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Friday, December 29, 2017

Too much pressure increase

Reading an RV forum thread on TPMS usage. One comment jumped out at me.

"I have a serious concern with the G rated tires on my 5th wheel trailer. When I run the cold max pressure at 110 PSI, I get TPMS readings up to 134 PSI rolling down the road. Lately, I have been running 95 PSI cold and am getting 115-119 PSI rolling. The tires and rims are rated for 110 PSI, I can live with 120 PSI, but the 134 PSI concerns me."

I replied that inflation ratings for wheels are based on a "cold" pressure. Increase in pressure due to operation is considered by manufacturers and I would not be concerned as long as the proper cold inflation is used along with appropriate limits on load and speed.
Tire wording "Max Pressure" can be confusing but in reality the pressure stated on the tire sidewall is the cold pressure needed to support the stated load and that load is the maximum load the tire should be subjected to. I advised the owner that he should NOT underinflate his tires and plan on operation temperature to increase the pressure. "Cold" pressure is the only pressure you should be concerned with.

Tire pressure increases by about 2% for each increase in temperature of 10F. If you don't remember the Science from High School you can read THIS post.. If you are seeing a 21% increase in pressure (110 > 134) that means you are seeing about a 100F increase in internal tire temperature, which I would consider a bit excessive for normal tire operation.

If you are seeing a 25% increase in pressure (95 > 119) then you are getting a 125F increase in temperature which indicates you are working the tires even harder . This extra "work" that is generating a greater temperature increase is not good for long term tire life. You are "aging" the tire rubber faster. Some might want to review this "Key Point" of tire life as covered in THIS post.

I cover these points on Temperature, Inflation and Aging in various posts on my RV tire blog.

Your temperature increase indicates you are possibly overloading your tires and also possibly driving faster than desirable for your tire loading.

You need to confirm your pressure is 110 psi AND that your gauge is giving an accurate reading at that level.

The poster then responded:
"Today drove 250 miles and my 95 PSI tires were running 115-119 PSI and the tire temps were at 20F above out side temp, 50 outside and 71 tire readings. I still contend that 134 PSI is way to dangerous for tires to run on 110 rated tires."

So I responded back:
 Few people realize that the pressure increase as a function of temperature is based on well established and confirmable Physics and that a TPMS is not reading the actual tire temperature  but is actually reading the temperature of the brass valve stem and the metal base of the TPMS itself, which is being cooled by outside air.

Air is a very good insulator and if you think about it, you have a small column of air running up the inside of the valve stem which makes it difficult for the heat to travel up the center of the stem and past the valve core itself all the while the valve is moving rapidly around being cooled by the outside air.

I am aware of laboratory tests that goes against what "common sense" might indicate. That being that the air inside a tire is not uniform in temperature but it is always cooler than the hot spots of a tire and it is the hot spots that can result in tire failing if hot enough for long enough.

I have no doubt that the TPMS was indicating only 20F above the cool 50F outside air temperature. If you are still concerned about the hot pressure of 134 on tires that have a cold pressure rating of 110 for its max load capacity rating.

As a tire design engineer with 40 years experience, I trust the science of the "Gas Law" and knowledge that air is an insulator and metal conducts heat from a hot source to a cooler one.
I don't know what to advise other than to decrease the operating load and speed and to confirm your hand gauge is accurate and to always inflate the tire when cold to 110psi, as continued operation at current load and speeds will certainly result in pressure reading that are above the cold pressure of 110 psi.

Friday, December 22, 2017

Quick post on Dual tire air balance systems

Read a thread on an RV Forum on connecting two tires in dual position, with hoses to "balance" the inflation pressure. There were some claims made about performance improvements but IMO the conditions necessary to deliver those improvements are pretty extreme and improvements are tenuous.

The information on the function of this pressure balance equipment also ignores the potential for damage to the RV when one tire loses air but the driver is not immediately informed of the loss. There is even talk of serious air loss in excess of 25% of the pressure needed, only resulting in faster wear.

I found no mention of operating a tire that low possibly resulting in a catastrophic failure (coming apart and potentially doing thousands of dollars' worth of damage to the RV).

