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Tires and gear ratio


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Redid the front end on a 1086. Long story short is it has always been too fast, which led to it's demise. Next step down was to a 5.43 instead of the 4.88. it now is pushing the front tires because it's too slow. For every 13 revolutions in the rear, the front barely did not make 13, like a 12.75. the 4.88 is nearly impossible to find for it. So, which leads me to discuss tires. It has 16.9-28 on the front and 20.8-38 on the rears. One place says we have to go down a tire size so it has less circumference to make up, and the other place claims we would need a 14.9-38 on the front to catch up. The confusing part is we are keeping the ground speed the same, so it makes sense to go to a smaller tire because it needs to get to the same point sooner. The rear end tires have a larger circumference to make up for, but in aspect of say a lever, the outside is actually "faster", more velocity than the inside. 

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31 minutes ago, MinnesotaFarmall said:

Redid the front end on a 1086. Long story short is it has always been too fast, which led to it's demise. Next step down was to a 5.43 instead of the 4.88. it now is pushing the front tires because it's too slow. For every 13 revolutions in the rear, the front barely did not make 13, like a 12.75. the 4.88 is nearly impossible to find for it. So, which leads me to discuss tires. It has 16.9-28 on the front and 20.8-38 on the rears. One place says we have to go down a tire size so it has less circumference to make up, and the other place claims we would need a 14.9-38 on the front to catch up. The confusing part is we are keeping the ground speed the same, so it makes sense to go to a smaller tire because it needs to get to the same point sooner. The rear end tires have a larger circumference to make up for, but in aspect of say a lever, the outside is actually "faster", more velocity than the inside. 

Number of revolutions x circumference of the tire should be approximately equal.  Assuming the front should be slightly higher or faster. 
 

what are your revolutions on the front axle compared to your back axle?  You say 13 to 12.75.  If that is true you would need bigger tires in the front than the back.

what is your rear tire size/rolling circumference?

if you have these 2 you can calculate the needed front tire circumference/diameter.

 

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6 minutes ago, MinnesotaFarmall said:

The front is not making the full 13 revolutions. It's at about 12.75. the rear makes the full 13. Our tire guy said that front should be about 13.1-13.4. care to elaborate on the formula for rolling circumference?

Sure.  You need both the front and the rear to travel the same distance on the ground… approximately.  To get distance, you would take revolutions and multiply by circumference.  That gives you the distance traveled along the ground for front and rear.  As mentioned they should be close to the same distance.

how are you turning the rear axle?  If you have it jacked up and turning one wheel then it would double due to the differential.  So you would be 6.5 on the rear and 12.75 on the front.  This makes more sense as fronts are about half the diameter/circumference log the rears.

someone should confirm the true half speed when spinning one tire thru the differential.  I believe that to be true, but have never done it myself.

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10 minutes ago, Mudfly said:

Number of revolutions x circumference of the tire should be approximately equal.  Assuming the front should be slightly higher or faster. 
 

what are your revolutions on the front axle compared to your back axle?  You say 13 to 12.75.  If that is true you would need bigger tires in the front than the back.

what is your rear tire size/rolling circumference?

if you have these 2 you can calculate the needed front tire circumference/diameter.

 

16.9-28 on the front 20.8-38 on the rear.

Screenshot_20220712-211133.png

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Just now, Mudfly said:

Sure.  You need both the front and the rear to travel the same distance on the ground… approximately.  To get distance, you would take revolutions and multiply by circumference.  That gives you the distance traveled along the ground.

how are you turning the rear axle?  If you have it jacked up and turning one wheel then it would double due to the differential.  So you would be 6.5 on the rear and 12.75 on the front.  This makes more sense as fronts are about half the diameter/circumference log the rears.

someone should confirm the true half speed when spinning one tire thru the differential.  I believe that to be true, but have never done it myself.

Driving on flat gravel driveway with the front engaged. I finally found my scratch pad. This is the results. 

KIMG1365.JPG

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1 minute ago, MinnesotaFarmall said:

Driving on flat gravel driveway with the front engaged. I finally found my scratch pad. This is the results. 

KIMG1365.JPG

Would that be 13 revs on the front and 10 on the back unlocked?

and 12 front, 10 rear locked?

I don’t quite follow what 13:10 means.

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A little google search says that a 16.9-28 is 56.5 inches tall.

your rears are 73 inches

56/73 = .767

10/13 = .769

so that is your ratio unlocked

when you lock it you are 10/12

that mean you need a tire 10/12 x 73 = 60.8 inches tall. 

18.4-28s are 58.1 So you would need something taller yet.

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A little research says 18.4R30s are 61 inches (1550 mm).

12 revolutions x 61 inches tall x pi = 2298.5 inches of front wheel travel

10 revolutions x 73 inches tall x pi = 2292.2 inches of rear wheel travel.

that would give you a slight lead on the front end.

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2 minutes ago, jass1660 said:

18.4-38’s on rear. 

That would be close too.  12/10 x 56.5 = 67.8 rears. 
 

looks like agribibs 18.4 -38s are 69 inches.  It would be closer, but might still push a bit.

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All the bologna math in the world can make things look good, real world says, if your front is going 12.7 revolutions, your rear 13, it's obvious that you need to go smaller tires in the front to get closer to a 13 revolution.  I've done this plenty of times in swapping axles in trucks. Not rocket science here 

Mark

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8 minutes ago, td9inidaho said:

All the bologna math in the world can make things look good, real world says, if your front is going 12.7 revolutions, your rear 13, it's obvious that you need to go smaller tires in the front to get closer to a 13 revolution.  I've done this plenty of times in swapping axles in trucks. Not rocket science here 

Mark

My fronts go 12 revolution for every 13 on the back with them locked in. 13 to 10 with the front unlock. The smaller tires was my thinking also. You would think a smaller tire is going to get to the same point quicker, one is has less circumference to travel, and two, we are leaving the ground speed the same and have a fixed speed. What ed leaman says too. But when we call our tire guy, he says what they say above, but different tire again. We have a smaller set of tires we could easily swap on, just would need to block the tractor up we take them off of. I figured it's easier to ask.

