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Tuesday, July 26, 2016

Unhappy with the quality of the tires on your RV?

Post on an RV forum where there was a thread about "poor tire quality". There was this interchange:

"ST tires may be approved by government regulatory agencies. They still have a 3 year life. When I had tread separations, documenting the event for a government agency was not my focus. Getting new tires and going camping was my focus."


I responded...

First off, there is no government agency that is "approving" tires. The DOT does publish performance regulations that the tire company is "certifying" all the tires they make are capable of passing the requirements. If a tire company wants to game the system and make tires that do not meet the regulations there is no way for the DOT to learn this unless they conduct an investigation. The DOT will not start an investigation without justification or they could be accused of wasting taxpayer money. A major part of that justification is the number of complaints on file.

Sending a complaint to NHTSA not being your focus after a tire failure is clearly understandable. But if you never bothered to record the DOT serial number (S/N) of your tires and simply disposed of them, then you will never be able to file an actionable complaint. [Editor: The link to NHTSA is for filing a complaint regarding a tire.]

No actionable complaint =>  No Investigation =>  No findings of low quality =>  No recall => No improvement in quality of ST tires.

IMO there are some in the business of low cost tire production and sales who know the average RV owner will never complain. They are playing the odds that there will never be a recall, so with no future penalty there is no incentive to improve quality.

Even if you can't focus on filing a complaint at the time of the failure you certainly could file it a few days later, but that would mean you had made the initial effort to record the S/N for your tires.

If RV owners can't make that minimal effort of recording the S/N and spend the few minutes it takes to file a complaint,  I simply do not understand why they feel they can take the moral high ground and complain about poor tire quality.

Filing a complaint only takes a couple of minutes. Maybe less time than what many are willing to spend complaining on an RV forum.

Have you recorded the DOT S/N for your tires?


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Tuesday, July 19, 2016

Another post on Cold Inflation Pressure and "Ambient" temperature.

Some clarification from a tire engineer. 

First off, I am not going to address what the correct Cold Inflation Pressure ("CIP") is for your RV. We will assume you have read my other posts on how to learn that number. If you haven't, simply look at the list of labels on the left side of this blog and you can review them.

There are different guidelines for Trailers vs. Motorhomes. I want to focus only on setting the CIP

Some people want to refer to a Temperature compensation chart.

Tire Rack was off a bit till they updated their tech page last year after my input. Also Wikipedia definition for Cold Inflation Pressure was also almost correct till I added a clarification aimed at RV owners. The 1 Psi for 10 F is OK if your base inflation is near 40 psi but many RVs use 80 to 110 psi.
The correct "Rule of Thumb" to use is 2% for each 10F.

Tire pressure is not based on any laboratory standard temperature (some claim 70 F) but is based on the tire not being warmed from either use, i.e. being driven in previous two hours, or from being in the Sun for previous two hours. Even partial sun can affect the reading.

Classical "Temperature in the Shade" is the "Ambient" tire engineers are talking about. Not temperature in a theoretical laboratory.

So regarding a situation of setting the pressure when the tires and air is 50 F. That would be fine and we would expect the pressure to increase by about 8% if the Ambient increased to 90 F even without driving or Sun exposure.

It is correct to say, "The ONLY time to check CIP is FIRST thing in the morning BEFORE the day's temp has had a chance to increase and BEFORE the sun has had a chance to shine on the tires and BEFORE you have used the vehicle."

However, if you are driving from the campground on top of Pike's Peak and stop for lunch for two hours in the shade in Flagstaff where it is 90 and check your air, you might find a change of a few psi. You could adjust your pressure before continuing to Phoenix, where it is 120 F, but I don't bother to adjust inflation by the 1 to 3 psi variation I observe day to day. In my mind that is too much work.

NOTE: My personal CIP is  75/80  F/R on my Class-C MH.  Both of these pressures are more than 10% above the minimum pressure needed to support the measured load on each tire so I have a "cushion".

I usually wait till I am home and am getting ready for the next trip before I adjust my inflation to my personal CIP, so I simply monitor the running inflation pressure which goes up and down as ambient temperature, driving and Sun exposure changes the inflation. My TPMS will warn me of air loss, so all is good as I motor down the highway.


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Tuesday, July 12, 2016

Change Tire size

When considering a change in tire size there are many different areas you should consider.

Tire load capacity. You need to be sure you go with tires that have equal or greater load capacity than original.
It is possible that the original tires delivered a very small load capacity such that the coach is easily overloaded. Recalls may have been ordered or initiated but sometimes a manufacturer went bankrupt so no recall action took place or no new tire certification label was provided.
So it is up to you, the current owner to do your homework and get the facts.

