Go Bottomfront rear ratio for kubota b7800
 | crashey
Join Date: Mar 2010 Posts: 11 mass 
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2010-04-07 169838
my last post regarded what tires and rims (turf) would fit a b7800. what I've learned , so far:
L series often have the same size tires but the rims are different
Not all tires have the same rolling circumference at the same posted size
this is important when using 4 wheel drive, tho not as critical if it's slippery
so i've found a used set of tires and rims from a b2710; oddly, the rims will fit and both tractors use the same R1 and R4 size but not the same size R3 (turf)
The b2710 uses 23x8.50x12 front/ rear 13.6x16
the b7800 uses 24x8.50x14 front/rear 13.6x16
If we assume that both tractors started with a positive slippage ratio(greater than 1) then how does the b2710 go smaller in turf in the fronts? If we assume a 1.60-1.60 front rear ratio for the b7800 then the 23 inch fronts would have a negative slippage ratio. Very weird!
The facts: Titan multi trac rolling circumferences:
23x8.50x12 is 66 inches
24x8.50x14 is 72 inches
13.6x16 is 113 inches
the formula is circumference of the front times the front rear ratio divided by the circumference of the rear
The question? what is the front rear ratio for the 2710 and the b7800?
And how can the 2710 use a smaller turf front when it's other sizes are the same as the b7800 (and the 2910, 3030,3200)?
I'm flumoxed here in mass. can any of you non new england eggheds give me an explanation?
thanks
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 | earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan 
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2010-04-07 169841
I wouldn't focus so much on rolling circumference as it doesn't really affect the gear ratio necessarily. What is more critical is the distance from the center of the wheel to the contact point. This changes with air pressure, tire and air temp and load; two tires of different circumferences could have the same axle-to-contact point dimension simply by altering the pressure. Conversely two identical tires with different pressures would have different ACP dimensions. ....
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 | Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2010-04-08 169852
There's really only two points that matter.
1) If you are only using turf tires in 2WD, the size doesn't matter squat.
2) As long as the rear tire is 1.6 times the rolling circumference of the front tire, everything will be fine.
Another member who isn't so active here anymore had much the same issue, but found that a pair of large off-road tires for a pickup worked better, and had better load capacity than the turfs did. You might want to look into that too.
Best of luck. ....
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 | kthompson
Join Date: Oct 2005 Posts: 5275 South Carolina
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2010-04-08 169858
Boy like the question!
EW, realize you are much older and wiser than me but the circumference has to matter or I am the one running in circles? Notice I did not post this until the Professor made reference to the circumference. ;-) It would seem as the circumference increases so does the height the axle is from the ground.
....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2010-04-08 169860
Kenny BTW I'm younger than you, boy. By about what 7-8 years?
Anywho, take a dozer track--it might be 30 feet in circumference but only 24" tall when installed. Git my drift? ....
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| magicheater
Join Date: Apr 2009 Posts: 29 Wisconsin
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2010-04-09 169866
I went through this on a B7800. Wanted wider fronts but could not find any for a 14" rim that were correct. Ended up buying 12" rims and buying a 10.5" wide R-1 that came within 2 percent of the 14" Titans. I like the solution I found.
Earthwrks: You are comparing apples to oranges. ....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2010-04-09 169869
Really. How is that?
Set me straight.
2 percent is a huge gap when talking rolling radius or circumference. ....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2010-04-09 169870
If you're referring to the track analogy, Kenny is a former crane salesman and thought he could relate to it better. Of course a track is not comparable to the rolling radius as tire but the concept is the same. ....
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| Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2010-04-09 169878
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Let's see, if the track is "30 feet in circumference but only 24" tall" that means the track is ~14' long.
That dozer must be a real PITA to turn around. ;)
Best of luck. ....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2010-04-09 169881
UknowwhadImean Murf. It was an analogy--poor one at bet--but one nonetheless. ....
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| magicheater
Join Date: Apr 2009 Posts: 29 Wisconsin
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2010-04-09 169882
With all do respect, the track analogy does not apply. Not sure how many drive wheels you have but probably don't have two with different ratios. The radius is important when sizing fronts to rears. When I did my first tractor I contacted Kubota as the dealer was no help. They gave me a formula AND a test to do after the math was done to see if it was correct. It involves measuring the distance the tires circumference travels in 4WD and 2WD and comparing results. Mine passed so was good to go. On the second tractor I just trusted my math. On your tracked unit your height would not increase but the circumference of the track might which I imagine would be adjustable. ....
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| kthompson
Join Date: Oct 2005 Posts: 5275 South Carolina
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2010-04-09 169884
Old or if I must Ole Jeffery, I don't agree your thinking. If wrong you know it will not be even the 32nd time.
