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The Doctor's Archives

Inflation . Tire Application . Tire Facts . Tire Maintenance
Tire Performance . Tire Specs . Tire Wear . Unidirectional Treads

Dear Tire Doctor,

On a medium duty tire, such as a 11R22.5 that is speed rated (PR),
what would be the speed rating if the tire has a nail hole repaired
or it is retreaded?
view reply >>

I have a 1972 MACK Log Truck DM 800 with 11.00 -24 tube type bias tires, load range G. These tires are mounted on the older style split rims. The truck has Dayton wheels. My problem is that I can't find rims and radial tubeless tires to fit the truck. Do you have any suggestions for a source of tires and rims? I run this truck in my timber business. view reply >>

What is the difference in construction of a 14-ply tire and
a 16-ply tire? And why is the air pressure so much different if
the tire is made stronger?
view reply >>

We need advice on preventing tire fires in commercial vehicles.
While I understand it has to do with maintaining proper pressure,
pre-tripping the unit to make sure brakes are releasing properly,
not overloaded, I'm not able to find a definitive item dedicated
to preventing tire fires/what to do if they happen. 

Do you have thoughts? view reply >>

I am looking at the differences in rolling resistance between driven and undriven tires.

Obviously, there are factors that are more significant (tire design and load), but I was wondering what the difference was between a driven and an undriven tire as an approximate rule of thumb. view reply >>

I have a customer who wants to know how many pounds of steel are in an average over the road truck tire. view reply >>


Dear Tire Doctor,

On a medium duty tire, such as a 11R22.5 that is speed rated (PR), what would be the speed rating if the tire has a nail hole repaired or it is retreaded.

Thanks, Jeffrey

^ back to top

Dear Jeffrey,

Thanks for e-mailing us with your question.

Truck tires sold for use in the U.S. generally are not "speed rated" like passenger tires.  The mark you identified, "PR", is not a speed symbol or rating but stands for ply rating, a calculated measure of tire casing strength.  Truck tires have a published "max speed," which is determined by the manufacturer. These max speeds are not affected by repairs or retreads.

This information is available from your servicing dealer. Or, you may reference the Truck Tires button on the left-hand side of this screen. Choose the tire you want from the Tire Selector. The speed rating is in the far right-hand side of the specifications chart.

Best regards, Tire Doctor


Dear Tire Doctor,

I have a 1972 MACK Log Truck DM 800 with 11.00 -24 tube type bias tires, load range G. These tires are mounted on the older style split rims. The truck has Dayton wheels. My problem is that I can't find rims and radial tubeless tires to fit the truck. Do you have any suggestions for a source of tires and rims? I run this truck in my timber business.

Thanks, Robert

^ back to top

Dear Robert,

Locating these tires/wheels is becoming more and more tough.  We recommend you contact some local dealers and get their advice on where you might look.  The more established locations should be able to help, sorry we couldn't.  If you need the names of some dealers in your area, call our Dealer Locator toll-free at 1-800-815-9793.

Good luck! Tire Doctor


Dear Tire Doctor,

What is the difference in construction of a 14-ply tire and a 16-ply tire? And why is the air pressure so much different if the tire is made stronger?

Thanks. David

^ back to top

Dear David,

Let us address the second part of your question first.

We need to remember that it is the air contained within the tire that carries the load, not the tire components. If a 16-ply tire and a 14-ply tire of the same size are run at the same PSI, they have the same load capacity.

You can increase the load capacity of a tire in two ways: (1) Increase the volume of air in the tire (increase the tire's size) for a given air pressure, or (2) increase the pressure of the air in the tire (increase PSI) for a given volume (size).

In the "old days," tires were constructed using cotton cord material.  When designers needed to add air pressure capability to increase the load a specific size tire could carry, they would simply add more plies of cotton cord. Therefore, a 16-ply tire had 16 actual plies of cotton cord, while a 14-ply tire only had 12 plies of cord. This enabled the 16-ply tire to be run at a higher PSI and thus carry more weight.

Of course tires are no longer made with cotton cord. Modern truck tires only have one casing ply of steel. This is the reason the industry prefers to use "Load Range" rather than "Ply Rating." But since old ideas die hard, ply rating is still with us today.

Now for the first part of your question regarding the differences between a 14- and 16-ply tire.  There is not a hard and fast answer to this question, since tires of different brands or pattern may use different means to enable the extra air pressure to be used, and thus gain extra load capacity.

While one tire pattern may use a cord body cable with a higher wire count in the 16-ply than in the 14-ply, another may have the same wire count but use a heavier wire. Some tires may use identical wire cables in the casing but use different rubber compounds in some components to help dissipate the increased heat generated by the higher loads of the 16-ply tire. And there are other possibilities as well. So, the only way to really answer the question is that the designers did whatever they thought was needed to a particular tire to allow it to function properly at the higher PSI and load capacity.