- You need to learn the real load capability of the axle by contacting the manufacturer if there is no plate on the axle with GAWR info.
- You also need to learn the real loads you are placing on your tires with your coach fully loaded. Here is a worksheet with instructions on the steps and calculations you need to do.

Now load capacity is not the only information you need. You also have to be sure the tire will never rub and contact any portion of the coach or its suspension. part of this is obvious and can be discovered by rolling on your back with a flashlight to confirm sufficient clearance around all your tires. Don't forget to check both front tires with wheels turned completely both directions. I have read that 3" on all sides is a reasonable clearance.

Rim width - There is a list of approved widths for each tire size. You must stay with the dimensions listed by the tire company

One final clearance check that is sometimes overlooked is the clearance between dual tires. This requires more than just looking at the rear tires. In tire company specification charts there will be a dimension called "Minimum Dual Spacing" for the tires you may be considering. This dimension is controlled by the wheels. Sometimes this information may be marked on the wheel but most likely you will need to contact the wheel manufacturer to learn the specification. This is CRITICAL, as too small a clearance can result in tire damage or even failure.


I can't address if a specific size will fit but simply going up one size would be reasonably close to the 1/2" width increase.

For example if your current size was an ST215/75R15 going to run ST225/75R15 would give you a tire that was approx 10mm ( about 3/8" wider over all)
Now it is important to remember that tire dimensions are approximate and if you change brands the actual with may be a little different.

Also don't forget that the OD would also increase which means the clearance between the tread and wheel well or the companion tire would change also.

You should be able to find published OD and width dimensions from your tire companies web page.

One other item is that a change in wheel width (as published in the literature) will affect the tire width by about 40%. So a tire with a published width of 10" on a 6" wide rim would be about 10.2" on a 6-1/2" wide rim (40% of the 1/2" rim change = about 0.2")


 But in some cases on older coaches tires may have been changed in size or even the size designation such as an old 7.50-15.  If you find yourself in that situation you might consider dropping me an email so I can help walk you through the things you need to consider.



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Sunday, July 3, 2016

How you might run ST type tires at 75 to 80

RE Speed Ratings and operation speed.

My comparison to engine redline seems to me to be the easiest for many to understand. It is possible to run an engine right at redline or even above for a short time but I can't find anyone that advocates operating an engine at the rated max speed for any significant time or distance.

High Speed test is not a DOT regulatory test but is a test based on Society of Automotive Engineers testing. To be marked with a speed symbol a new tire needs to run for 30 minutes on a smooth drum at the stated speed.

There is no requirement for a tire to be "conditioned" with a few thousand miles at lower speed over potholes and up on curbs or with 110% of the rated load or for the tire to be able to pass DOT regulatory durability tests after running the SAE High Speed test.

Yes, an increasing number of ST type tires now come with a speed symbol molded on the sidewall. The primary reason for this seems to be to avoid import duties. What many want to ignore is the fundamental truth that the load capacity of ST tires is much higher than an LT type tire based on the premise from 1970 that the ST tire would be on a single axle trailer that was limited to 50 mph operation speed.

Molding the letters "ST" on a tire is not magic. Physics still applies. If people want to drive at 75 or 80 while towing as they would with their LT type tires and not have failures, then I suggest they pay attention to the Physics and limit the actual tire load as if it were an LT type. This is easy to do.

First simply look for an LT type tire with the same dimensions i.e. ST235/75R15 > LT235/75R15 in the Load tables and limit their measured load to the number found in the LT tables.

If you do that you will probably see a significant reduction in tire failures. Of course this also means you are not in the 50+% of RV owners that operate your tires under-inflated.

If you ignore the Facts and Physics of reality you will have to live with the consequences no matter how much you want to believe otherwise.

Bottom Line. If you want durability and life more like LY type tires then treat them as if they were LT type tires.

IMPORTANT NOTE and WARNING
As with the engine in your RV or tow vehicle it may be possible to run faster but it does not mean you will avoid all problems. This post is just about tires and not about safe and reasonable operation of your RV. Personally I think 70 is too fast to drive a "big rig" and 75 is certainly too fast for towing. I have heard comments about an increase in truck tire failures due to increased speed limits in many states. A MAX of 75 is stated in some tire information guides published by major tire companies and as with any maximum, the closer you are to it the more likely you will have some negative consequences.


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