Tire Question - New Tires - B7200 - Can't Find Answer

[85Hokie]

Well, first I am told that if I have 9.5-16 rear tires that I have to run smaller 5-10 fronts. Now I am told that I need to have a larger 5-14 front. Where is this information that everyone keeps giving out coming from? I know that just because something is a "Factory Spec" that does not mean that it is correct. Manufacturers do sometimes send out wrong specs. With that being said, can you tell me why you believe the factory specs are wrong? Kubota only specs a 6-12 ag tire for the front of a 4wd B7200D with 8-16 or 9.5-16 rear tires. Kubota does not recommend a 5-10 or a 5-14 front tire. I have done some math to determine the gear ratios for the front and rear of a B7200D. I use Kubota's online parts lists for all gear specs except for the 12 tooth rear jackshaft gear that I needed to find in my workshop manual. If anyone finds my math or specs to be incorrect please let me know and i will correct it. This is how I calculated the ratios. The rear trans shaft transmits power to the rear jackshaft through a 6 tooth pinion and a 37 tooth ring gear(6.16667 ratio). The rear jackshaft transmits power to the rear axle through a 12 tooth and 55 tooth set of spur gears(4.58333). The overall rear axle ratio is (6.16667 x 4.58333) 28.26388. Now comes the difficult one, the front axle. The rear trans output transmits power to the front trans output through a 16 tooth and 24 tooth set of gears(1.5 ratio). The drive shaft transmits power to the front axle through a 6 tooth pinion and 37 tooth ring gear(6.16667 ratio). The front axle transmits power to the bevel shaft through a 13 tooth and 14 tooth set of bevel gears(1.0769 ratio). The bevel shaft transmits power to the wheel hub through a 16 tooth and 31 tooth set of bevel gears(1.9375 ratio). The total front axle gear ratio is (1.5 x 6.16667 x 1.0769 x 1.9375) 19.3005. Front to rear axle ratio is (28.26388/19.3005) 1.4644. Tire specs very by manufacturer so I am using those supplied by Carlisle because that is what is on my tractor. A 9.5-16 tire is 33.5" tall, a 8-16 is 31.4 tall and the 6-12 is 23" tall. If you divide 33.5 by 1.4644 you get a tire size of 22.876 for the front(error of .54%. If you divide 31.4 by 1.4644 you get a front tire size of 21.442" (error of 7.266%). Given the math, the 9.5-16 tires are what should be on the tractor. The standard 8-16 rear tires give you almost 15 times the ratio error as the "optional" 9.5-16. This information is for the B7200-D. I do not know if it would be different if you have a B7200-hst. It may be the same but I did not verify it.

What tires do you have on IT now? and what are the rolling circumferences at this point of each tire? Do the test while someone else watches. Chalk marks at 6 o'clock on both tires and in 4wd - roll one rotation of rear nice and straight on something not firm, see where fronts end up, then repeat with only 2wd, it will show you the pushing or pulling of the fronts vs the rears.

the internal ratio is fine to know but it really boils down to what the ratio is of the tires on the ground. With this info you can go by any tires you wish, as long as it falls into the magic % of the ratio, aint nothing gonna be perfect, but at same time, 4wd should never be used when you are on a non slip surface. Even if the ratio was perfect, making any kinds of turns - changes the game and ratios went out the window!

 

[CJN8]

How come Carlisle does not provide rolling circumference? I am getting ready to get new 6-12s for my 7200HST and noted Carlisle does not provide this information.

Nate

 

[85Hokie]

How come Carlisle does not provide rolling circumference? I am getting ready to get new 6-12s for my 7200HST and noted Carlisle does not provide this information.

Nate

Sad that they dont! , and even once new, the rolling circumference changes as it wears down!

 

[ka24deroadster]

Every thing you said might be all well and good but to me it doesn't matter!

There is one set of magical #'s to worry about RC (Rolling Circumference),
As long as you keep those #s right, everything will be fine!

