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Tire Basics - Tire Forensics

Written by Police Fleet Manager Staff

One of the most popular sessions of any Police Fleet Expo is the Tire Basics – Tire Forensics presentation. The class is taught by the tag team of Goodyear’s Rick Wendt, backed up by engineering support from Joe Dancy, and Firestone’s T.J. Tennent.

This PFE session is not a simplistic, “how-to read tire sizes” and “this is what aspect ratio means” kind of presentation. Instead, it is a hard and focused look at police pursuit tires in actual police use: how to get the best tire wear; how to get the best fuel economy; how to prevent a “blow-out.”

 

Leading Cause of Failure

The single most important aspect of tire use and maintenance is also the simplest: proper inflation pressure. For almost all police and special-service package vehicles, that means 35 psi, cold. And “cold” means first thing in the morning, or not driven for the past two hours, or driven for less than 2 miles. Heat builds up while the tire is being driven. This heat raises the air pressure inside the tire. For example, 35 psi (cold) may be 40 psi (hot). Check the tire pressure cold.

Proper tire pressure is so important that it is the very first diagnostic step when any tire concern whatsoever is raised. Under inflation reduces the load carrying capacity of the tire. Think 450 pounds of cargo in the trunk of your Ford CVPI and tires inflated to only 30 psi, which is just 15 percent under inflated.

Under inflation increases the sidewall flex as the vehicle is being driven. That causes excessive internal heat build-up. Heat is the number one enemy of any tire. Under inflation also increases the rolling resistance of the tire, which greatly reduces fuel economy. Under inflated tires greatly affect the handling and driving dynamics of the tire. That also means braking ability. Under inflated tires produce longer stopping distance in emergency situations.

You reduce fuel economy by 33 percent running on 20 percent under inflated tires. You reduce tread life by 50 percent by running on 20 percent under inflated tire. Tires lose 1 psi of pressure every month. Tires lose 1 psi of pressure for every 10 deg F drop in ambient temperature. Today’s 17- and 18-inch police tires have such low profile sidewalls, not even a tire expert can tell if they are under inflated just by looking at them. You must use a tire gauge.

Rotate tires every 3,000 miles. When you change the engine oil, check the tire pressure. When you check the oil level, check the tire pressure. Regardless, check the tire pressure at least once a month.

Under inflation is the leading cause of tire failures. This is easy to visually identify. Heavier wear on the outside tread and lighter wear on the center tread is a clear sign of under inflation. Heavier wear in the center of the tread and lighter wear on the outside is a sign of over inflation. How about a properly inflated tire with “feathered” tread blocks? Suspension wear, alignment issues...or extremely aggressive driving.

Of course, under inflation also increases tread wear and reduces tire life. Tire performance and tire failures aside, a properly inflated tire both saves gas and delays buying replacement tires. Simply put, it lowers operating costs.

 

Nitrogen

Remember when filling tires with nitrogen was an interesting topic but there was no compelling reason to use it? It was new, and perhaps a gimmick. Opinions were divided. That was then – before Tire Pressure Monitoring Systems. This is now – every new police car has TPMS in every wheel.

Moist, humid air attacks the TPMS, and these are expensive. If the air dryer on the air compressor is broken, fix it. If you can manage to fill the tires with nitrogen, do it. The jury is in on nitrogen. It allows the tire to hold its proper inflation pressure much longer. It won’t corrode or attack the TMPS.

 

Valve Stem Caps?

Yep, these are actually very important. Tires without valve caps lose air pressure. It is not the cap that keeps pressure in. instead, it is the cap that keeps dust and debris out. Dust and debris get into the valve seat and allows the valve to leak, which lowers tire pressure.

If it is not a constant, slow valve leak, dirt and dust in the valve seat may allow the valve to “burp” as the tire is being driven. A little air leaks as the tire hits every railroad track, every pothole, every section of rough pavement, every rib on a washboard road. Again, the result is lower tire pressure—the number one cause of tire failure.

