Posted: Dec 30 2008 at 2:14pm | IP Logged

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Nice answer Boomer! here is an article I wrote a while back trying to cover several questions at once.

TIRE PRESSURE up date 07-15 - 07 not copy righted

NOTE ! All the tires on a Vehicle should be aired up cold and in the shade. If the vehicle is parked in the sun, the sunny side tires can be 5 or more pounds UNDER inflated at the same gauge reading due to the added heat.

Air pressure requirements for tires will vary as the load each carries is changed.

… There are about 14.7 lbs. of "air pressure" at SEA LEVEL so our tire air pressure gauges are set to 14.7 lbs. equals 0 lbs. Almost every thing on the earth’s surface is floating including our 4x4s. We trap more than 14.7 lbs of atmospheric pressure into soft airtight rubber "donuts" and float our 4x4s on a cushion of air. How much additional pressure does it require? The answer is what does your 4x4 weigh and how big is the foot print of all the tires on the ground? A large foot print, as in square inches, means less air pressure. As the weight of the 4x4 goes upward more air pressure is required to support the 4x4 on the same size foot print.

So, foot print area, times the air pressure = weight of 4x4. The bigger the tire (with bigger foot print) will require less air pressure (that’s pounds per square inch) at the same weight loading.

40 sq"x 4 = 160 sq" x 32 lbs/sq" = 5120 lb.

40 sq"x 4 = 160 sq" x 28 lbs/sq" = 4480 lb.

… Basically a very light weight 4x4 with large tires can air down to maybe 6 lbs. or less but a heavy 4x4 would crush the tires flat with the rims at that same pressure.

Unfortunately a lot of trucks unless loaded carry up to 65% of the weight on the front tires and this will slightly complicate the figures. Allowances should be made for weight distribution in actual practice. Most manufactures do not make allowances for weight distribution on tires. Altitude and heat are also major players in the tire pressure calculations.

… Some tires are rated at a given MAX air pressure at a given MAX load. If the MAX load rating for the tire is 1600 lbs. and the gross load you are putting on the tire is 1250 lbs. then the MAX air pressure shouldn’t be mandatory. Operating tires at below proper pressure at higher speeds will make them run hotter due to increased flexing and often leads to tire failure.

… Wheelers continually talk about tire diameter and it is important as far as clearance is concerned. Most tires are NOT usually the actual (call out) size they claim to be. It is equally or more important to think about ‘static radius’ and this is the dimension from the ground footprint to the axle (center) which is LESS than half the diameter of the tire. The ‘static radius’ increases with additional air pressure and also increases the AXLE clearance. Axle clearance is a good reason NOT to ‘air down’. As the static radius decreases with less air pressure the distance traveled per revolution decreases. As the static radius of the tire lessens so does the effective over all gear ratio which is an off roading plus. A soft tire will be less stable.

… Climbing obstacles is more difficult for vehicles with hard tires. A soft tire can form over the higher parts of an object and that means the vehicle did not have to climb as high to get over the object.

Hard tires with small foot print areas will sink deeper into sand, snow, and mud, and in some cases getting better traction. In sand it seems the tires are always trying to climb and push the sand out of the way. A larger tire footprint helps the vehicle to float higher in the sand and use less power. Large smooth foot print areas can cause hydroplaning at a lesser speed on wet roads.

1. Airing down increases the contact patch and many people think that is what increases the traction between the tire and the rock. Technically by airing down you have actually changed the friction coefficient (Mu) of the tire taken as a whole by allowing increased flex of the tread elements. The flexible tread elements can now get a better grip by molding over objects. This doesn't mean the friction coefficient between the rubber and the road has changed, since that is a physical property of the rubber.

2. Airing up. This decreases the contact patch. Sometimes touted as a means to decrease rolling resistance and improve gas mileage. But the energy savings is not due to the decreased road contact, it is due to better energy recovery as the tire goes from being round at the top to flat contact with the road. The tires side walls flex to make this work. It has to compress, extend, compress, extend many times per mile. Energy is lost in the form of heat due to internal friction. The more air in the tire, the more efficient the spring-rate. The less air, more heat is generated and heat comes from friction. Added heat can break down the rubber and eventually cause catastrophic tire failure.

3. Shear, tensile strength, other physical properties. You could put skinny tires on a Top Fuel dragster and the friction would be exactly the same as the big slicks. In theory anyway, and we'll stick with that. As long as the rubber compounds are the same, the Mu is the same, the Ff is the same. But the Top Fuel dragster will leave two distinct black streaks the length of the track. Which is the clue to the answer. Those two streaks are rubber that has been sheared off the tire.
… Contact patch area does improve when you go to big tires. Increasing the area of the shear will reduce the amount of the shear. Increasing the shear area allows you to take full advantage of the Ff that the specific tire/surface conditions will allow. Should traction still be a problem, then you have to use a softer compound to change the Mu. Using a softer compound will have lower shear strength, and it brings you are right back at the starting point.

... The chalk or liquid methods are touted as ways to calculate tire pressure for your truck. This sometimes works well for the front tires but it is a bit more difficult doing the rear tires. On the tread of the tires draw many heavy chalk lines across the running surface. Drive the truck straight checking often to see where the chalk is being removed from the tread. If the chalk is worn off the center of the tread we are supposed think there is too much pressure in the tire and should reduce pressure and test again. Conversely wear on the outer edges of the tires would mean more pressure. All this may be true. This system should work well with perfect conditions, with the perfect tires, and with the perfect alignment.

... Toe-in and camber distort the cross section of the front tires as the truck is driven and it is a contributor to tire wear. The amount of distortion is related to the air pressure in the tire. Toe-in and camber settings cannot self adjust for different compounds of rubber, varied side wall construction, or sizes of tires.

Air pressure is just one of many governing factors of tire tread wear, fatigue and fuel mileage.