QuestionHi, I have had troubles with some new ST tires I had installed. Just various problems such as blow outs. The size is ST205/75r15. I always check tire pressure and inspect/check tires frequently and I even went from load range C to D's. I have a Terry 26ft. tandem axle travel trailer 4600lbs dry 6700lbs max. I have read online that many people are changing from ST's to LT's and even some toy haulers now come with LT tires instead. Do you have any thoughts on going from ST to LT? I have read a lot on the differences but just seems like there is so many problems with ST's even though they are built for the trailers abuse. THANK YOU for your time!
AnswerSeth,
The derivation of ST tires is an out-growth of leftover molds when the tire sizing changed. There was a need for inexpensive tires for trailers, and the tire manufacturers had these molds that would otherwise be thrown away.
But this is more about load carrying capacity:
An ST225/75R15 Load Range C has a load carrying capacity of 2150 # @ 50 psi.
AN LT225/75R15 Load Range C has a load carrying capacity of 1875 # @ 50 psi.
That's a difference of 275# (9%) in favor of the ST. However, LT's and ST's are built very similarly, so there isn't any real difference, except to say the ST's are more highly loaded.
BUT
If folks are smart, while they are migrating from ST's to LT's, I'll bet they are also doing 2 things that will have a major impact on overall performance:
1) They are switching up in load range and using more inflation pressure - which has the effect of reducing heat buildup for a given load.
2) They are switching to a major brand. ST tires tend to be built by niche tire manufacturers, while LT's tend to be built by major tire manufacturers. Needless to say, a major manufacturer is going to be more up to date on materials and design techniques - and that will translate into better performance.
BUT
An ST205/75R15 Load Range C has a load carrying capacity of 1820# @ 50 psi. 4 tires would have a max load carrying capacity of 7280#. Your trailer say the max GVW is 6700#.
Assuming a 10% tongue weight, then the tires actually see about 6090#.
So on the surface, there appears to be sufficient load carrying capacity in the tires. But the speed limitation is 65 mph. To get a speed capability between 66 and 75 mph, the inflation pressure must be increased 10 psi, and for speeds between 75 and 85 psi, not only is the pressure to be increased the previously mentioned 10 psi, but the load must be reduced 10%.
Plus, one of the lessons from the Firestone / Ford situation was that the tire needs to have more unused load carrying capacity (The Explorer didn't have any!) I like to see a 15% reserve beyond all these things we've previously mentioned.
So I think that 4 ST205/75R15 Load Range C's have a usable load carrying capacity of 5755# - which is less than the 6090#.
It would be a good idea to weigh the trailer fully loaded - with everything you can possibly think of that you would put in - a worst case. That also means all water tanks full - gray, white, black. I know that is not how this usually works, but there is always the possibility that this might happen so it needs to be considered.
Weigh the trailer tire by tire if possible, but at a minimum, axle by axle. This will give you a good idea of what the actual load on the tires are.
But you posted another question to me that relates to using a 7" rim - and I'm not sure how long you have had them and whether it has had any change in your overall tire performance.
But just to be clear, the term "Blowout" covers a lot of territory. The way I use the term only includes a rapid loss of inflation pressure, which is always due to impacting an object that in turn damages the structure of the tire to the point where it can no longer contain the air pressure - which a resulting explosion.
But some folks use the term to mean a tread separation - and some use the term to mean the result of running a tire with little or no inflation pressure - more properly called a "run flat" - and that's usually the result of a puncture.
Only the tread separation should be considered linked to the tire size and load carrying capacity. The other 2 are road hazard and should be completely discounted when trying to solve a problem of this type.
