Rear Tire size change on 4WD

85Hokie

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I think you meant the width of the wheel.

Now that I have a viable option, I just have to find some at a good price.
What I really meant was a tire is a tire period- the rim size, other than fitting the correct tire and fitting the bolt hole pattern makes little difference when talking a bout RC -

if you look at some of these........well, without insulting a group of people.....the cars with 40 series tires on a 22" rims, there RC is almost the same as the diameter of the rim!!! Or the truck with 33" on a 15" rim.....

if you find a rim the fits the tractor, a tire that fits THAT rim with the correct RC....then you are golden!:)
 

North Idaho Wolfman

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So, I have been looking around and have hit on Titan Tires website. It appears that the 14.9-24 R4 in 6,8 and 12 ply all have a Rolling Circumference of 143.

Taking the into account the following:

Front tires: 10-16.5 R4 - RC = 91
Rears: 420/70/24 R4 - RC = 137
Ratio = .6642

Substitute in the 14.9-24 R4 and you get this:
Front tires: 10-16.5 R4 - RC = 91
Rears: 14.9 - 24 R4 - RC = 143
Ratio = .67132

This is only a 1/2% difference.
I'm really not trying to pick on you, but sorry to tell you that your math a little off: :eek:
Just off the top of my head I could tell that 6" could not equal a 1/2 of a percent difference.

Front tires: 10-16.5 R4 - RC = 91
Rears: 14.9 - 24 R4 - RC = 143
Ratio = .63636
Differance of 2.8%

While it's not a real bad difference, it's pushing 3% and you would be way better with 0 to -2%, again anytime that number drops below the original ratio your pushing the front end around, you're better to go the other way.
 
Last edited:

clay45

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I don't see where anyone is using the tractor's drive train ratio in their calcs. Don't just rely upon printed factory tire sizes to establish the correct ratio and then use different profile wheels to boot. Presuming you can establish the actual factory rim dimensions and have the DTR then try using static load ratio (SLR) in lieu of Diameter or Rolling Circumference. I found all this stuff immensely aggravating and at least Titan provided a lot of specs to make sizing up easier.

Talk with a Kubota tech directly and seek their assistance. I found them wonderfully helpful.

Regards,

Clay
 

knightgang

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Georiga
I sent Kubota an email expecting a phone call to discuss tire sizing options and drivetrain ratios. Instead I get back an email that lists tire size options and nowhere on the list is the first R4 tire mentioned.

I called Kubota last night and initially the first person that I talked to wanted to refer me to dealers until I told her that I have the same information dealers have and they do not have the information that I am looking for. She gave me another number to call with extension and would you believe that the person that answered that call was the same guy that sent me the email. When I mentioned that he never addressed an R4 tire, he said that he had sent the issue further into service engineering and I may get a call next week.

Not real sure how this will end up turning out.
 

clay45

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Knightgang,

It went like that for me but an engineer did call me with specs on my tractor.

They were very helpful once contact was established.

Good luck,

Clay
 

tiredguy

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I don't see where anyone is using the tractor's drive train ratio in their calcs. Don't just rely upon printed factory tire sizes to establish the correct ratio and then use different profile wheels to boot. Presuming you can establish the actual factory rim dimensions and have the DTR then try using static load ratio (SLR) in lieu of Diameter or Rolling Circumference. I found all this stuff immensely aggravating and at least Titan provided a lot of specs to make sizing up easier.

Talk with a Kubota tech directly and seek their assistance. I found them wonderfully helpful.

Regards,

Clay
And there in lies the problem knightman and Wolfman pointed it out to you
correctly.
You CANNOT use any formula based on the radius diameter or using pi etc to get the correct and factual number that you need which is:
ROLLING CIRCUMFERENCE period. Each manufactures specifications are different though some or should I say most familiar OE name brand tires such as Firestone Goodyear and Titan are nearly identical and the reason that's the case as the tractor manufactures may have to switch brands for any number of reasons ( fire at factory strike raw material problem ) which may even include the price based on each suppliers contract bid etc. Rim size will change rolling circumference so that does add a grain of salt into the wound
process of buying replacement tires.

