Pressure, Stopping Distance and Causality - Part I
By: Patrick Sundby, Accident Investigator
Specializing in Low Speed and Catastrophic Crashes
Mark Studin DC, FASBE(C), DAAPM, DAAMLP
There is a lot of information regarding tires, so much so this information goes way beyond the recommendations and reviews on the megastore website. Here we will discuss, from a post-collision perspective, some vehicle specifications, standard tire information, and how tire pressure monitoring systems (TPMS) work. We will then examine how tire pressures relate to automotive collisions in helping to determine causality.
Vehicles sold in the United States have a placard in the driver’s door jamb or inner door. This placard contains some information we need to have to explore the tires including the vehicle manufacturer recommended tire size, load rating, and tire pressure. Here is an example:
(There is a second placard specifically for tires but this should be confirmed against the above placard as the second one doesn’t contain any vehicle identifying information such as a VIN. In this photograph the last six digits of the VIN have been omitted.)
The majority of modern tires have writing on the sidewall which explains the tires dimensions and other important characteristics. What does it mean? In the tag above the tire sizes for front and rear are listed. The 265 is the width, in millimeters, of the tread face. The next number, 70, is the height of the tire sidewall as a percentage of the tread face (in this case 70% of the 265). The “R” makes the tire construction a radial. Finally, the 17 is rim diameter in inches.
Notice the listed tire pressure is supposed to be cold. Tires have to sit for at least eight hours out of direct sunlight before they are considered cold enough for taking a reading. Gases expand as they are heated and the minimum cold pressure is set so the tire will be at the optimal pressure once at operating temperature; accordingly, if a tire is at or below the minimum and is at operating temperature, the pressure was even lower when the tire was cold.
Tire Pressure Monitoring System (TPMS)
The TPMS became a mandated standard after the fallout of the Ford Explorer & Firestone tire event. The federal government wanted a system which would alert drivers to “low” tire pressure(s). There are two types of systems. The first type is called “direct measurement” and it uses a sensor inside each tire which wirelessly relays the pressure. The second type is called “indirect measurement” and it uses the anti-lock brake system to determine if a tire is spinning faster than the others. A tire with less air pressure will have a smaller diameter and therefore will spin faster; the anti-lock brake system can calculate this difference.
The gap in either system comes when we examine how this system decides to warn the driver. Because the pressures in a tire can vary for a few reasons (we just discussed how temperature is one of them) the TPMS doesn’t look for a single pressure, but rather a range or minimum pressure. The parameters setup within the vehicle’s computer only illuminates the warning light if a tire’s pressure is beyond the preselected specifications.
Many studies by the federal government, independent organizations, and tire manufacturers all support substandard performance of tires where the tires are below the recommended pressure. The studies referenced at the end of this writing have three points of discussion.
71% of drivers check tire pressure less than once a month.
More than 1/3 of passenger cars surveyed had at least one tire at or below 20% of the placard.
Only 36% of vehicles tested would get a warning light at 20% or more below the placard.
The first point isn’t a surprise. The lack of regular tire pressure maintenance is part of why the federal government mandated the TPMS system. The second point is also not surprising. If the majority (71%) doesn’t regularly check tire pressure, it should be expected tires are below the recommended pressure. The final point is the one we want to focus on. We want to focus on this fact because the majority of passenger vehicle pressures are 30 PSI; 20% less is 24 PSI.
If 100 passenger vehicles were on the road, 36 of them would have at least one tire at least 20% below the placard pressure. Of the 36 vehicles, only 13 of them would have a warning light. (For the record it’s not much better for the light truck / SUV category.)
So now we know a third of the vehicles on the road have an underinflated tire and further only a third of those vehicles have a warning light. Now the question is does 6 PSI matter? Yes, it does. Testing done by Goodyear and the NHTSA both confirmed a 20% reduction in pressure results in greater stopping distances, reduction in handling, increase in blowouts, lower fuel economy, and increased tire wear.
Putting it all together
The National Highway Transportation Safety Administration (NHTSA) also routinely studies tire related collisions. One study found approximately 9% of all collisions are tire related. In 2012, out of the 5.6 million police reported collisions, 504,000 were tire related.
For simplicity, we will assume all of the collisions involved one vehicle making the total 5.6 million. If we use the percentages from the above table, over 2 million would have at least one underinflated tire, 725,000 would have the warning light. Increasing the total number of vehicles only increases the subsequent statistics.
When determining causality, there are 504,000 tire related collisions as reported above and this misunderstood and frequently overlooked fact is historically omitted when attempting to determine the culpable party. It is for this reason that maintenance that tire pressures should be determined immediately post-accident and not just focus on skid marks (although they are equally important in the equation important) as demonstrative evidence when attempting to reconstruct accidents in the quest of determining causality.
In Part 2 we will discuss how these factors influence tire performance that further gives demonstrative evidence to the accident reconstructionist, accident investigator and lawyer.