Different size tire that OEM on 4wd tractor

[halfbean]

I recently bought a M9540 4wd that has 16.9-34 tires on the rear. It also has cast wheel centers. From the factory it ships with 18.4-30 on the rear according to my owners manual. The front tires are original 12.4-24.
Will this hurt the transmission in 4wd and would you run it this way? On my place if I do use 4wd it is on loose soils. I don't do much plowing either.
The rolling circumference is 185" on 16.9-34 and 182" on 18.4-30 and height if 63" and 61.7" respectively.

Thanks
Hal

 

[Rustyiron]

It would bother me in terms of "the long run", and I'd be alert to the available traction. (tires really hooking up vs some slick mud or clay)
I wonder how you'd do the math to figure what size front tire/wheel combo to make it good? I guess you'd be into ratio's.

 

[halfbean]

I can't find the drive ratio for a M9540 anywhere. Maybe the cast center wheel models have 16.9-34 tires and the regular rim models have 18.4-30 tires. Does anyone else with a 9540 have cast center wheels with 16.9-34 tires?

 

[Rustyiron]

I have the cast centers with the 18.4-30.
After some operating in 4wd, is it difficult to get the front dis-engaged?

 

[art]

Not getting to in depth with lead lag ratio's both size rear tires the 30" and 34" still use the 12/24 fronts! Naturally the 30" will give you more lead!

 

[LD1]

Wouldn't bother me to run it given the small difference.

But did you buy it from a dealer. What do they say?

Does it engage and disengage difficult? Does it act like it's binding it 4wd

Put in on a surface that ensures good rear wheel traction. Mark a front tire and drive in a straight like with it in 2wd and count the revolutions. Do the same in 4wd.you want the front to turn more by a little when in 4wd. (Lead).

Does it?

 

[Rustyiron]

Put in on a surface that ensures good rear wheel traction. Mark a front tire and drive in a straight like with it in 2wd and count the revolutions. Do the same in 4wd.you want the front to turn more by a little when in 4wd. (Lead).

Does it?

That only works if your able to watch (and count) driveshaft revolutions, like in a p-u truck. On a tractor with different diameter tires, without knowing the final drive ratios of both differentials.......
The rears may turn once for 2-3 revs of the front, that difference is made up in the dif gearing.

 

[LouNY]

There are a lot of variables that come into play,
the important one is the loaded rolling radius or circumference,
by loaded I'm referring to how they are on the tractor (pressure) and how the tractor is loaded ( front and rear weight).
What you can do is on undisturbed dirt, drive it in 2wd, get out and look at your tracks, now,
put it in 4wd drive it again when you stop release your brake, now place it in 2wd, if it lurches it is getting into some bind,
look at your tracks see if you are pushing your tread mark ahead or actually pulling it, you can correct it quite a bit
by varying your air pressure a few pounds up or down in the front or rears untill you get a good balance, and yes a little bit of lead on
the front does help especially in turns. Your lead / lag will change as your tires wear and as your axle loads change, heavy loads being carried on the
3 pt lift arms will squat the rears and change the loaded radius of the rear tires, a heavy load in a loader will load the front tires and reduce there
loaded radius and actually unload the rears and increase there loaded radius. Air pressure is NEVER a set value as loads increase the air pressure
needs to increase, your foot print is the critical factor,keeping all your tread in contact with the ground and working for you.
Have fun
Lou

 

[LD1]

That only works if your able to watch (and count) driveshaft revolutions, like in a p-u truck. On a tractor with different diameter tires, without knowing the final drive ratios of both differentials.......
The rears may turn once for 2-3 revs of the front, that difference is made up in the dif gearing.

It works. But you have to make sure the rear tires have good traction.

IE, if you have nothing on the back of the tractor, and not loaded tires, and have a loader mounted, the rear tires are gonna be the ones slipping and the fronts will turn the exact same.

But no loader, and an average weight implement on the back, makes sure the fronts are gonna be the ones slipping and not the back.

Then mark a starting line. Mark a front tire. Count however many revs you want. Call it 10 revs. Mark the stopping line.

Now go back, engage the 4wd, go the exact same distance but count the revs. Should be something a little more than 10 revs for a lead ratio.

Not have to get exact with it, and this method wont give you an exact ratio. But knowing the rear tires are actually a little smaller than OEM specs, if you have any lead at all, like 10.25 or 10.5 revs when in 4wd....then I wouldnt worry. However if you are in a lag condition, I'd correct the situation

 

[Rustyiron]

I "got ya", I usually don't question your posts and was worried that I mis understood you, and I did.:thumbsup:

So your doing a "run" in 2wd, then one in 4wd and comparing?