Anyone with the BFG KO2 39x13.5r17?

[SFLJEEPER]

Yup.

What are you doing for a spare?

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[BLK HOLE]

What are you doing for a spare?

Using a 37" KO2 under the bed. If I get a flat air the 39down an inch or so to match the 37". Really just need to air down an inch.

 

[Levi.Butler]

Using a 37" KO2 under the bed. If I get a flat air the 39down an inch or so to match the 37". Really just need to air down an inch.

What would airing down do? The circumference is all that matters to a differential.

 

[Levi.Butler]

I managed to snag a set of these recently and got them mounted this past weekend. My 37 KM3s are going on the wife's Wrangler build.

So far, I love them. We'll see how big of a difference they make in the rocks.

Also, I've got a Rock Krawler 4 link going in the rear in the next few weeks... I suspect the 39 will fit in the stock spare location after that.

 

[BLK HOLE]

What would airing down do? The circumference is all that matters to a differential.

That’s not true at all. What matters is the measurement from the ground to center of the axle shaft. Which is why you don’t measure the entire tire when resetting the speedo - you measure the ground to axle shaft center. I don’t care about the diff (I have open diffs)…tricking the speed sensors is the goal. Don’t think of it as one tire is larger than the other. All that matters is centering both axles

 

[DocMcStuffins]

I managed to snag a set of these recently and got them mounted this past weekend. My 37 KM3s are going on the wife's Wrangler build.

So far, I love them. We'll see how big of a difference they make in the rocks.

Also, I've got a Rock Krawler 4 link going in the rear in the next few weeks... I suspect the 39 will fit in the stock spare location after that.

Looks great! What wheels/backspace? Also what lift are you adding the RK 4-link to? Long arm?

 

[Levi.Butler]

That’s not true at all. What matters is the measurement from the ground to center of the axle shaft. Which is why you don’t measure the entire tire when resetting the speedo - you measure the ground to axle shaft center. I don’t care about the diff (I have open diffs)…tricking the speed sensors is the goal. Don’t think of it as one tire is larger than the other. All that matters is centering both axles

Are you trying to say that the speed sensors somehow operate based on whether the axle is level or not? Does your open diff axle not have ABS or traction control?

When one tire is larger than the other, one wheel will be spinning faster than the other at the same speed. You can't trick a speed sensor (or an ABS sensor) with air pressure. The only thing that sensor is reading is rotational speed.

That being said, as long as the two tires aren't MASSIVELY different, you can get away with it for a short period of time.

 

[Levi.Butler]

Looks great! What wheels/backspace? Also what lift are you adding the RK 4-link to? Long arm?

Thank you sir!

The wheel are KMC Machete Crawls, 17x9 -38 offset.

I am sticking with the Mopar springs but I am adding the RK adventure pro control arms and I've got some RadFlo shocks on order to go with it.

 

[BLK HOLE]

Are you trying to say that the speed sensors somehow operate based on whether the axle is level or not? Does your open diff axle not have ABS or traction control?

When one tire is larger than the other, one wheel will be spinning faster than the other at the same speed. You can't trick a speed sensor (or an ABS sensor) with air pressure. The only thing that sensor is reading is rotational speed.

That being said, as long as the two tires aren't MASSIVELY different, you can get away with it for a short period of time.

I’m saying all that matters is the measurement from the ground to the axle shaft center. Which is why your Speedo is measured that way when you get new tires.

 

[Levi.Butler]

I’m saying all that matters is the measurement from the ground to the axle shaft center. Which is why your Speedo is measured that way when you get new tires.

Well... as much as you've been told that. It's not true. Not in this case. That's just the easiest way to calibrate a speedo because a 37 isn't actually a 37 and you're making an assumption when doing it this way that all 4 tires are the same size.

If you understand how these systems work, you would know that airing down a 39 so the axle is level when paired with a 35 on the same axle WILL NOT affect the circumference of the tire and thus, not affect the speed that the reluctor ring (that's feeding information to the ECU through the ABS sensor) is spinning at a given speed of travel.

Airing down the larger tire so the axle is level will lessen excessive tire wear from the truck wanting to pull toward the smaller tire but, I suspect that effect will still be there because of different contact patches from side to side.

At the end of the day, this stuff isn't work bickering back and forth over and what you're talking about will get you home on a short trip but for anyone else who reads this, it's not true. Don't do this unless it absolutely has to be done... and if you have a limited slip differential, don't do it at all, it WILL destroy a differential in short time.

 

[BLK HOLE]

Well... as much as you've been told that. It's not true. Not in this case. That's just the easiest way to calibrate a speedo because a 37 isn't actually a 37 and you're making an assumption when doing it this way that all 4 tires are the same size.

If you understand how these systems work, you would know that airing down a 39 so the axle is level when paired with a 35 on the same axle WILL NOT affect the circumference of the tire and thus, not affect the speed that the reluctor ring (that's feeding information to the ECU through the ABS sensor) is spinning at a given speed of travel.

