QuestionQUESTION: Can you tell me how tires are speed rated? What is the testing procedure and who performs the testing (each individual tire manufacturer?)? Reading through some of the previous posts sparked my interest. Where can you find that type of information (like what testing procedures are for different vehicle components)? Or is something like that even available?
ANSWER: Chet,
I can't tell you what testing procedures there are for other vehicle components, but SAE (Society of Automotive Engineers) might be a good place to start. Just be aware that most vehicle component specifications are written by the vehicle manufacturer and any testing to those specifications would be proprietary. However, some things would be standardized for the industry and SAE would be the organization doing the standardizing. This would include things such as horsepower rating, bolt standards, and other things the are more or less independent of the manufacturer.
But you asked about Speed rating tires. This is something that has been standardized - except there is some slight differences between several organizations as to the exact test procedure. What I am about to paste is a summary I made of the European standard, but the other standards are very similar.
The ratings are based on an ECE test. I think ECE stands for Economic Commission for European, but part of what they do is set testing standards for a wide variety of products.
The test consists of a tire mounted in a fixture at the rated inflation pressure and applied to a wheel at the equivalent of the rated load in a room at 77F. To pass the test (and receive the rating) the tire must sustain the speed without failure for 10 minutes. There is a warmup procedure that is tied to the speed rating desired.
Please note: This is a “testing to standard” kind of thing. It is known ahead of time what the intended usage is (the rating desired) and the test is conducted with the goal of “reaching the standard”. Needless to say, tire manufacturers generally run the test beyond the “standard” to assure ALL tires would pass the test. How and what their testing procedures are varies with each tire manufacturer, and I’ve heard of some very interesting extensions beyond the rated test condition.
A number of important things come out of this. I’ve put a plus or a minus sign in front of each to indicate how the speed rating test relates to real world conditions.
+ Rated Load – normally a tire is not used at the rated load, so the actual speed capability would be greater than indicated.
- Rated Inflation – High inflation pressures improve the speed capability, so using less than the rated inflation pressure would reduce the speed capability, even if the load is appropriate for the pressure.
- 77F – Any time the ambient temperature gets hotter than this, the speed capability is less. BTW there is an SAE speed rating test that is run the same way, but at 100F.
- 10 minutes – not a lot of time at the speed, but it would take a long time to develop any sort of long term durability issue with the tire.
- Recently produced tire – While the test doesn’t specify a recent production, that is in fact what gets tested. Tires do degrade over time and speed capability is certainly affected negatively.
The net effect of this is that there are many more negatives than positives, and so this should be taken into account when choosing tires.
I wrote this up about a year ago as a result of a discussion I had over the internet.
I hope this helps.
---------- FOLLOW-UP ----------
QUESTION: You said that the procedure is standardized. By whom is it standardized? Is there written info on the subject (or a listing of the minimum standards) any where? Also, is the procedure the same for all types of tires (car, light truck, RV, heavy truck, etc.)?
AnswerChet,
You asked:
"You said that the procedure is standardized. By whom is it standardized?"
I think in the part that I pasted I indicated it is an ECE test.
"Is there written info on the subject (or a listing of the minimum standards) any where?"
Yes, and you have to find a copy of the ECE test procedures, which might be a bit difficult unless you know where to look. I'm guessing that you would be hard pressed to find it by searching the net. It's more of a thing that only tire testing folks would be interested in and they would generally work off hard copies.
"Also, is the procedure the same for all types of tires (car, light truck, RV, heavy truck, etc.)?"
That is clearly delineated in the test procedure, but my memory says they are substantially similar.
