Episode 173 – Tire Pressure, Tire Contact Patch, and Armored Vehicles

In this week’s episode, we will cover a range of tire topics. Specifically tire pressure, the tire contact patch, and armored vehicles and tires.

Anyone who has been to an old Scotti School or over the last 18 years to a VDI training program, knows the importance we place on tires. The ISDA is constantly monitoring the tire industry.  We spend the time and effort researching tires because no other vehicle component is susceptible to time and the environment as the tires.

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As a Security Driver, tires are the decision-making component of the vehicle. A decision that adds to the safety and security of your principal.

Tire Pressure Basics

Maintaining correct tire pressure helps optimize tire performance and fuel economy. Proper tire inflation pressure allows Security Drivers and their passengers to experience tire (ride) comfort, durability, and performance designed to match their vehicles’ needs.

Every time you move the controls, you are shifting weight throughout the vehicle.  These shifts are all eventually felt at the tires.  The tires’ ability to bear that weight is dependent on the tire pressures.

Tires don’t support the weight of your car. It is the air pressure inside the tire that supports the car. The tire is basically a rubber container that holds the air the car rides on. Setting the correct tire pressure is required for good handling, traction, and durability.

For example, a tire that requires 32 psi but is inflated to 24 psi loses 10 percent of its handling capability.

That is another way of saying that low tire pressures decreased the amount of force drivers can place on the vehicle’s center of gravity and still maintain control.

10% may not seem like a lot. But, in a prior ISDA podcast entitled Safety, Security and the Science of G’s, we explained that a decrease in the amount of force (G’s) a driver can place on their vehicle, the less time the driver has to react to an emergency. Also, that 10% decrease means a slower speed will be needed to drive around a corner or into an off-ramp.  It also means the driver will need more distance to stop their vehicle. I  could go on and on. But I think you get the idea. The lower the tire pressure, the less time and vehicle capability you have to avoid emergencies – or even to perform simple driving tasks like braking. Low tire pressure equates to a lower safety and security level the driver is supplying to their passengers. That is unacceptable not only for a security driver but also for anyone driving.

Here is where the problem begins; you can’t just set the tire pressure and forget it. Road conditions and temperatures play a significant role in the tire’s performance and safety. For every 10 degrees Fahrenheit change in ambient temperature, your tire’s inflation pressure will change by about one psi. It will go down with lower temps and up with higher temps. The typical difference between summer and winter temperatures is about 50 degrees F. This results in a loss of about 5-psi and will sacrifice handling, traction, durability, fun, and safety.

The Proper Tire Pressure?

How can you determine the proper tire pressure? In a simple phrase, “It’s not easy.” Go by what the tire manufacturer recommends. The tire type, the load on the tire, and environmental conditions determine the proper tire pressure. As mentioned previously, the condition that is literally deadly is low tire pressure combined with a heavy load on the tire.

The amount of weight you can support with a tire depends on the air in the tire. The more air in the tire, the more weight the tire can support. The less air in the tire, the less weight the tire can support. The max load is stamped on the side of the tire. The stamping will give a tire pressure number at a particular weight. As an example, “45 psi at 1450 lbs.” The Tire Company is telling you that if you have 1450 lbs. on that tire, you must have 45 psi in the tire. You can measure the tire pressure with a tire pressure gauge, that’s easy enough, but how do we determine the weight on the tire?

The ISDA suggests a simple mathematical procedure that can compute each tire’s weight.

Go to the owner’s manual and find the maximum weight the vehicle can accept and the weight distribution front to rear. From that, you can compute the weight on each tire.

Let’s take a hypothetical car that weighs 4000 lbs. It has a weight distribution of 60/40. 60% of the 4000 lbs. is on the front tires and 40% of that weight is on the rear tires.

A quick calculation tells us that 2400 lbs. is on the front tires (4000 lbs. x 60%), and there are 1600 lbs. (4000 Lbs. X 40%) on the rear tires.

Another quick calculation will tell us that 1200 lbs. on each front tire (2400 / 2) and 800 lbs. on each back tire (1600 / 2). We have determined that the car, loaded, has 1200 lbs. on each front wheel and 800 lbs. on each back wheel through the magic of math.  Now go back to what is stamped on the side of the tire. If the stamping indicates 38 psi at 1200 lbs., you need 38 psi in those tires.

We recommend that you check tire pressure at a minimum of once a month. Once a week would be better. If you are responsible for company vehicles, supply the drivers with a tire pressure gauge. Don’t overload the cars. Know the load-carrying capacity of the vehicle, especially SUVs and pick-up trucks.

Keep in mind that those numbers represent the static weight, meaning at rest, required on each tire.

The Tire Contact Patch

As we previously mentioned, all vehicles are supported by a cushion of air contained in four flexible rubber tires. 

The following excerpt is taken from an article on the VDI website

If you could place a car on a glass floor and look at it from below, you would see four patches of rubber; most folks are surprised at the size of these four patches, depending on the vehicle – each patch is a little smaller than a hand.  These are the only points of contact between the vehicle and the road.  Each of these four small patches of rubber is known as the “contact patch.” These four patches create traction, which makes the vehicle – go – stop and turn. These four patches send the feedback back to the driver, and it is these four patches the driver has to manage. Consider them to be a source of information needed to control the vehicle.

