Inter Axle Ratio for the B1750 and B7300

I made in inquiry to Kubota Corporate regarding the inter axle ratio for the B1750 and B7300. Their response:

"Robert, what you are asking for is the Drive Train Ratio. For the B1750, it is 1.528. For the B7300, it is 1.475. Thank you for contacting Kubota Tractor Corp."

Hope this helps someone else searching for this info.

I ran the calculations for the lead/lag ratio for the B1750, using the inter axle ratio furnished by Kubota (see post above), and rolling circumference for the factory Firestone Turf and Field tires, as furnished by Firestone. The calcs are as follows:

For the 29x12.50-15 rear tires (with a RC of 86 inches) and 20.5x8.00-10 front tires (with a RC of 61 inches) -->

61 x 1.528 = 93.208

93.208 / 86 = 1.0838.

Therefore, with 29x12.50-10 rear tires, the B1750 comes from the factory with a 8.38 percent front lead or overrun.

For the 31x13.50-15 rear tire option (wth a RC of 93) and 20.5.8.00-10 front tires (with a RC of 61 inches) -->

61 x 1.528 = 93.208

93.208 / 93 = 1.0022

Therefore, with 31.13.50-15 rear tires, the B1750 comes from the factory with essentially a zero lead/lag ratio.

If the inter axle ratio furnished by Kubota is correct, and the rolling circumferences furnished by Firestone are correct, it appears that Kubota is not overly concerned about maintaining a 1 - 5 percent lead for the front tires. Of course, I'm not a mathematician and could have crunched the numbers incorrectly.

For the B7300, the results would be:

61 x 1.475 = 89.975

89.975 / 86 = 1.0462

Therefore, the front wheel lead for the B7300 would be 4.62 percent, using factory tires.

I don't know what to make of these calculations. But, it appears to me that the Kubota engineers don't always follow the customary "2 percent lead is ideal " design philosophy.

Note that both the B7300 and B1750 came from the factory with Firestone Turf & Field 20.5x8.00-10 tires in the front. For the rear tire, the B1750 specifies either 29x12.5-15 or 31. 13.5-15. For the B7300, the only rear tire option is the 29x12.5-15. Perhaps this makes sense because when I run the calcs, the 31 inch rear tire on the B7300 would result in a substantial front lag situtation, which appears to be very bad.

Finally . . . I intend to replace all 4 turf tires on my B7300. They still have plenty of tread, but there is too much dry rot. As noted above, Kubota has advised me that the inter axle ratio (or Drive Train Ratio) for my B7300 is 1.475. I want to stick with turf tires, and the Firestone Turf and Field tires that came with the B7300 are no longer available for a reasonable price. The only turf tire I could find that is similar to those that came with my B7300 is the Carlisle Multi Trac C/S tire, size 29x12.5-15 (for the rear), which Carlisle tech specs indicates has a rolling circumference of 90.2 inches (as opposed to the 86 inches for the Firestone tire that came with the Kubota). Carlisle also makes a turf friendly tire, denominated as the USA Trail in size 20.5x8.00-10 with a rolling circumference of 62.5 (load range B = 910 pounds). Running the calcs:

62.5 (rolling circ. of front tire) x 1.475 (inter axle ratio) = 92.1875

92.1875 (see above) / 90.2 (rolling circ. of rear tire) = 1.0220

Therefore, I would expect this combination to provide a 2.2 percent front tire lead, which should be nearly ideal. The same front tire is also available in load range C, D, & E. But, the rolling circumference is slightly different for each tire. The load range B furnishes the best lead/lag ratio, with load range E a very close second.

Anyone see any problem?

