4WD Front to Rear Tire Ratios

[ovrszd]

This is nitpicky but might apply to the current discussion.

True 4wd tractors are center pivot tractors that are constantly in 4wd, no way to disengage, and none needed. They do not bind when turning or going straight. They are match geared. Only selectable mechanism is diff locks if equipped.

MFWD or FWA tractors on the other hand are selectable. They also bind in certain conditions. All of them do. Some guys will say their tractor does not bind when driving straight. Okay, but it will bind when turning. Some guys will say their tractor does not bind when turning. Okay, but it will bind when going straight. No way around it in Utility size and down tractors. Some high end, large AG tractors are electronically controlled and can disengage when turning to avoid binding.

So to apply this. Driving your tractor with FWA engaged all the time puts unnecessary stress on the driveline. That stress will show in tire wear for sure. That wear will mostly show in the front tires. They are making more revolutions than the larger rear tires and they have the added stress of guiding the machine. That stress can show in other ways but less likely.

To those that operate their tractor with FWA engaged all the time, I would ask, do you do the same with your 4wd truck??

 

[dragoneggs]

To those that operate their tractor with FWA engaged all the time, I would ask, do you do the same with your 4wd truck??

No, but I don't drive my 4wd truck on unpaved steep slopes! For the 1% of the time when I do go on pavement I sometimes shift out of 4wd just to work the system.

 

[samofsweden]

Hello Sam, Here's how to proceed with the numbers the dealer gave you:

You have that internal gear ratio for the 7040 which was given to you by your dealer: I believe it was 1.438 : 1.000. This is the basic F/R ratio that is set by the number of internal gear teeth. Unless you are to change to a different front or rear differential this will never vary.

From the Kubota web site on "build your own 7040" I looked up the tire options for the M7040 and came up with the chart here:

Agricultural Tires (R1)
FRONT - 9.5-24 R1 Titan Tru Grip Lug
REAR - 16.9-30 R1 DURA TORQ 17 mm holes
FRONT - 9.5-24 R1 Goodyear Dura Torque
REAR - 16.9-30 R1 Goodyear Dura Torque Cast
FRONT - 9.50-20 R1 Bridgestone Farm Service Lug M
REAR - 16.9-24 R1 Titan Hi Traction Lug TL 2.8"offset
Turf Tires (R3)
FRONT - 9.50-24 R3 Goodyear All Weather TL
REAR - 18.4-26 R3 Firestone All Non Skid (ANS) 17mm holes
FRONT - 29x12.50-15 R3 Titan Multi Trac 8stud
REAR - 21.5L-16.1 R3 Firestone Turf & Field
FRONT - LSW305-521R3 Titan Grizz LSW Soft Turf
REAR - LSW570-648 R3 Titan Grizz LSW Soft Turf
Industrial Tires (R4)
FRONT - 14-17.5 R4 Titan HD-2000
REAR - 16.9-24 R4 Titan Industrial Tractor Lug

So now we go get the the information needed to complete the calculations. As an example, I'll use the default tires (first option) and begin by going to the Titan Tire site and looking up the rolling circumferences for those tires .
(Titanｮ Tire catalog)

On the Titan site we see that they spec 124 inches as the rolling circumference for the 9.5 x 24 R1 fronts.
and we find that the 16.9 x 30 DURA TORQs on the rear have a rolling circumference of 175 inches - also from the Titan site.

Next we calculate how far the front tires roll for every single revolution of the rears and it is: 124 x 1.438 equals 178.315 inches.
The rear tire calculation is easy; for one single revolution the rears travel : 175 x 1.000 which equals 175 inches.

So already as we glance at the numbers we can see that the fronts are covering about three more inches of ground each time the rears rotate once. This is good! We are feeling confident that we have described the problem correctly.

Your actual ratio is : 178.315 divided by 175 or 1.0189:1 . If we just round it off for convenience we have 1.02:1 ..... or to say the same thing as a percentage we multiply that ratio by 100 and say the ratio is 102:100 or alternately we can just say the fronts are traveling about 2% faster than the rear tires.

In my design experience that is about exactly what you want. It would be interesting to go through the other tire options they offer and see how those options compare with their default choice. In the past I've found that tires with less traction can get by with a bit more difference in ratio; probably because they can slip easier and so take the stress off of the drive train.

