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Old 05-07-2016, 06:10 PM   #1
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Same size tire with different circumference.

I replaced all the tires on my 2013 Rockwood 8329SS about 18 mounts ago. The original tires were junk and started to fail after only 12 months of service. I bought 4 new Power King Tow Max ST225R7515tires. The new tires are the same size at the old ones. But, the circumference of the new is larger than the old. This has resulted in the tires rubbing the underside of the trailer. I had a tire fail recently and discovered the rubbing. I replaced a failed Tow Max with a Goodyear Marathon, same size, and noticed the difference in circumference. The trailer is equipped with Lippert Torsion Axles. I mention this because they are unique and the trailer sits lower to the tire than conventional leaf spring axles. Does it make sense that tires with the same size rating would have a different diameter and circumference? Seeing the two tires side-by-side, it becomes obvious one is larger than the other.
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Old 05-07-2016, 08:13 PM   #2
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There can be some tolerance in the actual size as compared to the stated size, but it shouldn't be a large difference.

How much difference in the diameter is it?
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Old 05-07-2016, 11:27 PM   #3
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I saw the same thing with my tire size. I was very aware because I have very close spacing between the two tires on each side.
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Old 05-08-2016, 08:59 AM   #4
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Diameter should be given by a simple linear relationship:

diameter = rim size + (2 times sidewall height)

For 225/75r15 that should be 15 + ((2 x 225 x .75) / 25.4) = 28.28 inches

Goodyear has charts at http://www.goodyearrvtires.com/tire-selector.aspx

Tires increase diameter substantially if there has been an internal belt failure but I wouldn't expect all 4 to go at the same time. Maybe the Chinese mislabeled their sidewalls.
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Old 05-08-2016, 09:27 AM   #5
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Quote:
Originally Posted by DaveSchwartz View Post
Diameter should be given by a simple linear relationship:

diameter = rim size + (2 times sidewall height)

For 225/75r15 that should be 15 + ((2 x 225 x .75) / 25.4) = 28.28 inches

Goodyear has charts at Goodyear RV Tires €“ Tire Selector

Tires increase diameter substantially if there has been an internal belt failure but I wouldn't expect all 4 to go at the same time. Maybe the Chinese mislabeled their sidewalls.
Dave gave the correct formula on how to approximate tire diameter according to size, as I have also posted this before. Although there can be a slight difference in actual diameter depending on how the manufacturers measure, number rounding, and their sidewalls, it is usually fairly small.

Comparing the charts that Dave posted above for the Goodyear Marathon and this one below for the Towmax however, they both list the same as 28.3 inches.

Towmax STR Tires | Own The Road

This is odd, and sight unseen I would think perhaps the numbers may be being kicked, and you perhaps may have a 205/75R15 somewhere in the mix. Is there anyway you can take a pic of the tire label you most likely have on the side of your RV. It is most likely a yellow tag.

Also pics of the tires sizes on the tire themselves (the Towmax and the replacement Marathon) would help....before researching further.


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Old 05-08-2016, 09:35 AM   #6
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BTW: you may have just rediscovered why it is recommended that you not mix tires of different size (obvious) or even from different manufacturers on the same axle (and since RV tandem axles are so close, maybe even all 4 should be the same size and manufacturer if at all possible)
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Old 05-08-2016, 11:53 AM   #7
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Tired of tires!

Hi, had the tow max on my crusader. It lives under covered storage, but the part with the spare gets morning sun for about 2 hours in the mid morning. So, I remove the spare to the garage when not traveling. Within 2,000 miles, 18 months old according to sidewall date, (trip from the factory to Az, plus my first trip of 150 miles), all four separated. Always had to add 5-10 lbs air due to loss. Even replaced valve stems and internal Schrader valves. They all looked ok, just grew larger, and had started rubbing each other, not the frame/body. Had to replace all 4 in Payson. Not much choice in tire types up there. Took what I could get. Fortunately, they are a mid-grade tire.

Just completed a 4,000 mile trip on the new set, with no pressure loss issues, or problems of any kind. By the way, my trailer has the upgraded 16 inch, load range E tires and heavier duty optional axles. My total trailer loading is very light at 10,600 lbs as weighed fully loaded with clothes, etc. No food. (Est. add 1-200 lbs) Deduct 1,800 pin weight when hooked. (With full water, and all holding tanks 3/4-to filled, which I never travel with)

Fast forward to my next trip. Getting ready to leave. The tow max spare, never on the ground, (kept in garage when not in use) also grew in size, with the tread separating from the tire itself. Fortunately, all five tires had warranty for the tire itself. Checked pressure regularly. Always had to add air.

