A way to deal with uneven tires.

[1 Lucky Texan]

Being of a practical as well as scientific nature , I decided a little research was in order. Choosing the rear driver's-side wheel of my '99 OB (with Michelin XW4 tires), I put a height reference mark very near the center. I then took measurements from my garage floor to that reference with 24, 28, and 32 psi inflation (recommended pressure and +/- 4 psi. The variation (change in radius) was about 1/16 inch for each 4 pound change, or 1/8 inch over the range of 24-32 psi. That would yield an effective circumference change of 2*pi*r, over 3/4 inch difference between 24 and 32 psi inflation (naturally, 3/8 inch for a 4 psi change). These are static determinations; a rolling tire will behave somewhat differently, but probably not by much.

 

I'm in the rust/salt belt, and my alloy rims leak at the tire bead area (yes, they've been "resealed", to no avail). The above is one reason why I check tire pressure frequently. I don't know to what degree a Subaru AWD drive train suffers due to tire size/inflation mismatches, but I'd prefer not finding out through personal experience.

 

But it's a conformable item. Not a solid disc. Reducing the air pressure just means it is shaped more like a 'track' on a military tank. The actual perimeter in contact with the road is the same - I guess!

[seattlelegacy]

All above aside, I would be more concerned with the tire company's response to your issue. I have always bought my tires from LesSchwab. They HAVE replaced all four of my tires as one was faulty. They should do the same for anyone else, and you should have had all four replaced.

 

Jordan

[Olnick]

The concept of "uneven tires" has intrigued me for a long time so I can't resist throwing in my $.02.

 

As I see it, the circumference of a radial can't change. Steel belts cannot stretch or shrink by 1/8" or 1/16" or any amount! What does change is the functional geometrics of the tire.

 

Consider the extremes: An overly inflated tire will contain the extra pressure by ballooning the sidewalls while a flat tire will collapse the sidewall and practically allow the rim to touch the ground.

 

The contact patch has varied wildly--but the circumference hasn't changed.

 

Measuring the distance from the center of the hub to the ground is not really measuring the radius because the tire is not a "perfect circle" (if it ever was!)

 

Thus the only uneven factor in a tire is the thickness of the rubber in the tread.

 

Does this make sense to anyone else or is my logic faulty?

 

Aloha.

[1 Lucky Texan]

The concept of "uneven tires" has intrigued me for a long time so I can't resist throwing in my $.02.

 

As I see it, the circumference of a radial can't change. Steel belts cannot stretch or shrink by 1/8" or 1/16" or any amount! What does change is the functional geometrics of the tire.

 

Consider the extremes: An overly inflated tire will contain the extra pressure by ballooning the sidewalls while a flat tire will collapse the sidewall and practically allow the rim to touch the ground.

 

The contact patch has varied wildly--but the circumference hasn't changed.

 

Measuring the distance from the center of the hub to the ground is not really measuring the radius because the tire is not a "perfect circle" (if it ever was!)

 

Thus the only uneven factor in a tire is the thickness of the rubber in the tread.

 

Does this make sense to anyone else or is my logic faulty?

 

Aloha.

 

If only I was that articulate!

 

A fun exercise might be to take said vehicle (3 older tires and one new one) to a large parking lot. mark the ground contact point on all 4 tires. Roll/move the car carefully staright forward for - what 20-50 revolutions and stop with one of the worn tires 'mark' on the bottom and see if the new tire's mark is at a different position.

Change the tire pressure and repeat. Who knows, we may be able to confirm that it works AND quantify the pressure change required! (I have my doubts.)

 

I still wouldn't know if it was a big practical problem or not. Just don't want a warranty claim denied i guess.

[Olnick]

Thanks, Tex!

[OB99W]

But it's a conformable item. Not a solid disc. Reducing the air pressure just means it is shaped more like a 'track' on a military tank. The actual perimeter in contact with the road is the same - I guess!