6-12 RC 69
8-16 RC 93
=.7419

5-14 RC 74
9.5-16 RC 99
=.7474

They are a close enough match

I agree with you that you need to keep the ratio of the front tire to the rear tires correct. If you want to use the rolling circumference instead of the tire O.D. we can do that. The rolling circumference is probably a more accurate way to compare ratios compared to tire od. Going with the numbers you supplied, if you have 6-12's and 8-16's you have a ratio of 93/69 or 1.3478 . If you have 5-14's and 9.5-16's you have a ratio of 99/74 or 1.3378. The numbers are pretty close and you did a good job of finding a front tire for the 9.5-16 to keep the ratios the same as the 6-12 8-16 combo. The problem is that the gear ratio between the front and rear axle is 1.4644 and not 1.34. Using your numbers, if you have a 6-12 and a 9.5-16 your ratio is 99/69 or 1.4347. This number is much closer to matching the gear ratio of the axles. I think that too many people have seen the specs listed on websites like "Tractor Data" and re-posted earlier in this thread and think that the 8-16 is the standard tire that is perfectly matched and the 9.5-16 is just an optional size. If the 8-16 is the standard ,according to that website, you could calculate the correct ration between the front and rear tires by using the 6-12. The problem with this is "Who" determined that the 8-16 is the standard and the 9.5-16 is the optional. Kubota does not state this in my manual. They list both 6-12 8-16 and 6-12 9.5-16 as the tire combos but do not recommend one as being preferred over the other.Tractor Data says the 9.5-16 is optional if you have 5-10 fronts. I have seen it repeated over and over that you need the 5-10's in the front. Is there really anyone still arguing that if you increase the rear tire diameter that you should decrease the front tire diameter? The specs on Tractor Data is incorrect according to the specs by Kubota. Either tire combo that Kubota specs should work. They both should be within whatever allowable tolerance the manufacturer determined was ok. If you want to change you tires to sizes that are different than the two combos recommended by Kubota, you should be able to as long the tire ratio matches your Axle gear ratio. If the tire ratio and axle ratio match, neither axle will push the other. With that being said, of the two tire combos approved by Kubota, the 9.5-16 6-12 is the closest tire ratio to the axle ratio and in my opinion will work the best. As to putting chalk marks on the tires to compare their travel between 2wd and 4wd , that is a good quick sanity check to check if one axle is pushing the other. One problem with the method is it can not be done until after you buy the tires and mount them on your tractor. I have never found a tire store that will allow you to return a set of tires after they are mounted , installed on the tractor and used. The method will work good though for checking how your current tires are working together. As the tires wear the ratio between them will shift. Since i can not afford to buy multiple tire sizes just to do the physical rolling test, I will stick with Kubota's recommendations and math. Math says Kubota's 9.5-16 6-12 combo is the closest to the tractor gear ratio.

 

[tiredguy]

How come Carlisle does not provide rolling circumference? I am getting ready to get new 6-12s for my 7200HST and noted Carlisle does not provide this information.

Nate

Carlisle did include that critically important rolling circumference in the very first data book they put out then neglected to do so after that. I've been working on the company very hard so that hopefully soon it will be out again including ALL of their R4 and turf tread tires too. That's what I was promised would happen and I've had many discussions with those directly involved in the sales of those tires. I've also given them information and suggestions that all future tire plans as far as sizes and tread designs go be well thought out and targeting having the exact same rolling circumference for each tread design so people like us would have a simple way of converting ( even converting back and forth ) from one design to the other eliminating the huge problems of switching designs.

That said I'll throw this out there as Wolfman has correctly pointed out:
The only thing that matters is that the correct rolling circumference of tires equals the correct ratio needed period. That's PERIOD end of all the researching to death figuring out complicated formulas that when all is said and done don't mean jack $hit. I'm not all that computer savvy or I'd figure out how to post one of those special "sticky's" or whatever it's called on this tire part of the forum so that it was easy to access and easy to understand.
It's simple to figure out the ratio by measuring the rolling circumference on the current tractor you have with an extra person, a piece of chalk or marker, and a tape measure. Mark the tires at 6 O'clock at the bottom along with point A ground/garage floor/driveway drive the tractor so that the tire makes one complete revolution ending back at the 6 O'clock position ( the extra person will stop you or you them ) point B and measuring the length of travel between the 2 points and that my friends is the rolling circumference of the tire. Plain and simple no guessing no data that can't be found done deal.
Al