Got a tire pressure question...... - JeepForum.com
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post #1 of 14 Old 07-24-2021, 04:39 PM Thread Starter
Rapidrob
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Got a tire pressure question......

I was given last year a 1998 GCL 4.0 with brand new Pathfinder All Terrain tires.
They are 30x9.50R15LT's 6-ply...load range "C"
The tires say to inflate to 50 PSI
The door sticker says to 36 PSI, which I have them.
The ride over bumps is not fun. The rear end wants to go its own direction. The ride is very firm over any bumps or deep cracks.The shocks and bushings are new.

I realize 6-ply is not going the be smooth ride,but the Jeep bouncing over bumps and going its own direction is not safe at Free-way speeds.
On the way home I hit a heavy rain squall. The Jeep handled just fine at 65 MPH on the wet road...and the tires were well soaked and went over the bumps/seams with no issuers now they were "softer" from being wet.

It is not the suspension or the wheel alignment, ( it tracks dead bolts straight with hands off of the wheel) it is the tires being very stiff.
Should I lower the PSI? if so to what PSI?
With the tires at 36 PSI,the foot print on the pavement is not much compared to the tire width.
I value your first hand experience.


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Last edited by Rapidrob; 07-24-2021 at 05:02 PM.
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post #2 of 14 Old 07-24-2021, 04:46 PM
HighLonesome
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well I6 is a bit lighter than 5.2, so try 30lbs all around. Maybe go to 32 in the front. Just a starting point.
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post #3 of 14 Old 07-24-2021, 04:48 PM
PolkaPower
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Got a tire pressure question......

Just lower then a few pounds at a time until it feels better. You aren't going to get that nice ride with stiff tires. I only use C load. Been down that road before.
What are your shocks like? Old?


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post #4 of 14 Old 07-24-2021, 05:03 PM Thread Starter
Rapidrob
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Brand new shocks and bushings. Nothing fancy but designed for this Jeep.

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post #5 of 14 Old 07-24-2021, 05:16 PM
GraniteState
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Check out “tire chalk test” on YouTube, that is the way.
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post #6 of 14 Old 07-24-2021, 05:55 PM
PolkaPower
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Quote:
Originally Posted by GraniteState View Post
Check out “tire chalk test” on YouTube, that is the way.

To find correct tire pressure yes but he's trying for more comfort, not correct tire pressure.


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post #7 of 14 Old 07-24-2021, 06:34 PM
riiz
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The PSI on the tires is the maximum pressure, not the recommended pressure. Use the door label and reduce from there.
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post #8 of 14 Old 07-25-2021, 07:36 AM
yjsaabman
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I'm running E load range and I run them around 32 front, 30 rear like was mentioned above and they aren't too bad. I used to run the C load range tires on my 4 cylinder YJ around 30 psi all around.

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post #9 of 14 Old 07-25-2021, 07:54 AM Thread Starter
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Thank you for the replies. I will drop the pressure down and see how the Jeep handles.

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post #10 of 14 Old 07-25-2021, 11:57 AM
RedRiverT
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Hopefully you can find a happy medium by changing pressure. Keep in mind, your door sticker says 36psi, but it also says P225/75/R15, or something slightly larger if it had an upgraded suspension package new. (Maybe Upcountry or whatever would have had 16" wheels.)

You have more sidewall height now, a little under 1", and a heavier built tire since it is a Light Truck tire. It should have a recommended pressure range listed on the side, not just a maximum pressure. Your trade-off for low pressure and softer ride is lower gas mileage due to rolling resistance and possibly less life from the tires than what they advertise, 50,000 miles or whatever. I would opt for the softer ride myself. The gearhead-geek way to do it absolutely to the T would be to change springs and/or shocks to match the tire you want to run.
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post #11 of 14 Old 07-25-2021, 06:12 PM
Mr. Puddles
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Also remember that the door sticker is a COLD tire temp. Industry average is 1.5-2miles of driving to reach HOT tire temp. The difference is about 3 psi on standard air. So if you are going for that 36 psi after you have driven to the air pump you'd set to 39 psi. Just a little bit more info to keep in mind.

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post #12 of 14 Old 07-25-2021, 06:26 PM
CJ7-Tim
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28 - 32 psi.

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not to overthrow the Constitution but to overthrow the men who would pervert the Constitution” Abraham Lincoln, 1859.
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post #13 of 14 Old 07-25-2021, 08:49 PM Thread Starter
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I dropped the front tires down to 32 PSI and the rear to 30. The handling is much,much better now. The ride is still on the stiff side by smoother than the 36 PSI ride that also allowed the rear end to brake loose on road bumps at 65 MPH.
I have not noticed the rear end wanting to break free as before.
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post #14 of 14 Old 08-21-2021, 11:33 AM
Delta0
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Tyres comes with something called "Static Load Radius."

This tell you the height the centre of the wheel should be above a smooth road surface.
Your tyres probably have a static loaded radius of 322mm

If you measure the distance across your wheel, it's probably about 380mm
If it is 380mm, then its radius is 190mm.

Take 190mm from 322mm and you are left with 132mm
If my educated guesses are right, a good pressure for you tyre will be the pressure that gets the rim132mm from a smooth level road surface.

Up to you to measure your wheel and to find the exact distance across your wheel.

That SLR bases sum ahs given me good handling on all but one car.

Up to you how much you increase pressure to get the handling you want.

Tyre pressure are very much seat of the pants jobs.
The SLR method gives you a starting point.
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