Tires: Trailer Tires - 6 ply vs. 8 ply, st205 75r15, chevy camaro

Question
Hello Barry (nice name by the way), I have a 5x10 ft tilt-bed flat bed trailer.  I changed out the rims when I purchased it 8 years ago as I had a set of 15" Chevy Camaro rims and did not like the 14" tires/rims it came with (used car tires).  I purchased a set of Douglas P225/75 15 tires for the rims and they are now 8 years old.  One problem with getting the 225's is that when pulling heavy loads, the fender well would make a groove in the tire from pressing down.  I want to go with a smaller tire, and with -present tires being 8 years old, feel now is the time.  I have read that I would be better going with an "ST" rated tire which is recommended for a trailer.  I received a response from a company telling me they would recommend a ST205/75r15 size.  This would be an inch narrower and should give me some more clearance.  They recommended a C Load range (6Ply) tire, but also stated they have an 8 ply version as well.  My question is what is the difference between a 6 ply and an 8 ply tire?  My thought is due to the weight of the load, possibly an 8 ply would be better, but I wanted to ask the expert.  Thanks in advance, Barry

Answer
Barry,

The terms "6 ply" and "8 ply" are confusing because many folks think that means that is the number of plies.  For that reason I prefer the term "Load Range" - and that's what I am going to use in this discussion.

The functional difference between a Load Range C and a Load Range D is that the Load Range D carries exactly the same amount of load at the same pressure as the Load Range C, but use can use higher pressures to gain even more load carrying capacity.

Now for some math:

We don't know what the original tires were on the trailer - right?  If we did we would start there.

But a P225/75R15 has a maximum load carrying capacity of 1704 # at 35 psi.

An ST205/75R15 has a maximum load carrying capacity of 1820 at 50 psi for a LR C and 2150 # at 65 psi for a LR D.  Please note that at 35 psi both LR's have a maximum load carrying capacity of 1480 #.

Another way to look at this is that you would need 45 psi to get the same load carrying capacity as the P225/75R15's.

The problem here is that the stock rims might not be strong enough for the pressures used by the ST tires.

But in the big scheme of things, the rims will start to crack  before they fail - so you will have some warning before that happens.  Most likely the cracks will appear at the base of the flange and be circumferential in direction.  In other words, the flange would be pushed out, and the cracks will appear where the flange transitions from horizontal to vertical.

Hope this helps.