I've done a fair amount of research and now I believe I understand why you cannot rely on the "outside diameter" measurement of a tire in selecting tires for a MFWD tractor. This had me stumped at first. Now I believe I understand why you REALLY do need to know the "rolliing circumference." The outside diameter measures the diameter of a UNLOADED, mounted tire. The rolling circumference measures the distance a LOADED tire travels in one revolution. One would think that you could calculate the "rolling circumference" of a tire by the simple equation: Circumference = pi x d. However, in the world of tires this doesn't work because the sidewalls of a loaded tire defect, resulting in a small but significant change in the rolling circumference. See the attached illustration (notice that the defection -- labled as "f" -- of the sidewall under load changes the circumference of the tire). This is why tires with the exact same outside diameter can have small differences in their rolling circumference -- they vary in the amount of sidewall deflection due to the way they are made. These small differences, in turn, can have a small, but significant effect on the calculations that determine whether a given tire provides the proper lead/lag ratio for your tractor. I don't claim to be a tire expert, but I'm now convinced that nothing but the rolling circumference will give you an accurate estimate of whether a proposed tire will work. You also need what Kubota calls the "drive train ratio". There is little margin for error, and the effect of making the wrong choice can be expensive. Fortunately, once you have the correct data (for the proposed tires and your tractor), the calculations are very straightforward.

Today I could not be more pleased with Kubota! I just got off the phone with a technician/customer service representative from Kubota (from Suwanne, Georgia). He called me (and this was unexpected) to follow up on my earlier request for information concerning the final drive ratio for my B1750 and B7300. He was extremely helpful and generous with his time. We went through all of my calculations. First he helped me understand precisely what the final drive ratio represents -- e.g. for my B1750, it means that for every complete rotation of the rear tire, the front tire rotates 1.528 times. He also explained that Kubota actually recommends that the front wheel "lead" fall within the range of 3 - 7 percent (as opposed to the 1-5 percent you see everywhere on the internet). And, he stated that while using the rolling circumference is acceptable, the best measurement to use in your calculations is the "static loaded radius" (it will give the most accurate result). And, he asked me whether my tractor has a FEL (I said it didn't). He said that helps because the added weight of a loaded FEL makes it much more important to have the correct tires on your tractor. Finally, he gave me some good tips regarding replacement tires. One, he suggested that I contact Titan tires (1-800-USABEAR) because Titan purchased Firestone/Bridgestone's ag tire business and may very well have replacement tires. Second, he said that the front tires (on both the B7300 and B1750) could be replaced with Kubota part number BR8735. This is a tire and wheel assembly -- the wheel is 12 inches rather than 10 inches, but the bolt pattern is the same and the rolling circumference of the tire is identical to the factory installed front tires. He said the list price (from the sales department) should be about $139 -- and Kubota presently has about 30 of them in stock. I'm definitely impressed with Kubota's customer service in this instance. Oh, he also recommended performing the "rolling test" after installing the tires to confirm that you have tires that furnish the proper lead/lag ratio -- don't simply rely on your calculations. Spent nearly 30 minutes on the phone -- excellent customer service!

Forgot to mention one thing. The technician confirmed that the final drive ratio is the same whether I have an HST or gear drive model (at least when you have a B1750 or B7300). And, he said if you can't obtain the BR8735 tire/wheel assembly from your local dealer (for the front tire replacement), you could try Lashley Tractor or Messicks. He said Kubota is well aware of the problem customers are having finding replacement tires for some older Kubota models and is doing everything it can to help. Hate to repeat myself, but I'm a happy camper -- thought I would have a difficult time obtaining customer service for a 20+ year old tractor, but was very pleasantly surprised.

I took notes during my conversation with the Kubota technician. In reviewing my notes there in one more thing I should mention. He said you can determine what tire you need for the rear (assuming a given front tire) by using the following formula:

(drive train ratio / 1.05) x front tire rolling circumference = proper rear tire rolling circumference.

For the front tire, you would use the following formula:

(rear tire roling circumference / drive train ratio) x 1.05 = proper front tire rolling circumference.

He explained that the constant (1.05) was selected because it falls in the middle between 3 percent and 7 percent front wheel lead. He said you can also run these calculations using 1.03 and 107 as the constant. This will give you the outside range of the tire you are looking for, i.e., the shortest acceptable rolling circumference and the longest acceptable rolling circumference.

Sorry about the multiple posts -- just trying to capture as much as possible from the conversation.