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Old 06-21-2013, 09:25 AM   #1
buzzcop63
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Tires

Tires:
Looking at Power King Towmax STR web sight I found the tires on my 2012 Cougar 27' TT, (model 24RKSWE) ST225/75R15LRD are rated at Dual Max Load 2,200 Lb at 65PSI. My trailer max weight is shown at 7,200Lb by Keystone. If I want 20% safety margin for my 7,200Lb trailer I would add 1,440Lb, then I would need four tires who's combined max rating would be 8,640Lb and the Power King Towmax four tires combined are 8,800Lb. I have my safety margin for my unit.
However if you look at two additional TT models in Keystone 1/2 ton Series that are heavier then my unit, 25RLSWE and 26BHSWE which utilize the same tire we find those two trailers have a max weight rating of 7,800Lb. 20% of 7,800 would be 1,560Lb or 9,360Lb combined total of the four tires when they are only rated for a max of 8,800Lb this would be about 13% safety margin over max trailer weight! I suspect that for 5th wheels this could well be the same, the safety margin drops as the same tire is used on heavier units.
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Old 06-21-2013, 09:52 AM   #2
raytronx
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Are you taking in to account that some of the total weight is going to be on the truck's tires. My fifth wheel loaded up is 9000 lbs but only 7200 on the trailer axles. I ran the OEM ST225/75R15LRD Towmaxx tires for 25,000 miles no issues, and when replaced for wear went to the same brand but E Load range and have 15,000 on those.
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Old 06-21-2013, 09:55 AM   #3
CWtheMan
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Quote:
Originally Posted by buzzcop63 View Post
Tires:
Looking at Power King Towmax STR web sight I found the tires on my 2012 Cougar 27' TT, (model 24RKSWE) ST225/75R15LRD are rated at Dual Max Load 2,200 Lb at 65PSI. My trailer max weight is shown at 7,200Lb by Keystone. If I want 20% safety margin for my 7,200Lb trailer I would add 1,440Lb, then I would need four tires who's combined max rating would be 8,640Lb and the Power King Towmax four tires combined are 8,800Lb. I have my safety margin for my unit.
However if you look at two additional TT models in Keystone 1/2 ton Series that are heavier then my unit, 25RLSWE and 26BHSWE which utilize the same tire we find those two trailers have a max weight rating of 7,800Lb. 20% of 7,800 would be 1,560Lb or 9,360Lb combined total of the four tires when they are only rated for a max of 8,800Lb this would be about 13% safety margin over max trailer weight! I suspect that for 5th wheels this could well be the same, the safety margin drops as the same tire is used on heavier units.
The 2200# dual rating for your tires is only applied when they are being used in a dual position like the rear tires on dually trucks. For your RV trailer axles they are rated at 2540# at 65 psi and have a 65 MPH maximum speed restriction set by their manufacturer. That means they are at their maximum load capacity range at 65 MPH. When operated above that speed they will degrade rapidly, especially when near their maximum load capacity.

According to your trailerís specs it has a GVW of 6670#. Keystone has most likely used 3500# axles and rated them somewhere from 3335# - 3500#. The exact figure will be on the trailerís certification label.

Note: The published hitch weight and certified total GAWR MUST exceed the certified GVWR.

CW
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Old 06-21-2013, 10:09 AM   #4
buzzcop63
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Very good observation and I bet their are going to be others who see things that I have not considered. What I hoped to show is that buyers of trailers that have worries about the tires the trailer rides on should be made aware that as they increase the trailer size and weight they decrease the safety margin that the tires provide, if the same tire is used on the heavier models. Would be interested in the safety margin on load that our trucks provide in comparison to that provided by the tires on our trailers?
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Old 06-21-2013, 10:40 AM   #5
buzzcop63
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Side of trailer GVWR shows 7,200 Lb, GAWR each axle is shown as 4,400Lb.

Corrected math using Single Max Load. Looking at Power King Towmax STR web sight I found the tires on my 2012 Cougar 27' TT, (model 24RKSWE) ST225/75R15LRD are rated at Single Max Load 2,540 Lb at 65PSI. My trailer max weight is shown at 7,200Lb by Keystone. If I want 20% safety margin for my 7,200Lb trailer I would add 1,440Lb, then I would need four tires who's combined max rating would be 8,640Lb and the Power King Towmax four tires combined are 10,160Lb. I have my safety margin for my unit and then some.
However if you look at two additional TT models in Keystone 1/2 ton Series that are heavier then my unit, 25RLSWE and 26BHSWE which utilize the same tire we find those two trailers have a max weight rating of 7,800Lb. 20% of 7,800 would be 1,560Lb or 9,360Lb combined total of the four tires when they are only rated for a max of 10,160Lb this would be about 30% safety margin over max trailer weight. I suspect that for 5th wheels this could well be the same, the safety margin drops as the same tire is used on heavier units.
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Old 06-21-2013, 05:28 PM   #6
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Buzzcop,

You are correct, the "max safety margin" decreases the heavier the trailer gets even if equipped with the same tires. I may be missing what you're asking, but ST tires are designated to be used on trailers only. They are not rated or "certified" the same as LT or P series tires. There is no requirement to have a "safety margin" on ST tires used on a towed RV. Theoretically, the manufacturer could put 3500 lb axles under the trailer, equip them with tires rated at 1750 lbs each and if the tongue is carrying 1000 lbs, assign a GVW of 8000 lbs. That's 7000 for the two axles and 1000 on the tongue. Yes, this is an extreme example, most RV's are equipped with axles/tires that give some slight leeway to a safety margin, but in the interest of building RV's with the "least amount of different parts" you will find several models built on the same Lippert frame/axle assembly. Some may be 24 ft long and weigh 4800 lbs dry and the other end of that "frame model run" may be 30' long and weigh 6500 lbs dry. You may well find all the models built on that frame have the same tires/axles under them. This will allow for more "leeway" in maximum tire load capacity on the lighter RV's built on that frame, and barely "squeek by" on the heavier units.

Since there's no "requirement" for a safety margin, the manufacturer builds and we buy........
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Old 06-25-2013, 08:20 PM   #7
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IMHO you have zero safety margin no matter which Chinese Towmax tires you use.
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