Tires: horse trailer tires, worst case scenario, factory tires

Question
QUESTION: I have a 2005 Gore trailer with tire size ST205/75R151.  These are original factory tires, that have dry rot on two of the tires.  The service station where I got it inspected recommended replacing only the two tires.  1) what size tire do you recommend. (I think there is a tire for heavier load range).  2) is it safe to replace only the two tires showing wear?

ANSWER: Patsy,

First, you say you are only replacing 2 tires because of dry rot, then you ask if you can replace only the 2 worn tires.  Are they both worn and dry rotted?  If so, then you need to figure out why you are having different rates of wear.  The wear likely indicates a difference in the load that the tires are actually carrying - and perhaps the dry rot is a clue that this is the case.

If the 2 tires are only dry rotted, you have to figure that part out as well.  Is it because the pair are on the same side of the trailer and it's that side that is exposed to the sun when the trailer is stored?

Basics:

1)  For a 2 axle trailer, it is important that the load on each tire be nearly the same.  So you need to weigh the trailer, at least axle by axle, but if it would be better if you could figure out each individual tire's load.  

BTW, you ought to weigh the trailer in a worst case scenario:  Everything you can possibly think of that you would put in the trailer.

2)  The amount of space available and the width of the rim are going to limit what the maximum tire size will fit.  If you have the opportunity, go for the higher load range and use the higher pressure.  Trailers are notorious for being  ...... ah...... how best to say this .......  marginal when it comes to tire load carrying capacity.  My experience is that there ought to be a 15% unused capacity in the worst case.

3)  Unlike cars, where there is a rotating part that connects to 2 sides, a trailer has 2 tires close together on each side that work in tandem.  I'd recommend you keep the same tires on the same side of the trailer.

---------- FOLLOW-UP ----------

QUESTION: Two tires are dry rotted - one on either side of the trailer.  The other two tires are good.  What size tire are you recommending, and is there a special tire for heavy loads i.e horses?  Is that the higher load range?  Unfortunately, I usually only haul one horse and he will be on one side of the trailer - the driver's side.

Answer
Patsy,

First, you need to figure out why only 2 tires have dry rot.  If you can't, then pay close attention to everything about these 2 wheel positions.  The tires are trying to tell you something and you should be listening.

Your trailer is going to limit the size of tire that will fit on it.  So I can't make a recommendation on a tire size that is different, because I would need to know how much room there is.  The "ST" in front of the tire size means "Special Trailer" and those tires are designed specifically for small trailer use.  But going for a higher load range would be helpful.

However, if you change to a different load range you should change all 4 tires.

BTW, I still recommend you weight the trailer fully loaded