RTH: Different tread depths - same axle (1 Viewer)

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I have 3 BFG KM tires with approx 30% tread left. The 4th has sidewall damage. I use these for summer only. I'm looking at a brand new tire of the same make and model for a wicked deal. Only one is available.

Do you foresee any issues running mostly city, with occasional highway travel with 3 tires at 40% and 1 at ~100%? Should I mount the new one up front or in the back?

Tire size is 35x12.5x15 and it's full time 4WD.
 
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Just to add to this, I may grab another new matching tire and place it on the same axle as the other new one. How would this scenario work out? 30% tread on one axle, 100% on the other? Will I grenade my center diff?
 

ardentyota

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I would personally run it on the rear and not worry about it. There will be enough slip in your open diff to allow for the slight difference. Running it on the front is more likely to affect your alignment and create pulling when hard braking.
 
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I would personally run it on the rear and not worry about it. There will be enough slip in your open diff to allow for the slight difference. Running it on the front is more likely to affect your alignment and create pulling when hard braking.

My rear diff is limited slip. Does that make a difference?
 

ardentyota

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Yeah, that could definitely make a difference. Run it up front like Ebag said. Just be careful if you have to slam on your brakes.

Best thing would be to get two and use the 5th for your spare.
 

tirewater

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I think you're worrying too much about it. :) Tires don't wear evenly anyway, unless you're diligent about rotating them. In addition the difference in diameter due to uneven wear is probably less than what most people have between their tires due to differences in tire pressures.

Your LSD rear has to be able to turn corners, the differences in corner speeds of the inner and outer tire is *much* higher than any difference in rotational speed due to uneven tire wear.

Say if you're turning 360 degrees, w/ about a 5' radius... the inner wheel travels ~31'. So, assuming about a six foot wheel base, the outer wheel travles ~69 feet. So the outer wheel rotates over twice as fast as the inner one in a turn - and the LSD generally accommodates that.

I think you've got quite a bit of leeway between tire differences for a LSD...
 

CreeperSleeper

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Chris is right... And seriously, you are talking about a height difference of around 1/2". Not going to be a big deal. Now if you bolt up a 31" tall spare while running 37" tires on the other 3 sides, then you may have an issue! My bet is you would even notice the difference and neither would the truck.
 
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In addition the difference in diameter due to uneven wear is probably less than what most people have between their tires due to differences in tire pressures.

While I think you're correct about the LSD, I disagree with the difference in tire pressure.

Regardless of the tire pressure, a certain tire with a certain tread depth will have a certain number of revolutions per mile. The actual height of the tire has nothing to do with it, you could run it at 5 PSI or 50 PSI and it'll still have the same number of revolutions per mile despite being different tire heights.

But the revs per mile in this case is probably something like 598 to 600. Probably not going to hurt anything, but it wouldn't hurt to run it in part time mode to be on the safe side.
 
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I don't have the tires here to measure overall diameter. Assuming new is 35" and the ones I have are worn down 0.5". Then they would measure 34" in diameter. That's a 2.9% difference in diameter and circumference. I would say that's pretty significant on a single axle. I would also say this is worse case scenario.

What's the general rule of thumb when it comes to tire diameter difference?

BTW: I do not want to run in part time.
 

tirewater

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I think we need a test! :)

With a low pressure tire, the effective radius of the wheel is less, but the circumference remains the same. If the circumference is the same, then revolutions should be the same as you stated.

Although, in my experience *extremely* low tires will slip a little when the other tires are fully inflated, so there's a line somewhere between how low it'll go before traction reduces and the tire spins faster.

End result, I'll agree with you ebag. :)



While I think you're correct about the LSD, I disagree with the difference in tire pressure.

Regardless of the tire pressure, a certain tire with a certain tread depth will have a certain number of revolutions per mile. The actual height of the tire has nothing to do with it, you could run it at 5 PSI or 50 PSI and it'll still have the same number of revolutions per mile despite being different tire heights.

But the revs per mile in this case is probably something like 598 to 600. Probably not going to hurt anything, but it wouldn't hurt to run it in part time mode to be on the safe side.
 

