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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.

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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  1. Totally agree. So many overload their rigs, under inflate their tires and speed off down the interstate at higher than recommended speeds only to face disaster....and then blame the tire manufacturer.

  2. It seems that no one ever says this if the best brand of tire to use on an RV. We run Michelin tires that cost too much. We always have to replace them due to age and cracks rather than wearing them out. Surely there is a creeper tire for a 36DC Itasca Horizon.

  3. Those of us who grew up in farm country know that tires will take a lot more load than their ratings. We overloaded trailer tires almost every time that we used them. The most surprising thing was that tires failed just as often on an empty trailer as they did on an overloaded one. We just figured that the tires had picked up a nail and gone flat at speed. We would have gone broke if we had purchased fancy tires with high load ratings. I am not trying to challenge science, just sharing the experiences of a lifetime with trailers.

    1. Looks like you have fallen for the common idea that the operating conditions at the instant of failure are why a tire fails. In reality the damage from overload, under-inflation and over-speed are cumulative. Every revolution run at those conditions consumes part of a tire's "life". At some point even normal or under-load operation is the final straw so the tire fails.

  4. Any person that runs their rigs at 70 mph, never mind 75-80 mph, had better give their head a shake. Most are retired so what's your hurry. We travel the 6/300 method (6 hrs or 300 miles, whichever comes first). We arrive at a site, set up, prepare the supper and are sitting down, relaxed and stress free, having a glass of wine by 4. If you are not retired, choose a spot closer to home and travel the same way. You will feel better.

  5. Ken, James & Busy
    I agree that running above 65 is not needed but many insist. Best I can do is to address the tire durability question.


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