Tire Pressure
Tuning pressure for street use:
Tuning pressure for street use is not too difficult of a task. Learning to adjust your tire pressures for the different terrain you wheel on is a great tool to have. The correct pressure on the street is required as it ensures long tread life, decent ride quality, and predicable handling. Pressure at either extremes on the street will cause issues. Pressure too high and you'll wear the center of the tread off the tires very quickly and this will also produce a jarring ride. Pressure too low and the tires will build up heat, this will kill the tire quickly and cause adverse and unpredictable handling. Many people look at the rated pressure on the side of the tire and assume that is the running pressure, ITS NOT. The pressure listed on the sidewall is usually maximum pressure. Do not exceed the max pressure. The max pressure is directly connected to the maximum load if your tires are rated for 2,000 lbs each. So for an example you have a set of tires that read max pres 35 psi and max load 2000 lbs. If your vehicle weights 8000 lbs loaded you may well need to put 35 psi in the tires, but hopefully that does not apply to you (if it does you need to evaluate what you are bringing with you). Inside the door jam of most vehicles you will find a label that has a recommended tire pressure, that's great with the stock wheels and tires but once they have been modified those numbers are useless.
Here is a common procedure for finding the correct street pressure for your tires.
Find and empty parking lot.
Inflate your tires to the max pressure listed on the sidewall.
Mark the tires across the tread with a piece of chalk or crayon whatever really.
Drive straight forward several feet, be sure not to turn at all.
Look at the mark across the tread and see which part of mark has worn off.
If the mark has worn off at the center but not at the edges you have too much pressure (duh i already told you max pressure would be too much). Drop the pressure and repeat steps 2-5 until the desired pattern is achieved. Be careful too low of a pressure will rub the mark so always start with too much and work your way down. Loading will play a large roll in your quest for the perfect tire pressure. Normally the front will be slightly more loaded than the rear as a result of the positioning of the engine, likewise a fully loaded rig ready for 2 weeks in the woods may balance that out and require more pressure in the rear then normal. Atmospheric pressure and temperature play roles too. Changes between the seasons can effect optimum tire pressure. A 10 degree drop in temp will drop your cold tire pressure by 1 psi. Most tires will lose about 1 psi per month due to porosity of the rubber compound in the tires and slight leaks at the bead. Also altitude makes a difference pressure will increase slightly with a gain of altitude due to the reduced atmospheric pressure. Final determination of proper tire pressure is seat of the pants, go test drive it and play around for a while.
** If this method will not achieve the correct pattern you have selected the incorrect tire width/ wheel width combination.**
Tuning Pressure for Trail:
Tuning pressure for trail use is a little more difficult of a task. Almost every experienced offroader knows to reduce tire pressure as soon as thy reach the trail. Airing down will create traction on many surfaces by increasing contact patch (the amount of the tires tread in contact with the ground) also by allowing the tire to conform to irregularities, usually the trail isn't perfectly flat. Also consider that while you air down you decrease the ground pressure (pounds force per square inch across the contact patch) and your ground clearance, so don't over do it. From my experience what i have seen on the trails is a pressure between 5-15 psi depending on the tire/wheel combination and vehicle weight. Tuning for the trail is more experience than anything else there are just too many variables at play to give a definitive number, have fun experimenting though!