We hope this answers your question to your satisfaction.

Best regards, Tire Doctor


Dear Tire Doctor,

We need advice on preventing tire fires in commercial vehicles. While I understand it has to do with maintaining proper pressure, pre-tripping the unit to make sure brakes are releasing properly, not overloaded, I'm not able to find a definitive item dedicated to preventing tire fires/what to do if they happen. 

Do you have thoughts?

Thanks, Dave

^ back to top

Dear Dave,

Thank you for allowing Bridgestone to be of assistance.

Tire fires occur when an external heat source is present that results in the tire reaching combustion temperature. The most common source of this heat is malfunctioning brakes - the brake drum transmitting the heat to the tire.

It is common for the fire to not occur until the vehicle comes to a stop, since airflow over the drum and tire is no longer available to remove much of the heat.

In general, operating temperatures of radial tires can be anywhere between 'ambient plus 60ºF'  (150 -180ºF.) Under severe conditions the operating temperatures will range in the 200+ºF area. 

We know that when heat is applied gradually to a tire, the tire will go through several stages. In general, as a compound is heated, molecular activity within the compound increases. At temperatures in the 250ºF range the tire starts to lose strength and the influence of other stresses becomes greater. 

Sometimes, with time as a factor, these other stresses will cause a tire to be damaged.

During the manufacturing process, tire vulcanization occurs at 250 - 320ºF.  Depending on the factors mentioned above, tire reversion also begins around 250 degrees. Of course, during the manufacturing process, these factors are highly controlled so we can produce a high quality product. As heat is, again with time as a factor, the molecular bonds disintegrate and the tire starts to physically come apart (reversion). It is this physical destruction that leads to the "two halves" of the tire that we sometimes see on vehicles. 

We do not remember any occasion where a tire started on fire as a result of this sequence.

However if you continued to apply external heat to the tire into the "over 600ºF (> 300ºC)" range you would see the tire catch on fire. Once a tire begins to burn, it will generate enough heat to continue the combustion process.  Time and rate of heat application are factors that help determine if a tire destructs or combusts.

Always the heat source is outside the tire envelope, possibly the brake system or other sources. Usually a tire melts (reverts) before it burns.  However, once the combustion threshold is reached, the rate of time and temperature are two factors that determine how quickly the tire catches on fire. 

The best way to prevent fires that involve tires is proper brake maintenance. In certain types of commercial vehicles, such as refuse disposal trucks, Electro Magnetic Retarders  - Frictionless Braking Systems - have proved helpful.

 The best way to extinguish a fire that involves a tire is with a steady fog stream of water.

The most important factor is to eliminate the external heat source.

Portable extinguishers are commonly of little use in these situations, since they do not have the ability to remove the heat source and thus can allow re-ignition.

Best regards, Tire Doctor


Dear Tire Doctor,

I am looking at the differences in rolling resistance between driven and undriven tires.

Obviously, there are factors that are more significant (tire design and load), but I was wondering what the difference was between a driven and an undriven tire as an approximate rule of thumb.

Sincerely, Matthew

^ back to top

Dear Matthew,

Thank you for the opportunity to be of assistance.

Yes, a driven and an driven tire do have different dynamics in terms of rolling resistance and contribution to fuel economy.

This is due to the driven axle tires being subject to torque.

Bridgestone testing has found that on a standard 18-wheel tractor trailer @80,000 lb. the values of weight vs. fuel consumption are as follows

Steer axle - weight carried = 16%,
fuel consumption = 17%

Drive axles (tandem) - weight carried = 42%,
fuel consumption = 31%

Trailer axles (tandem) - weight carried = 42%,
fuel consumption = 52%

We hope this answers you question.

Best regards, Tire Doctor


Dear Tire Doctor,

I have a customer who wants to know how many pounds of steel are in an average over the road truck tire.

Thanks, Mike

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Dear Mike,

The relationship between steel and other materials in a tire obviously varies significantly based on the specific size and design.  Nevertheless, we'll do our best to respond using an 18/32" rib tire in the 295/75R22.5 size, which would have an overall weight of around 110 lb. as an example.

Total weight of 4 steel belts = 19 lb. (17%)

Total weight of body ply = 13 lb. (12%)

Total weight of bead bundles = 8 lb. (7%)

Total weight of wire reinforce/chafers = 2 lb. (2%)

Total weight of steel components = 42 lb.
(or 38% of the total tire weight)

Hope that answers your question, Mike.

Tire Doctor

The Doctor's Archives > Inflation . Tire Application . Tire Facts . Tire Maintenance
Tire Performance . Tire Specs . Tire Wear . Unidirectional Treads

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