How fast can an ST tire run?

Originally Posted by SNOKING

So Roger, can you explain how Carlisle has raised the max speed of their ST tires like the Radial Trail RH? Chris

It's easy. Just run a drum test for a few minutes and you can claim the tire "passes" the requirement for the speed symbol.

My point is that the fundamental basis for the load formula was developed back in the '60s & '70s when the National Speed Limit was 55 mph. To my knowledge there have not been any substantive changes to the formula since then, even though highway speeds are now much faster than 55. It is also important to remember that in 2002 other tires like LT and P type had a major overhaul of the regulatory requirements to make radials more robust, but it is my understanding that the RV industry opposed the application of stricter, modern requirements be applied to ST type tires.

DOT regulatory test requirements for ST tires are over 40 years old and the only requirements for "High Speed" are as follows from FMVSS 571.109:

"S5.5 High speed performance.

S5.5.1 After preparing the tire in accordance with S5.4.1, mount the tire and wheel assembly in accordance with S5.4.2.1, and press it against the test wheel with a load of 88 percent of the tire's maximum load rating as marked on the tire sidewall.

S5.5.2 Break in the tire by running it for 2 hours at 80 km/h (50 mph).

S5.5.3 Allow to cool to 38° ±3 °C (100° ±5 °F) and readjust the inflation pressure to the applicable pressure specified in Table II.

S5.5.4 Without readjusting inflation pressure, test at 121 km/h (75 mph) for 30 minutes, 129 km/h (80 mph) for 30 minutes, and 137 km/h (85 mph) for 30 minutes."

NOTE the above applies to a new tire only and the inflation in "Table II" is 2 psi below the max on the tire sidewall. Once a tire completes the above without visible failure it is considered to have "passed" the test. You will note the relative short cumulative time and that the test is on a new tire that has never hit a pot hole or curb, or been exposed to sunlight for weeks, months or years.

SOURCE Here

LT tires must meet the following under 571.139
"S6.2.1.2 Test procedure.

S6.2.1.2.1 Press the assembly against the outer face of a test drum with a diameter of 1.70 m ±1%.
S6.2.1.2.2 Apply to the test axle a load equal to 85% of the tire's maximum load carrying capacity.
S6.2.1.2.3 Break-in the tire by running it for 2 hours at 80 km/h.
S6.2.1.2.4 Allow tire to cool to 38°C and readjust inflation pressure to applicable pressure in 6.2.1.1.1 immediately before the test.

S6.2.1.2.5 Throughout the test, the inflation pressure is not corrected and the test load is maintained at the value applied in S6.2.1.2.2.
S6.2.1.2.6 During the test, the ambient temperature, measured at a distance of not less than 150 mm and not more than 1 m from the tire, is maintained at not less than 32°C or more than 38°C.
S6.2.1.2.7 The test is conducted, continuously and uninterrupted, for ninety minutes through three thirty-minute consecutive test stages at the following speeds: 140 (86mph), 150 (93 mph), and 160 (99 mph) km/h.

S6.2.1.2.8 Allow the tire to cool for between 15 minutes and 25 minutes. Measure its inflation pressure. Then, deflate the tire, remove it from the test rim, and inspect it for the conditions specified in S6.2.2(a).
S6.2.2 Performance requirements. When the tire is tested in accordance with S6.2.1:
(a) There shall be no visual evidence of tread, sidewall, ply, cord, innerliner, belt or bead separation, chunking, open splices, cracking, or broken cords.
(b) The tire pressure, when measured at any time between 15 minutes and 25 minutes after the end of the test, shall not be less than 95% of the initial pressure specified in S6.2.1.1.1."

Source here.

Note the test pressure in 6.2.1.1.1 is about 5 psi below the max on the tire sidewall.

Also remember that an LT225/75R15 LR-D carries 15% less load than an identically sized ST type tire. I would ask what part of physics allows an ST to carry more load at the same inflation as an LT if all other conditions of use are identical?? Just making the container stronger doesn't result in increased load capacity even if it was the container that supports the load, WHICH IT IS NOT. It's the air pressure that supports the load else why have "Load & Inflation" tables. If it was the tire weight or mass we would have tire weight vs load tables.

If you want to learn more about the speed rating test tire companies are using I suggest you read This post written by another tire engineer.

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##RVT791

Question of running year old tires or Running less than max psi

Question from Chris

Hi Roger,

I came across your website by way of referral from a member on a Keystone RV forum.  I have a concern regarding upgraded tires and wheels for a Keystone trailer I own.  

