How is this possible?

[ScaldedDog]

The only way the circumference will change with air pressure is if you put so much in that you gain back some of the crown that is normally lost when the tire contacts the ground. Handling will be affected.

Also, the average 35 has a rolling diameter of 33.75", while the rolling diameter of the average 315 is 33.5. (This is due to the aforementioned crown under no load at max pressure, which ends up adding about 0.5" to the radius, and 2x that number is what manufacturers advertise as a tire's size.) The exact rolling diameter of a new tire can be calculated from the revolutions/mile number provided by the manufacturer, and their new tread depth can be used to calculate the difference between that and your worn ones. Dave's numbers are likely close enough, but the math is easy if you choose to do it.

Lastly, some tire shops will shave down a new tire to match old ones. This is more common on street tires for sports cars, but they might do it for your ATs.

Mark

 

[Corbet]

View: https://youtu.be/K4AlRtexS5E?si=OuUTLAwkzj7DKrR3

 

[rushthezeppelin]

View: https://youtu.be/K4AlRtexS5E?si=OuUTLAwkzj7DKrR3

Dude that thing sounds downright mean with that turbo spooling!

 

[Hulk]

Corbet, your truck sounds bad ass but that doesn't look like Durango.

 

[Corbet]

mine is lacking about 14psi from that one. No burn outs. Maybe if I pulled the front drive shaft.

 

[LARGEONE]

Interesting to hear all of the crazy stories of items found in tires! I was hoping this thread would lead to some of that since it makes me feel better.

I am definitely interested in whether the tread difference really equates to a difference in circumference with the tires at different air pressures. I think that it has to....just thinking about airing down to get into my garage But maybe a science experiment is in my future.

 

[DaveInDenver]

Interesting to hear all of the crazy stories of items found in tires! I was hoping this thread would lead to some of that since it makes me feel better.

I am definitely interested in whether the tread difference really equates to a difference in circumference with the tires at different air pressures. I think that it has to....just thinking about airing down to get into my garage But maybe a science experiment is in my future.

The side wall is bulging more. Grab a coffee and stare at your tires (OK, maybe this is only something I'd do). But the tire isn't changing length, just shape, around the circumference. But it's also not round where rubber meets the road. That's the whole point of airing down, to increase contact patch. You can't change how wide the tread is but you can change how much is touching in length. The sidewall changing shape is what allows it.

Here's a graphic showing how a tire is made. It's (at least) a two layer steel and fiberglass mesh doughnut with the rubber serving to contain the air and provide a frictional surface.

This is why they're called "belted". It's because decades ago as vehicles improved and speeds increased tire would expand and pull off the rim. Centripetal force FTW! So they literally wrapped them in a circumferential belt just like those of us getting old need to squeeze our guts into slacks. Better tires got "steel belted" instead of fiberglass or poly but all tires have them.

Another thing that might help with the mental exercise. If the tire could change it's circumference it would be impossible to balance them. The part on the road would be different than the unloaded top part and that would seriously throw the center of mass off.

 

[LARGEONE]

Ahh...now I've got it. So basically I am helping NOTHING by letting some air out of the tire with more tread depth. I just measured, and the tread depth on the "mostly unused" tire is 17/32s, and the depth on the used tire on the other side is 13/32s.

 

[wesintl]

acceptable tire depth on 2 different tires is 2-6/32. That's what an open diff is for. Let it eat.

 

[LARGEONE]

acceptable tire depth on 2 different tires is 2-6/32. That's what an open diff is for. Let it eat.

And this is coming from a guy who has never seen a tire he wouldn't run on his truck :0 haha! "that crack doesn't look like it's all the way through...GTG!"

 

[wesintl]

Does it hold AIR? GTG! I once ran 36" bias swampers with a rope plug.

 

[ScaldedDog]

An eighth of an inch difference? Oh, yeah, run it.

WRT Dave's great explanation above, lay a ruler across the top of your tire. Though most look flat, you'll see that the average 12" wide tire has about a half inch crown. To make a meaningful difference in circumference with air pressure you have to add enough to gain back some of this crown at the contact patch. Not impossible, but the unintended consequences can be high.

Great story, BTW.

My wife drove through a bunch of sheet metal on I25 the other day and sliced the sidewall of one of her run flat tires. She was able to drive to the dealer and $450 later was back on the road.

Mark

 

[DaveInDenver]

I cut a sidewall on a tire like Mark shows driving on the shale and volcanic tuff road from St. George to Toroweap on the north rim of the Grand Canyon. That shale especially is like big arrow heads, knife-sharp arrow heads. I was able to get to the Discount in Flagstaff. They "found" a small (I'd be darned if I could see it, wink-wink) bubble defect in the other tire on the axle so that I was able to drive out that day with two brand new BFG ATs on the rear axle of Imelda thanks to the road hazard certificates.

 

[subzali]

We once did a 2-week 2000 mile road trip to the Pacific Northwest. Got a flat 2 miles from home at about 11pm as we were pushing to make it back home. Had to unload the whole back of the truck to get the jack out to change it. Fork in the tire.