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Friday, December 28, 2018

"Safety Factor" or "Reserve Load"

"Safety Factor"   The dictionary offers this "the ratio of the maximum stress that a structural part or a piece of material can withstand to the maximum stress estimated for it in the use for which it is designed."
While that sounds reasonable it really only works when talking about pieced that fail from simply increasing the load placed of the component.

Items like tires do not really have a "Safety Factor" as tires generally do not fail from simply increasing the load too much. In a non-rolling situation, I would not be surprised if we could load tires to 200% or maybe even more than 300%  of the load marked on the tire sidewall. However as soon as you introduce rolling or time or operating temperature the maximum load before failure is much closer to the max load molded on the tire sidewall. The exception to Max speed is affected by temperature time and load. With zero load many tires can probably handle 200+ mph but again for how long and at what temperature?

Since tires are basically a structure made of "organic" components tire and temperature can have a significant impact on the maximum load capabilities of the tire.

If we think of non-organic items like a steel girder or maybe even a stone block as used in the pyramid we can see that time and normal atmospheric temperatures have essentially no impact on the long-term maximum strength. The exception would be if we were to allow steel to rust or stone to be exposed to water and freeze/thaw cycles.

Tire Engineers prefer to use the term "Reserve Load" when talking about the load capacity of a tire. Here we would find Tire Engineer definition as the difference between the tire's maximum capacity when inflated to the stated level for the specific application (the inflation on the tire Placard) and the actual load to be placed on the tire.

Here are a few comparisons: First some normal car and truck applications.

Next a larger 5th Wheel RV

When you compare the reserve load percentage of the different groups you can easily see the different level of reserve load.

What should the reserve load be for your RV?  Currently, RVIA considered 10% to be the minimum Reserve Load. However, the few actual Tire Engineers that are posting on RV forums are suggesting a MINIMUM Reserve Load of 15%  with more being desirable.
I know that on my Class-C I am running closer to 20% Reserve load based on actual "4 corner" weights i.e. individual tire position scale readings.

What is your actual Reserve Load?

Friday, December 21, 2018

Video showing why Interply Shear happens

As we head into the Christmas Holiday, most of us are not thinking about our tires.

Here is a quick post that might help people with a better understanding of how and why tires can develop tread/belt separations. The video is focused on multi-axle RV trailers but ALL radial tires have this force that is called "Interply Shear." It's just more obvious on these trailers.

Now go finish your shopping. One gift suggestion I can make is a good digital pressure gauge. The gauge I use as my "Master" is the Accutire MS-4021-B digital tire pressure gauge. Learn more or order on


Friday, December 14, 2018

Better Fuel Economy for Class-A

Smartway is a program from the EPA that is supposed to help truckers select tires that have better Rolling Resistance (the way tires can be measured) for better fuel economy.

Review the information provided and you will see that most truck/bus tire companies have a line of tires available that meet the Smartway standards.

Will selecting one of these tires get you 20 mpg on your 40' DP  No it won't but it might get you a 5% improvement which could be like getting $ 0.15 off a gallon of fuel which
I bet anyone would like.

While we are on the topic of fuel economy here are some tips from RVTechLibrary

ß Every 2% reduction in aerodynamic drag results in approximately 1% improvement
in fuel economy.
ß Above 55 mph, each 1 mph increase in vehicle speed decreases fuel economy by
0.1 mpg.
ß Worn tires provide better fuel economy than new tires, up to 7% better fuel economy.

ß Ribbed tires on the drive axles provide 2–4% better fuel economy than lugged tires.
ß Every 10 psi that a tire is underinflated reduces fuel economy by 1%.

ß Tires make biggest difference in mpg below around 50 mph; aerodynamics is the
most important factor over around 50 mph.
ß The most efficient drivers get about 30% better fuel economy than the least efficient
ß Idle time is costly. Every hour of idle time in a long-haul operation can decrease fuel
efficiency by 1%.