QuestionQUESTION: I've read your earlier response to a question regarding LT vs ST tire applications on RV's. Your answer cleared up several questions that I had.....thanks!......but I have a couple follow-up questions. Your explanation that LT tires are designed for better "side load" handling than ST's creates a question of whether the two types are constructed differently, and if so,how? Also, are ST tires with an E weight rating designed to handle more weight than LT tires with the same rating? Due to a recent blow-out incident on my 5th wheel while vacationing in Montana, I replaced all 4 tires - made in China - with Michelins. ST application tires were not available at the dealer in the size that I needed, so I followed the advice of their tire professional and installed LT in the same size and E rating. Even though the purchase is past history, I'm trying to determine whether I made a correct choice. Thanks again for your help!
ANSWER: Bob,
I have to be very careful in my wording here - because I don't think I ever said "that LT tires are designed for better "side load" handling than ST's". If I did, I may have meant it in a different way.
So your question: ".,.....whether the two types are constructed differently, and if so,how?..." - well, let's just say that I currently think they are constructed similarly - and sufficiently close enough to be considered "the same".
Your next question: "Also, are ST tires with an E weight rating designed to handle more weight than LT tires with the same rating?...." Not in the sense that you mean.
The load carrying capacity of a tire is a function of many things. Most people are aware that size and inflation pressure make a difference, but they are NOT aware that speed and the road surface also make a difference. In my view, ST tires are LT tires that have been "tweaked" for trailer usage at 65 mph max speed. and by tweaked, I mean small changes that trailer usage can allow - like narrower tread widths, and less tread depth.
You also said this: "......I followed the advice of their tire professional and installed LT in the same size and E rating....." I'm going to bet they went up a size from ST235/80R16's to LT235/85R16 (80 vs 85!!).
And if you read my website, you'll see that that is the advice I give:
http://www.barrystiretech.com/sttires.html
- BUT -
You need to do the other things I mention - weighing the trailer, using 85% of the weight capacity (and in this case it starts to get a bit complex because of the different way ST and LT load carrying capacities are calculated.), but I can help you with that.
---------- FOLLOW-UP ----------
QUESTION: Hi Barry,
Thanks for your quick response to my question. You're correct in that the tire dealer did go from an 80 to an 85 series tire. I followed the link you provided to your website and viewed the spreadsheets regarding ST > LT tire replacement options. I may be incorrect, but my understanding is that even though I didn't exactly gain load capacity by going from a ST235/80R16 to a LT235/85R15, I didn't really lose any. Did I misunderstand the chart?
Lastly, you mentioned that load capacities for ST and LT tires are calculated differently. Is there a "Reader's Digest" explanation for that, or is it dependent on multiple and/or varying factors.
Again, I appreciate your expertise and willingness to help us "tire- challenged" RV'ers.
AnswerBob,
Remember I said "....The load carrying capacity of a tire is a function of many things. Most people are aware that size and inflation pressure make a difference, but they are NOT aware that speed and the road surface also make a difference......"
That is the short version of why ST and LT tires have different load calculations.
