Air pressure with 37s?

[Jtphoto]

22 psi would be fine if you never had to steer abruptly or corner hard. Tread stability already sucks on a 37" tire on a 17" wheel. But low pressures make it worse. Compromises, lots of compromises.

Consider that a 3500 Ram on the same tire at 50psi would have the same contact patch (tire bulge) as a 4,800lb Jeep at 25psi. That’s why we have tire inflation formulas that take vehicle weight into account.
https://www.toyotires.ca/sites/default/files/guide_application_of_load_inflation_tables_2020.pdf

Single tire weight on a 4800lbs Jeep is 1200lbs which the chart doesn’t go down that far (2240lbs) for 25psi.

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[roaniecowpony]

Consider that a 3500 Ram on the same tire at 50psi would have the same contact patch (tire bulge) as a 4,800lb Jeep at 25psi. That’s why we have tire inflation formulas that take vehicle weight into account.
https://www.toyotires.ca/sites/default/files/guide_application_of_load_inflation_tables_2020.pdf

It's interesting that they specify cold pressures. So 25 psi is more like 28 warmed up. That table provides only the load bearing limits. Appropriate pressures for a specific vehicle on a specific wheel width are other factors that deserve consideration. Compromises.

 

[Terrymo]

It's interesting that they specify cold pressures. So 25 psi is more like 28 warmed up. That table provides only the load bearing limits. Appropriate pressures for a specific vehicle on a specific wheel width are other factors that deserve consideration. Compromises.

You would think there would be an online tire pressure calculator that took into account all the parameters and gave a ballpark answer. A quick google search I got no real results. But then I would need to sctually weigh the Jeep fully loaded…like stepping on the scale after the holidays.

 

[Jtphoto]

It's interesting that they specify cold pressures. So 25 psi is more like 28 warmed up. That table provides only the load bearing limits. Appropriate pressures for a specific vehicle on a specific wheel width are other factors that deserve consideration. Compromises.

All the tire is concerned about is the pressure per vehicle weight. The heavier the vehicle the higher the pressure must be to maintain the same contact patch.
The inflation formulas use vehicle weight per tire, max tire inflation (as specified on tire) and maximum single tire load (as specified on tire). Vehicle type or wheel width doesn’t change the load rating. I agree Wheel width can change the contact per pressure. Another reason to do a chalk test.

 

[roaniecowpony]

All the tire is concerned about is the pressure per vehicle weight. The heavier the vehicle the higher the pressure must be to maintain the same contact patch.
The inflation formulas use vehicle weight per tire, max tire inflation (as specified on tire) and maximum single tire load (as specified on tire). Vehicle type or wheel width doesn’t change the load rating. I agree Wheel width can change the contact per pressure. Another reason to do a chalk test.

Fair enough. If the tire's concerns are met, it will carry the load without damaging the tire. I'm concerned with tread stability/steering response too. Less pressure equals less tread stability/steering response.

 

[Jtphoto]

You would think there would be an online tire pressure calculator that took into account all the parameters and gave a ballpark answer. A quick google search I got no real results. But then I would need to sctually weigh the Jeep fully loaded…like stepping on the scale after the holidays.

Your Jeep won’t be that far off weighted. Even at 6000lbs loaded that’s only 1,500lbs a tire. It’s still important to do the chalk test.
I usually air down when doing the chalk test untill I get to the lowest pressure that still provides the proper chalk test then decide from there what will be the mid pressure. Usually you have about 3psi variance.

 

[Terrymo]

Your Jeep won’t be that far off weighted. Even at 6000lbs loaded that’s only 1,500lbs a tire. It’s still important to do the chalk test.
I usually air down when doing the chalk test untill I get to the lowest pressure that still provides the proper chalk test then decide from there what will be the mid pressure. Usually you have about 3psi variance.

Same. I feel like a chalk test, is a close enough start for me but am open to other ideas

 

[JeepinPete]

On my D rated 37" KO2's, I found 25 psi to be about right as far as tread contact was concerned. 30 miles at 70 mph showed no indications of overheating. That being said, I did not like how they handled, too mushy. I've bumped that up to 28psi and am happy. Definitely seeing more center tread wear than the edges as a result. This set has 45k on the Jeep, another 15k on other vehicles. They will be replaced before winter.

 

[roaniecowpony]

Same. I feel like a chalk test, is a close enough start for me but am open to other ideas

Find a big vacant parking lot and do some abrupt manuevering, starting at fairly low speed. I did some "lane changes" quickly, starting at 30 mph and going up. Keep realistic expectations and chose a tire pressure that gives you the qualities you value most while keeping a balance of things like wear, contact patch, etc.