Interesting theory, but it doesn't work quite that way. The belts in a tire aren't like rigid metal bands (segmented or not), and the tread and sidewalls are even less so. Neither is a tire running on a road surface similar to a rack-and-pinion. Rather than get into a long-winded discussion of how a tire deforms and squirm/slip/scrub vary with inflation pressure, let me give you some food for thought:

http://auto.howstuffworks.com/question97.htm

http://www-nrd.nhtsa.dot.gov/vrtc/ca/tpms.htm

 

If the effective radius/circumference doesn't vary with inflation pressure, then how does one suppose the wheel-speed discrepancy needed for these flat-detection systems to function occurs?

 

By the way, I chose the post to respond to somewhat at random; my reply is meant to be a general one on the topic, not directed to anyone personally.

[Setright]

Radial tyres have a very stiff metal mesh underneath the tread. This is very reluctant to change shape or size. That's why it keeps a good grip on the road, it won't deform and lose contact patch like a cross-ply does.

 

The radial bit is actually referring to the direction of the weave/mesh that makes up the sidewalls. This is rather flexible, allowing the tyre to offer ride comfort. More air will straighten these walls out and usually give less ride comfort because the flex is reduced.

 

The stiff mesh around the perimeter doesn't alter shape or length because of that.

 

Sure, a tyre pumped to 45psi may stretch in perimeter compared to a non-inflated tyre, but the difference is minimal.

 

 

With respect to tyre-pressure sensor systems: They haven't done too well yet. Porsche did it right with the true pressure sensors on the 959 - the rims had hollow spokes to allow the sensor to live in the center and not cause balance problems!

[Olnick]

Wow! Fascinating stuff, OB99W--although I confess I didn't read every word!

 

If the engineers & scientists can base safety systems on wheel speed based management, who am I to argue with them?

 

It must work. My only question is, why?!!

 

Don't mean to impinge on your time and good nature, but care to venture even a super-simplified explanation?

 

Thanks.

[Zephyr]

If tire inflation was that big of a deal, wouldn't Subaru explicitly mention the adverse affects it may have on the transmission because of it?

 

In this world of idiot signs and cautions, I would imagine Subaru would have informed us better about this possible problem. At the very least, if they made explicit warnings about it, that would legally be an "out" for them. "Well, the damage you did to your diff is due to unequal tire pressures."

[friendly_jacek]

All you neysayers, just lift up your butts and do an experiment yourself.

 

But seriously, if you are all worried about constant cicumference due to the steel belts you are totally missing my point of measuring the ROLLING diameter and not the actual diameter of the tire. You see, the load of the car makes the tire deflect, and the amount of the deflection is proportional to the load on the axis and reversly proportional to the tire pressure. In other words my pressure manipulation changes the degree of tire deflection only.

 

C'mon guys, this is very basic physics. I know schools are bad in US but can't be that bad!

[1 Lucky Texan]

Interesting theory, but it doesn't work quite that way. The belts in a tire aren't like rigid metal bands (segmented or not), and the tread and sidewalls are even less so. Neither is a tire running on a road surface similar to a rack-and-pinion. Rather than get into a long-winded discussion of how a tire deforms and squirm/slip/scrub vary with inflation pressure, let me give you some food for thought:

http://auto.howstuffworks.com/question97.htm

http://www-nrd.nhtsa.dot.gov/vrtc/ca/tpms.htm

 

If the effective radius/circumference doesn't vary with inflation pressure, then how does one suppose the wheel-speed discrepancy needed for these flat-detection systems to function occurs?

 

By the way, I chose the post to respond to somewhat at random; my reply is meant to be a general one on the topic, not directed to anyone personally.

 

below is quoted from the second link;

>>>>Through its testing, NHTSA found that systems that use sensors to directly measure tire pressure (pressure-sensor based systems) were better able to detect underinflation, had more consistent warning thresholds, and were quicker to provide underinflation warnings than the systems that infer tire pressure from monitoring wheel speeds (wheel-speed based systems).<<<<

 

Could this be BECAUSE the actual amount of rotation IS very nearly the same?

