Proper PSI for 10.5 33 15 on stock 5.5 rims

[Trickydicky]

Hi,
I am rebuilding my 1981 BJ42. Just put on some 33 10.5 15 on my stock 5.5 rims at about 30 psi I notice significant crowning. Does anyone have a recommended PSI for street or highway driving that would minimize this. Thanks for the help.

 

[gaijin]

Look at the sticker on your Driver's door jamb that gives you the GAWR for front and rear axles.

Add the Front GAWR and the Rear GAWR, multiply by 1.25 (25% safety margin) and divide by 4. This will give you the load capacity (with a 25% safety margin) required by each tire.

Example:

Front GAWR ........ 3595 pounds
+Rear GAWR .... +4300 pounds
= Total ................7595 pounds
Multiply by 1.25 = 9,493.75 pounds
Divide by 4 ..... = 2,373.44 pounds load capacity per tire.

Assuming you are running the 33x10.5R15LT Load Range C tire which has a maximum load capacity of 2,535 pounds @ 50 psi, then the appropriate tire pressure would be about 44 psi chosen from the following values from the standard Load & Inflation Tables:

Pressure/Load capacity(pounds)

25psi/1630
30psi/1855
35psi/2040
40psi/2260
45psi/2445
50psi/2600

The safety margin, 25%(1.25) in the example, can be anywhere from 25% to 33%(1.33) which means the correct pressure for this example could range from approx. 44psi to approx. 47psi.

 

[StaleAle]

If he is crowning at 30psi, it would probably help to lower the air pressure, not increase it as the above post appears to imply. Those are very skinny wheels for that tire, and I'm sure that is contributing to the crowning effect. Unfortunately, it's just about your only choice if running 33's on stock 15" wheels.

 

[Pacer]

While I agree the the OP's solution would be to reduce tire pressure (or possibly add a body), I'd like to point out that gaijin was using an arbitrary GAWR in his suggested formula. Simply plug in your own corner weights and chug along.

Very nice looking build by the way Trickydicky.

If he is crowning at 30psi, it would probably help to lower the air pressure, not increase it as the above post appears to imply. Those are very skinny wheels for that tire, and I'm sure that is contributing to the crowning effect. Unfortunately, it's just about your only choice if running 33's on stock 15" wheels.

 

[StaleAle]

Good point, I didn't pay attention to the weights he was using in his example, just the psi. The fact that he doesn't have the body on it yet is probably the biggest reason he sees "crowning" on the tires in the pic. Somewhere between 25-28 psi will probably be okay after he gets on the road.

 

[gaijin]

Please note that the "crowning" the OP is talking about is on the tread area of the tire which is NOT in contact with the ground. This has no relationship to the evenness of tire pressure across the contact patch. Decrease or increase tire pressure all you want and the "crowning" observed by the OP will still be there.

 

[Trickydicky]

Thanks for the information and possible solutions. I initially notice the crowning because of the dust on the tires. So at 28-30PSI the crowning on the thread was all the way around the tire, even where it is in contact with the pavement. Adding the body might help, combined with lower PSI, perhaps. I will try to experiment. Ultimately, I will look for 15 inch rims that are at least 7 inches wide. Thanks for the feedback and especially the formula!