Let's talk tires

Ha ha.. It's common I see meet strangers here on trails and become friends. One time we met at a trail where many got stuck and it was like a unplanned strangers party. We were sharing food having a nice chat laughing. LoL.

 

The four tire contact patches must be equal to the car weight, so the pressure would be given by: P=MgAP=MgA - where MM is the car mass and AA is the total contact patch area.

 

A chalk test seems less scientific than trusting the calculations.

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What I'm saying is that I don't understand why 2 tires of the same size require different air pressure to carry the same load safely. I understand that there are differences between LT and SL tires. I just don't understand why that causes the different required pressure. The contact patches should be the same.

 

Let me know if you get a good method how to determine other than chalk test. According to the Tiresize site I have to run 26psi on my Limited which I don't think its a good idea. It will give me low tire pressure warning as well.

 

My thought would be to run the OEM pressure minimum. Have you tried 26, though?

I was referring more to the difference between SL and LT tires of the exact same size. I understand that there is a higher recommended pressure for the LT Tire. I just don't understand what reasoning there is for that. It seems like the same size tire should carry the same size load at the same pressure regardless of how many Plies it has.

 

No I didn't go down to 26. I'm little scared that it would over heat. I keep33-34ish.
And every time I take it for service Toyota puts 38ish psi and I drop to 33-34ish.

 

I agree. I can understand why a truck carrying a heavy load (with an LT tire) would require a higher tire pressure to maintain the same tire profile. I don't understand why an unloaded truck like our 4runners would require higher pressure with the same tires.

 

I’m not a smart man, i have been using the chalk test for more years than i like to admit. I guess the psi calculation works, but if i can actually see the contact patch on the road then to me that works. I see by the chalk test, or dirt test that my tires are not making full contact with the road then I adjust the psi until it does. For me, with 285/70/17 load range E Falken MT’s i run 44 psi in the front, and 34 psi in the rear. I have adjusted for load, and all is well. But hey, I’m an old guy that has gone through many vehicles, and even more tires with this method………..fuck have i been doing it wrong all these years?

 

Tires are not equally made, construction might differ, nevertheless, the variation in Psi is small, look at rigidity of the side walls, tire tread, directional, symmetrical, asymmetrical, and directional/asymmetrical all might be different. So, the air pressure in the tires multiplied by the four tire contact patches must be equal to the car weight, like shown in my equation.

In case of the 4R no matter which brand tire, 265/70/17 - 36/38 Psi for C rated is the norm - 275/70/17 E rated 38/40 Psi variation 6 ply versus 10 ply. Hence, (this has little to do what your tire is rated for; i.e. C rated 60 Psi - E rated 80 Psi only means that is the maximum Psi you should use for this tire). Maybe I'm over explaining.

 

A lot depends on the climate your living. The rule of thumb is for each 10°F (5.6 °C) decrease/increase in temperature, the tire pressure will drop/rise by 2 psi for most 4R tires.

 

According to the calculator on Simpletire.com, 38 is the appropriate psi for a 285/70/17 on our 4runners for load range C or E.

I obviously understand that there are slight differences between manufacturers and tire styles. That is not what I am confused about.

You are digging way too deep into my question. The only thing I am confused about is why the exact same size tire should require a different amount of air pressure to support the exact same load on the exact same vehicle just because it is a different load range

 

The dealer puts them at 38 because they assume they are LT tires without even looking. LT 285 / 70 / 17 Tire tires are recommended to run at 38 PSI on our 4Runners.

26 does seem awfully low to me. If I were sticking to an SL Tire I would start out at 32 and then adjust from there as needed.

As much as I like to have faith in the simple Tire calculator, I ran 35 PSI on LT 265 / 70 / 17 ko2s and had excellent wear.

So, it almost seems like it's a crapshoot. I don't blame anyone for relying on the chalk test.

Exactly! This is what I'm getting at. I wish I understood the physics behind it.

Are you suggesting that people should run lower pressure in the winter? Or are you just pointing out that as the temperature cools the tires lose pressure? I always measure mine cold as recommended.