 

And I hope I did not give the impression a FLAT tire would necessarily roll it's full circumference -I bet most of us have seen how the bead can slip or the tire can 'in-fold' on itself and 'skip' when flat.

 

fun stuff to consider

 

 

 

[OB99W]

[...]The stiff mesh around the perimeter doesn't alter shape or length because of that.

 

Sure, a tyre pumped to 45psi may stretch in perimeter compared to a non-inflated tyre, but the difference is minimal.[...]

Perhaps my usage of the word "effective" preceding "radius/circumference" was missed. In no way did I suggest that the "perimeter" varies much with inflation, just that the vehicle weight causes the tire to deform/deflect and have an effective change in radius. A "non-inflated" tire would have a significantly lower effective radius once it was bearing several hundred pounds.

 

Olnick, does that do it for you?

[forester2002s]

Juan said:

Still, you would think that somebody could come up with a fluid that wouldn't solidify at high temp, or, some sort of emergency cooler to prevent solidification

I think that you're missing the point here. That's how Subaru's AWD works; the fluid stiffens on purpose when it detects differential rotation between wheels.

[OB99W]

below is quoted from the second link;

>>>>Through its testing, NHTSA found that systems that use sensors to directly measure tire pressure (pressure-sensor based systems) were better able to detect underinflation, had more consistent warning thresholds, and were quicker to provide underinflation warnings than the systems that infer tire pressure from monitoring wheel speeds (wheel-speed based systems).<<<<

 

Could this be BECAUSE the actual amount of rotation IS very nearly the same?[...]

It's because a certain percentage of change in tire inflation pressure isn't reflected as nearly that percentage change in effective radius (and hence wheel speed). In other words, it's not as sensitive a method of detecting the problem. However, that doesn't mean that the change is trivial. Keep in mind that for purposes of the original discussion, a fraction of an inch out of about 83 inches (very roughly what my OB's tires measure in circumference) is enough to be outside Subaru's recommended maximum difference.

[1 Lucky Texan]

It's because a certain percentage of change in tire inflation pressure isn't reflected as nearly that percentage change in effective radius (and hence wheel speed). In other words, it's not as sensitive a method of detecting the problem. However, that doesn't mean that the change is trivial. Keep in mind that for purposes of the original discussion, a fraction of an inch out of about 83 inches (very roughly what my OB's tires measure in circumference) is enough to be outside Subaru's recommended maximum difference.

 

Well, I think we may be close to agreeing on something here - not sure. The 4EAT engages more RWD based on wheel speed sensors. And Subaru CLAIMS this could be triggered by as little as 1/4 inch difference in circumference. So, the them at least, it is not trivial. I personally do not know at what point it becomes a PRACTICAL concern.

 

I found this (of course, he might be wrong too);

http://64.233.179.104/search?q=cache:4i6AxlLXLt8J:www.forwardlook.net/mml-archive/msg04853.html+circumference+tire+underinflated+distance+speed+rotation&hl=en

 

Oh, and to the other posts, the manual tranny's center diff is viscous LOCKING. Not pure viscous coupled/limited. I guess the fluid heays up and LOCKS to 50/50 until the fluid cools again.

 

I dunno

[tcspeer]

Chevy wagons, what a great car those were. I wish the full size wagons were still around. I think the last full size wagon I seen was the Buick Centry in the early 90's.

I learned to drive on the mail route from Greenville Maine to Rockwood. My uncle taught all the family kids there. We might go five miles without a mailbox , then at the end of a road you wouldnhave several, then maybe one every half mile. I think your route might be similar but with more heat and less 20 below zero weather?

The route sure took a toll on the old Chevy wagons my uncle favored. You got a bit of high speed, a lot of stop and start, and wear on everything.

