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Old 07-18-2014, 03:31 PM   #1
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Tires and Rim Load Range Capability--Please Help

The tag on my 1979 Sovereign says the following:

GVWR 7100
GAWR Front 3200
GAWR Rear 3200

15 x 6 Rims at 40 PSI cold
7.00 x 15LT-LRC or 7.00 x 15xCA-LRC

When I purchased the trailer about five years ago, the seller put new tires on prior to my traveling to pick up the trailer. The shop he took the trailer to installed ST205 75 R15 6-ply radial tires. He also said the tire shop that installed them said the rims are marked as Load Range C.

While the tread depth on the tires is still pretty much good as new, and the sidewalls show no dry rot and appear good as new as well, I have an upcoming trip and am reluctant to drive on the tires since they are 5 years old. I took a tire to two tire shops and was informed by both that the rim is 5.5" wide (not 6 like the trailer tag shows) and that I can run 225 75 R15s no problem even though the rim is only 5.5" wide. They both said a 225 is specified for a 6 inch rim, but that a quarter inch less on either side isn't a big deal. I was told 235s would be pushing it, but may be possible. When I mentioned that I believed the load range of the steel rim was a C, both shops said I could run whatever load range tire I wanted as long as the tire wasn't too wide for the rim. When I asked about the increase in PSI being too much for the rim, both shops said there was no concern. They also both said that if all my tires looked like the tire I brought in, that they would go ahead and use them for my upcoming trip despite them being 5 years old.

Do I get four new tires, or use these for one more trip? After this upcoming trip (about 5 hr drive one way), it will probably be at least 9 months before I pull the trailer again.

If new tires are in order, do I need to pay attention to the Load Range of the tire? Would a Load Range D be alright even though the rim is marked as Load Range C?

Should I go with the 225 75 R15 instead of the 205s?





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Old 07-19-2014, 07:27 AM   #2
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OK, some reality checks on the information you provided:

1) 40 psi for the rims does not make sense. At 40 psi, the original tires would not have had enough load carrying capacity for the GAWR's.

2) The rims would not have been marked with a Load Range. Load Ranges are for tires. If the rim was marked, it would have been marked with a maximum load.

3) According to the standards, an ST225/75R15 has an allowable rim width range of 6" to 7". While you could get away with a 5.5", it should be avoided.

4) Yes, it is true that you can run any tire you want on the rim, it is always a good idea not to exceed the rim specs. So to try to put this in the terms you are using, you can mount a Load Range D or E on a Load Range C rim, but you should run the tire to Load Range C specs. (Remembering, rims don't come in Load Ranges!!)

5) And lastly, you didn't tell us if the tires are Load Range C. I'll bet they are.

So to answer your questions keeping in mind the 5 items above.

1) You live in Kentucky, where it doesn't get so hot. I am of the opinion that you are probably OK for this year. If you lived in Arizona, or Texas, I would have a different answer. Others may disagree.

2) I think you ought to verify all the info on the rims. The info is usually on the back side of the wheels, so all you need to do is dismount the wheel and turn it around. I'll bet the wheels will be good for at least 65 psi.

3) You would be OK with the ST205/75R15's but if you have the opportunity to upgrade to a Load Range D, that would be good. (And if you do, use the Load Range D specs - 65 psi)

4) If you have a conflict with the rim's maximum pressure and the tire's maximum pressure, it is my opinion that the rim's pressure specs aren't that important and you should follow the tire specs. (But that is a whole 'nother discussion. Let's discuss that when you after you find a conflict - which I am not expecting)

5) You should use the max tire pressure listed on the tire. It will be in small letters near the rim line. It will be either 50 psi for a Load Range C or 65 psi for a Load Range D.

6) And lastly, I don't think you need to go to an ST225/75R15.
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Old 07-19-2014, 08:45 AM   #3
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The current tires are Load Range C. My understanding is that the shop told the prior owner that Load Range C tires were all the rims could handle. So perhaps they weren't stamped as LRC, but the maximum PSI stamped in the rims was equivalent to LRC.

What would be the concern with the 225s? In terms of height, the 225s would be closer to the original 7.00 x 15s. The 205s are about an inch shorter than the 225s. Of course the section width of the 225s is .5 inches more. It is strange that the rims measure 5.5" whereas the tag on the trailer indicates 6 in rims. The rims, including the spare, do appear original.

Are there any times when the PSI of a trailer tire when COLD should be below the maximum PSI?


