of 86 inches, or 31 inch tires having an RC of 93 inches. This is straight out of the owner's manual, and the RC's are straight from the Firestone tire specs. The "magic formula" isn't something I invented -- a Kubota factory technician advised me this is the formula Kubota uses to evaluate whether a proposed replacement tire is acceptable.

Your formula is a shortcut, and I believe it will work under most, but not all, circumstances. Using your method, you first take the RC of my B1750's original front tire (61 inches) and divided it by the RC of my B1750's original rear tire (86 inches) and come up with a ratio of .71. According to this method, any combination of tires that produces a ratio within 2 percent of the result (.69 to .73) is acceptable.

So, let's look at a replacement tire that is slightly larger than the original rear tire, say a tire having an RC of 87 inches. Using your method, you arrive at a ratio of .70 --> (61/85 = .70). Looks good. And, when we run the numbers through the "magic formula":

61/87 x 1.528 = 1.071 (indicates front lead of 7.1 percent which is okay)

So far, so good. But, let's also take a look at a rear tire that is slightly smaller than the factory tire, say one with a RC of 85 inches (rather than 86). Your method would lead me to conclude this tire is also acceptable:

61/85 = .7176 (.7176 is within 2 percent of .71).

But, when I use the "magic formula" I find the tire is actually unacceptable:

(61/85) x 1.528 = 1.096 (9.6 percent front wheel lead). Selecting a smaller tire is clearly heading the "wrong way" -- if anything, I need something larger. Your method doesn't tell me this. I believe the reason why your method leads to a bad result in the second example is because the factory installed 29 inch rear tires are at the very margin of the acceptable range (on the small side). Going any smaller and you are heading into the danger zone. At the end of the day, you need to know your tractor's final gear ratio to insure that replacement tires are going to work properly.

 

[tiredguy]

Tiredguy,

Per my factory manual, my B1750 came with either 29 inch rear tires having an RC of 86 inches, or 31 inch tires having an RC of 93 inches. This is straight out of the owner's manual, and the RC's are straight from the Firestone tire specs. The "magic formula" isn't something I invented -- a Kubota factory technician advised me this is the formula Kubota uses to evaluate whether a proposed replacement tire is acceptable.

Your formula is a shortcut, and I believe it will work under most, but not all, circumstances. Using your method, you first take the RC of my B1750's original front tire (61 inches) and divided it by the RC of my B1750's original rear tire (86 inches) and come up with a ratio of .71. According to this method, any combination of tires that produces a ratio within 2 percent of the result (.69 to .73) is acceptable.

So, let's look at a replacement tire that is slightly larger than the original rear tire, say a tire having an RC of 87 inches. Using your method, you arrive at a ratio of .70 --> (61/85 = .70). Looks good. And, when we run the numbers through the "magic formula":

61/87 x 1.528 = 1.071 (indicates front lead of 7.1 percent which is okay)

So far, so good. But, let's also take a look at a rear tire that is slightly smaller than the factory tire, say one with a RC of 85 inches (rather than 86). Your method would lead me to conclude this tire is also acceptable:

61/85 = .7176 (.7176 is within 2 percent of .71).

But, when I use the "magic formula" I find the tire is actually unacceptable:

(61/85) x 1.528 = 1.096 (9.6 percent front wheel lead). Selecting a smaller tire is clearly heading the "wrong way" -- if anything, I need something larger. Your method doesn't tell me this. I believe the reason why your method leads to a bad result in the second example is because the factory installed 29 inch rear tires are at the very margin of the acceptable range (on the small side). Going any smaller and you are heading into the danger zone. At the end of the day, you need to know your tractor's final gear ratio to insure that replacement tires are going to work properly.

We have a misunderstand of sorts I think I've figured out why and also that my formula is something I learned from the Firestone Farm Tire engineers. My formula is not a short cut it's been figured out by tractor and tractor tire manufacturers so that everyone's on the same page and being certain to use the rolling circumferences from the manufacturer as you have most correctly done.

I should have made myself very clear on the 2% allowance AND added the formula for that too as it's a MUST to be sure it's correct. The 2% allowance isn't figured by using what we started with as the .71 ratio target being ok
anywhere from 69 to 73 because it IS NOT CORRECT and WILL NOT WORK.
If you subtract that 69 from 71 then divide it by the 3 difference it's 0.028
which we round up to .03 which is 3% and not acceptable. I apologize for not doing so before.

As I said before I'm only putting it out what the simplest way there is to figure it out because it's hard enough to get people to understand what
rolling circumference is much less everything else. I use the KISS theory
of Keep It Simple Stupid because it requires no more than the correct tire
rolling circumferences and straight forward simple math. It's never failed me to come up with the correct answer and it's very easy to teach other how to do it so they can be sure they don't have any problems. Problems
are expensive to solve especially when it comes to anything on wheels that
is 4 wheel drive.
Al

 

[clay45]

For this to be such a critical set of data I am amazed that every tire spec sheet doesn't have to display this, that every tire doesn't have it molded into its sidewall, and that every tractor manufacturer of 4wd tractors doesn't cast this in large letters on the front axle of every tractor.

How is Kubota going to give ALL of us this information?

Is it now critical to replace all tires at the same time or can we replace the fronts as long as the rear tires "look good"?

 

[D2Cat]

Man, after you guys get this all hammered out, it makes me glad I live in the country and don't drive my equipment on hard surface roads.

I've got a built in fudge factor, dirt, clay, rocks and water! Oh and grass and manure.

 

[clay45]

Man, after you guys get this all hammered out, it makes me glad I live in the country and don't drive my equipment on hard surface roads.

I've got a built in fudge factor, dirt, clay, rocks and water! Oh and grass and manure.

Same here. But I find tractors and their maintenance expensive enough without inviting a problem.

 

[Long Gone]

Hello all,
I've enjoyed reading this post, and really value all the expertise here.
Everything I've read usually prescribes 10% wheel slip (overall) for 4wd tractors, and 13% to 15% for 2wd. In order to achieve this, I understand that we should target 120lbs/hp to 145lbs/hp for our mfwd tractors, biased toward the lighter end of the scale for higher speed applications, and the heavier side for low speed high-draft applications. According to a University of Kentucky at Lexington study, from their department of Agriculture, we should target a 35% front/65%rear tractor weight distribution for our mfwd tractors used in towing service, and 40%front/60%rear weight distribution for mounted implements. 2wd tractors should be ballasted for 25%front/75%rear (towing), and 35%front/65% (mounted implement) in comparison.
The report goes on to say, that we need to consider tire pressure as well, using a the least pressure possible to support applied weight of the tractor... less air pressure improves adhesion but reduces overall carrying capacity of the tire. Apparently there should be a tire load limit at cold air pressure table available from the tire manufacturer for each of our specific tires that we can consult.
I say all of the above in order to push back against the original poster's assertion that we should be concerned with a 3%-7% lead (or lag) on our tractors. This seemed a little restrictive when I read it, and I can't find anyting in writing to support it. In contrast, I can remember the targeted 10% wheel slip goal stated in my first MFWD tractor owner's manual (it was green, not orange) and have seen the 10% wheel slip goal multiple times elsewhere, and best explained in the article from UK at Lexington.

http://www2.ca.uky.edu/agc/pubs/aen/aen93/aen93.pdf

I believe some wheel slip is a good thing in order to protect the driveline. I also think Tiredguy's formula simply maintaining the factory ratio front/back should work fine. If you're not able to acheive the 10% wheel slip in mfwd, you should use 2wd. Wheel slip protects the driveline, but wear tires. I'm ok buying tires... it's a simpler repair for me anyway.

Work safe, have fun,

LongGone