As most manufacturers continue to cut costs in value models, wheels and tires seem to be ground zero in their savings.  So, to help mitigate tire failure on my coach, I've upgraded my china bomb OEM 14" tires sized ST205/75R14 LR-C to ST225/75R15  LR ? and purchased new wheels to accommodate the new rubber.  My wheels are Sendel wheels that support 2540 lbs of load each. 

I purchased Carlisle Radial HD tires in LRD from Discount Tire.  Unfortunately, they received LRE tires in instead of the LRD tires.  The Discount Tire salesman quickly resolved the error and within 2 days, had LRD in stock.  I returned to the store to have them balanced and mounted on the wheels. The stock they received in had DOT date code of 1516. He told me that was the newest in all his warehouses. I didn't want year old tires so I rejected the sale.  

Then, I ordered the same tires from Walmart.com.  Same story, the DOT date code was 1616 and 2016.  Rejected that sale.  Finally, I ordered them from Amazon.com.  They arrived today.  Again, tires are around 1 year old and I'm sending them back.

My first attempt at ordering tires from Discount Tire resulted in LRE tires. They were much newer around Nov 2016. I think my quest for load rated D tires from Carlisle in size 225/75/15 is futile.

So now, I'm considering purchasing the LRE tires as I think the local stock is much more fresh than I can ever find with LRD. The only limit I have is my wheel capacity at 2540 lbs. If the LRE tire max inflation pressure is 80 lbs supporting 2830 lbs, will it compromise the tire if I inflate only to 65 lbs (in order to stay within the specs of the wheel)? The OEM tires were LRC on 14 inch wheels supporting only 1760 lbs at 50 lbs. Weight is not a concern as I will never come close to max weight limits with my trailer on either LRD or LRE tire. 

Another thing that worries me is the stiffness in the LRE rubber. Comparing the Carlisle LRD to the OEM Trailer Kings is night and day. The tire is just so much more beefy. Don't really want to rip apart the trailer because it's rolling on unforgiving rubber. 

Any opinions and recommendations is greatly appreciated.

Thanks in advance.

Chris M.

Concerned RV owner

+++++++

My answer:

OK you have a couple different issues.

You need to know that tires are made in batches. For relatively small markets it's possible that a company may only make an item once a year or even less frequently. While tire age is of concern when the tires are on the ground and exposed to sunlight and heat, it's less of an issue when in the factory warehouse.

One way to let your mind rest a bit is to ask when the warranty clock starts ticking. Some companies clearly state on day of application or first retail sale. Others say the warranty is based on DOT serial. If it's based on date of sale and clearly stated as such in the literature, I would be less concerned and just be sure that the sales receipt has the date and DOT serial of each tire identified and keep both warranty and receipt in a safe place.

Now, since you have taken the step of going from the ST205/75R14 LRC to ST225/75R15 LRD, you have increased the load capacity from 1760 @ 50psi to  2540 @ 65 psi. You have increased the reserve load capacity by over 50%, which should deliver very reliable service. While I do not have any data to go on, I think that when inflated to the same psi the LR-D and LR-E would have similar stiffness. It's the air that supports the load and it's the air that provides the majority of the tire stiffness.

For your application I would think there would be no problems with running LR-E tires at the wheel limit of 65psi.

So, bottom line, I think you can go with the year-old LR-D tires or run the LR-E at 65.

Why isn't my TPMS accurate?

Comment on an RV forum
"I just bought a new TST 507  TPMS (tire pressure monitoring system) and  I gotta say they could improve the documentation.
Definitely confusing as setting up is a bit complex. At one point you are told to press a button and wait for the second beep (not the first!) before setting the pressures or whatever.
But once that's over, it works as advertised. It does seem to take a while, once turned on, for all the sensor readings to be displayed. After that it just cycles through them every 5 s, and beeps when one is over or under.
Not sure how accurate the psi readings are. Will have to get out my pressure gauge and compare."
+++++++++++++++=
My reply

TPMS are primarily designed to WARN of a loss in pressure and are not intended to be your primary source of accurate pressure measurement. In my experience ALL TPMS will have one or more sensors reading a psi or two different than a hand-held digital gauge. For that matter even hand-held gauges are seldom accurate to +/- 1.0 PSI based on the calibration checks I offer at my tire seminars.

IMO best practice is to set your tire pressure as close as possible to the desired CIP (cold inflation pressure) using your personal, master digital hand gauge and then install the TPMS sensors. Wait about 5 min for pressures and readings to stabilize, then do a pressure check on the monitor. You will note slight variation, but all should be within the specified range (some spec +/- 2%  some spec +/- 2 psi) You will find that after a few days use you will get comfortable with the pressure readings showing some variation from each other. With a little experience you will get comfortable and know the range of normal variation.