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Old 07-19-2014, 09:24 AM   #4
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You should also be concerned with the speed rating of the tires. If the speed rating is only 60 mph and you want to run at 70 to 75 mph Then you may end up like a friend of mine. He had a tire tread separate in North Carolina on his way home to Ohio. The tread acted like a big tree stump chipper and ripped out the wheel well and side of his SOB (some other brand) trailer.
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Old 07-19-2014, 09:33 AM   #5
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Yes, damage to the trailer is something else that has concerned me about running these 5 year old tires.


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Old 07-19-2014, 09:45 AM   #6
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Tires and Rim Load Range Capability--Please Help

As far as the stamped card on the Airstream specifying 40 PSI, I wonder if that isn't so far off given the weight of the trailer. The trailer weighs roughly 5,000 lbs (Airstream actually says 4800) so let's say when packed up it's at 6,000. According to a link HoweE made reference to in another thread, at 40PSI my current tires would be able to handle a load of 6440lbs. I realize the stamp on the trailer is referring to 7.00 x 15s, whereas the load table is refers to trailer tires. Nonetheless, it appears 40PSI would not be so low afterall.

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf


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Old 07-20-2014, 07:04 AM   #7
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KY,

You need to look at the back side of the wheels before we go any further. It's important that we get that info correct.

Why not 225's? Because they exceed the specs for the wheels (assuming they are indeed 5.5", which is why you need to look at the backside of the wheels!)

40 psi? No, not possible. That would mean that Airstream violated a Federal Law as the GAWR's are larger than the tire load capacity. I am sure that is NOT the case!
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Old 07-20-2014, 08:40 AM   #8
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Originally Posted by KYAirstream View Post
As far as the stamped card on the Airstream specifying 40 PSI, I wonder if that isn't so far off given the weight of the trailer. The trailer weighs roughly 5,000 lbs (Airstream actually says 4800) so let's say when packed up it's at 6,000. According to a link HoweE made reference to in another thread, at 40PSI my current tires would be able to handle a load of 6440lbs. I realize the stamp on the trailer is referring to 7.00 x 15s, whereas the load table is refers to trailer tires. Nonetheless, it appears 40PSI would not be so low afterall.

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf


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The original steel wheels that Airstream used back then were rated at 2800 pounds each.

If load range "C" tires were used for years, then you cannot increase the rating to a "D".

To do so, encourages the wheels to split. as per the manufacturers of wheels.

The best thing you can do, is purchase new wheels, that are now rated at 2600 pounds each and install "D" tires.

The correct tire size is ST 225/75R15 load range "D".

Generally speaking, load range "E" tires are over kill. To properly air them, creates a rougher ride for the trailer.

Andy
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Old 07-20-2014, 08:44 AM   #9
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Tires and Rim Load Range Capability--Please Help

I did check the back of two wheels and didn't see any markings showing maximum PSI. I cleaned through a bunch of rust, but still saw nothing. As far as the tag attached to the trailer indicating 40PSI, I can take a pic of it next weekend if you like.

I think I found a solution that will get me out of the ST tires and into a LT tire that is slightly larger(although mininal), but will still work with the rim. Yokohama makes the Geolander in a LT215 75 R15. It is Load Range C, but that's no different than my current 205s.

I appreciate all the help. Any concerns with the Geolander? They have a max load of 1765, or 7060 for the 4. Not a huge margin above my GAWR for two axles (6400), but still past it.


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Old 07-20-2014, 08:48 AM   #10
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Originally Posted by KYAirstream View Post
I did check the back of two wheels and didn't see any markings showing maximum PSI. I cleaned through a bunch of rust, but still saw nothing. As far as the tag attached to the trailer indicating 40PSI, I can take a pic of it next weekend if you like.

I think I found a solution that will get me out of the ST tires and into a LT tire that is slightly larger(although mininal), but will still work with the rim. Yokohama makes the Geolander in a LT215 75 R15. It is Load Range C, but that's no different than my current 205s.

I appreciate all the help. Any concerns with the Geolander? They have a max load of 1765.


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No way. Those tires would blow probably on your first trip.

Goodyear Marathon's load range "D", are rated at 2540 pounds each.

It's wise to stay with a major brand tire, so that should you have a problem, it's easy to find help when traveling.

Andy

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Old 07-20-2014, 09:48 AM   #11
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Originally Posted by KYAirstream View Post
I did check the back of two wheels and didn't see any markings showing maximum PSI. I cleaned through a bunch of rust, but still saw nothing. As far as the tag attached to the trailer indicating 40PSI, I can take a pic of it next weekend if you like.