Link – http://vehicledynamics.com/the-tire-contact-patch/

How Much Rubber Does Your Vehicle Have?

In ISDA’s opinion, if you are a training provider, you have to know the size of the contact patch your vehicle creates. Here is how VDI does it – jack up your vehicle, put ink on the bottom of the tire, and let the car down on a piece of paper (VDI uses fingerprint ink). Have someone keep their foot on the brake as you let the vehicle down; it keeps the tire from rolling, and lower the tire on the paper – the tire will leave a mark on the paper representing the tire contact patch. The paper should be outline paper divided into one-inch blocks; it makes measuring the contact patch easier.

Once done, you have a “picture” of the tire contact patch and can easily measure the size of the patch in square inches (width of the patch times the height of the patch). If you take that number and multiply it by four, that is how much rubber is on the road.

For example, the older police package Crown Vics with P225/60R16 tires with 32 PSI, there is 36.75 square inches per tire, and all four tires will give a static weight contact area of 147 square inches of rubber touching the road. The size of the patch is a function of the load on the tire. Using the science of Vehicle Dynamics, a trainer can simulate various loads on the patch for different scenarios. From the analysis, you acquire the information needed to coach the student through the scenarios.

Armored Cars and Tires

Other than the armor, tires are the most important part of an armored vehicle. Tires are one of the very few components the driver can easily modify that will produce substantial changes to the vehicle’s performance and safety. ISDA experience tells us that most problems with armored vehicles start with the tires (Specifically the front tires) not able to handle the vehicle’s additional weight but how is the weight on the tire determined -that’s the job of the armorer.

Manufacturers Responsibility

The armorer needs to weigh the vehicle to determine the weight on each tire; considering the cost of the armored vehicle, this is not asking for much.

The static weight (the car motionless) on each tire is good to know; it is only a starting point. A vehicle is a weight transfer machine. When the brakes are applied and/or the steering wheel is turned, weight is transferred side to side and/or back to front. This applies additional weight to the tires and specifically to the front tires.

Since most vehicles have more weight on the front tires than on the rear, the front tires need attention.  The additional weight on the front tires from weight transfer is a function of wheelbase and Center of Gravity height and needs to be measured. The armorer should give you the weight on the front tires static and when the load is transferred under hard braking. This requires some high school math and a $150 G Meter. Again, I don’t think it is too much to ask for what you pay for the vehicle.

Explanation of Tire Load

A tire with the designation of P195/60R15 87S – The load index (87) is the tire size’s assigned numerical value used to compare relative load carrying capabilities. In the case of our example, the 87 identifies the tire’s ability to carry approximately 1,201 pounds.

The higher the tire’s load index number, the greater its load-carrying capacity.

 Suggestions

  • Don’t ask; demand that the manufacturer measures each tire’s weight – before and after armoring.
  • Ask for the additional weight on the front tires due to weight transfer under braking.
  • The additional weight on each tire has to be addressed by the manufacturer.
  • Constant measuring of tire pressures is suggested especially in environments that have poor road conditions.
  • As a reminder – low tire pressure plus weight combined with weight above the center of gravity is a major problem.

The Problem

In an emergency maneuver – either accident avoidance or vehicle violence – the driver will be required to apply 80 to 100 % of the armored vehicle’s weight pushing on the vehicles’ CG. 

The concerns:

  1. The CG height – The higher the CG – the more weight will be transferred to the contact patch.
  2.  The weight above the CG – the majority of the armored vehicle’s weight comes from the windshield – the heavier the windshield and the greater the distance from the CG to the windshield– the more weight transferred to the contact patch.

Questions to Ask the Manufacturer

  • Question to ask on CG Height – Has the armorer computed the CG height? Although this may seem difficult, there are some simple methods that can be used to determine CG.
  •  Question on Weight Transfer – Have the spring and shock absorbers been modified to compensate for the additional weight and weight transfer? Not just add stiffer springs and more shocks – has the manufacturer computed/measured the weight transfer and thoughtfully compensated for the additional weight transfer.
  •  Question on Braking – An increase in weight also decreases brake life. The energy the brakes must absorb and dissipate is a function of the vehicle’s weight and changes in speed. What has the manufacturer done to compensate for the additional energy the brakes must absorb?

As consumers of an armored vehicle, ask the manufacturer what they have done to solve these concerns. Also, keep in mind that armor changes the vehicle characteristics, and you have to accept those changes for the protection that armor provides.


If you have an interest in going much deeper into these types of topics, I invite you to check out the International Security Driver Association’s website ISDACenter.Org and consider joining the membership to gain access to the encyclopedia of executive protection and secure transportation – The ISDA knowledge center. The knowledge shared encompasses a wide range of Executive Protection and Secure Transportation focused topics with resources, information, and metrics.

For more information on all of the member benefits, head over to https://isdacenter.org.

For Load Vs. Tire Size this is the site to go to

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35&s_kwcid=TC|21263|tire%20speed%20rating||S|b|8733580933&

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