Rubicon - Wrangler Tires

I don't have anything to back it up. Someone mentioned it to me once and being a physics major in college it made perfect sense.

The max PSI of a tire at max load gives a certain contact patch- which may or may not be ideal. It is just what you get from whatever the tire manufacturer considers the maximum 'safe' load for the tire at speed. This formula gives you the same contact patch at your own selected load.

I start with this pressure and use the chalk to test until the edges don't rub off anymore. That has always been 3-5psi above what was calculated on every vehicle and every tire I've run.

The contact patch the formula gives is always to big. I guess the manufacturers rate the max load to be as much as they safely can at the expense of tire wear. It would be nice if they published numbers for an 'ideal' contact patch so you could calculate the exact pressure for your own load every time without using chalk.

As a rule of thumb this makes sense to me. I think I'll run the math on my Cherokee and see what it comes out to. My light truck tires have a max rating that is wildly different from the door sticker and recently I was wondering which number was more nearly right.

Lee:

You should always go by the pressure on the door sticker if the tires are similar in type and size to the originals. The pressure stamped on the side of the tire is the maximum pressure for that exact tire, NOT for your vehicle. That tire can be used on many vehicle applications, some which are much heavier, and require higher pressure.

Tom

snip

NEVER load a tire to the specs shown on the sidewall. There is no way you will enjoy the ride at 80 psi because you will have nowhere near the weight limits that they suggest as the max capacity of the tire.

Take a look at ANY tire and it will have similar information on it. This information has nothing to do with real life applications. It is the MAXIMUM weight the tire can carry, and the max pressure yo can put in at that weight.

The rest of the time, put in somewhere between 25 and 30 psi, and leave it at that.

Do not over inflate the tires in a misguided attempt to improve the mileage.

I beg to differ. One should ALWAYS use the sticker on the door jamb at least for a starting point, then adkust the pressure as needed to balance the treadwear to the desired pattern. The sticker on the door jamb will provide an accurate tire pressure no matter what tires are on the car, as long as they are the right size.

Set those tires to about 28 psi, and you will be much closer to the right pressure.

I would have thunk you knew this because you have found that your 9.50s prevail where guys with 12.50s have trouble. Your 9.50s are better for almost exactly the same reasons as overinflating.

The 9.5's don't have the lift off factor though. The wide tires over inflated can just lift up off the ground due to the open space between the tread and road.

Mike

CRWLR wrote:

Thin tires will not hydroplane as easily as wide tires, and wide tires that are over inflated are narrower than wide tires that are properly or under inflated.

I think I get your point though. If the wide tires are over inflated, then there is an opportunity for some portion of the tread to be lifted on a thin film of water, setting up the conditions for hydroplaning perhaps even earlier than might otherwise happen.

Yes, I have had it happen on 'performance' 60 series tires on a Volvo GLT that would just fly with proper pressure in the tires, 200 kph+.

The wider the tire, the more the middle bulges out with over pressure, the more the side tread edges are exposed to a water or snow film lift.

Mike

CRWLR wrote:

... except of course for higher load range tires that need higher pressure due to stiffer sidewalls, particularly when noted by the tire manufacturer--presumed to be somewhat knowledgeable on their own tires in most instances.

Tire construction has a lot to do with this. e.g. a steel belted radial with several tread plies that also has the tread belts wrapped with another belt won't do this much. On the other end of the scale, a wide bias ply tire with only a couple tread belts and no wrapping will bulge like a donut. Tread design and compound has even more to do with tendency to plane under normal or overinflate conditions.

A couple good/bad examples would be the old Firestone Wide Oval that even managed to hydroplane in the Utah Desert...pretty much the poster child for a hydroplaning tire at the slightest bit of moisture. Replacing these with a set of even wider, but good tread design, steel belted Dunlop Sport D series cured the planing.

The size generally is reflective of the load range. That is, when going from a 235/65/15 (for example) to a 30/9.50/15, the size is esentially the same (assuming I picked the right numbers from my ass), but the Load Range will be different.

The higher load range tires can take more air pressure, but this does not mean they demand more air pressure.

If the door jamb sticker gave the pressures for the equivelent of a 30" tire, and one chose to use a 33" tire instead, then the door jamb information might not apply anymore. But, if the sticker defined the pressures for a metric equivelent of a 30" tire, and one opted to actually use a 30" tire as a replacement, the door jamb sticker would be reasonably close, even though the load range has changed.

I run a 32x11.50x15 at between 25 and 30 psi on the street. I run the same tires at between 5 and 10 psi on the trail.

Not really. Perhaps for a given tire make or so, but not as any general relationship. As is pretty much nothing related to a particular tire generally related to all other tires.

Again not really. Some tires with a higher load capability only need door pressure or a tad higher--since door pressure is typically too low for maximal tire wear in the first place [with the disclaimer that this is true for passenger cars and not necessarily for Jeeps]. Other high load range tires need a higher pressure due to the stiffer sidewalls of that particular construction.

Not necessarily.

In the context of the original poster, never fill any tire to the numbers printed on the sidewall.

Prezackly.

maybe your eyes were misguided and didn't see this:

NEVER load a tire to the specs shown on the sidewall? why are these specs there?

how do you know this?

nothing to do with real life applications? i still don't understand. maybe your real life and REAL life are different!

long-winded way of saying 1000psi @ 10,000lbs?

have a bad day at work? this last line is all that was necessary to convey your opinion!