ROLLING CIRCUMFERENCE of a tire is the actual measured distance the tire travels in inches on a tractor when making one complete revolution. To further understand what this means think of this: mark the tire at the bottom 6 O'clock position and the driveway/road/garage floor then drive the tractor ahead with someone spotting you until the tire makes that revolution and comes back to the 6 O'clock position. Mark the surface as you did when you started then measure the distance in inches from start to finish and that answer is the Rolling Circumference of that tire. It will change with wear of course and the front and rears both do that at a slightly different rate. So the 2% allowance allowed really is to cover that as the fronts on a 4x4 tractor wear out faster than the rears.
I did not go back and figure out the answer you came up with that showed you at the 2% on those different sizes because I didn't go searching for nor did I know what brand of tires you were using for comparison. I also thought
that those numbers weren't from any manufactures specs but do know from
Wolfmans answer ( he's very good at this! ) that he 99% of the time did go out and research to come up with his answers.

I unlike most on here do sell tires for a living and have NEVER sold a single tire to anyone on the board yet because that's not why I'm here. I'm here to learn from others experiences that we collectively share and the results that we all hope for is to avoid making mistakes to save us all a bunch of money so we can continue to enjoy our toys WHOOPS! I meant to write tools! for many many years and use the money saved to add to the implements we can attach to them to make our lives easier or to do a much better job at what we do. I have learned a bunch saved a bunch and haven't had to learn it all by making the mistakes I avoided by others advice much I might add they did learn the hard way and posted it immediately so others wouldn't suffer the same bad education. If you buy the wrong tires based on false/faulty information one thing I can guarantee you is that it will be a very expensive mistake if it doesn't work out as planned AND after spending money to fix a broken tractor you'll be spending the money too to buy the correct tires that you should have bought in the first place.

$1000.00 difference in a set of 4 new tires is cheap in comparison to buying all new front differential parts..gears bearings differential carrier axles etc
plus the labor or at least your own time to change it all if you're capable to do it which lots of people are not. Oh and don't forget that extra $1000.00 will then be spent on top of it all.
Al
 

knightgang

Member

Equipment
Kubota L2950
Aug 20, 2015
192
2
16
Georiga
And there in lies the problem knightman and Wolfman pointed it out to you
correctly.
You CANNOT use any formula based on the radius diameter or using pi etc to get the correct and factual number that you need which is:
ROLLING CIRCUMFERENCE period. Each manufactures specifications are different though some or should I say most familiar OE name brand tires such as Firestone Goodyear and Titan are nearly identical and the reason that's the case as the tractor manufactures may have to switch brands for any number of reasons ( fire at factory strike raw material problem ) which may even include the price based on each suppliers contract bid etc. Rim size will change rolling circumference so that does add a grain of salt into the wound
process of buying replacement tires.

ROLLING CIRCUMFERENCE of a tire is the actual measured distance the tire travels in inches on a tractor when making one complete revolution. To further understand what this means think of this: mark the tire at the bottom 6 O'clock position and the driveway/road/garage floor then drive the tractor ahead with someone spotting you until the tire makes that revolution and comes back to the 6 O'clock position. Mark the surface as you did when you started then measure the distance in inches from start to finish and that answer is the Rolling Circumference of that tire. It will change with wear of course and the front and rears both do that at a slightly different rate. So the 2% allowance allowed really is to cover that as the fronts on a 4x4 tractor wear out faster than the rears.
I did not go back and figure out the answer you came up with that showed you at the 2% on those different sizes because I didn't go searching for nor did I know what brand of tires you were using for comparison. I also thought
that those numbers weren't from any manufactures specs but do know from
Wolfmans answer ( he's very good at this! ) that he 99% of the time did go out and research to come up with his answers.

I unlike most on here do sell tires for a living and have NEVER sold a single tire to anyone on the board yet because that's not why I'm here. I'm here to learn from others experiences that we collectively share and the results that we all hope for is to avoid making mistakes to save us all a bunch of money so we can continue to enjoy our toys WHOOPS! I meant to write tools! for many many years and use the money saved to add to the implements we can attach to them to make our lives easier or to do a much better job at what we do. I have learned a bunch saved a bunch and haven't had to learn it all by making the mistakes I avoided by others advice much I might add they did learn the hard way and posted it immediately so others wouldn't suffer the same bad education. If you buy the wrong tires based on false/faulty information one thing I can guarantee you is that it will be a very expensive mistake if it doesn't work out as planned AND after spending money to fix a broken tractor you'll be spending the money too to buy the correct tires that you should have bought in the first place.

$1000.00 difference in a set of 4 new tires is cheap in comparison to buying all new front differential parts..gears bearings differential carrier axles etc
plus the labor or at least your own time to change it all if you're capable to do it which lots of people are not. Oh and don't forget that extra $1000.00 will then be spent on top of it all.
Al
You are exactly right, which is why we ( and I) are here to bounce these ideas and thoughts before we make costly mistakes.

