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Tractor Talk Discussion Forum

Info on matching front to rear tires on FWA tracto

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DAN9-Midwest

09-17-2006 06:24:09




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I recently learned way too much about replacing tires on FWA tractors and front-to-rear tire size relationships and thought I would pass it along for others. Take it or leave it I'm just trying to be helpful. I found little on this site about it. It is very important.
Every FWA tractor has a "front-to-rear gear ratio" and the manufacturer states this in it's manuals or website info. For one tractor, it was .741
The inverse of this is 135 (1.00 divided by .741 = 135) Some state the ratio this way and most are in the 125 to 150 category.
Multiply this number by the rolling circumference "RC" (get from published tire data) of the front tire size.
Mine was 137--- so 135 times 137 (front RC) equals a given number (1849). Divide this number by the RC (rolling circumference) of the REAR tire (179) and that's the number we're looking for expressed in percent. It is 135x137=1849 divided by 179= 103.3%. It's gear ratio times front RC divided by rear RC. The example above gives 103.3 % and means the front tire is turning "pulling" 3.3% faster (positive) than the rear and this is OK. A number below 100 would be a "negative" number and indicate the front was pushing against the rear and is very bad. Positive good, negative very bad. Suggested is positive 2% to 5% and negative never suggested. Negative will "load" and probably damage a transmission, front gear case and grind up the tires, especially on harder ground.
If one gets a negative number they need to change rolling circumference (tire sizes) until the right poitive relationship (+2% to +5%) is found and then change tires. For the correct RC of a particular tire, you have to check THAT tire's website because an 18.4-34 from one source may not be the same RC as the 18.4-34 from another. This especially matters with smaller compact tractors I was told. The above exercise answered a question I have wondered about for years and is a combination of advice you can get at the Firestone and Titan websites. I also spoke with a tire engineer on this and THAT sounds like a fun job unless you did the Explorer tire. Hope this helps.

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RodInNS

09-17-2006 17:17:48




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 Re: Info on matching front to rear tires on FWA tr in reply to DAN9-Midwest, 09-17-2006 06:24:09  
I still think you're overthinking this subject. Unless you changed out a set of tires to an entirely different size, I would not worry about this. Period. These are farm tractors, not F1 race cars. So long as the tires are the same size that the manufacturer intended, I would not worry about the profile differences of various different tires unless they were wildly out of sync. So long as you're happy with what you've got, not much else matters.

Rod

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Hugh MacKay

09-17-2006 11:09:26




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 Re: Info on matching front to rear tires on FWA tr in reply to DAN9-Midwest, 09-17-2006 06:24:09  
Dan: Are you telling us you didn't know all that before you bought the tractor? Back in the 1970s I looked at 4x4 a good many times. Decided they weren't worth the hassel unless manufacturer used all tires the same size. I've seen so many different tire profiles, just not worth the hassel. Seems like you go back in two years and they've changed the specs.

Yes I did buy a 4 wheel drive tractor and all tires were 18.4x38

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Wardner

09-17-2006 10:56:04




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 Re: Info on matching front to rear tires on FWA tr in reply to DAN9-Midwest, 09-17-2006 06:24:09  
You seem to key on "rolling circumference". I think that builds in an error right away. You should be doing the calculations with "loaded radius". Multiply that by 2 and again by 3.14 to get "true rolling circumference" for the situation. Of course, you don't need to do the additional math. Develop the ratios based on the distance the centerline of the axles are above the ground. A large rear tractor tire at 8 lbs could have a 2-3" shorter radius than one inflated to 20 lbs. That will effect revolutions per mile. The tire mfgs have no control over inflation pressures and weight so they have to use rolling circumferance based on unknown user set-ups.

I always get a kick out of 4x4 backhoes and some of the Kubotas around here. The front tires always wear quicker than the rears particularly when the owners don't bother to disengage the front axle when it is not needed. There is a tremendous amount scuffing in turns. I ask them why they want to stress the drive train and increase tire wear. They don't know what I am talking about.

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