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Wednesday, October 23, 2013

Breaking News - Max Speed on ST type tires

Finally chased down the answer. Getting to the bottom was a challenge because I ran into "customer service" people who could read from a script but had no idea why they were giving me the answer they were.

Even got to an engineer at Tire & Rim Association and based on his answers it was obvious TRA had not thought about ST type tires or the unique nature of most RV trailer applications for many years so I believe he acted a little surprised when I identified the engineering analysis that indicated that the current loads too high for many trailers and that the load capacity should be decreased by 15% to 25% or more if the failure rate was to be decreased to a more tolerable rate rather than the 5 or 10% or higher some are reporting. This is not his or TRA fault as I understand it is the responsibility of tire company representatives to TRA to present and review technical matters.

Anyway here is the bottom line.

For Goodyear Marathon tires only:
There is a Goodyear  Tech bulletin   PSB#2011-13 that uses information from TRA.   
The tech bulletin says that the normal max speed for Marathon ST type tires is 65 mph. This speed can be increased up to 75mph only if the inflation pressure is increased by 10 psi. This new inflation can be 10 psi higher than the pressure molded on the sidewall of the tire that is associated with the tire max load.
The load does not increase beyond the load molded on the tire sidewall.
Goodyear does not support or condone operation above 75 at any time.
The trailer owner is responsible to confirm the rim is rated for the new inflation level.
While TRA indicates a further increase in speed might be achieved Goodyear has decided not to adopt this option.

The person I talked with at Tire Rack said they had already changed their web site to indicate this applied to Goodyear tires only and that they were attempting to learn if any other ST tire manufacturers were willing to make a similar change. As of Saturday Oct 19, 2013 they had not identified any other tire manufacturer willing to make an increase in max speed above 65.

Info added 10/26/13 10:51 AM EDT
If you have a GY Marathon ST225/75R15 LR-D it is rated for 2540# @ 65psi and a max speed of 65 mph. If you increase the cold inflation to 75psi the tire would now be rated for 2540# @ 75psi and a max speed of 75 mph.
The example currently only is OK for Goodyear Marathon ST tires as I have not seen a published document from any other tire manufacturer. 

The pressure increase must not exceed the max pressure capability of the wheel. You need to find a max pressure rating stamped on the wheel or get something in writing from the wheel MANUFACTURER.

My observation
This Max speed is like your engine red-line. While it might be possible to exceed this speed for short periods it will significantly reduce long tern durability.

I have sent a copy of this post to both TRA and Goodyear. If they ask for corrections or additions I will edit this post accordingly.

TPMS batteries, Changing tires, Tire Size info, ST type tire speed rating and RV control with tire Blowout

Few quick topics in this post.
Saw a few posts on RV forums on TPMS sensor replacement.
It seems some people have had their TPM systems long enough that they need new sensors
because their batteries are low. So some folks are confronted with spending $200 to $300 on a set of 6 sensors.  I suggested that they look into the TireTraker system as it has low cost watch batteries so the cost is only a few bucks for a new battery rather than $35 to $50 each sensor. Full disclosure. TireTraker is a sponsor of this blog but I did buy my TPMS from them at a rally a couple years ago based on their features.
I suggest you do a "Life Cysle" cost comparison based on 5 years. Include the initial system cost, the cost of replacing the batteries if your system allows that or the cost of replacing all the sensors if you can't replace just the batteries. You might also want to consider the length of the initial warranty.


Another person said he had 8 years on his Class-A tires and was asking "What brand should I buy"
This of  course immediately started a flurry of "I have had great success with Brand-X" or I had a failure with
Brand-X so will never buy another one of their tires, etc.
I asked the poster why he was considering changing brands if he had had 8 year  good service from the tires he had? When it comes time to consider new tires I suggest you make a list of the Pros and Cons of your current tires.

 Be sure not to include things like the puncture with the roofing nail as that can happen to any brand tire.

 I often tell people you are buying a tire company and their dealer network not just a set of tires. If you get a great price from Billy-Jo-Bobs Cheap Tire Emporium and Bate Shop but there are only a handful of dealers in the country where you can get a replacement and you have to pay shipping back to Billy-Jo if you want to make a warranty claim I doubt that the total price of owning that set of tires is as good of a deal as you first thought.


