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Old 03-19-2019, 08:44 AM   #1
Bostongone
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Running D rated tires on C rated rims

Looking at some new Carlisle ST205/75R15s for the trailer and I find that I have C load rated tires on 15x5 inch rims on 3500 lb axles. The C rated 6 ply Carlisle tires are rated at 1820 lbs at 50 psi while the D rated 8 ply Carlisle tries are rated at 2150 lbs at 65 psi. First question is does it make sense (or can I) put the D rated tires on the 50 psi rims and run them "derated" at 50 psi. I have seen some derating tables but the derating variance was only around 7 psi.

Second the rims are supposedly 5.0 inch bead while the Carlisle tires are stated to be 5.5 inch bead. Is that a potential problem or not a big deal?
Have not priced out 65 psi 15x5.5 in rims yet.
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Old 03-19-2019, 10:47 AM   #2
rjrelander
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Your current rims should have a weight carrying capacity stamped on them. You might need stronger valve stems for the extra PSI but I don't think the rims themselves are rated for PSI, just the weight carrying capacity. I don't believe it's recommended to derate pressure so I would stick with the 65 PSI. The 3500 lb axles limit it to 1750 lb per tire. Assuming that going to the D rated 2150 lb tires is only for extra safety margin and normal degradation over time. The rim width should be based on the section width, 205mm in this case, which as you stated, Carlilse recommends 5.5" width.

Not sure what was on the trailer originally, but if you need to go to a wider rim anyway, you should probably find one that matches the 2150 lb. weight on the new tires even though (don't get anyone started) you would never be loading anything more than what the OEM tires were capable of carrying, especially with 3500 lb axles. Also, make sure that the offset on the new rims is correct so the side clearance between the tire and the wheel well doesn't change.

EDIT: Bigger problem might be finding 5.5" wide rims. Most 15" rims are going to be either 5" or 6" width. Are you sure the current rims are 5" width? If the old tires are ST205/75R15 LRC then you might have 5.5" wide rims already.
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Old 03-19-2019, 12:40 PM   #3
CWtheMan
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Here's a new Carlisle ST tire spec sheet. Both the sizes you mention are for the 5.5" rim width.

https://www.carlislebrandtires.com/o...adial-trail-hd
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Old 03-19-2019, 02:42 PM   #4
Bostongone
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rjrelander, as you surmised am considering the D rated tires for the safety margin as my GVWR is 7740 and I not planning to exceed that. So your thought that 65 psi should be ok as long as I don’t exceed the 1820 lb rating of the rims (same as C rated tires) and use steel valve stems? Plan on taking spare and one of the trailer tires to my local shop and have them broken down to verify bead. Strange that Carlisle and Greenball only seem to offer this tire in 5.5” recommended bead, of course these rims are 10 years old so not sure what was popular back then?
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Old 03-19-2019, 07:46 PM   #5
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The rim width range for the Carlisle ST205/75R15 tire is 5 - 7". The measuring rim width was 5.5".
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Old 03-20-2019, 05:33 AM   #6
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CWtheMan, thanks for the information. I might have guessed at maybe 5 to 6 inch range but out to 7 is interesting. What do you think about running them at full 65 psi or would running them at 60 psi be acceptable since folks are always airing their tow vehicle down from 80 psi to 60 psi when not towing?
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Old 03-20-2019, 08:06 AM   #7
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Quote:
Originally Posted by Bostongone View Post
CWtheMan, thanks for the information. I might have guessed at maybe 5 to 6 inch range but out to 7 is interesting. What do you think about running them at full 65 psi or would running them at 60 psi be acceptable since folks are always airing their tow vehicle down from 80 psi to 60 psi when not towing?

You will have to first verify the pressure ratings of the wheel and it's valve stem.

It's best to just change the valve stems to metal. They are normally recommended for all RV trailer applications.

Some wheels have PSI ratings and some do not. The safest way to verify those without any markings for PSI is to call the manufacturer and provide them with the wheel PN & MN. The safety warning the tire industry gives is; "never exceed a wheel's PSI rating". Like tires, that's a recommended cold PSI.

NOTE: Many of the trailer industry OEM providers have easily found links on-line. In the links they normally have the specs for all the wheels they provide to the RV trailer industry.

Here is a link to a large OEM provider of RV trailer wheels/tires.

http://lionsheadtireandwheel.com/Wheels


Here is another link to a very sturdy steel valve stem.

https://www.etrailer.com/s.aspx?qry=...s_and_Wheels#/

Most tire retailers don't have steel valve stems on hand. Automotive parts stores do. They will also carry replacement cores. I keep those in my tool box. Very handy to have a couple on hand. Sometimes when checking your air pressure one will not seal properly or it's just worn out.
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