Using what I think you said you have a set of tires front and rear that match ratio for 4 wheel drive. But for some design reason they build that tractor in a high clearance model, standard clearance and then a low clearance model..all with same gear ratios. The set of wheels that work for one tractor should work for either of the other two also, IF the gear ratios are all the same.
If you don't follow me on that: I have a four wheel drive pickup and decide to adjust the set up like some of you youngins do and lower drop the rear springs to make it squat in the rear. You don't change the tires or wheels as the gearing ratio has not changed just by lowering the bed.
If that were true then you would have a major problem with a four wheel drive pick as you change the load in the bed and it lowered you would have to keep swapping out tires.
As I said could be way off here and would not be the first time but think the circumfernce is what matters.
As to the age difference...it will soon be two years since I hit the double nickle. ....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2010-04-09 169887
Kenny if you had a truck that was loaded and the tires were showing sings of that load, that effectively changes the loaded radius which does change the gear ratio front to back. This would be noticeable IF the drivetrain did not have a center differential and would result in driveline binding. A center diff allows difference for side to side and front to back.
In the case of having a limited slip rear differential, if the two tires did not have the same loaded radius the axle fluid will overheat and the clutch material will wear even though there is a diff between the two.
And I'll say it again: circumference is not anywhere as critical as rolling radius when it comes to gear ratios with respect to having front and rear tires spinning at the correct RPM. ....
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| magicheater
Join Date: Apr 2009 Posts: 29 Wisconsin
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2010-04-09 169889
glad we are back to static loaded radius vs. tracks! this is where the rubber meets the road and what is required in Kubotas formulas. Can't disagree with you now Earthwrks! ....
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| crashey
Join Date: Mar 2010 Posts: 11 mass
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2010-04-09 169891
Thanks for the comments. What is still puzzling is the goodyear tire site that claims mfwd tractors can not alter the rolling circumference to create a negative slippage less than 1 -and the margin is 5%. So if that is the case (IF!) then why does my 2005 manual have the 7800, 2910, and 2710 listed together with the approved tires that include a 23x12 and a 24x12 and a 24x14, in turf. The 23x12 would exceed the 5% rule by a small amount-so does Kubota 4 wheel drive have greater slippage front to rear than other tractors making that possible? Or do they enjoy rebuilding drivelines? Planned failure... ....
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| earthwrks
Join Date: Dec 2003 Posts: 3853 Home Office in Flat Rock, Michigan
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2010-04-10 169896
Maybe botas have a center diff or viscous coupling to compensate?
I know when my blue got a low rear tire (before I got fixed) and I drove it on pavement I could hear the engine lug so I would pull off the road and let it slip then continue. ....
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 | Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York
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2010-04-12 169934
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A track unit only has one driver so you don't have a fair comparison to a ractor that you are trying to match two different drive sources.
Kubota's drives are solid or direct. I'd say they switched axles between the models. ....
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| hardwood
Join Date: Dec 2002 Posts: 3583 iowa
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2010-04-12 169935
OK, I knew this at one time but have forgotten.
R-1 tractor tires that are meant to be used mainly in soft soils.
Is the circumference of an R-1 tire measured at the base of the lugs on the top of the lugs? ....
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| hardwood
Join Date: Dec 2002 Posts: 3583 iowa
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2010-04-12 169936
Sorry, I meant to say ON the top of the lugs, not OR the top of the lugs.
My brain to finger circut blows a breaker now and then. ....
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| hardwood
Join Date: Dec 2002 Posts: 3583 iowa
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2010-04-12 169937
WOW, still got it wrong.
Better go cut the grass or something simplier than typing.
You guys will figure out what I maeant. ....
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| Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2010-04-12 169938
I think at this point we're designing a nuclear bomb to kill flies here.
Even if you drastically overloaded the tires I doubt you'd change the rolling circumference by more than 1 or 2 percent, certainly nothing to be concerned over. Bear in mind, we're talking about relatively stiff agricultural tractor tires here, not soft automotive radials.
If you look in the operators manual of nearly any MFWD tractor it will give a caution not to operate the machine in 4WD on a hard surface such as asphalt or concrete because of the lack of tire slip.
Best of luck. ....
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| kthompson
Join Date: Oct 2005 Posts: 5275 South Carolina
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2010-04-12 169941
This past Saturday as I was using my TWO wheel drive tractor and looking at my FOUR wheel drive parked under shelter a thought hit me on my comments...my brain likes simplier ideas. So I hauled and dumped another load of dirt and went back for another. Weather was great,so this is why have equipment, to play in the dirt...I mean work the dirt. ....
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| Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York
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2010-04-14 170002
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Been here before!!! They are measured on top of the lugs not the casing so on soft or tilled soils this can be a driveline killer!!! This can happen between turfs and industrial tires or the ag tires. My awakening was between R-1 and R-3's which is where I had the trouble and it did mean drive shaft u-joints within thirty minutes of operating time turning grease to mighty hot oil!!!! ....
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