The topic for this week’s episode is a basic outline of the effects of the Vehicle Center of Gravity, Tires, and Run Flats on an Armored Vehicle.
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Before we get into the outline, here some basic definitions
The center of gravity
The center of gravity (CG) is that point on the vehicle where all the weight is balanced. It is measured from the ground to a point in the vehicle’s height and width—all forces acting on a vehicle act through the CG.
A car’s wheelbase is the distance between the centers of the front and rear wheels. It’s, therefore, a smaller number than the car’s full length, as measured from its front and rear end. The length of the vehicle’s wheelbase affects weight transfer; the shorter the wheelbase, the more weight is transferred while braking and accelerating.
A vehicle’s track, or track width, is the distance between the centerline of each of the two wheels on the same axle on any given vehicle. On most vehicles, the track width will differ between the front and rear axles since they perform different functions. The length of the vehicle’s track affects weight transfer; the shorter the track, the more weight is transferred while turning the vehicle.
There are three ways that weight transfers can occur: Acceleration. Deceleration. Steering. Hence, every time the driver accelerates – brakes – moves the steering wheel, there is weight pushing on the vehicle’s CG.
Height of the center of gravity
The height of the CG relative to the wheelbase determines weight transfer between the front and rear and rear to front. The same is true of the CG height relative to the track width determines the weight transfer side to side.
Weight above the center of gravity
In an armored vehicle, the weight above the CG is a crucial vehicle characteristic. The weight above the CG determines the vehicle’s tendency to roll over and is a significant factor in the vehicle’s emergency maneuvering capability. The more weight above the CG, the more weight will be transferred.
The Tire Contact Patch
All vehicles are supported by a cushion of air contained in four flexible rubber tires. Each of these four small patches of rubber is known as the “contact patch.” These are the only points of contact between the armored vehicle and the road. These four patches create the traction that makes the vehicle go, stop, and turn and send the feedback back to the driver. It is these four patches the driver has to manage. Consider them to be a source of information needed to control the vehicle. The contact patch is sensitive to weight – all modifications made by the armorer affect the contact patch.
Center of Gravity (CG)
Armored vehicles, in general, and armored SUVs specifically are sensitive to the vehicle’s weight in relation to the vehicle’s center of gravity. There are vehicle characteristics that an armored vehicle driver needs to understand and know how they interact. They are:
- the height of the vehicles CG
- the vehicles wheelbase and track
- the weight above the CG
- the vehicle’s weight transfer characteristics – and tires and tire pressure.
The worst-case scenario is a vehicle with a high Center of Gravity with an increase in the weight above the CG and a short wheelbase or track or worse yet, both, and tires that cannot handle the additional weight of the armor. Not only in the static position (when the vehicle is not moving) but in the condition of emergency maneuver weight transfer (when stuff happens) – and add to all this – low tire pressures.
In an emergency maneuver, either accident avoidance or vehicle violence, the driver will be required to apply 80 to 100 percent of the vehicle’s weight pushing on the vehicle’s CG. The concerns:
The CG height
The higher the CG, the more weight will be transferred to the contact patch. The weight above the CG – in an armored vehicle, the majority of the 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.
A non-armored windshield glass could weigh around 25 pounds. A windshield armored to B4 weighs between 150 – 250 pounds depending on the vehicle, while a windshield armored to B7 could weigh 500 pounds or more. This makes a significant difference in the amount of weight transferred when braking – turning or accelerating.
Armored Cars and Tires
Other than the armor, tires are the most important parts of the armored vehicle. The weight of the armor and the tire’s characteristics cannot be separated. The 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. 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.
The amount of weight the tire can support depends on the air in the tire. The more air in the tire, the more weight the tire can support. Stamped on the sidewall of all tires is the amount of air combined with the weight the tire can support. The stamping will give a tire pressure number at a particular weight. Such as a “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. Tire pressure can be measured with a tire pressure gauge, but how is the weight on the tire determined -that’s the job of the armorer.
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. As we mentioned, 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 $190 G Meter. Again, I don’t think it is too much to ask what you pay for the vehicle.
Explanation of Tire Load
Anyone who operates an armored vehicle needs to understand a tire’s limitations; it starts with learning how to read a tire. 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.
If the threat is at a point that an armored vehicle is needed, then the armored vehicle needs to have Run-Flat tires. But there is some confusion concerning the types of run-flat systems that are available.
There are two primary types of run-flat tire systems: The Self-Supporting System and The Run Flat Insert.
The Self-Supporting tire systems feature reinforced sidewall construction that will continue supporting the vehicle in the event of air loss up to the speed and distance specified by the manufacturer.
Run Flat tire insert systems employ a ring of hard rubber or another structure material that can support the vehicle’s weight in an air loss condition.
Tire blowouts are still possible with Self-Supporting tires. If the puncture occurs on the sidewall or the tire hits a large object, the tire will deflate.
There are two types of run-flat inserts – one works by preventing the tire or what’s left of the tire not to separate from the rim.
A Run Flat system supports the tire-tread, so the wheel rims do not damage the sidewalls. This system is securely mounted to the rim and has no contact with the tire during regular vehicle operation, and is only in contact with the tire when it is deflated.
The other Run-Flat systems employ a ring of hard rubber or another structure supporting the vehicle’s weight in an air loss condition. Another system is a polymer-type liner that fits into the tire. The driver can drive for a longer time, and they are much better for rough terrain.
The disadvantage is that they are not easy to install and remove. Most often, they come with a special piece of equipment that makes that task easier.
There are pictures of Run Flat Inserts included in an article from a VDI test on Run Flats
VDI conducted Run-Flat test – Just How Far Will a Run Flat Run, If a Run Flat Must Run
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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.
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