 

Tire Mounting

Checking the tire pressure in a cross-section of the fleet is the first diagnostic step related to tire failures. The second step is the proper method of tire mounting. The correct liquid lube must be used on the tire bead during installation and seating. This may seem like a small, picky issue, but it is directly related to the problem of under inflation.

A properly lubed bead will both slip under the wheel lop and fully seat during tire mounting. The use of dried-out lube, or no lube at all (water is not lube), may result in sections of the tire bead being torn or getting small cuts during mounting. The result is, of course, slow leaks and under inflation.

 

Use Only a Patch/Plug

The only proper tire repair, regardless of the puncture, is a patch / plug combo. Not a patch-only. Not a plug-only. Why not use a patch-only? After all, a patch pretty well seals the inside of the tire. The problem is that holding air is just one aspect of a proper tire repair. Keeping moisture out of the belt package is the other aspect.

If moisture gets into the belts, they may corrode. However, the moisture will certainly cause the belts to bubble, raise and separate slightly. The resulting belt slip will cause friction, heat and finally complete belt separation and tire failure.

Why not use a plug-only? A couple of reasons. First, the plug-only may not completely or entirely seal the damaged area. For example, it may seal a perfectly round hole, but not a hold that has slight tears around the puncture. These micro-cuts are hard to see, but they will allow air seepage. Just as importantly, tires that get a plug-only repair frequently do not get an inspection of the interior of the tire. The number one reason for the mandate of a patch / plug combo is to force a tire dismount and a proper tire inspection.

For the record, according to Rubber Manufacturer’s Association and the Tire Industry Association, a tire that has been run 20 percent under inflated has irreparable internal damage. It permanently ruins a tire to operate a police vehicle on tires inflated to 28 psi or less. You cannot pump it back up to 35 psi and think all is well.

How about really small punctures? Like that from a finishing nail? Won’t a patch work the best? After all, that is a very tiny hole to pull a plug through. It doesn’t matter. A patch / plug combo is the only acceptable repair. The trained tire tech may run a carbide cutter (drill bit) into the tiny puncture to allow it to accept the appropriately sized patch / plug combo. Drill out the tiny puncture hole? Make the puncture bigger, just so it can accept a patch / plug? Right.

 

Engage the Belt Package

Most people know you cannot repair a tire with a puncture in the sidewall. Some people know you cannot repair a tire with a puncture in the shoulder, i.e., the edge where the tread meets the sidewall. In general terms, the shoulder includes the outer tread blocks and extends all the way to the first groove. It is widely assumed you can make the repair as long as the puncture is on the tread, i.e., the rubber that actually meets the road. Not true! The shoulder has a great deal of flex.

As a rule, the belts cover the face of the tread from just beyond one outer groove to just beyond the other outer groove. The whole idea is to engage that belt package with the patch / plug. That way the belts will grab and hold the patch / plug in place, and the patch / plug will keep moisture out of the belt package.

The only way to be sure the plug portion of the patch / plug combo gets inserted into a belt is to limit repairs to the portion of tread inside the two outer grooves. Instead of being able to repair a tire with a puncture anywhere on the footprint, in reality, only the center section of the footprint can be repaired – just 80 percent of the footprint.

Tires that have been punctured are usually driven for some time either under inflated or actually flat. These almost flat tires can suffer internal damage to the inner liner and ply cords. This damage is not visible from the outside. The only way to determine if the tire can be repaired and placed back in service, or if it must be replaced, is the removal of the tire from the wheel and a competent visual inspection of the tire.

If properly inspected and repaired with a patch / plug combo (maximum of one repaired area), most tires retain their original speed rating. On the other hand, many agencies do not allow repaired tires on pursuit-oriented, enforcement-class vehicles. Instead, these properly repaired tires are used on admin and support vehicles. Or they are simply discarded. A high-speed tire failure may simply be not worth the risk…a new tire is about $100.


Published in Police Fleet Manager, Jul/Aug 2012

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