Determining Correct Tire Size

[Kelvin]

To all,
Rather than go back to an older thread, I'll start a new one to finish up this topic.

When correct 4wd tractor front-rear tire size pairings are unknown or unavailable, I have come up with a simple test to determine if questionable substitute tire sizes are acceptable AFTER they are installed on the tractor. While I have an engineering degree, my statement is far from scientific or educated so please feel free to debate this issue.

Kelvin's Questionable Test: To determine if your tractor's 4wd gearing is compatible with the installed wheels & tires, do the following test with the tractor engine shut off and transmission in neutral (this is easiest on smaller tractors):

(1) Put the tractor in 2wd mode and see how easy the tractor is to push straight forward or backward.
(2) Put the tractor in 4wd mode and then see how easy the tractor is to push straight forward or backward.
(3) If there is resistance in 4wd mode that was not encountered in 2wd mode then the tires size may be a problem.

In my specific case, the Kubota B6100/B7100 tractors call for 20.5x8.00-10 front turf tires which are no longer available (there are golf car and trailer tires available in that size, but they (1) don't have the correct tread and (2) supposedly are not the same size as a tractor turf tire bearing the same numbers).

With my AG tires, my improvised test was successful. With turf tires mounted (correct rear tires with 20.0x8.00-10) the test failed (I had a lot of difficulty pushing the tractor in 4wd).

While comparing the B7100 to the B7100's replacement (B7300) I noticed that all of the recommended Ag and Turf tire sizes were the same except for the front turf tires; instead of 20.5x8.00-10 the B7300 recommended a 21.0x8.00x10 which is a tire that is available. I bought a pair, mounted them and performed the above test. The tractor rolled equally easy in 2wd and 4wd mode.

So is the above mentioned simple test valid? If so, whomever reads this may want to pass the tire size information to anyone with "turf tire-equipped 4wd B6100s or B7100s" that the 21.0x8.00-10 is an acceptable substitute front turf tire.

 

[MChalkley]

Your test sounds like it should work well enough on that size equipment to tell you if you've got a go/no-go situation.

Suggestion: call Kubota and ask them what the mechanical ratio between the front and rear axles are. With that number, you can calculate the rolling circumference (RC) of the front tires by multiplying it by the actual RC of the front tire. The result should be 0-3% larger than the RC of the rear tires. Then, they only thing you'll need to worry about is whether they'll physically fit or not.

Mark

 

[rbargeron]

This may be old news to many readers but the reson manufacturers of 4wd pickups say not to engage it on dry pavement is that there is some difference in front-rear tractive speed by DESIGN. Same difference is true in 4wd tractors. Mark's comment about 0-3% is certainly true in theory - maybe in practice there is more like 5%.

If the tire sizes are correct, you may not be able to push it easily in 4wd. If you can, there may not be enough difference to get the full 4wd effect in loose going.

Some simple evidence I've noticed: 4wd tractors on dry pavement are in "synch" when turning sharply. Reason is the front tires have a slightly longer travel length (along the arc) than the rear tires do. You can verify this by putting the 4wd lever in and out while moving - it's easy to disengage when turning (no gear load) but it's hard when going straight.

 

[WVBill]

Kelvin:
I need to replace the Ag tires on my B6100D. I would like to put R4's or Bar-turf tires on is as I need the traction but want a "wider" tire.

I read your post and understand that 21.0X8.00-10 are correct for a front turf tire. What size/type tire do you have on the rear? Also, what manufacturer.

I'm guessing that if I look up the specs for these tires and find out what their rolling circumference is that I can calculate the front-rear ratio and then go find a set of R4s with the same ratio.

Thanks.

Bill

 

[Kelvin]

Bill,

First I'll re-visit the numbers for the turfs and then go into the Bar-turfs and R-4s discussion.

Front Turf Specs: From the owner's manual the front turf tire specs for both the B6100D and the B7100D are 20.5x8.00-10. As mentioned before this tire is no longer produced in a farm turf and I have found the 21.0x8.00-10 to be an acceptable substitute on the B7100.

