[moorecomp]

All,

While looking for hard evidence in regards to the question of over/hyper inflation of tires, I came across a technical report from the National Highway Traffic Safety Administration. Report number DOT809297 titled "An Evaluation of Existing Tire Pressure Monitoring Systems". Does'nt sound as if it would say much about overinflation, but they evaluated TPMS's based on direct pressure and also indirect systems. The indirect systems detect a low tire based on the decreased rolling radius (circumference) of a tire that has lower pressure. Twice in this report (Section 2 - Objectives - page 2, paragraph 1 and Section 3 - Systems Tested - page 27, paragraph 3) it was stated "It should be noted that though tire over-inflation leads to accelerated tire wear (in the center of the tread pattern), it is not known to be a significant safety issue." Later in the report, when talking about Wheel Speed Based systems, Section 9 - Discussion - page 102, "This can be attributed to the fact that the rolling radius of modern, standard- profile radial tires will only change about 1mm when the tire pressure drops from 2.1 to 1.4 bars (30 to 20 psi) (See Figure 9.1)." The figure 9.1 shows a straight line correlation between pressure and rolling radius, so it would correspond that a 1mm increase would be seen with a 10psi increase in pressure. 1 freaking mm increase in circumference! Any center tread wear would be hardly noticable! It's not like we are talking about would happen to a bias ply tire.

As far as the person who wants to know why race cars have wide tires, no one has mentioned that tires have different compounds. Racing tires are sticky. They have a higher coefficient of friction than street tires. Dragsters do a burn out to get them up to temp and make sure they are as sticky as they can be. The only problem with that is that sticky means soft and they wear much quicker. A narrow, soft tire would wear out very quickly, so to make them last as long as practical, they must have more material, a larger surface area to spread the load and heat. It is a balance between traction and durability. Think about different street tires and the wear ratings. Some tire are rated for 45000 miles, and some are rated for 65000 miles. The higher rated tires are a harder compound, and wear longer, but have to sacrifice traction. High performance street tires are not the best choice for us hypermilers, even if you overinflate them.

As for inflation pressures, car manufacturers and tire manufacturers have the same balancing act, comfort, durability, safety, performance, even asthetics. Is putting more air in a tire any different than removing weight from your car and running the tire at the "normal" pressure? Both reduce the rolling resistance. Running a tire 10% over the sidewall rating is a compromise that I choose, I don't load my vehicle to max, I know I "may" have sacrificed some resistance to road hazard damage or reduced handling. I drive them like I would if the road was wet all the time. I don't follow close and look ahead. I live in Northern Michigan and we have enough practice in the winter!

There is a big difference in simply putting air in a tire until it explodes, and putting a tire on a car, overinflating it and loading at or beyond its rated capacity, and then running until it explodes. I see it as long as we stay well below the load rating, we can safely run elevated pressures. Tires that are overinflated generate less heat due to sidewall flex, and heat is the number one killer of tires.

Lastly, from the report - "Alerting a driver to the condition of overinflation may also be beneficial. Overinflation causes excessive wear to the center of the tread pattern and shorter tire life. This condition is not known to be dangerous; however, avoiding overinflation and ensuring expected tire life would increase consumer satisfaction. Since overinflation is not known to present the same danger that underinflation does, a different visual display should be used to indicate overinflation."