I can't understand why some would think that lowering the hot tire pressure (decreasing load capacity) would ever be a good thing to do.

Someone will probably point out that some of the systems that connect the air between a set of duals limit the amount of air transferred, but that still leaves a problem.

If you have two tires set to 80 psi and one gets a puncture and starts to leak, the "good" tire bleeds off air to the leaking tire until 5 or10 psi has been transferred. Now both tires are at 70 to 75 psi and are just starting to be overloaded.

But the leak continues and the tire going flat transfers its load to the "good" tire, resulting in the "good "tire seeing ever-increased the level of overload.

This can continue until one tire is flat and one tire is 100% overload. Now you are driving down the road with no knowledge you have a flat and a tire at 200% of rated load.

If you are lucky, you stop and discover the problem before the "good" tire fails, too, leaving you with two scrap tires.

This system does make airing up tires easier in that you only have one valve for the pair of tires, but I'm not sure if that "benefit" is worth the cost.


Friday, December 15, 2017

Leveling Blocks - Only support one dual?

I received this question the other day.

"I have some questions about leveling with ramps, specifically on vehicles with rear dualies.

The obvious temptation is to use only one ramp -- and the obvious response is that this is a 2x overload condition and should be avoided.

HOWEVER -- since tires and wheels are clearly designed with safety factors, and since the overload is a very static situation with none of the dynamic variations of a moving scenario, what do you think are the real risks involved?

In weight-sensitive designs like aircraft, I know that a typical safety factor is 2.5x. But I don't know what's in tires and wheels.

It's easy to say "just carry four ramps" to level the rear of a dually coach, but for the smaller rigs, that kind of bulk is pretty impractical.

I'm curious, but I'm cautious too. I'd appreciate your thoughts.


My reply
First "Safety Factor".
I understand the concept for aircraft and for some other items but the concept doesn't translate to tires very well. Unlike the metal used in aircraft, there is a lot more variation in the raw materials and the finished products
Tires are designed and manufactured to meet and exceed a set of requirements. The requirements are such that all tires must be capable of meeting or exceeding the standards but there is no advantage to exceeding them beyond their design goals as you can't make a claim that some tires in a line might deliver x thousand miles if all tires in the line can't and with hundreds of thousands of tires in a production run it is essentially impossible.
In your aircraft example, there are thousands of subsystems that can each be replaced or upgraded individually and while there may be a goal of a 2.5 safety factor I doubt that each and every part manages to wear out at the same point in its life.

Tires don't have replaceable parts, so while some parts may last a long time and even longer than advertised, when any one component simply meets its design goal but doesn't exceed it, the tire's life is over.

Now to your question on ramps. The answer for ramps is the same as for basic supports used to keep tires out of water when parked for the winter months.

The #1 priority is to support the entire footprint (contact area) uniformly. Never just half the tread width or half the tread length. Even only supporting 80 or 90% of the contact area is not good for the tire structure. Technically the 200% load is not a serious issue but it will definitely stretch a sidewall greater than normal. This increased stretch could, in the long term, lead to localized sidewall cracking after long-term use. RV application can also add other issues such as the tire sidewall losing some of it's "stretch" properties. RV tires are not designed to ever be loaded more than 100% of the load stated on the tire sidewall and inflated to the level associated with that stated max load.

THIS post has some pictures of what you should NOT do. It also shows an example of a tire "footprint" after parked on grass for a couple days.  A 2x4 under part the tread is particularly bad. When you use a block or ramp that is not as wide or long as the tire contact patch the edge that is not fully supported will have localized bending that can overstress that part of a tire.

Yes, tires are built to tolerate abuse and overload and overspeed and under-inflation but in each case, there will be some damage done to the structure. The damage is cumulative and tires do not repair themselves.

I carry 3 leveling blocks in my small Class-C. Here are two set up for my dual tires.  I can use just one if I need to raise a front tire. These ramps can raise the tire(s) 1-1/2" or 3". If I need more I will move the RV to a more level location.

If I need to raise one side of the rear I always use two under the duals.  If that isn't enough, I move the RV to a different spot.  

Your tires are not likely to fail simply because you only supported one of the duals overnight but I can say that putting 200% load on a tire is not a good idea if you can avoid it.