[OB99W]

Well, I think we may be close to agreeing on something here - not sure. The 4EAT engages more RWD based on wheel speed sensors. And Subaru CLAIMS this could be triggered by as little as 1/4 inch difference in circumference. So, the them at least, it is not trivial. I personally do not know at what point it becomes a PRACTICAL concern.

 

I found this (of course, he might be wrong too);

http://64.233.179.104/search?q=cache:4i6AxlLXLt8J:www.forwardlook.net/mml-archive/msg04853.html+circumference+tire+underinflated+distance+speed+rotation&hl=en

 

Interesting link, but I think you've made my case, not yours. First, we've got the fact that low-inflation detection can be done by monitoring wheel speed, as I've previously mentioned; the low tire has to rotate faster to make up for its lessened effective radius relative to the others. Excerpted from the link you gave: "The effective radius (shorter than the ideal radius) has nothing to do with distance travelled per revolution. It will affect the effective gear ratio, however. ... Also, the greater the difference between the effective and ideal radii, the greater the tangential speed of the tread at the top of the tire." As I said, the low wheel has to speed up, with the attendant drive train concerns.

 

Secondly, have you ever noticed that if you've got a solid axle with wheels of different diameters at each end, that it doesn't want to roll straight (in the direction perpendicular to the axle)? Well, anyone who's had a tire go down (and isn't totally oblivious to it) has experienced the car pulling to one side. Want to guess what the primary factor is that causes the pull?

 

If anybody is still unconvinced, I give up . By the way, Jacek, physics doesn't baffle all of us schooled in the US.

[Olnick]

 

Yup! By George, I've got it. Of course the tire's perimeter stays constant . . . but the effective radius changes.

 

Dang, I wish that horrid US school education wouldn't keep getting in my way!

 

Thanks OB99W

[1 Lucky Texan]

Interesting link, but I think you've made my case, not yours. First, we've got the fact that low-inflation detection can be done by monitoring wheel speed, as I've previously mentioned; the low tire has to rotate faster to make up for its lessened effective radius relative to the others. Excerpted from the link you gave: "The effective radius (shorter than the ideal radius) has nothing to do with distance travelled per revolution. It will affect the effective gear ratio, however. ... Also, the greater the difference between the effective and ideal radii, the greater the tangential speed of the tread at the top of the tire." As I said, the low wheel has to speed up, with the attendant drive train concerns.

 

Secondly, have you ever noticed that if you've got a solid axle with wheels of different diameters at each end, that it doesn't want to roll straight (in the direction perpendicular to the axle)? Well, anyone who's had a tire go down (and isn't totally oblivious to it) has experienced the car pulling to one side. Want to guess what the primary factor is that causes the pull?

 

If anybody is still unconvinced, I give up . By the way, Jacek, physics doesn't baffle all of us schooled in the US.

 

Well, it may be my poor public school education, but I don't see any agreement from my link to the idea that any distance smaller than the actual perimeter of the tire is covered in a single revolution of an 'under inflated' tire.

 

quote from my link;

>>>>

The centerline of the axle is now

lower (closer to the ground) than the ideal condition where the contact is a

line. Note that the length (circumference) of the tread hasn't changed,

only the shape has. It doesn't matter if its distorted into an ellipse,

triangle, square, or any other shape, the length of the perimeter doesn't

change. This is the distance that the tire travels in one revolution of the

axle. <<<<<

 

 

As for 'pulling' that could also be from increased drag and altered suspension geometry. Though I suspect the decreased radius is at least partly responsible - even if it isn't really a 'radius'!

 

I think we can agree to disagree for now. Though it is fascinating, it still may be of less than practical concern as regards the fairly small 1/4" number from subaru. I've read of other 4WD manufacturers suggesting 1.5"! (though, if those are larger tires the actual spread in terms of percentage may be less extreme than at first glance. still........?)

 

thanx guys

[OB99W]

Well, it may be my poor public school education, but I don't see any agreement from my link to the idea that any distance smaller than the actual perimeter of the tire is covered in a single revolution of an 'under inflated' tire. [...]