I tried to contact Kubota so that they could recommend a size/manufacturer combination in another size that might work, or at the very least give me a more commonly available rear tire size with a matching front size that I could plan a total sizing swap on my tractor and everything still be good.

Here is the response that I received:

"Eric, Kubota Tractor will not recommend another size tire other than what would come on it from the factory. These tires are matched to the front tires and will not harm the tractor's drive train. If you do go to another size? You have 2 options. 1st. Keep it in 2wd. all the time and never use 4wd. or 2nd., look at replacing the front wheels (this includes rims) to match what the rear tires are. Your local dealer should be able to assist you with this. Thank you for contacting Kubota Tractor Corp.."

They tell me that if the rear tire size changes, then the front size needs to change as well, but they do not give me any suggested tire size combinations to use. What great help that was...
 

Optaylor

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Dec 1, 2015
8
0
0
Va
I guess it depends on who you get to talk to at Kubota. After sending them the tire sizes on a tractor that I have recently purchased they proceeded to send me a list of sizes that are recommended for the front and the rear. They then sent me the actual gear ratio of the tractor and told me what size RC I needed to try to find for the front to keep everything in the 3-7% overrun/lead or whatever you prefer to call it. Then the tech actually called me and explained everything he had corresponded on to make sure I was clear on what RC i needed to shoot for and as he told me it doesn't matter what kind of tire it is, R1, R4, or even a truck tire, as long when the RC of the tire is plugged in and the numbers are run I should be good if it falls into the suggested range. I give an A+ to the service tech I dealt with.
 

knightgang

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Equipment
Kubota L2950
Aug 20, 2015
192
2
16
Georiga
I was under tractor today trying to start taking it down due to the issue with the oil plug falling out while it was operating. Anyway, in reference to this thread, I noticed that the casting on the front axle has L3650 on it. Did all in this series have the same front axle, or would this mean that an axle swap has been done. One other thing, does this change the RC ratio if the front axle was off of a 3650 vs a 2950?
 

North Idaho Wolfman

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I was under tractor today trying to start taking it down due to the issue with the oil plug falling out while it was operating. Anyway, in reference to this thread, I noticed that the casting on the front axle has L3650 on it. Did all in this series have the same front axle, or would this mean that an axle swap has been done. One other thing, does this change the RC ratio if the front axle was off of a 3650 vs a 2950?
Nope, they were all the same gear ratio, 7 Tooth pinion gear 34 Tooth ring gear.

My 3450 also has a front Differential case cast with L3650.

The L3650's were in production a year before the other L's of that series.

There was actually 2 Differential cases made for the L3650, They changed the case to allow for hydro-static steering.
 

knightgang

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Kubota L2950
Aug 20, 2015
192
2
16
Georiga
Well, lets drag this one back up... I heard back form Kubota today... Here is the information (which does not all quite make sense...)

Kubota said the drivetrain ratio for an L2950 is 1.55842 and that the overrun front to rear should be between 2-7%

The stock specs of the tractor show the following static load radius and rolling circumferences:
Rear: SL Radius-23.0 RC-150
Front: SL Radius-15.15 RC-90

Then he gave me the following formula to calculate the SL Radius or RC, whichever one you use.

Largest Rear Tire - DT Ratio / 1.03 x Front SLR or RC
Smallest Rear Tire - DT Ratio / 1.07 x Front SLR or RC

Funny thing is that using this formula and the stock spec RC he provided, I don't arrive at the stock specs of the 150 RC that he provided for the tractor. I get a rear RC of 131 to 136. If I use the SLR of 15.15 that he provided, then I get a SLR of 22.06 to 22.92 which fits into the specs for the 420/70R24 and the 16.9R24 tire.

Boy, this leaves one stumped. When I look at the Titan website, a 420/70R24 spec shows that the 16.9R24 is the metric equivalent and vise versa. They also show the same RC and SLR specs. Funny how the math works one way but not the other.

Going by the DT Ratio and the front RC that he gave, then I should have a slightly smaller tire on the rear, but using the SLR instead, then they are correct.

Alot of help Kubota was, more questions than answers. I wonder if a 16.9R24 truly is the proper size replacement tire for this machine being a metric equivalent.

When I drive the tractor in 4WD and turn the wheels, it feels like it is trying to push and not pull, otherwise it feels fine so I just don't know.

Good news is that I finally got that rear tire tube to hold air.... so not really searching for tire right now.

Bad news is I am still digging into what got damaged in the engine.