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Friday, March 15, 2019

How much Reserve Load or "Headroom" is enough?

Following an RV forum discussion on How much extra load capacity or "Reserve Load" or "Head Room" is enough. Some folks had moved from LR-C to LR-D  others to LR-G but were concerned about running the tire sidewall pressure as it would be too hard on the trailer.

There was also some confusion about the "Maximum" allowable inflation.  I said:
"The inflation on a tire sidewall is the MINIMUM inflation pressure needed to support the MAXIMUM load (also molded o the tire sidewall).
There is a sound scientific reason to run higher inflation when running tires on multi-axle trailers. It is called "Interply Shear". This force is what can result in belt/tread separations. I have covered this in detail along with references to technical papers on the topic.
IMO you need a minimum of 15% "head-room" of load capacity over the measured load of the heavier end of an axle. (CAT scale only gives the total on an axle and very few RVs have 50/50 load split. Some folks have discovered unbalance of 500 to 1,000#)
Most cars come with 20% to 40% tire load capacity headroom. I run 25% on my Class-C.
"
Which resulted in this reply:
"So if a given pressure (below the tires maximum) still provides the necessary headroom over the measured load, it's okay to run it, correct? Or am I not understanding something? "

This leads me to respond:
"I wish it were that simple but all radial tires have the Interply Shear force. It's a question of how much "tire life" is being "used up". The force results in microscopic cracks forming between the two steel belts at the outer edge. Once cracks form they do not repair themselves but can only grow.
No good tool or math model can turn Shear Force into Miles of tire life is "consumed" as there are just too many variables.
Finite Element models simply indicate that higher inflation can result in lower shear force every time you turn a corner or every time a tire completes a revolution. 

About the best you can do is to do a close visual inspection (Free Spin) to look for signs or out of round or side to side runout which are strong indicators of internal structural tears. I have a video and pictures of the autopsy I did on a tire where you can see the separations running across 50% to 70% of the tread width. This large separation might run a thousand miles or under certain circumstances might fail tomorrow.
Think of it like your Blood Pressure. Higher is worse. So the question is at what number can you expect to have a fatal attack?
If you consult your owner's manual you will see a suggestion that 3 to 5 years is the max expected life but obviously someone that runs at or above Max Load and above 65 mph and drives 4,000 mi a year is more likely to suffer a failure than someone that only drives 2,000 mi, never faster than 62 and at only 80% of tire load capacity.
Moving up one Load Range C>D or D>E can probably expect longer tire life if they also go from 50 to 65 or 65 to 80 psi. Going to LR-G in a Commercial grade all-steel tire should result in longer life even if you don't run 110psi.
Since load capacity is shown in the Load/Infl tables then I would suggest trying to get to 20 to 25% Reserve Load would be a good move. You still need to watch max speed and minimize sharp turns (as seen when backing) and of course, do a good free spin inspection once a year or once every 2,000 mi whichever comes first. Plus run a TPMS to confirm you never run lower pressure than your goal
."

I realize this is a long winded post but hopefully, you can think about the ideas and concepts we are discussing.  Easy questions do not always get simple answers, Especially when talking about a complex topic.

##RVT888

 

Friday, March 8, 2019

Are ST tires the "Best" tires made?

One fact that many choose to ignore, or just don't think about, is that very few RV have the axle load evenly split between axles on trailers or end to end on any one axle. Some owners have learned that there can be upwards of 500# to over 1,000# load unbalance.

It takes more work and effort to learn the reality of YOUR RV tire loading. I and others have covered the how and where to learn the loads on each individual tire (usually not on CAT or similar truck scales).

Regulations are written based on the assumption that RVs have close to perfect weight balance but few owners will make the effort to learn the actual tire loads. Also, those sacred regulations fail to tell the trailer owners that they can expect to need to replace ST tires at 3 to 5 years usage based on the tire DOT serial.  You will see numerous complaints about tire failures but most can be traced to overload/underinflation and over-speed or other external damage.  There is no "magic" rubber in tires with "ST" on the sidewall but many seem to want to believe there is.

As a tire engineer I have yet to have anyone explain why or how ST tires should be expected to perform better than any other tire of the same size on the road, yet that is what some would have you believe based on their insistence that simply because a tire company makes such an ST tire with greater capacity than their premium LT line.  Where is the tire company that makes an ST tire, that will offer a warranty on their ST tires comparable to what they offer on their premium P or LT tires?

Friday, March 1, 2019

Optimum, Appropriate, Required Inflation ?

I have seen people use a variety of words to describe the inflation numbers on Vehicle Tire Placards.  These numbers are arrived at by different methods for different vehicles.

For cars and Light Trucks (1/2 ton), Tire pressure is almost always arrived at based on on-vehicle testing and evaluation by the engineers of the vehicle manufacturer in conjunction with the tire company engineers. There will be a number of evaluations done using prototype vehicles and requests will be made to the tire company to adjust ride, handling, noise and a dozen other objective and subjective qualities of the tires. Eventually, the desired compromise is arrived at with one of the variables being the placard inflation.

For most cars, the inflation is significantly higher than the normal expected GVWR. Many cars have reserve load 0f +20% to over + 30% of the normal vehicle loading. You will note that the tire placard for passenger vehicles do not show GAWR or GVWR.
Lightweight trucks (1/2 ton) will normally be treated more like a car but as you move the heavier vehicles Federal requirements kick in and by the time you get to HD 3/4 ton and certainly 1-ton units the GAWR & GVWR ratings become a higher priority.

RVs have different DOT requirements and also different Tire Placard requirements. Most RVs have the inflation specified based on just one criterion, that is load capacity.

DOT requires that the inflation specified to provide the load capacity of the tires when inflated to the specified level, be able to support half the GAWR.
RVIA in 2017 upped their requirement to requires to be capable of supporting 110% of the GAWR (IMO this is better but not enough).


So the bottom line is that for most RVs the inflation is basically the level needed to support 100 to 110% of the GAWR. But many RVs are not loaded to that level which means you can lower the inflation to achieve better ride as long as you don't end up with overloaded tires.

DOT doesn't trust the average driver to know the actual weight of the RV. They have done studies and discover that a significant portion of the population thinks they are OK but actually have tires in overload. So the DOT doesn't publish alternative inflation guidelines and RV companies follow what the government does.

It's just that simple.

##RVT886