Just to be certain, I reread my previous posts on this topic; I can't find anywhere I said "that any distance smaller than the actual perimeter of the tire is covered in a single revolution of an 'under inflated' tire". It's the wheel speed that changes. I found yet another article touching on wheel speed versus inflation pressure:

http://www.findarticles.com/p/articles/mi_qa3828/is_200209/ai_n9119387

Excerpted from the above:

>>The wheel-speed-based (WSB) or indirect method of monitoring tire pressure uses the wheel signal from the ABS sensor. As the tire's inflation decreases, so, too, does its radius. The tire then has to speed up to keep up with the other tires. Since most late-model vehicles already have ABS and a body computer, the main cost of adding this system is the need for a display device on the dash to give the warnings. NHTSA estimates the manufacturing cost to be in the area of $13. In contrast, the direct-reading system is expected to add as much as $200 to the cost of a vehicle.<<

 

Since I enjoy this banter (to a point limited by my free time), I'll try once more to convince the remaining disbelievers. I'm not willing to get my CAD program running right now, so this is an exercise left to those who can picture the following in their heads, or are willing to draw it themselves. Here goes:

 

Draw a circle (tire tread). Mark its center point (axle). Draw an angle like this /\ with the apex at the circle's center point and the opposite ends at the circle's lower circumference (tread). Let's call that angle a partial rotation of the axle. Note the distance around the circumference (tread "segment") that the angle intersects (distance traveled). Now draw a second circle of the same size, with its center point vertically aligned with the first one, but placed just a bit higher than the first one (axle offset from center, as when a tire goes down). Note that the angle now intersects a smaller segment of the tread. In other words, even though the axle is going through the same angular change, the bottom of the tire doesn't move as far. Therefore, to keep up with the other 3 wheels, the low one has to speed up. I hope that was detailed enough to evoke a "picture". (Yes, I know that my drawing "exercise" doesn't take into account the flattening of the tread at the road contact patch. That changes the ratio of the distance/speed change a bit, but not nearly enough to overshadow my point.)

 

To further complicate things (believe it or not), even when a vehicle is going straight down a flat road at a constant speed, the tire tread isn't; the tread's tangential speed is always changing (accelerating and decelerating). Why? Because the effective radius is smaller at bottom of the tire than the top due to weight bearing. Interestingly, due to the ply orientation, even when radial tires are properly inflated the "squat" they take increases the tread-speed variation over stiffer-walled bias-ply tires.

 

Rereading Carl's (1 Lucky Texan) link in light of what I said might clarify things. If not, I don't fault US schools, it's not an easy topic to get your head around.

But this time I really mean it , no more .

[1 Lucky Texan]

Just to be certain, I reread my previous posts on this topic; I can't find anywhere I said "that any distance smaller than the actual perimeter of the tire is covered in a single revolution of an 'under inflated' tire". It's the wheel speed that changes. I found yet another article touching on wheel speed versus inflation pressure:

http://www.findarticles.com/p/articles/mi_qa3828/is_200209/ai_n9119387

Excerpted from the above:

>>The wheel-speed-based (WSB) or indirect method of monitoring tire pressure uses the wheel signal from the ABS sensor. As the tire's inflation decreases, so, too, does its radius. The tire then has to speed up to keep up with the other tires. Since most late-model vehicles already have ABS and a body computer, the main cost of adding this system is the need for a display device on the dash to give the warnings. NHTSA estimates the manufacturing cost to be in the area of $13. In contrast, the direct-reading system is expected to add as much as $200 to the cost of a vehicle.<<

 

Since I enjoy this banter (to a point limited by my free time), I'll try once more to convince the remaining disbelievers. I'm not willing to get my CAD program running right now, so this is an exercise left to those who can picture the following in their heads, or are willing to draw it themselves. Here goes:

 