 

[C.Sco]

I run 30 psi cold (~ 32-33 PSI warm) on the street with my 37x12.50x18 Ridge Grapplers.

But they are E load rating. If I had proper C-rated tires I'd probably run a tad higher, but probably still not more than 35psi.

Fuel mileage be damned, I don't commute in it so I don't care. I don't want my teeth to rattle out.

 

[Jtphoto]

I run 30 psi cold (~ 32-33 PSI warm) on the street with my 37x12.50x18 Ridge Grapplers.

But they are E load rating. If I had proper C-rated tires I'd probably run a tad higher, but probably still not more than 35psi.

Fuel mileage be damned, I don't commute in it so I don't care. I don't want my teeth to rattle out.

Especially with E rated tires you should consider bringing that down to 25-26psi. If even just to try it. It will ride so much nicer and wear more even. I notice a big difference between 25 and 28psi.

 

[zouch]

i'm not familiar with all the off brands; but all the tires i've had on my J**ps and other vehicles have been crowned (at least at full pressure), with the exception of some of the tires i used Autocrossing. having any other tire profile means that you're coming off of your full tire contact patch as soon as you're turning which is exactly when i'd want my *most* contact patch.

it would be interesting to hear how much higher hot tire pressures are after an hour or more at speed on some of the low cold pressures people are led into with these 'chalk tests'. if you're seeing more than about a 10% rise between cold and hot pressures, the sidewalls are creating so much heat from the excessive flexing that we're increasing the likelihood of them damaging themselves. remember the Root Cause of the Firestone/Explorer debacle? by far the highest incidence was found to be sidewall failure caused by damage from low tire pressures at speed.
(sidewall damage due to heat caused by under-inflation might be less of an issue for people that never drive far enough for their tires to heat up, but they'd still have the same sub-optimal handling.)

Not all tires are crowned. Haida tires are crowned as well as Patagonia, which is indicated on the tread by orange and blue inflation (contact patch) stripes on the tread. So the chalk test should be done to pattern to those stripes rather than full tread.
All my tires other than Haida 869 have been full contact patch tires, hence chalk test has been done to that.
My full tread chalk tests provide me a warm tire pressure of 27psi at most. I rotate every 5000klm and measure tread for wear every rotation.
If you were to follow tire inflation formulas that take into account vehicle weight and tire max weight capacity etc. you would see inflation numbers for 37s in the neighborhood of 22psi cold.
Following photo of Haida 869 with blue and orange contact stripes.

 

[Nokones]

I contacted a MT tech rep and gave them all my vehicle info and got this answer. So I’m going to need to do a chalk test.
“ The correct pressure for your new Baja Boss MT 37x12.50R17LT's would be 33 psi cold for normal street driving.”

For my 23 Wrangler Rubi 2 Dr. running Load Range E 35s on Methods 106 Beadlocks with a street weight of 5400 pounds which is not my trail weight, I was told to run 35 PSI.

On the trail, my weight is 5900 pounds and I air down to 10 PSI.

 

[Nokones]

For my 23 Wrangler Rubi 2 Dr. running Load Range E 35s on Methods 106 Beadlocks with a street weight of 5400 pounds which is not my trail weight, I was told to run 35 PSI.

On the trail, my weight is 5900 pounds and I air down to 10 PSI.

The chalk method is not the most accurate way of checking tire pressures. It would be best to take several tire temps across the tread block with a probe type pyrometer. That is what we autocrossers and road racers do to see how the tire responds to various road/driver inputs so you can adjust your tire pressures or your alignment settings.

 

[Jtphoto]

i'm not familiar with all the off brands; but all the tires i've had on my J**ps and other vehicles have been crowned (at least at full pressure), with the exception of some of the tires i used Autocrossing. having any other tire profile means that you're coming off of your full tire contact patch as soon as you're turning which is exactly when i'd want my *most* contact patch.

it would be interesting to hear how much higher hot tire pressures are after an hour or more at speed on some of the low cold pressures people are led into with these 'chalk tests'. if you're seeing more than about a 10% rise between cold and hot pressures, the sidewalls are creating so much heat from the excessive flexing that we're increasing the likelihood of them damaging themselves. remember the Root Cause of the Firestone/Explorer debacle?

What do you mean by “At Full Pressure” the max pressure stated on the tire?

It’s funny that people think the PSI on the door jam is the correct tire pressure for the vehicle. It is not, it is to meet the EPA. There is absolutely no way a factory 35” tire on my XR should be 37psi same as 285/70-17 on a regular Rubicon.

Running 24psi cold will give me rough 27psi maybe 28psi on an hot day after an hour on the highway. As stated above the same tire on a 8,000lbs truck would see the same contact patch/tire bulge at 2x the psi.

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