I was looking at a premium tire that had an "H" speed rating but a "B" temperature rating. That seemed a bit unusual as normally I expected to see a "A" temp. rating; unless it was a "T" or lower speed rating.
I guess I still do not understanding the Temperature Test and saw your explanation below. Could you put some numbers as an example? For example if a tire had a load rating of 1,200 lbs. and a 44 psi sidewall rated max pressure then I believe the tire would be tested at 44 psi ? However, I don't understand how or why the load may vary at 88% vs. 93% of rated capacity - not clear on the difference ?
Perhaps more importantly, since one normally is running at a lower than the max sidewall pressure, say 32 - 34 psi or ~10lbs.; does that provide a margin of safety so that a "B" temp. rating is not normally a concern over the life of a tire?
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Temperature Rating Test:
- The test is run at 88% of the rated load
- The pressure varies according to the type of tire, but the net effect is the tire is run at 93% load for the pressure.
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And, should one generally be concerned with a "B" temp rated tire if they are being used in the Southcentral and Southwest where speed limits are 80 mph and air temps can be 100F in summer (I suspect asphalt surface temps would be much higher)?
Cressida,
You've hit upon one of the reasons I don't like the temperature test. This is already confusing, and that test just makes things worse.
Now for some clean up: I want to apologize because I have been saying the speed rating tests were conducted at the wrong pressures. H speed rated tires are tested at 41 psi, and V and higher at 44 psi. - and it's actually more complicated than that!
Barry's Tire Tech: Speed Ratings
And here's a link to the UTQG webpage:
Barry's Tire Tech: UTQG Ratings
Allow me to clear up the load thing first.
Taking your example of a tire with a max load of 1200# with a max pressure of 44 psi.
What is the rated load? 1200#. What is the rated pressure? 35 psi (assuming a Standard Load P metric). Notice it is NOT the max pressure of 44 psi.
There would be load table associated with the tire and it would look something like this:
Pressure ....... Load Limit
35 psi...............1200#
32 psi...............1000#
29 psi............... 800#
(Note: all values made up!)
So if I test at 88% of the rated load, that's 1056#.
If the test pressure is 32 psi, then I am testing at effectively 105.6% load.
That's why the confusion on the load. The test is NOT conducted at the rated pressure, but is conducted at some percent of the rated load. which is different than the load limit for the given pressure. This happens at lot in tire testing.
So now let's look at the 2 tests:
.................Temperature ......... Speed
Room Temp.....95F..................77F
Effective Load...93%..............109%
B Temp = 100 mph minimum
A Temp = 115 mph minimum
H speed = 130 mph minimum
Notice that the tests are challenging in different ways, so it's not clear how a tire would perform in both tests.
My guess is that you've encountered a tire that barely passes an H rating, and doesn't pass the A Temp, so it is B Temp. Normally H rated tires will get into the A rating.
So what about driving in the southwest and south central?
I think this has more to do with the construction - specifically cap plies. H speed rated tires generally have cap plies, but you've encountered a tire that I wonder if it has full cap plies. It might be the B Temp rating is because it only has cap strips over the belt edges.
The states of interest are AZ, CA, NV, TX, and FL. There is data to support the idea that S and T rated tires are prone to failure in these states - less so in neighboring states.
What is MO? Not one of those 5. It might be OK.