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Friday, December 25, 2020

How are tires "Designed"?


How tires are designed and why is there always a compromise?

First off lets be clear on the word "design". For this post I am not talking about the exterior "look" of the tread or sidewall but about the complete structure and how the tire performs.

Passenger, LT, ST type tires would be considered "consumer " tires. the 19.5 and 22.5 size tires used on Class-A RVs are really "Commercial" tires designed for the truck market. Some may be marketed to the RV owner but in general they are intended for Commercial (truck) applications.

 When a tire is being designed for a specific vehicle manufacturer such as Ford, Chevy, Toyota, or BMW, there will be a number of tires submitted by competing tire companies all trying to deliver the best overall compromise in performance characteristics. Please note than all original equipment vehicle manufacturers have slightly different requirements but all make similar requests for performance improvements in many areas. In the future I will use the term "OE" to include these car and pickup manufacturers.

Compromise: Now is a good time to talk about some of the various trade-offs the tire design engineer is faced with when trying to meet conflicting goals and customer wants. I am sure we would all like an RV that has all the interior space and amenities of a 40’ diesel pusher but gets 25 mpg and can be driven down crowded city streets without knocking off our mirrors. Oh yes, it should also cost under $30k. Well Bunkie, that just ain’t gonna happen in real life.

The same goes for a tire that handles like an Indy tire, is as quiet as the proverbial mouse, has great off-road traction, is good for 100k miles, and costs $25. One thing few people realize is that most if not all performance characteristics are a compromise. For example: if you improve wet traction you probably hurt fuel economy unless you use a special type of rubber that costs double per pound and is more difficult to process. If you improve handling you might hurt ride and noise. When you improve noise you can significantly increase the cost of making the molds used in manufacturing. The cost of a tire mold can be as low as $10,000 and can approach $100,000 each. Depending on the production volume needs, a tire manufacturer could need 30 or more molds. The list of trade-offs goes on and on.

Tire design engineers usually will have a couple dozen tires made for 3 to 4 specifications. Each "Spec" has a number of minor variations as we never know the exact trade off the OE customer is willing to accept as usually they will only say things like I want a one step (in a 10 point scale) better steering response but will accept a 2 step loss in snow handling. But since the test for each of a dozen different parameters are subjective and depend on the seat of the pants "feel" of the evaluation engineer, we tire engineers can't be 100% certain how important each performance parameter really is as they all seem to start off as all being the "Most" important.

The competition for a tire application might start three or more years before scheduled start of delivery with two to five tire manufacturers competing for the contract, knowing that only one or two will end up being selected to actually provide tires. The costs associated with building and testing special prototype tires can run in the hundreds of thousands of dollars and are absorbed by the tire company. The only way a tire company can afford this type of activity is by landing a contract for a few hundred thousand tires so the costs can be spread out and recovered over years of tire production.

Unlike “OE”, an RV manufacturer may only need a couple thousand tires so a custom tire, designed for a specific RV would be cost prohibitive. Since the RV manufacturer won’t be trying to get custom tires, it doesn’t have staff engineers working on developing specifications for such tires. The RV company will in all likelihood either take what comes already on the cut-away chassis as used on most Class-C RVs or the bare truck chassis for Class-A vehicles. In the case of trailers, the RV company may simply specify the lowest cost tire in a given size and Load Range as load capacity is one performance feature the RV company must meet along with a delivery schedule.

For RV applications the one thing that is in the control of the manufacturer is “Reserve Load”. This is the difference between the load placed on each tire with the RV normally loaded and the load capability of the tires at specified inflation.



Friday, December 18, 2020

Can tire failure start a fire?

 The quick and short answer is: Yes it is possible but is also highly unlikely.

After reading a statement about RV fires being started by tire fires that were themselves were started by a tire failure I felt compelled to jump in with some facts.

While it is true that tires can burn and be difficult to put out once they are on fire, it is also true that it is difficult to start a tire burning.

According to Wikipedia "Tire fires are normally the result of arson or improper manipulation with open fire. Tires are not prone to self-ignition, as a tire must be heated to at least 400° Celsius (750° Fahrenheit) for a period of several minutes prior to ignition. 

Now I can almost hear some saying: "Ya but what about a tire blowout?" Well lets consider a tire failure. You are driving down the road at 50 MPH or more and there is a sudden, loud "BANG". You are startled but with luck and skill, you can pull off the road to look at the damage. Most likely you find a tire in multiple pieces along with some damage to your RV. What you do not see are pieces of burning rubber strewn along the road. If we consider the process of a tire coming apart it may be easier to understand why we don't see burning rubber when we stop because of a tire failure.

Considering the two major reasons for a tire to fail. 1. The belt separation where the belt and tread detach or become separate from the body of the tire. and 2. The sidewall fails because of excessive flexing while being driven at extreme low inflation (usually with an 75% or greater air loss).

If we look closely at the sidewall flex failure of a regular Passenger, LT or Trailer tire we will see that the body cord is usually Polyester surrounded by various rubber compounds. The process of over flexing or over bending while running at highway speeds the same time can result in the polyester overheating. This can result in the Polyester actually melting as seen in this picture of fused Poly cords.

 and in this picture


If you stopped right away they tire might look like this where we can see the "Melt line" running 360° around the tire sidewall.

 Think for a moment what you do when you cut a piece of Nylon or Polyester rope. You get out a lighter or match and fuse the end.

Polyester looses half it's strength at 300 to 350°F and melts at about 480°F. This is a far cry from the 750°F ignition point of tire rubber.

The second mode of tire failure, Belt separation

takes many hundreds or even thousands of miles to result in a tire coming apart and if the tire has a Nylon cap ply and you occasionally may see signs of some of the Nylon melting at 430°F we see that the rubber that is holding the belts together fails by "de-vulcanizing" or reverting to it's uncured state and looses almost all of its strength . This occurs at about 348°F.

Having covered the examples of why it is unlikely that a tire failure can start a fire, the question remains: Is it impossible? The answer to that is Yes it is possible to start a tire on fire but this usually occurs when the fire started with a brake or bearing failure that ignites the axle grease,  or even the brake fluid which both have an ignition temperatures of about 550°F.

Someone is probably thinking about their steel body radial on their big Class-A RV. Yes they do not have Polyester or Nylon that can melt but the rubber in those tires can still revert and lose all it's strength and still be 200°F under the ignition temperature.

A final observation: We need to remember that tires are being run 24/7 at tire plants around the world with most being "run to failure" yet we simply do not see tire test labs suffering tire fires initiated by the failed tires.

I hope you understand why I would say "Please do not spread the false narrative that tire failures start tire fires. The facts just do not support that claim.


Friday, December 11, 2020

Can I "Fix" my flat?

 This is a reasonable question but the answer is not a simple Yes or NO. Here is a video from the Tire Information Association that covers the use of simple "plug" type repair.

This video shows an example of "hidden damage" and here is what that "saw tooth" can turn into.

My post in 2017 has pictures of three examples of tires with internal damage that were improperly "plugged".

Obviously if just doing an external plug is improper then squirting "oatmeal" into a tire in the hopes it will block the air leak is not a good option. 

It may look like this.