I hope you have better luck with your tow max than I did. Good luck, and watch them close!
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Old 05-08-2016, 12:29 PM   #8
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In 2013 I had a tire rubbing issue with my new Windjammer. Turns out the factory installed the wrong size mounting bracket for the axles which did not allow proper clearance. FR replaced the axles with ones that have the proper clearance. During this process I discovered from Dexter that they recommend at least a 3 inch clearance to allow flexing when the trailer is underway. You might check with Lippert to see what they recommend.
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Old 05-08-2016, 12:47 PM   #9
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My OEM C rated Duro tires of the same size as yours were 28.8 inches. Four years later I replaced them with Hercules E rated tires which are 28.3 inches. So in my case yes, the same size tire can be a different diameter.


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Old 05-08-2016, 12:56 PM   #10
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Yes, tires of the same size can vary greatly. We went through a spell a few years back when people wanted to replace three of the bf Goodrich tires on their trailblazers and use the matching spare for the fourth. Started having issues with the automatic four wheel drive kicking in and out on dry roads. Turns out that the OEM version was a little shorter than the replacement BFG.
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Old 05-09-2016, 10:00 PM   #11
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I have posted the tire diameter difference on a previous post, but no one responded. Two mounted tires, the same inflation, with virtually the same numbers on the sidewall had a OD difference of nearly an inch. It was so visible that I thought it was a different size tire.
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Old 05-09-2016, 11:39 PM   #12
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Quote:
Originally Posted by bamawig View Post
I replaced all the tires on my 2013 Rockwood 8329SS about 18 mounts ago. The original tires were junk and started to fail after only 12 months of service. I bought 4 new Power King Tow Max ST225R7515tires. The new tires are the same size at the old ones. But, the circumference of the new is larger than the old. This has resulted in the tires rubbing the underside of the trailer. I had a tire fail recently and discovered the rubbing. I replaced a failed Tow Max with a Goodyear Marathon, same size, and noticed the difference in circumference. The trailer is equipped with Lippert Torsion Axles. I mention this because they are unique and the trailer sits lower to the tire than conventional leaf spring axles. Does it make sense that tires with the same size rating would have a different diameter and circumference? Seeing the two tires side-by-side, it becomes obvious one is larger than the other.
The picture depicts how much a tire can grow before it fails from tread separation.

http://www.irv2.com/photopost/showfull.php?photo=27625

New tires of the same size and load range can differ from manufacturer to manufacturer, especially when the tread depth is larger/smaller. The measuring rim determines the actual height of the tire. narrower or wider rims can cause height changes.
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Old 05-13-2016, 10:47 AM   #13
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Quote:
Originally Posted by bamawig View Post
I replaced all the tires on my 2013 Rockwood 8329SS about 18 mounts ago. The original tires were junk and started to fail after only 12 months of service. I bought 4 new Power King Tow Max ST225R7515tires. The new tires are the same size at the old ones. But, the circumference of the new is larger than the old. This has resulted in the tires rubbing the underside of the trailer. I had a tire fail recently and discovered the rubbing. I replaced a failed Tow Max with a Goodyear Marathon, same size, and noticed the difference in circumference. The trailer is equipped with Lippert Torsion Axles. I mention this because they are unique and the trailer sits lower to the tire than conventional leaf spring axles. Does it make sense that tires with the same size rating would have a different diameter and circumference? Seeing the two tires side-by-side, it becomes obvious one is larger than the other.
What were the original ones that failed, I'd like to avoid them.
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Old 05-23-2016, 09:42 AM   #14
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Picture in AIRDALE post shows a tire with poor design (too narrow or weak belt package) or that is in process of failing. Max Grown OD by industry guidelines for ST225/75R15 is 28.86". (Best if you measure OC). NOTE new tire is suppose to be near 28.31 If your tires are much larger than that I think you have good case for returning them to dealer for adjustment.
IMO they either are in process of failing or have manufacturing defect. There are a number of posts on RV blogs & forums showing the grown OD as evidence of probable internal structural failure.

