Allow me to clarify:
All Season tires are designated by the letters "M" and "S"m, with perhaps a symbol between them, such as "/", "-", "+", but a symbol is NOT required. The tire itself has to meet some tread geometry criteria, such as a "luggy" shoulder (my term!), and some amount of sipes and cross grooves.
A summer tire is one that does not meet the criteria above.
A winter tire has to pass an actual test and can use the "Mountain/Snowflake" symbol. Prior to the creation of this test/symbol, there was no criteria and it was up to the tire manufacturer to decide what did and did not constitute a winter tire - and I think we are long past the point where the transition between those 2 systems has taken place.
So what about the actual construction and tread compound?
Construction wise, summer tires TEND to be H and higher speed rated, so their construction generally has a cap ply or 2. They also tend to be lower aspect ratio.
Compound wise, summer tires TEND to be better grip/ faster wear - however, that is NOT a defining characteristic and you may find some summer tires with decent wear characteristics - although I don't think it makes a lot of sense for a tire manufacturer to do this.
The construction of All Season tires is geared towards the speed rating - and that can range from S all the way to Z (although I can't think of any Z rated All Season tires!) Tread compounds also are wide ranged, but TEND to be better wearing / lower grip than summer tires.
Winter tires are constructed appropriate to the speed rating. Compound wise, they use colder weather compounds (ones that are more flexible at colder temperatures), but that is also not a defining characteristic.
At this point it might be useful to point out that there is a category of tire that seems to be developing - the All Weather tire. That's a tire that has a Mountain/Snowflake symbol, but is compounding for hotter weather, and, therefore, can be used year round. Please note that I am NOT saying All Season tires can not be used year round, but that a common complaint about All Season tires is the poor snow traction (that an All Weather tire addresses.)
And lastly, I want to address the issue of a trailer tire's traction:
Obviously the trailer must be slowed down by either the brakes on the tow vehicle or the brakes on the trailer - preferably both. What you do NOT want is for the trailer tires to lock up as that causes the trailer to try to get in front of the tow vehicle - very bad!
So I think the traction of a tire on a trailer is fairly important - one worthy of consideration.
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