 

Speaking of tires, I just took off the Yokohama mud terrains and put the Toyo AT3's back on and took it around the block and it is like driving on a cloud! LOL

 

I just drove on a cloud too

 

No, I'm not suggesting that at all, and no the tires don't lose pressure; the air in the tires get condensed when the temperature drops, so, it takes up less space.

 

That's what I drive 38 Psi.

Well, I answered your question, nevertheless, maybe my writing is a bit confusing. I think Discount tire explains your question at matter in a more clarifying way. https://www.discounttire.com/learn/load-range-load-index

 

Sure they do. Go fill up your tires when it's 80 degrees out and then measure them again when it's 30° out. They will have lost pressure. They will not have lost air. But, they will have lost air pressure.

Not really. You gave me a math problem. I fully understand that there is math involved in calculating tire pressure. What I am trying to figure out is why the math changes when you switch from an SL Tire to an LT Tire. If the required to contact patch changes somehow because it is an LT Tire or an SL tire then that would have been an answer.

 

My bad, I meant air, I said "air in the tires get condensed when the temperature drops, so, it takes up less space." So, stands to reason pressure drops. Nevertheless, I made a mistake writing.

No, I did not gave you a math problem, I showed the equation and explained the math behind the problem. Well, is it not common knowledge that SL & LT tires are constructed different?

 

I agree. I never understand why somebody driving a relatively lightweight vehicle that came stock with passenger tire suddenly needs to run 50 psi because it's an E load. Just because it says 50 PSI max or whatever on the sidewall doesn't mean that tire pressure is required.

If anything, I would think one would want to run LESS pressure in an E load to compensate for the stiffer sidewall.

 

So, with the chalk test, what’s the accuracy and granularity of this method?

In other words, say you’ve found the ideal tire pressure for a tire where the chalk is all rubbed off across the tire tread. How much of an increase in pressure would it take to “fail” the test? Is it as little as +/- 1PSI? 2 PSI? 5PSI?

It seems rather subjective to be very accurate.

What is your experience?

 

I was just nitpicking.

Yes, I understand that. But that doesn't explain why a 31.6" × 10.4" tire needs ~35% more air than another 31.6" × 10.4" tire to carry a given load. That's all I'm curious about.

I know LT and SL tires are different. But, why does that change the amount of pressure needed so drastically?

 

Off course not and I explain that in my posts above, It means that E load can handle more weight because of difference in structure, it doesn't say that you need to inflate your tires to that pressure. Most of the time their is a little sticker in the door of your vehicle that tells how much pressure you need, meaning according to your vehicle weight.

 

This picture explains what I'm talking about. Same size. Same vehicle. One SL, one LT. Massive increase in pressure for the LT.



I don't believe anyone has suggested that we should drive around at max inflation.

 

I think because of the structural characteristics. That's why I said from the beginning "the air pressure in the tires multiplied by the four tire contact patches must be equal to the car weight, like shown in my equation."

If you do so, you have a very good picture, if the specs are right or wrong. Maybe the factory instructions are a misprint.

 

12 Psi difference between an SL & LT tire, this not out of the extraordinary for an LT tire 44 Psi on the 4R. Like I said the structural characteristics are different. Btw. this is not max inflation.

 

I understand that. That doesn't explain why the different pressure is required.

Just saying that it's different because the tires are built differently doesn't explain anything. I would like to understand why the exact same size tire would require different pressure to support the exact same vehicle just because it is built differently. What I'm saying is I would like to understand the difference is and what causes it. I know that there are differences. I would like to understand them

I KNOW! I didn't say it was. Nobody said it was.

 

Just for the heck of it, two buildings, one in concrete the other one in plywood, both exactly the same measurements with a window at the same place. both window's break and at the same time a 150 Mph wind comes in what happens?

 

This still doesn't explain anything about why the exact same size tire requires higher pressure in an LT tire. According to your equation the pressure is based on the cars weight and the size of the tire. This is incorrect. It is based on the car's gross weight. And it is also based on the size of the tire, and the load rating. Otherwise it would be exactly the same whether it was an LT or an SL Tire

 

What does this have to do with tires at all?

 

Come on, seriously, what happens to that LT tire with specs of 44 Psi on the 4R and you put 32 Psi in it. Explain to me what does that do?