If dealer wants to claim tires are OK then ask for something in writing that there will not be a belt separation in the next 2,000 miles
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Old 05-25-2016, 05:16 PM   #15
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Originally Posted by Tireman9 View Post
Picture in AIRDALE post shows a tire with poor design (too narrow or weak belt package) or that is in process of failing. Max Grown OD by industry guidelines for ST225/75R15 is 28.86". (Best if you measure OC). NOTE new tire is suppose to be near 28.31 If your tires are much larger than that I think you have good case for returning them to dealer for adjustment.
IMO they either are in process of failing or have manufacturing defect. There are a number of posts on RV blogs & forums showing the grown OD as evidence of probable internal structural failure.

If dealer wants to claim tires are OK then ask for something in writing that there will not be a belt separation in the next 2,000 miles
Before taking a 1,700 mile trip, we replaced a trailer tire. The shop did not have a balancing machine and stated that it wasn't necessary. Because we do balance all of our tires, we took it off to have it balanced when we replaced the TV tires. When attempting to spin balance the trailer tire, it was found to be severely out of round. Even the balance machine was shaking. We replaced the tire and balanced it. The faulty tire was returned for refund. If for no other reason, the tires should be spin balanced to assure they are not defective.The defective tire would not have lasted 1,700 miles.
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Old 05-26-2016, 12:38 PM   #16
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Before taking a 1,700 mile trip, we replaced a trailer tire. The shop did not have a balancing machine and stated that it wasn't necessary. Because we do balance all of our tires, we took it off to have it balanced when we replaced the TV tires. When attempting to spin balance the trailer tire, it was found to be severely out of round. Even the balance machine was shaking. We replaced the tire and balanced it. The faulty tire was returned for refund. If for no other reason, the tires should be spin balanced to assure they are not defective.The defective tire would not have lasted 1,700 miles.

If you are dealing with a "tire shop" that doesn't have a balance machine I would look for a real tire store rather than deal with "Billy-Jo-Bob's Cheap Tire Emporium and Bait Shop".

Have to wonder what brand tire "failed" the initial attempt at balance. Tires made by Tier 1 or 2 manufacturers are inspected with most checked for uniformity and out or round before they leave the plant.

List of Tier 1 - 4 from trade publication HERE. IMO if not on the list, i.e. Tier 5, I would only use on lawn mower or wheel barrow.
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Old 05-26-2016, 01:19 PM   #17
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If you are dealing with a "tire shop" that doesn't have a balance machine I would look for a real tire store rather than deal with "Billy-Jo-Bob's Cheap Tire Emporium and Bait Shop".

Have to wonder what brand tire "failed" the initial attempt at balance. Tires made by Tier 1 or 2 manufacturers are inspected with most checked for uniformity and out or round before they leave the plant.

List of Tier 1 - 4 from trade publication HERE. IMO if not on the list, i.e. Tier 5, I would only use on lawn mower or wheel barrow.
The tire was purchased at a trailer parts and repair shop that has been in business for a long time. In Florida, they cater to local businesses using flatbed and cargo trailers and feel they don't need tire balancing for local use.

We went to a national tire chain for TV tires and changed from the original P to LT tires. As for the trailer tire, I watched them take the tire directly to the balancing machine. When they tried to balance and noted the tire to be out of round, they called me out into the shop to visual see it was bad. They also checked the rim. When they mounted the new tire and balance, they again called me into the shop to see the difference. We actually got a better price on the tire.

The defective tire was returned for a refund with no hesitation. I would be guessing at the faulty tire brand. It was a known brand, but with making a major move, it became irrelevant after the return.
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Old 05-26-2016, 07:03 PM   #18
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If you are dealing with a "tire shop" that doesn't have a balance machine I would look for a real tire store rather than deal with "Billy-Jo-Bob's Cheap Tire Emporium and Bait Shop".

Have to wonder what brand tire "failed" the initial attempt at balance. Tires made by Tier 1 or 2 manufacturers are inspected with most checked for uniformity and out or round before they leave the plant.

List of Tier 1 - 4 from trade publication HERE. IMO if not on the list, i.e. Tier 5, I would only use on lawn mower or wheel barrow.
Sorry but the correct link for tire company Tier level is HERE.

Clipboard screw-up.
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