Bottom Line
Anything you use to support a tire must be at least as wide and as long as the tire contact area or "footprint". If your "blocks" aren't wider than the tire or if you do not drive upon them such that they are centered you are doing some damage that could contribute to premature failure that you probably would never associate with the use of improper sized leveling blocks.

Friday, December 8, 2017

4-Corner weights and a "Meaningful" measurement.

I was following a forum thread on using closed state scales to learn your "4 corner weights" This is possible in WA & OR and some other states. 
Some commenters felt the practice was not wise as the area around the platform scale might not be perfectly flat. I posted my thoughts on the topic.

Not sure if I understand the concern over the accuracy of using a large platform scale to try and get a handle on your side to side loading.

I have a blog post on "Measurable vs Meaningful" that some might want to review.

When setting the inflation based on the published Load & Inflation tables, you should always "err on the high side." This means if your measured weight were 2,005# you should go UP to the first weight that exceeded 2,005# and maybe even higher.

Let's assume the table for your tire gave 90 psi at 1,900#  and 95psi at 2,210#. I would recommend you select 95psi as your MINIMUM inflation.  But what would you do if the chart for your tire showed an inflation for 2,010# or 2,005# or 2,000#? Would you cut things so fine as to select the inflation for these loads?  I hope not. What might happen if 50 minutes after getting the tire loads you top off the fuel and buy some groceries? You are now most certainly over the measured 2,005#. Are you gonna run back to the scales?  I wouldn't, and I would not need to as I have a built-in cushion of load capacity over the minimum needed to support my actual load.

We haven't addressed the question of your tire pressure gauge accuracy. Unless you have access to ISO Certified laboratory gauges as I do, you probably need to assume that your gauge is off by at least one increment reading of the pressure scale. For some truck type stick gauges that means 5 psi. What is your cushion that covers your gauge accuracy?

My "cushion" results from a few actions.
1. I always go up in the charts to the next inflation level and if my measured load is within 100# of a level, I will go up TWO levels of inflation which would be 10 psi.
2. I also add 10% to the selected table inflation and again round up to the next "0" or "x5" pressure

The additional 10% means you can avoid having to chase the Cold Inflation level every morning when the temperature drops and pressure drops a few psi.

When I checked my "4 corner weights" on the state scale was the area beside the platform exactly level? No, it wasn't, but it looks close enough that even if my weight was off by 100# my other adjustments would more than compensate.


Friday, December 1, 2017

Are you smarter than your tire salesperson? With Michelin LTX you may need to be.

As an RV owner, I believe that in most cases it is important for us to know as much as possible about our tires. Sometimes this means knowing more about our tires than the average salesperson.

Here is a real-life example:

An owner of an older Travel Trailer posted
"Please correct me if I'm wrong... The Michelin Defender LTX M/S 235/75R15 is a true LT tire and does not need to be derated?

Shopping for tires on my Airstream 2017 23FB. I'd love to just go with the 16" SenDel S02/Michelin combo but not sure if the clearance is there for them."

Just before the above post it had been pointed out  the correct designation is P235/75R15 XL
The "XL" is the tip-off as that stands for "Extra Load" which is only found in Passenger type tires.

LT tires have Load Range C, D, and E
P-type have Standard Load (no special marking)  and XL which is lower in inflation than an LT-C.

Your tire dealer should have made clear the type tire they are talking about.  Sometimes RV owners need to know more than the salesman if you want to get the tire you need or want.

I had also previously posted on that thread:
"Be sure you understand if your LTX tires are "LT" type or "P" type. If P you need to calculate the actual tire capacity when used on Trailer, SUV or PU-truck.

The adjustment is 
(Load molded on tire)/1.10 = Load capacity on the RV or truck or SUV application.

No load capacity adjustment is needed for LT type tires if placed on an RV, truck or SUV.
The LT in "LTX" does not make a tire an actual "LT type tire."

I don't know if the marketing folks at Michelin realized the confusion they were spreading when they came up with the name "LTX" and put that designation on BOTH LT type and P-Type tires. I would not be surprised if there aren't a good number of RV owners who think they have an LT tire when what they were sold was actually a Passenger type tire.