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  1. #1
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    May 2011
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    Dangers of Over Inflating Tires

    There have been many postings offering recommendations regarding increasing tire inflation to increase fuel economy. There have also been many questions about the hazards of over inflating, many inquiring about “blow out hazards.” In my opinion, most discussions on the topic of over inflation offered opinions and very little technical knowledge. I am going to offer a technical view of the topic. Increasing the blow out risk is a possibility, but the real hazards of over inflation are related to traction reduction and altered performance and handling characteristics. Traction and handling issues have a significant impact upon risk exposure.

    The key to understanding the issue of inflation pressure is to understand the adhesion patch. The adhesion patch is the surface area, or patch, of tire in contact with the road. Think of being in a deep hole that is covered by plate glass and a tire is parked on the glass. The portion of the tire tread that is pressed out flat onto the glass (road surface) is the adhesion patch.

    Imagine a trailer that carries a load sufficient to put a 1000 pound load onto that one tire on the glass. The tire is inflated to 30 psi. Now, 1000 lbs. divided by 30 pounds /sq. in. is equal to 33.33 sq. in. (the pounds cancel) of adhesion patch surface area. Now, double the load in the trailer. Does the tire pressure rise with increased load? No. The tire “squats” and increases the size of the adhesion patch. 2000 lbs. divided by 30 pounds /sq. in. is equal to 66.67 sq. in. The adhesion patch has doubled in size to support the doubling of the load.

    The adhesion patch is also changed by a change in tire pressure. Again, assuming the 1000 pound load but increasing the pressure to 50 psi results in 1000 lbs. divided by 50 pounds /sq. in. is equal to 20 sq. in. That’s a 66.7% increase in pressure resulting in a 40% decrease in adhesion patch size. Just as important as the total surface area of the adhesion patch is the shape of the adhesion patch.

    Some of us who live with seasonal snow and ice have discovered that “skinny” tires provide better snow and ice traction than “wide” tires. That means that an adhesion patch that is longer (front to back) than it is wide (side to side) provides better forward traction and stopping power on snow and ice that an adhesion patch that is wider than it is long. Increasing tire pressure decreases the length (front to back) of an adhesion patch faster that the width. In fact, it takes a massive increase in tire pressure to significantly change the width. The tire “lifts” the front and rear of the adhesion patch with increased tire pressure much more readily than it lifts the sides.

    Now, this combination of causing the adhesion patch to grow smaller at the same time that it is becoming shorter has a double impact on forward traction and stopping distance. So, as a rule of thumb, one should assume that a 10% increase in tire pressure above manufacturers’ recommendations results in a 20% decrease in forward traction and a 20% increase in stopping distance.

    There are other effects as well and those effects are not beneficial either. Radial tire designers stiffen portions of the sidewall and leave others flexible to keep the adhesion patch in contact with the road in a turn. As centrifugal force pushes sideways on the wheel the sidewalls of the tire will “roll” or “flex” to keep the adhesion patch on the road.

    In the old days, when we drove on bias-ply tires, it was not unusual to take a curve hard and fast and find oneself turning the steering wheel further than you thought you should to keep the car on the correct arc. This “over-steer” condition was a function of “tread slip angle.” Fundamentally, the sideways centrifugal force distorted the adhesion patch such that the treads deflected into an arc. The leading edge of the arc pointed out of the turn and we needed to compensate by over-steering. Push the turn too fast and the adhesion patch would break free of the road, with undesirable results consequent.

    In fact, early radial tire designs had sidewalls that were too stiff as tire designers tried to tune the tires to accommodate the soft suspensions and high roll centers typical of American cars of that era. The result was that the early radials would not “warn” the driver with an over-steer condition, it would just break free without warning. Since then, roll centers have been lowered as suspensions have become better tuned and tire designers have better balanced side wall flex characteristics.

    Over-inflating a tire alters the sidewall flex characteristics as well as the operating performance characteristics of the tire. Inflate too much and the vehicle might be described as feeling like it’s riding on bowling balls. This should be a fair warning that the tire performance characteristics are not balanced with the suspension designs. It should be expected, then, that traction, stopping distance and handling characteristics have been altered to a degree that increases the risk of losing control of the vehicle.

    The elevation of those risks increases the probability that the vehicle will not stop as expected and also increases the risk that the vehicle can become unstable in curves, especially on high speed roads and exit ramps.

    Most troubling of all is the increased vulnerability to hydro-planing to which an over inflated tire is exposed. The shorter and smaller the adhesion patch the more vulnerable a tire is to hydo-planing, as a short adhesion patch does not have as much “time” to “squeeze” water into the tread grooves. Instead, the water will ingress between the tire tread and the road surface, literally “lifting” the tire off the road. The effect is worse than driving on black ice. It is unnecessary to explain that the consequences of such an effect are unattractive.

    Do I raise the inflation of my tires beyond manufacturers’ recommendations? Yes. However, I do not over inflate more than 10%. In my FEH the recommended inflation pressure is 35 psi front and rear. That means that I inflate 110% of recommended inflation. 1.10 X 35 psi = 38.5 psi. Do I round that up to 40 psi? No. 40-35=5 and 5 divided by 35 is 14.2%, almost a 15% increase. That’s too much if I use my rule of thumb that stopping distance is increased, in percentage terms, by double the increase in air pressure, in percentage terms. Are you comfortable with a 30% increase in stopping distance? Are you comfortable with a 20% increase in stopping distance?

    There will be those who refute the risk exposure by offering that, as part of their hyper-milling routine, they “drive as if they had no brakes.” That’s fine, except in a panic stop event when a driver cuts you off by merging from a side road unexpectedly. That’s fine, except when a member of the family, who isn’t as skillful a “drive as if there were no brakes” practitioner, is driving the vehicle. That’s fine, except when the driver hits a wet patch and begins to hydro-plane. That’s fine, except when the tire breaks free without warning on a curved exit ramp, that the driver takes injudiciously fast as a component of the “driving as if there were no brakes” technique.

    In other words: don’t lie to yourself. If you are elevating risk by reducing traction, increasing stopping distance and unbalancing tire performance characteristics versus the suspension design – then you are increasing your risk. Steps taken within the driver’s control to reduce those risks do not mitigate the risk exposure fully. Keep in mind that a driver does not have full control of all of the risk conditions to which he or she is exposed. “Driving as though you have no brakes” is insufficient to mitigate all possible hazards.

    There are many steps that we can take to increase our fuel economy that do not increase our risks. Keep in mind, the driver is not the only entity exposed to those increased risks. Family members who drive or ride in the vehicle, other drivers, their passengers and pedestrians are also being put at higher risks.

    If that’s not enough to convince you then consider this: any serious accident will be investigated. If the investigating agents finds that tires are inflated more than 10% above manufacturers’ recommendations the over inflated condition will be cited as a factor contributing to loss of control or failure to stop. If your tires are inflated to 55 psi versus the manufacturer’s recommendations of 35 psi, you’re not in a good position.

    What most impressed me regarding the recommendations offered for increasing fuel economy were that many of them were based upon a technical examination of the characteristics of the Atkinson cycle engine and or approaches to more fully exploit the energy stored in the HIV batteries. When it came to discussing tire inflation, however, the technical approach seemed to be absent and recommendations were offered without a technical disclosure of the elevation of risk. Increasing fuel economy by trading off safety is, in this writer’s opinion, a fool’s economy.

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