Airing down the larger tire so the axle is level will lessen excessive tire wear from the truck wanting to pull toward the smaller tire but, I suspect that effect will still be there because of different contact patches from side to side.

At the end of the day, this stuff isn't work bickering back and forth over and what you're talking about will get you home on a short trip but for anyone else who reads this, it's not true. Don't do this unless it absolutely has to be done... and if you have a limited slip differential, don't do it at all, it WILL destroy a differential in short time.

Yeah - you’re wrong anyway. Top half of the tire is irrelevant when measuring tire size.

I have open diffs.

https://www.jkowners.com/threads/37s-with-35-spare.278057/

 

[Levi.Butler]

Yeah - you’re wrong anyway. Too half of the tire is irrelevant when measuring tire size.

I have open diffs.

https://www.jkowners.com/threads/37s-with-35-spare.278057/

Bro... did you really just cite another forum to prove your point on a forum? That's like quoting your mother and saying that alligators are so ornery because they got all them teeth and no tooth brush. Haha!

I will say that I agree with you that the top half of the tire doesn't matter when measuring tire size... BUT neither does the bottom half in this instance. We're not talking about measuring tire size. We're not talking about calibrating a speedometer. We ARE talking about putting 2 extremely different sized tires on the same axle on a Jeep.

It MIGHT be possible to pull this off if you have a big 35 and a small 37 and you put like 80psi in the 35... but a 35 and a 40, never gonna happen. I don't care what smooth brained dipshit told you otherwise... deflating a 40 will never change the circumference of the tire enough to be able to even remotely match the circumference of a 35.

Have you ever driven a Jeep with a steering wheel that's not perfectly centered when the tires are pointing straight? You get issues with the ABS system. Do you know why? Because your Jeep is expecting different rolling speeds between the inside and outside tire and assumes that you are losing traction to the inside tire because it's actually rotating the same speed as the outside tire.

If you are making the assumption that radius is all that matters (it doesn't), how do you think your vehicle would measure the radius of a tire? The answer is through wheel speed based on the input from the ABS system. An aired up 35 and an aired down 40 will 100%, without a doubt, have different wheel speeds at the same vehicle speed and they will undoubtedly cause issues with your ABS system.

Don't even get me started on how drastically the radius of an aired down 40 would change based on the centrifugal force that would be applied to it at different speeds... how would you account for that in comparison to an over-inflated 35?

You've got 39s now, you can prove this in real life. Go out to your drive way and put a small spare tire across from your 39 and air down the 39 until your axle is level, put a piece of tape at top dead center on each tire and move your truck one tire rotation. Those pieces of tape will not land in the same spot after a single tire rotation, not even close. Imagine that times multiple tire rotations per second. Your ABS system will have problems with that.

I'm not saying that if you've got 2 relatively close tires, like a 35 and a 37, that you won't be able to get off the trail so you can get a properly sized tire back on your truck. I am saying that if you think a 35 and an aired down 40 going down the highway aren't going to cause any issues because you have open diffs, you've got another thing coming and you should not in any circumstance be spewing that fake news to other humans on the internet.

I'm not going to respond to you again but to anyone else who is reading this, this dude is full of shit (at least in this circumstance) and don't take any advice from him.

 

[BLK HOLE]

Bro... did you really just cite another forum to prove your point on a forum? That's like quoting your mother and saying that alligators are so ornery because they got all them teeth and no tooth brush. Haha!

I will say that I agree with you that the top half of the tire doesn't matter when measuring tire size... BUT neither does the bottom half in this instance. We're not talking about measuring tire size. We're not talking about calibrating a speedometer. We ARE talking about putting 2 extremely different sized tires on the same axle on a Jeep.

It MIGHT be possible to pull this off if you have a big 35 and a small 37 and you put like 80psi in the 35... but a 35 and a 40, never gonna happen. I don't care what smooth brained dipshit told you otherwise... deflating a 40 will never change the circumference of the tire enough to be able to even remotely match the circumference of a 35.

Have you ever driven a Jeep with a steering wheel that's not perfectly centered when the tires are pointing straight? You get issues with the ABS system. Do you know why? Because your Jeep is expecting different rolling speeds between the inside and outside tire and assumes that you are losing traction to the inside tire because it's actually rotating the same speed as the outside tire.

If you are making the assumption that radius is all that matters (it doesn't), how do you think your vehicle would measure the radius of a tire? The answer is through wheel speed based on the input from the ABS system. An aired up 35 and an aired down 40 will 100%, without a doubt, have different wheel speeds at the same vehicle speed and they will undoubtedly cause issues with your ABS system.

Don't even get me started on how drastically the radius of an aired down 40 would change based on the centrifugal force that would be applied to it at different speeds... how would you account for that in comparison to an over-inflated 35?

