The humble tire

Feb 13, 2013 by

By Bill Sommers

Tires. They are everywhere. All vehicles that use roads have them with anywhere from 2 to 32 tires per vehicle, depending upon the size of the vehicle.

Have you ever really examined a tire, or given any thought as to what it does? Most of us think of it as a chunk of rubber on each corner of our car that sometimes goes flat, and sometimes wears out.  It is much more than that!

The tire must hold the weight of the vehicle and it’s contents. It must translate ALL forces from the car to the road. What transmits the power from a 600-horse power Corvette to the ground? A patch of rubber about the size of the palm of your hand is all that there is to do the job. What about the forces needed to turn your 4,000-pound car through a turn? That same patch of rubber. How about the forces needed to suddenly stop your car from our 80-mph speed limits. Yep, the same little patch of rubber is all you have.

Surprisingly, even in our modern mass-production methods, tires are still made with an element of handwork. That having been said, accidents that result from a tire-manufacturing defect number only about 1% to 2%. Tire failures do occur, but almost always they are a function of a lack of maintenance.

Tires are pretty much glued together. It is called “Vulcanizing”, but it amounts to joining the tire components with a heat-activated adhesive. This means that if heat can make a tire, it can also destroy a tire.

The sidewall of a tire will reveal a lot about the tire. It tells you where the tire was made, what week of what year the tire was made, what it’s resistance to heat, friction and wear may be.

The side of a tire has three categories listed: Temperature A, B or C, Traction A, B or C, and Tread wear 100+. 100 is the lowest rating. A 500 rated tire will wear about 5X longer than a 100 rating

When shopping for a tire, examine the sidewall and opt for an “A” if you can get it, “B” is O.K., but “C” is the bare minimum rating and should be avoided.

Near the rim there will be “DOT” and some numbers. These numbers tell you where the tire is made, and the last numbers (there are 4 numbers) tell you the week and year of manufacture. A number of 2611 means the tire was made in the 26th week of 2011. If you are going to buy a tire, buy a new one. Don’t allow the tire shop to mount a tire on your car that is over 6-months old. The same atmospheric gasses that affect a tire on the road will affect it in a warehouse.

Operating speeds also influence tire temperature, as well as the forces imposed upon the tire. The faster the tire is driven (spun on the wheel) the more the centrifugal forces are imposed upon the tire. Manufacturers have placed a code on the sidewall that tells one how fast a tire can be driven without being shredded by heat and forces. An examination of the sidewall will show that Markings such as “M” (81-mph), “N” (87-mph), “P” (93-mph), “Q” (99-mph), “R” (106-mph), “S” (112-mph), “T” (118-mph), “U” (124-mph), “H” (130-mph). “H” is the most common speed rating. Also seen are the Specialty tires “V” (VR) a 149-mph tire, “W” (ZR) a 168-mph tire, and “Y” (ZR) a 186-mph tire.

Have you ever seen a tire shredded or just the tread belt on the highway? Ever wonder why? Remember that tires are glued together using heat? Heat is a tires worst enemy.

Typically when you see a tire exploded all over the road it is evidence of CBU, or Casing Break Up. If tire pressure is not constantly checked and maintained, the tire pressure will drop through molecular migration, or worse, through a puncture. When the sidewall of the tire begins to flex more than it is supposed to, the sidewall breaks apart (the glue comes unglued). Sometimes the entire tire comes apart, and sometimes the tread separates.

“A modern tire can be run about a mile or two in a deflated condition before it degenerates into a shapeless tangle of cords and rubber ” (Investigator’s Guide to Tire Failure, p.78-8, R.L. Grogan, University of North Florida, publisher).

Tires should be matched for the vehicle they are used upon, the load they are intended to carry, and the speed at which they are to be operated. All of these factors influence tire break up. Assuming the tire and the operating conditions are matched, it now falls upon us, the operator, to make sure it is properly maintained.

Here in Utah we must also look for another bit of information: M+S, MS, or M&S. This means the tire is rated to be operated in snow and mud.

One thing we must always be aware of is the friction or traction available. While we have all heard about Hydroplaning, how many know of “Viscous Hydroplaning” or what hydroplaning really is. Simply put, a tire, when operated on a wet roadway, will lift off the road and be carried on a film of water at some speed. Tire pressure is critical in establishing the hydroplaning speed. Walter B. Horne, a NASA researcher established that the tire would hydroplane at a speed equal to the square root of the tire pressure multiplied by 10.34 (Horne’s Equation). A tire with good tread will begin to hydroplane at 51-mph with a tire pressure of 25 p.s.i. Thirty p.s.i. produces a hydroplane speed of 56-mph. Thirty five p.s.i. finds us with a 61-mph hydroplane speed. What about semis? Do they hydroplane? Consider that most semi tire pressures are about 110 p.s.i.: we see a hydroplane speed for semis of about 108-mph.

Viscous hydroplaning is a different matter. When a rainfall begins, the detritus on the roadway (oil, ground tire particles, dirt, etc.) lifts off of the road. This produces a slippery film, especially at intersections, that is unseen, and more dangerous than snow or ice. Generally these deposits are worse at stop signs or stop signals as the vehicles stopped there have an opportunity to drip onto the road. A person approaching a stop sign or stop signal in a fresh rain should be very careful as just when one needs a good friction surface to stop or turn, it is not there, and the danger cannot be seen. Tire type and condition have little effect on this type of roadway characteristic.

One other item: if you can afford to buy only a pair of tires, place them on the rear of the vehicle. The front can be steered, the back cannot. Tests by Dunlop show that sudden air loss (they used to call it a blow-out) on the front will not stop the car from being steered. A sudden loss of air on the rear, however, will result in a spin. Cars can be driven quite a distance without loss of control or even knowing the tire is low or flat with a flat on the rear, as the suspensions of cars today are so good. The minute the car starts to turn or make a lane-change, however, the car will spin, and a loss of control will result.

About the author:

Bill Sommers is a retired Peace Officer, an Iron County, Utah Sheriff’s volunteer, and an Accredited Traffic Accident Reconstructionist.

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