My hat is off to Kubota for designing a good default F/R tire ratio. I didn't really expect anything less of them - but it is nice to see the numbers confirmed. If it were my tractor I would use that 2% overdrive as a target when replacing tires.
Enjoy!
rScotty

rScotty
Thanks alot for helping me out, very very kind of you:thumbsup:
Merry Christmas to you and all you other great girls and guys on this forum

/Sam

 

[Cr555]

"rScotty
My local kubota dealer got back with numbers, he said that the front travel is 1,438:1 compared to the rear wheels. Now I have to admit that im not 100% sure if my dealer got it right...these figures say 43% "overdrive" on the front wheels or have I got it totally wrong?"

I believe your dealer did get it right. Kubota refers to this number (1.438) as the final drive ratio. As explained to me by a Kubota representative, this simply means that for every complete rotation of the rear tire, the front tire rotates 1.438 times. This is precisely the information you need to calculate whether a proposed tire will work on your MFWD tractor. I was further advised (by Kubota) that the proper calculation you want to run is as follows:

(rolling circumference of the front tire / rolling circumference of the rear tire) x final drive ratio = lead/lag ratio

In the case of my B7300, the rolling circumference of the front tire is 61 inches. The rolling circumference of the rear tire is 86 inches. The final drive ratio (as furnished by Kubtoa) is 1.475. Therefore, the calculation works as follows:

(61 / 86) x 1.475 = 1.0462. Any number greater than 1 means you have the front wheel leading the rear. Any number less than 1 means the rear is pushing the front (bad). In my case, I have 4.62 percent front wheel lead. What is also interesting is that the Kubota technician I spoke with (from the Suwanne, GA factory) advised me that Kubota recommends 3 - 7 percent front wheel lead. This differs from the 1 - 5 percent front wheel lead you find typically recommended for 4WD vehicles. You might want to confirm this with your own dealer, but I was speaking with a technician who said he was responsible for handling tire issues such as this.

 

[sixdogs]

Assuming this is correct, and it appears to be, we should chisel it in stone somewhere so we don't have to remember the math.

For my 5520 Deere, the "ratio" number I got was something like .74 and then I mathed things out using the rolling circumferences from there. As a double check it wouldn't hurt to math out the rolling circumference on several of the known and already Kubota tire choices so you can check the math and confirm front to rear tire lead to see what those percentages are.

Glad this is finally resolved.

 

[LD1]

The math and formula are easy.

The hard part is uncovering the actual final drive ratio of the tractor. It is not a very well published spec.

For my L3400, the ratio is 1.56. Gives the 7x16 and 11.2x24 ags a 5% lead.

Using a range of 4% to 7%, the front tires have to be between 86 and 88.5" Rolling circumference.

I have actually been looking into all this crap lately. Wish I could find a different front tire. I like my ags on the back, but would sure like a stiffer and wider front tire. Less prone to thorns and flats. And wider for less rutting doing loader work. I havent found anything yet that I like unless I want to swap the backs to 12.4-24's

 

[ovrszd]

My Ford 3910 is .768 or 1.3 depending on how you look at it. I figured that out by setting the tractor on jackstands, taping the wheels and rotating the rears 10 revolutions while counting the front revolutions as well. Then using the Tyre Learn chart I posted a link to earlier it is easy to pick tire combinations that will work.

I'll get energetic and jack up the Kubota and check it out and post what I find. I expect the results to be very similar. Using the Tyre Learn chart the Kubota's tires set up a .7425 ratio. Curious to see if the revolution count comes close to that. It has very very little pull or bind when running in 4wd. If running on hard surface it's just enough to cause it to not want to unlock. A seesaw of the steering wheel while moving unlocks it though.

 

[rScotty]

Then using the Tyre Learn chart I posted a link to earlier it is easy to pick tire combinations that will work.

I didn't find that link. Maybe in some other thread? Be sure to use the tire's spec for rolling circumference. Calculating circumference from diameter doesn't work well; it comes close, but not close enough.
rScotty

 

[ovrszd]

I didn't find that link. Maybe in some other thread? Be sure to use the tire's spec for rolling circumference. Calculating circumference from diameter doesn't work well; it comes close, but not close enough.
rScotty

Scroll to the bottom of the link to find the charts.

 

[PILOON]

Couple of years ago I went thru all that front/rear math as my fronts were finished and a rim was rotted out due to previous owner having filled with calcium.
Front rims were just too pricy.

I lucked out as a tire surplus outlet had a perfect rim pattern to match.
I ended up with a pair of rider mower rears that matched my bolt pattern exactly and were 2X wider with the exact same diameter so that I kept the front/rear ratio that I needed. I did however need to beef up the centers as they were thinner. (cut out the old centers and weld added them to the new rims)
Probably have over 500 hours on this upgrade.
Paid $75.00 for 2 rims and 2tires.