You've got 39s now, you can prove this in real life. Go out to your drive way and put a small spare tire across from your 39 and air down the 39 until your axle is level, put a piece of tape at top dead center on each tire and move your truck one tire rotation. Those pieces of tape will not land in the same spot after a single tire rotation, not even close. Imagine that times multiple tire rotations per second. Your ABS system will have problems with that.

I'm not saying that if you've got 2 relatively close tires, like a 35 and a 37, that you won't be able to get off the trail so you can get a properly sized tire back on your truck. I am saying that if you think a 35 and an aired down 40 going down the highway aren't going to cause any issues because you have open diffs, you've got another thing coming and you should not in any circumstance be spewing that fake news to other humans on the internet.

I'm not going to respond to you again but to anyone else who is reading this, this dude is full of shit (at least in this circumstance) and don't take any advice from him.

Using more words doesn't make you right...just makes you more wrong.

You can lead a horse to water...but, in this case...its just a stupid horse.

Bottom radius of my 39s = 19"
Effective diameter = 38"
Circumference = 38x3.14 (π) = 119.32"

For 37s I'll estimate, since I don't have them anymore (on the ground) -
Radius = 18"
Diameter = 36"
Circumference = 36x3.14=113.04"

For purposes of this exercise I need to just lower the pressure enough to get the rolling radius to 18". I've provide a link that will offer some education. Altho you've proven to be quite resistant to learning.

For the GEDs :

 

[Levi.Butler]

Using more words doesn't make you right...just makes you more wrong.

You can lead a horse to water...but, in this case...its just a stupid horse.

Bottom radius of my 39s = 19"
Effective diameter = 38"
Circumference = 38x3.14 (π) = 119.32"

For 37s I'll estimate, since I don't have them anymore (on the ground) -
Radius = 18"
Diameter = 36"
Circumference = 37x3.14=113.04"

For purposes of this exercise I need to just lower the pressure enough to get the rolling radius to 18". I've provide a link that will offer some education. Altho you've proven to be quite resistant to learning.

For the GEDs :

You've only done half of the equation, bud.

There is a TON of math in that 97% statistic that he just brushed over and you have also conveniently brushed over.

Here is a lengthy explantation of the math behind that --> https://www.automotorgarage.com/post/how-to-calculate-value-of-rolling-radius

Here is a more simple explanation of why the math you provided above doesn't pan out in the real world --> http://mgaguru.com/mgtech/tires/ti103.htm

And, here is a crusty old video of someone actually attempting to apply that math in the real world (spoiler alert: it doesn't work) -->

I invite you to go outside in the real world and conduct the same experiment. Post back with your findings.

I will absolutely concede that if the tire sizes are relatively close (2 inches is not relatively close, we're talking more like same size tire with drastically different wear amounts), you could probably make it work but I'd be willing to bet that over-inflating the smaller tire is going to net you much better results than airing down the larger tire. Either way, the ABS system definitely allows for a certain range of variance between wheel speeds and you'll more than likely be okay with tires that are stamped the same size.

I'm not sure what you do for a living, and this isn't a dig at anyone, but I'd be willing to bet you're in education of some sort. Academics appears to be the only thing that matters to you and you appear to be afraid of practical application of those academics. The old saying "Those who can't do, teach." is, more often than not, correct (there are rare exceptions).

If you prove me wrong (you won't), I will gladly record a video of myself eating a real crow and put it on YouTube so you can tell all your friends about the time you proved someone wrong on the internet.

 

[BLK HOLE]

You've only done half of the equation, bud.

There is a TON of math in that 97% statistic that he just brushed over and you have also conveniently brushed over.

Here is a lengthy explantation of the math behind that --> https://www.automotorgarage.com/post/how-to-calculate-value-of-rolling-radius

Here is a more simple explanation of why the math you provided above doesn't pan out in the real world --> http://mgaguru.com/mgtech/tires/ti103.htm

And, here is a crusty old video of someone actually attempting to apply that math in the real world (spoiler alert: it doesn't work) -->

I invite you to go outside in the real world and conduct the same experiment. Post back with your findings.

I will absolutely concede that if the tire sizes are relatively close (2 inches is not relatively close, we're talking more like same size tire with drastically different wear amounts), you could probably make it work but I'd be willing to bet that over-inflating the smaller tire is going to net you much better results than airing down the larger tire. Either way, the ABS system definitely allows for a certain range of variance between wheel speeds and you'll more than likely be okay with tires that are stamped the same size.

I'm not sure what you do for a living, and this isn't a dig at anyone, but I'd be willing to bet you're in education of some sort. Academics appears to be the only thing that matters to you and you appear to be afraid of practical application of those academics. The old saying "Those who can't do, teach." is, more often than not, correct (there are rare exceptions).

If you prove me wrong (you won't), I will gladly record a video of myself eating a real crow and put it on YouTube so you can tell all your friends about the time you proved someone wrong on the internet.

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