Rear Turf Specs: As for the rear turfs, the B6100D calls for a 27x8.5-15 whereas my B7100D calls for a 29x12.00-15.

Turf Availability & Pricing: Firestone makes all three (21x8.00-10, 27x8.5-15 and the 29x12.00-15) in a "Turf & Field" model turf tractor tire. Go to the Firestone site http://www.firestoneag.com/tiredata/tiredatabook.asp and enter 15 in the search box at the bottom (for 15" tires). A picture and tires specs will come up (no prices). I believe that the retail for the smaller front tire was around $88 and the rear tire was around $150...each. I bought my front tires discounted from a dealer whom I purchase a lot of stuff from.

Bar Turf: On the tractors that I've seen with Bar-Turf tires, they usually have Bar-Turfs on the rear and standard turfs on the front. Therefore, I would guess that the tire sizes for your B6100D would be the same as standard turf tires.

R-4 Tires: I don't know what the tire size would be required. The Firestone site listed above gave the dimensions of the turf tires (width, height, etc) from which you should be able to come up with at least one set of satisfactory R-4 tire specifications (whether they produce that set is another story). Comparing the ratio of actual heights of (1) your current AG tires and (2) the turf tire specs from the Firestone site should allow you to come up with a pretty close approximate ratio required for the R4 tires. I forgot to look for R4 tires while on the Firestone site but will look again later.

For my tractor, I noted that the front turfs were approximately 1" shorter than the advertised height number (the 20.5 was a little over 19" and the 21.0 was around 20" tall) before the tires were mounted on the rim. I haven't checked them after mounting.

Hope this at least points you in the right direction. I'll look around and if I come up with something else I'll pass it along.
Kelvin

 

[hayden]

The turf Bar tire do use regular turf tires on the front, but they are a different size since the turf-bar rear tire is a smaller diameter that a turn rear tire.

 

[Auto4]

Hey guys I have a Kubota L2350 four wheel drive tractor with front end loader that has 7.2-16 Ag tires on the front. Do you think I would have any problems if I switched them out with 8.3-16 Ag tires? I think it should be no problem but thought I should ask you guys to be sure. You seem to know a lot more about this topic than I do. thanks, Ron

 

[PILOON]

Hey guys I have a Kubota L2350 four wheel drive tractor with front end loader that has 7.2-16 Ag tires on the front. Do you think I would have any problems if I switched them out with 8.3-16 Ag tires? I think it should be no problem but thought I should ask you guys to be sure. You seem to know a lot more about this topic than I do. thanks, Ron

If the circumference (rolling distance) is the same you are good to go.
Go online as most tire manufacturers list the specs.

 

[Auto4]

If the circumference (rolling distance) is the same you are good to go.
Go online as most tire manufacturers list the specs.

Hmm now you've gotten me thinking. The circumference of the 7.2-16 tires is 92.63 (diameter of 29.5) and the circumference of the 8.3-16 tires is 98.91 (diameter of 31.5) so the rolling distance is 6.28 inches more but does it really matter? I mean the front tires on my tractor are worn as smooth as a new baby's behind so the diameter must be at least 2 to 4 inches shorter than when it was new. Assuming the tires lost just one inch of tread all the way around since they were new (I am sure it is more actually), the tires would have a diameter of 27.5 vs 29.5 when new which would mean that they have a circumference of 86.36 now vs 92.63 when new. The difference in the rolling distance then would be 6.28 inches, the same amount with the tire change I am considering. Am I missing something?

 

[rScotty]

Hmm now you've gotten me thinking. The circumference of the 7.2-16 tires is ......Am I missing something?

Yes. In your calculations you are missing the difference between calculated circumference - which you are doing, and "rolling circumference or RC" - which I believe is the spec that Piloon is guiding you toward.

Your question has an easy answer, and I believe that you will find that answer in the formal definition of rolling circumference. RC is the measurement spec. that manufacturers use in their 4WD designs.
good luck,
rScotty