CreeperSleeper

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I don't have the tires here to measure overall diameter. Assuming new is 35" and the ones I have are worn down 0.5". Then they would measure 34" in diameter. That's a 2.9% difference in diameter and circumference. I would say that's pretty significant on a single axle. I would also say this is worse case scenario.

What's the general rule of thumb when it comes to tire diameter difference?

BTW: I do not want to run in part time.

Sounds like you are convenced it is a bad idea... If that's the case, don't do it. I could care less! However, I still think you are making a mountain out of a mole hill... It's still not enough to hurt anything. :cheers:
 

ardentyota

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I don't have the tires here to measure overall diameter. Assuming new is 35" and the ones I have are worn down 0.5". Then they would measure 34" in diameter. That's a 2.9% difference in diameter and circumference. I would say that's pretty significant on a single axle. I would also say this is worse case scenario.

What's the general rule of thumb when it comes to tire diameter difference?

BTW: I do not want to run in part time.

34" Circ=106.76
35" Circ=109.9

That's 3.14 inches different in circumference. That's big to me too. It means that your differential needs to allow for 3.14 inches of different rotation each time.

34" rotations per mile= 593.48
35" rotations per mile=576.52

That's a difference of 16.96 rotations per mile. That's significant IMO.

:steer:
 
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That's a difference of 16.96 rotations per mile. That's significant IMO.

:steer:

17 tire rotations per mile is significant? Does that mean you only drive straight? THINK about what you folks are talking about. This makes ZERO difference at all. Differentials are there for a reason and that reason is to differentiate.

Also Ebag is correct about tire pressures and this test was done years ago on the 80sCOOL list in OZ. Tire pressure does not effect rolling circumference of the tire until the tread itself starts to fold at the bottom and is undriveable. On street tire rolling circumference is unchaged between 32 and 15 psi.
 
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I don't have the tires here to measure overall diameter. Assuming new is 35" and the ones I have are worn down 0.5". Then they would measure 34" in diameter. That's a 2.9% difference in diameter and circumference. I would say that's pretty significant on a single axle. I would also say this is worse case scenario.

What's the general rule of thumb when it comes to tire diameter difference?

BTW: I do not want to run in part time.

Well, to use some real numbers...

315 KM2's have 20/32" of tread depth, and a 34.8" overall diameter.

If the old tires are 30%, that means they went from 0.625" to 0.1875" of tread depth, or 0.4375" of difference.

So that means the new one is 34.8" while the used ones are 33.925" in size.

I would say that an inch is "pretty significant." At least, that's what she said. :flipoff2:


The danger, to me, is that you could travel enough that when turning the drive shafts have a large enough difference that the LSD or viscous coupler (if he has one) kicks in. Once or twice, not a big deal, but if it happens on a regular basis, not so good.
 

CreeperSleeper

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Well, to use some real numbers...

315 KM2's have 20/32" of tread depth, and a 34.8" overall diameter.

If the old tires are 30%, that means they went from 0.625" to 0.1875" of tread depth, or 0.4375" of difference.

So that means the new one is 34.8" while the used ones are 33.925" in size.

I would say that an inch is "pretty significant." At least, that's what she said. :flipoff2:

Still not enough to hurt anything... :cheers:
 

ardentyota

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HowStuffWorks.com said:
This type of LSD has all of the same components as an open differential, but it adds a spring pack and a set of clutches. Some of these have a cone clutch that is just like the synchronizers in a manual transmission.

The spring pack pushes the side gears against the clutches, which are attached to the cage. Both side gears spin with the cage when both wheels are moving at the same speed, and the clutches aren't really needed -- the only time the clutches step in is when something happens to make one wheel spin faster than the other, as in a turn. The clutches fight this behavior, wanting both wheels to go the same speed. If one wheel wants to spin faster than the other, it must first overpower the clutch. The stiffness of the springs combined with the friction of the clutch determine how much torque it takes to overpower it.

So the real question is how much faster one wheel must be spinning to engage the LSD. It may not be enough to cause damage to anything but the LSD, but it's still a risk I wouldn't take. It would be much different if it was a locked axle we're talking about- the LSD may have enough slip to be ok. This might only result in more wear on your LSD as it could constantly be fighting the slip.
 

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