Draw a circle (tire tread). Mark its center point (axle). Draw an angle like this /\ with the apex at the circle's center point and the opposite ends at the circle's lower circumference (tread). Let's call that angle a partial rotation of the axle. Note the distance around the circumference (tread "segment") that the angle intersects (distance traveled). Now draw a second circle of the same size, with its center point vertically aligned with the first one, but placed just a bit higher than the first one (axle offset from center, as when a tire goes down). Note that the angle now intersects a smaller segment of the tread. In other words, even though the axle is going through the same angular change, the bottom of the tire doesn't move as far. Therefore, to keep up with the other 3 wheels, the low one has to speed up. I hope that was detailed enough to evoke a "picture". (Yes, I know that my drawing "exercise" doesn't take into account the flattening of the tread at the road contact patch. That changes the ratio of the distance/speed change a bit, but not nearly enough to overshadow my point.)

 

To further complicate things (believe it or not), even when a vehicle is going straight down a flat road at a constant speed, the tire tread isn't; the tread's tangential speed is always changing (accelerating and decelerating). Why? Because the effective radius is smaller at bottom of the tire than the top due to weight bearing. Interestingly, due to the ply orientation, even when radial tires are properly inflated the "squat" they take increases the tread-speed variation over stiffer-walled bias-ply tires.

 

Rereading Carl's (1 Lucky Texan) link in light of what I said might clarify things. If not, I don't fault US schools, it's not an easy topic to get your head around.

But this time I really mean it , no more .

 

I think that's me lying on my back at the end of your posts! (please stop hitting me in the crotch!)

 

I have to admit, I'm gonna abandon my position and say you're right. Which means it is quite likely the original idea of running a 'new' tire with lower inflation could be a viable way to prevent triggering AWD engagement on hard surfaces. I was too focused on the static issues and it seems there is a dynamic way the axle speed is increased as lower pressure allows it to be closer to the ground (lower 'radius') I don't know HOW MUCH that distance would need to change to make the axle in question speed up to match the other, 'smaller tire' axles.(In my mind, it now seems that the axle speeds would be equal IF the actual distance from the ground to the center of the axles ['radius'] were all the same - regardless of tire configuration!) Still, it could be a good approach - even if some compromise may be necessary in ride/handling. The low tire is also gonna heat a little more. Still, the CONCEPT is good I now think and my position was wrong and I won't defend it. Both of Subaru's systems (all 3 - DCCD in the STI? also, I dunno how LSD could be involved either) are sensitive to axle speed and 'tire perimeter' is inconsequential to them. maybe the 4EAT would respond better to the technique? It may be that, if a new tire is put on, it could be inflated 2-3 pounds under and the other 3 inflated 2-3 pounds over to bring the AXLE SPEEDS near enough to prevent unnecessary AWD triggering. I dunno if it would be practical - but certainly possible.

 

Thanx for sticking with me OB - you're a gentleman AND a scholar!

[OB99W]

I think that's me lying on my back at the end of your posts! (please stop hitting me in the crotch!)

Sorry, didn't realize that was you! I'll stop now.

 

Thanx for sticking with me OB - you're a gentleman AND a scholar!

As are you, sir, even if we don't always agree.:cool:

[Suzam]

I think a good example would be drag racers, I was watching a show on the Speed Channel where they commented on the rise of the dragster as the wheels spin and the tire height changes due to centrifugal force. They actually take the tire into account for the final drive ratio because as the tire spins higher it acts as an "overdrive" to change the final ratio.

[1 Lucky Texan]

Here's one way to think about it. You have a platform with axles on 4 corners. (car-like)

If that platform is parallel to the ground and raised some distance above it such that the axles are all the same height, ANY non-slipping connection from the axle to the ground directly below it will rotate the axles the same speed if the platform moves straight forward mainatining the same height. One side could even be tracked with a belt front to rear like a tank and 2 wheels could be put on the other side. As long as the distance (radius?) from the axle to the ground is the same, the axle must rotate the same as the others.