What I do...
Each morning when I get up, I first turn on the TPMS monitor and go get my coffee. By the time I am done with the 1st cup - about 10 min - all the sensors have sent in a reading. I step through each tire on the system and based on experience, I know if the tires are ready to go as far as being properly inflated. Yes, the numbers vary, but as long as they all are in a range of a couple psi from what I got when I last set the tire pressure all is good.

With the above practice I am getting the "cool tire" pressure reading which is the only reading of importance. With my +10% over the minimum inflation needed for my Motorhome, I don't have to worry about a daily +/- 2 to 4 psi variation. However, if I have a few days in a row with the display pressures consistently showing a few psi lower than my CIP goal I know it's time to plan on a stop at the truck stop for a shot of high pressure air.

##RVT789

What is a good tire gauge?

You need a system, as any gauge can be damaged or fail.

Pressure gauge system
Over the past six years I have offered pressure gauge calibration check services. With well over 70 gauges of almost every description checked we have learned that many are not much better than door stops or for use as a tent peg.

There are some broad generalities we have learned.

Digital gauges

are usually the most repeatable and accurate with dial gauges.

Next are "stick"

or "pencil" gauges lagging far behind.

While it is also true that I have found some gauges of each type to be accurate, those tend to be those that are brand new. The stick gauges with the sliding bar tend to get worse with age. This is probably due to dirt and nicks in the slide stick causing inconsistent and inaccurate readings.

Before I get into my suggested "system" some are probably wondering how I can be confident that my master gauge is itself accurate.

Having worked as an ISO 9001 certification auditor, it was part of my job to understand how to establish systems and procedures that when followed could be relied upon to deliver accurate information. Even though I am retired, I still can gain access to tire laboratory test equipment and occasionally compare my gauges to current ISO certified laboratory gauges.

System:  You need at least two gauges. One is your own personal "Master Gauge". This gauge is not used to check the pressure in your tires. It is only used as a reference for your regular use hand gauge. The Master gauge should never be stored in your tool box but should be carefully packed away where it will not get dirty, wet, dropped or knocked around.

Calibration check procedure: Using one of your front tires as a source of high pressure air you would take a reading with your Master gauge and a reading with your hand gauge. Hopefully when you first set up your system they will both read within 1.0 psi of each other or better. If not you may want to check your gauges against other digital gauges to try and confirm if it is your master or hand gauge that is most accurate. Majority wins if you have two that match to within 0.5 psi. For your two gauges you should record the two pressure readings and the date. You can then take readings of all your tires using your hand gauge, recording the pressures as you go. You could also record the readings of pressure from your TPMS (tire pressure monitoring system). Now I would expect the TPMS readings to be within +/- 3 psi of your hand gauge.

Remember, TPMS are primarily a tool to warn when there is a change not to be used as an absolute accurate gauge. Again record the measured pressures. Once you have finished this initial check you can set your tires to their appropriate Cold Inflation Pressure (CIP) level using your hand gauge.

Now when you look at your morning TPMS CIP readings, you should see similar movement up or down of the pressure of all tires as daily morning ambient temperature changes. Remember the tire pressure will change by about 2% for each change of 10F ambient. I covered this change is THESE posts.

As time goes on you might see a slow decrease in pressure. This is due to air permeation through the tire carcass and O rings around the valve and TPMS sensor. Eventually after losing about 5 psi you should "top off" the tire pressure. At that time you would use your hand gauge to set the tire pressures. I would suggest that before setting the tire pressures you again do a gauge comparison to learn if there has been any change in the accuracy of your hand gauge. If the difference between the two gauges has changed then you need to learn why.

Reliability: It is very unlikely that both your master gauge and your hand gauge would both go out of calibration by the same amount and in the same direction at the same time.

My System:  I use an Accutire MS-4021B

Digital Tire Pressure Gauge that I bought from Amazon for $12 as my master gauge. (Note I confirmed this gauge is off by 0.5 psi.)  I also use one as my hand gauge (seen above). Since this gauge does not have a truck "dual foot"

head I also run hose extenders on my rear duals. 

I also bought a Tire Traker TPMS. 

I have not had any leaking problems as I check the valve stems, TPMS sensor and hose fittings with a spray of soapy water to ensure there are no leaks.

Some may want to save $12 and not bother with having a master they can trust. But I don't think it's too hard to imagine a gauge reading a bit higher with time as the internal springs lose strength, so you would get misleading high readings which means your pressure is actually lower. This can lead to a failure that will cost a lot more than $12.

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