When asking a question on an RV forum, it helps if you include the complete tire size designation. Some folks say nothing about the size but want specific answers on load capacity. Others provide only part of the size such as 235/75R16 and leave off if they are talking about a "P" type or "LT" type or "ST" type. The answer to these questions will probably depend on which type of tire we are talking about. It also helps if you include the Load Range as in LR-D or LR-F or whatever is molded on the tire sidewall.
Sorry but indicating that you have a Mountain Top Rambler RV doesn't help as there are just too many makes and models of RV out there for anyone to know all the tire options that migh be used.

ST type tires have a normal Max Speed of 65 mph. You should consider this like the redline on your engine. While it is possible to exceed the red-line it isn't good for long term durability. When covering this topic some point to the Goodyear Tech Bulliten that indicates you may increase this max speed rating up to 75 mph if you also increase the tire inflation 10psi above the inflation associated with the max load on the tire sidewall. While Goodyear may be willing to stand behind the warranty of their Marathon ST type tires at this higher speed you should not do the same for other brands of tires unless you get something in writing from that tire MFG. Tire company Tech Bullitens only apply to the brand and line of tires mentioned in the bulliten.

Finally I had a question on what to do if you have a  blowout on your RV.  Michelin has a good video on this topic covering Motorhomes. Similar driver reaction if you are driving a tow vehicle would probably apply, so there isn't much I can add. The question however asked about blowout on an RV trailer. Now I have not tested this myself but I have seen more than one example where the impact on vehicle control was so small the driver never knew he had a trailer tire failure till someone flagged the driver down.
As explained in the Michelin video the forces would be drag on the trailer which would keep it generally straight behind the tow vehicle with only a little side offset.
If your trailer did start to sway I would use the manual brake control to slow the trailer down. This should quickly stop the side to side sway and allow you to bring the tow vehicle and trailer to a safe stop.

Tuesday, October 15, 2013

Will you increase load capacity with higher Load Range?

The question of load capacity and Load Range gets asked a lot, so it seems to be a bit of a stumbling block for a number of RV owners. Here is an example:
"I need to replace the ST235/75R15 tires on my trailer. Currently have load range C tires which is sufficient for the maximum trailer weight when inflated to 50psi but allows almost no safety margin. I want to change to either load range D or E tires. My rims are only rated for 65 psi so if I went to the load range E tires I would only be able to inflate them to 65 psi. My question is this. Is there any advantage to a load range E tire used at 65 psi vs a load range D tire at 65 psi?"

Now just because he is asking about a 15" trailer application and considering a change from LR-C to LR-D it does not mean the general answer does not apply to others, even if they are considering a change from LR-G to LR-H on a Goodyear 295/75R22.5 Class-A tire.

Let me explain.

As some of you remember, I previously covered the fact that it is the air not the tire that carries the load. For those that still have doubts, here is a picture from a special test machine that measures force and delivers the answer in color. In this case, the lowest force is Blue which increases to Green, then Yellow and finally Red at the highest force. It is clear that the tire wants to be round because of the high inflation pressure but when you press the tire against a surface it becomes flat with  the highest force in the center of the tire. If it was the sidewall that carried the load, as some want to believe, then the Red (high force) should be at the outside shoulders.

Now when you go up in Load Range you are actually looking at the capability of the tire to carry more inflation pressure. Remember to gain in load capacity you need to increase the inflation. Years ago when tires used cotton for body cord we might have 4, 6, 8 or more layers or ply of reinforcement to retain the increased air pressure. Those increments provided steps in the Load/Inflation tables. Since the late 60's and early 70's when materials improved the actual number of ply decreased and the term Load Range was applied to those steps.

So going back to the original question, when increasing the Load Range you gain the possibility of increased load carrying ability but if you don't increase the actual cold inflation, the change in Load Range will not give you any increase in load carrying capacity.

The answer to the question is;
 He will gain a safety margin if he increases his inflation to 65psi with LR-D but if he stays at 65psi there is nothing further gained by going to LR-E as there is no difference in the capacity at 65psi.
The same thing would apply to the 22.5 example. When we look at the Goodyear tables we see that at 110psi both the LR-G and the LR-H are rated for 6,175# single or in dual application at 100psi they both are rated as 5,675#.

Special note: It is important that you look at the table on the web site of the manufacturer of your tires as not all companies give the exact same capacity number at every inflation level, even for the same size.