I think I found a solution that will get me out of the ST tires and into a LT tire that is slightly larger(although mininal), but will still work with the rim. Yokohama makes the Geolander in a LT215 75 R15. It is Load Range C, but that's no different than my current 205s.

I appreciate all the help. Any concerns with the Geolander? They have a max load of 1765, or 7060 for the 4. Not a huge margin above my GAWR for two axles (6400), but still past it.


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What is a maximum load?

Is it with the trailer standing still?

Or might it be when the tire/tires hitch a bump?

To me, a tire should never, ever, be loaded to even near maximum.

I believe that using tires that have a minimum of a 20 percent safety factor is ideal, and at the same time still providing a soft ride.

"C" tires are simply not enough for a large Airstream, nor are "E" tires needed.

Every thing exposed to use has a "comfort zone".

It's that comfort zone that provides safety instead of riding on the edge of a cliff.

Andy
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Old 07-20-2014, 08:15 PM   #12
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Andy- this trailer is a 1979 Airstream and the tag on the front of the trailer specifies Load Range C tires. The archive section of Airstreams website also shows Load Range C tires when looking at the 1979 specs. Did Load Range C tires carrier a heavier load in 1979 than they do today?




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Old 07-21-2014, 07:02 AM   #13
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Man, oh man, oh man.

We need to get your facts straight:

The load carrying capacity of a 7.00-15LT at 40 psi is 1610#. So I stand corrected. I was looking this up on as a radial tire. Sorry about the error.

So if the original wheels were rated for 2800, then why change to wheels rated for 2600#? Besides an ST205/75R15 Load Range D has a load carrying capacity of 2150#. So either of those wheels are capable of handling Load Range D.

Heck, even an ST225/75R15 Load Range E is only rated to 2830#, so even if the trailer is overloaded according to the GAWR, then that original wheel would be only the littlest bit overloaded.

And, yeah, that LT215/75R15 has more load carrying capacity than the 7.00-15LT did at 40 psi - 1765# @ 50 psi (and that is what I would recommend you use.) And the allowable rim width works: 5.5" to 7". It's a Q speed rated tire (99 mph)

I consider Yokohama a major tire manufacturer. You can find Yokohama tires pretty much everywhere. What is going to be a problem is finding unpopular tires - like ST tires and odd sized LT tires - and both of those things apply here.

So I am comfortable with the LT215/75R15's. They have 10% more load carrying capacity than the GAWR. They are Q speed rated (99 mph vs 65 mph for the ST's), so the actual load carrying capacity at 65 mph would be more than labeled.
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Old 07-21-2014, 08:32 AM   #14
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Man, oh man, oh man.

We need to get your facts straight:

The load carrying capacity of a 7.00-15LT at 40 psi is 1610#. So I stand corrected. I was looking this up on as a radial tire. Sorry about the error.

So if the original wheels were rated for 2800, then why change to wheels rated for 2600#? Besides an ST205/75R15 Load Range D has a load carrying capacity of 2150#. So either of those wheels are capable of handling Load Range D.

Heck, even an ST225/75R15 Load Range E is only rated to 2830#, so even if the trailer is overloaded according to the GAWR, then that original wheel would be only the littlest bit overloaded.

And, yeah, that LT215/75R15 has more load carrying capacity than the 7.00-15LT did at 40 psi - 1765# @ 50 psi (and that is what I would recommend you use.) And the allowable rim width works: 5.5" to 7". It's a Q speed rated tire (99 mph)

I consider Yokohama a major tire manufacturer. You can find Yokohama tires pretty much everywhere. What is going to be a problem is finding unpopular tires - like ST tires and odd sized LT tires - and both of those things apply here.

So I am comfortable with the LT215/75R15's. They have 10% more load carrying capacity than the GAWR. They are Q speed rated (99 mph vs 65 mph for the ST's), so the actual load carrying capacity at 65 mph would be more than labeled.
When a steel wheel has been in service for an extended period of time, increasing the tire to a larger size, with a higher air pressure usually leads to wheel failures.

Not me, but from wheel manufacturers many years ago.

History has certainly confirmed that issue.

Airstream unfortunately under rated the load range needed on the older tarilers. Note what they spec today.

Also, usung a smaller tire, is also a loss of proper traction. That certainly decreases the safety of towing on a wet or snow covered highway.

If anything, use a larger size, like 235 instead of smaller.

The cost of tires, is really extremely small, compared to the safety needed.

Andy
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