The Age Old Question, How Many Pounds Of Air Shall I Put In My Tires?
Well sir, I have never been accused of being terribly smart and politicaly astue, so here I go jumping from the frying pan into the fire on a subject that some have said in the forums is a dead horse and beaten to death. But there is more to the tire pressure story than just when will it fail or blow up. Or how much pressure to ad and when.
We all know that a tire that is under inflated will use more power to make it roll than a properly inflated tire. That is because of these factors:
- The tire is presenting a larger foot print on the ground increasing the rolling resistance.
- The distance from the steel or aluminum wheel to the tread is less on the bottom than on the top meaning that the wheel is constantly climbing up hill on the tire. The cords in a radial tire under the tread are wrapped around the tire like a bulldozer tread.
- The side wall flexes more with less tire pressure meaning that the cords in the tire are constantly rubbing together creating heat from friction. The side wall cords are wrapped from one bead of the tire up and over the tread.
OK, it is true that heat will increase the air pressure inside a tire but that is proportional and you will not make up the under iflation difference with that heat build up. But that heat build up can be enough to start to make the tires various parts loose their bond with each other. That is evidenced by the sightings of long strips of tread rubber beside the highway. They lost their bond and flew off. By the way all tires, new or recaps, are the same as the tread is laid on and vulcanized to the body of the tire called the carcass. The only difference is that the new tire the tread is vulcanized to new material and on the recap it is vulcanized to an old surface that has been ground down.
Proper tire inflation for the weight being carried means that the tire will flex within the design limits established by the tire engineer. This is the weight at so many pounds per square inch of pressure listed on the side wall. The engineer says to us that this tire will not flex it’s side walls exsessively at this ratio. He is also telling us that the tread pattern on the ground will be within acceptable size limits at these limits.
Under inflation is not only a problem when considering heat build up but can effect tire wear and wet driving traction. When a tire is under inflated the center part of the tread tends to be pushed up inside of itself causing more of the load of the tire to be borne on the shoulders of the tire. This will wear the two outer edges of the tread well before the center of the tread wears. In doing this the tread pattern is distorted and the gaps in the tread squeeze together. It is the gaps in the tread that provides an escape route for water on the road surface when it is raining. Under inflated tires will hydroplane, lift off the pavement and float on the surface of the water, at lower speeds than properly inflated tires.
Yes I know, the old “lower the air pressure when driving on sand” theme and that works because on sand you are not looking for traction you are looking to get the largest footprint on the sand to float the vehicle that you can. However the old lower the air pressure on snow has the opposite effect as it closes up all the nice biting surfaces of the tire tread that provides traction.
Over inflation can be seen in tire wear as the tread tends to wear more in the middle as the tire is ballooned out. Now with all of that said you are going to have wait until next week to see what I have to say about how to figure out the correct or recommended pressure, how tire pressure effects handling, and ride. I’ll be waiting for your comments.