4WD Front to Rear Tire Ratios

[samofsweden]

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?

(This numbers refering to my kubota 7040 ,2009)

 

[samofsweden]

Found this thread
http://www.tractorbynet.com/forums/john-deere-owning-operating/112648-front-rear-gear-ratios.html

 

[sixdogs]

You get the rolling circumference of the tire off the manufacturer's website. That's easy and the number is correct and not a guess. Then get the front axle ratio from a tire maker or ag dealer and math it out. You will come out with a 1% to maybe 3% lead of the front tire over the rear.

SamofSweden--I think the dealer got it wrong.

My tractor is a Deere 5520 and the ratio is .741. I have radial 18.4x30 and 12.4x24. The front is a 47.0" diameter (137 rolling circ) and the rear is 60.6" (179 rolling circ). I forget how to do the math but with the .741 ratio the fronts led the rears by 3.3%. Fool with the numbers and it will be evident.

If you can't get the front ratio, either use a tire combo already approved by the manufacturer or pick up the phone and call the company. I can't recall but think that's how I got the number the first time. Since I was switching to radials I wanted to make sure the numbers were correct.

The right answer is to use an already approved tire option since those options take into account the best match of horsepower and weight to the tire size and that affects the performance of the tractor and satisfaction using it.

 

[ovrszd]

I'm researching this issue now on a Ford 3910FWD. It has had the rears swapped from 28s or maybe 30s to 38s. It pushes real bad. I can look up the gear ratios with CNH's parts website. But all that does is confuse me.

I found this website couple days ago. Then I PM'd a friend on here that has a 3930 and asked him his tire sizes. Using the chart on this website I divided the front tire rci into the rear tire rci to get his ratio. Then I applied my front tire size (different than his) and got it's rci. Then I researched rear tire sizes until I found one that matched his ratio. I now know that I need 14.9x30s to get the best match. Next best match was 16.9x28s.

My point being. If you have a tractor with known tires sizes from the factory and all you want to do is upsize or downsize, this chart can help you figure that out.

Open the website, then scroll down almost to the bottom and you'll find two pages of rci numbers on a multitude of tire sizes.

Tyre Learn

 

[rScotty]

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?

(This numbers refering to my kubota 7040 ,2009)

Sounds like you are in the right ball game. I'm assuming that he is giving you a gear ratio and that we now need to apply that to the rolling circumference of the tires. Do you know the rolling circumference spec for the stock tires on the 7040?
keep after it....
rScotty

 

[samofsweden]

rScotty,
Well Im not 100% here. I mean, 1,438:1 seams like a strange gearratio, I know that the front axle on my 7040 is a portal axle that keeps another gearratio out in the hubs (my swedish is better than my english.. ) but still a bit "high geared ratio"

When christmas is over I will get back to my dealer and ask him if these numbers are correct.
Thanks for helping me out guys:thumbsup:

 

[samofsweden]

I'm researching this issue now on a Ford 3910FWD. It has had the rears swapped from 28s or maybe 30s to 38s. It pushes real bad. I can look up the gear ratios with CNH's parts website. But all that does is confuse me.

I found this website couple days ago. Then I PM'd a friend on here that has a 3930 and asked him his tire sizes. Using the chart on this website I divided the front tire rci into the rear tire rci to get his ratio. Then I applied my front tire size (different than his) and got it's rci. Then I researched rear tire sizes until I found one that matched his ratio. I now know that I need 14.9x30s to get the best match. Next best match was 16.9x28s.

My point being. If you have a tractor with known tires sizes from the factory and all you want to do is upsize or downsize, this chart can help you figure that out.

Open the website, then scroll down almost to the bottom and you'll find two pages of rci numbers on a multitude of tire sizes.

Tyre Learn

Richard,
Thanks alot:thumbsup:

 

[rScotty]

rScotty,
Well Im not 100% here. I mean, 1,438:1 seams like a strange gearratio, I know that the front axle on my 7040 is a portal axle that keeps another gearratio out in the hubs (my swedish is better than my english.. ) but still a bit "high geared ratio"

When christmas is over I will get back to my dealer and ask him if these numbers are correct.
Thanks for helping me out guys:thumbsup:

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® 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

 

[David Devuono]

My little B4200 is about 30 years old, has the same turf tires on it from when I bought it about 10 years ago, they may be originals? It has spent the last 10 years in 4x4 with 700 hours put on by me and 1,700 overall. The front tires are in good shape. Interesting that my 2wd Massey probably wears fronts out faster as in one of my 3 rib tires has no ribs left. As for my newest tractor (2011 b3200) it also has spent the past two years 250 hrs in 4x4 and there is noticeable wear on the front R4's. I may up the counter weight from 500 to 700 lbs. HST breaks traction real easy so there is lots of tire slip when doing loader work.

 

[dragoneggs]

There is no way with a mechanical driven system to eliminate ALL tire slipping between front and rear. Even on trucks, with identical ratios and identical tires, you will bind and try to slip when turning. Cause the front axle is traveling in a larger arc than the rears. Over driving the fronts helps this, like with tractors, but doesnt do good when going straight. Only way to eliminate it would be to have an open differential setup to split the front and rear halves of the tractor. Similar to the old 203 t-case in the full-time trucks of the late 70's era

Hey Scotty thanks for posting and an interesting topic (for some of us geeks at least). LD, I think you are spot on with your comment about arc difference between fronts and rears. I run 4wd 99+% of the time and I am convinced that the only thing helping me with the mismatch is the loose ground underneath.