Tires: front/rear ratio, I'm I doing the correct math?

[tractorin]

Hi everyone I am new here. I've been reading in various sites including this great one called tractorbynet :laughing: and would really appreciate if you guys can confirm something for me...it's a general question and not brand/model specific.

You see, if you want to make sure you have the correct tire sizes front and rear when your tractor is the type that has larger rears and is MFWD (not sure if that's what they call it)....lets say you don't know what the mechanical ratio (also not sure if called like that) and there is no way of knowing because its an old tractor....is it correct to do the rolling test (10 tire turns) in 2-wheel drive and then in 4-wheel drive and then check the difference in both measurements...the diff is the % of advance right? It needs to be let's say from 1% to 5% (+/- don't remember exact margin but you get the idea). My question is what can you do when the tires on your tractor are not the correct sizes to begin with? I think my rear tires are a little too small for the front size because the % is negative so the rear is pushing. From what I found out the fron has to be the right size on the tractor now...so just the rear is in question. Is there another way to calculate the mechanical ratio? My brain is a little fried from all the reading so forgive me maybe it's something so simple that I overlooked

 

[bindian]

Hi everyone I am new here. I've been reading in various sites including this great one called tractorbynet :laughing: and would really appreciate if you guys can confirm something for me...it's a general question and not brand/model specific.

You see, if you want to make sure you have the correct tire sizes front and rear when your tractor is the type that has larger rears and is MFWD (not sure if that's what they call it)....lets say you don't know what the mechanical ratio (also not sure if called like that) and there is no way of knowing because its an old tractor....is it correct to do the rolling test (10 tire turns) in 2-wheel drive and then in 4-wheel drive and then check the difference in both measurements...the diff is the % of advance right? It needs to be let's say from 1% to 5% (+/- don't remember exact margin but you get the idea). My question is what can you do when the tires on your tractor are not the correct sizes to begin with? I think my rear tires are a little too small for the front size because the % is negative so the rear is pushing. From what I found out the fron has to be the right size on the tractor now...so just the rear is in question. Is there another way to calculate the mechanical ratio? My brain is a little fried from all the reading so forgive me maybe it's something so simple that I overlooked

Tractorin,
Welcome to TBN.
Other than checking your tractor's operator's manual, you can call your tractor dealer and ask original tire sizes or check TractorData.com - information on all makes and models of tractors. I would think any tractor tire dealer worth his salt could tell you the original tires sizes.:thumbsup:

I think the term you are looking for is gear ratio. I don't know about the 10 tire turn equation you are asking about:confused2:. I am a mechanic, not an engineer.:thumbsup: If it is broke, I can fix it. But I can not figure out how much abuse it can take before it breaks.:laughing:

Oh yeah...........which tractor do you own?
hugs, Brandi

 

[Ford tractor]

Your test is correct. The target is for 5% faster front tires when in 4wd than in 2wd, but 2-10% is ok. If the number is SMALLER in 4wd than in 2wd-- that means the rears are TOO LARGE. Not too small. Too small would give a ratio much greater than target. You sure you did the math correctly? Unless you have a negative distance traveled the rolling ratio can't be negative!

If you have currently slower front tires you have some extra to do.

Put a tape measure around the outside of the front tires (middle of the tire) circumference measure. Do it in inches not feet and inches.
Do the same for the rears.
Go 10 revolutions of the REAR tires and COUNT the revolutions of the front tire in both 2wd and 4wd.
The revolution difference in 4wd will be the gear ratio difference and the diameter difference - but it will be a fixed relationship as long as you don't change the front tires. (this is just good information)
{The 4wd / 2wd is the ratio that should be 105% ideally.}
Take the measured FRONT tire circumference and multiply it by the counted number of revolutions the FRONTS did in 4wd when the rears did 10. This is your TARGET NUMBER.
Take the measured REAR circumference and multiply it by 10.
The front distance traveled is ideally 5% longer than the rear distance when in 4wd.

To find the best match tire, go to the tire specs and find the rolling circumference of several tires that will fit your rims. Multiply that number by 10.5. The one that is closest to your TARGET NUMBER is the one you want.

I know that when math is tossed in, it's easy to get lost. That's why I tried to explain everything in great detail. Not trying to talk down to anyone, just want to show the hows and whys so people get a better feeling for what's going on.