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2 minutes ago, td9inidaho said:

All the bologna math in the world can make things look good, real world says, if your front is going 12.7 revolutions, your rear 13, it's obvious that you need to go smaller tires in the front to get closer to a 13 revolution.  I've done this plenty of times in swapping axles in trucks. Not rocket science here 

Mark

Those weren’t his actual ratio.  He is 12 revs front to 10 rear revs.

you actually don’t want a 1:1 or 13:13 ratio on a tractor unless it’s an articulated 4x4 or a 2+2.  Trucks are totally different as they run the same size tires front and rear.  
ill put it another way, there is a reason there are different tooth count gears in a transmission.  If they were all the same every gear would be the same speed.  You need gear ratio to change the speed of the input shaft to output shaft ratio to get different output shaft speeds.

same thing here. 

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2 hours ago, MinnesotaFarmall said:

The front is not making the full 13 revolutions. It's at about 12.75. the rear makes the full 13. Our tire guy said that front should be about 13.1-13.4. care to elaborate on the formula for rolling circumference?

So am I missing something here?  He says the front is not making the revolutions of the rear, it's being pushed, so you say bigger tires so it gets pushed more?  I could be all wrong. Appologize if I am 

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2 hours ago, MinnesotaFarmall said:

it now is pushing the front tires because it's too slow.

This is all that really matters.  If this is true you either need to turn the front wheels faster or put a larger circumference tire on.

I have an MFD tractor that has front tires 2 RCI group sizes too large on the front.  When the MFD is engaged, the front axle pulls the rear axle.  I am very careful about using the MFD as this is a good way to burn up the MFD clutch.  Going to larger circumference tires on the rear would solve the problem.

 

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If your rear tires are pushing the front you need larger front tires or smaller rears. I am having this problem with my farmall 806.  Front diff had worn the back side of the ring gear away about 1/3 through.   I got another diff from Ed Leaman. Correct ratio. Put it in. Drove the tractor in a straight line on concrete (tires at the pressure they are normally run at). For 100 turns of the transfer case pinion the diff does 110 revolutions. (Larger front tires needed but not really an option as I would need to go from 12.4-24 to 16.9-28).  transfer gears are wrong and as far as I know the tractor has never been messed with in that area. When I bought it he commented traction was not great and it was hard to manoeuvre when FWA was engaged.  Try it with your tractor. The further you drive it the more turns you get with more accurate numbers. Front pinion faster need bigger front. Transfer case faster need smaller front.   The percentage difference in revolutions can be used to calculate correct tire size pretty easily. 
 

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If i read this right, you say the front is being pushed but your numbers say the opposite? I am confused on that. I would let some air out of the front tires and recheck it, let it squat a bit, that will tell you what way you need to go. Hope this helps.

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2 hours ago, jd4440 said:

If i read this right, you say the front is being pushed but your numbers say the opposite? I am confused on that. I would let some air out of the front tires and recheck it, let it squat a bit, that will tell you what way you need to go. Hope this helps.

The front tires make more revolutions than the back, but they are also not as large. As was done with the math calculations, the tires have to travel the same distances. Say you are pulling a boat trailer. The trailer tires are much smaller than those of the pickup. The tires all have to travel the same distance, but since they are different sizes, they need to make more or less revolutions to travel the same distance. The comparison is kind of apples to oranges I know, but you might get the idea. 

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This is probably just one more voice in the noise. I was contemplating a response to this last night as mudfly made his first response. As he touched all the questions I had and showed an understanding of what was needed. I passed. Checked back before I turned in and saw that he had solved the mystery of the rear tire revolutions. Ten not thirteen. He had also researched and calculated necessary tire size to correct problem. Without duplicating I have no doubt calculations are correct. I do doubt that size tire on front would make a satisfactory tractor. Would require either wide axle setting or wide turning circle or both. Also would raise front end changing steering geometry and lowering drawbar.

Returning to original gearing. You will probably find that original tire size is as close as you can get. What is needed is a little more air in rear tires and a little less in front. Remember front axle has to pull a little because it has to go further.

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2 minutes ago, snoshoe said:

This is probably just one more voice in the noise. I was contemplating a response to this last night as mudfly made his first response. As he touched all the questions I had and showed an understanding of what was needed. I passed. Checked back before I turned in and saw that he had solved the mystery of the rear tire revolutions. Ten not thirteen. He had also researched and calculated necessary tire size to correct problem. Without duplicating I have no doubt calculations are correct. I do doubt that size tire on front would make a satisfactory tractor. Would require either wide axle setting or wide turning circle or both. Also would raise front end changing steering geometry and lowering drawbar.

Returning to original gearing. You will probably find that original tire size is as close as you can get. What is needed is a little more air in rear tires and a little less in front. Remember front axle has to pull a little because it has to go further.

But if I air down the front and air up the rear, it would make the fronts shorter and the back taller. If I follow the calculations done, then, that would be doing the opposite of what "needs to be done" making the front taller. 

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1 minute ago, MinnesotaFarmall said:

But if I air down the front and air up the rear, it would make the fronts shorter and the back taller. If I follow the calculations done, then, that would be doing the opposite of what "needs to be done" making the front taller. 

That is with original gearing. Sorry if I am muddying the waters.

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