LRR (low rolling resistance) tires - Green Seal report & list

[hypermiler01]

According to Michelin themselves, the new HydroEdge is better than the Harmony. They won't tell me how either compares to the MXV4 Plus that I have on another car. Some blah-blah-blah about different speed ratings classes.

[NeilBlanchard]

Hi,

Originally Posted by hypermiler01

Some blah-blah-blah about different speed ratings classes.

This is the biggest load of crap in the tire sales business. Especially with women customers, they try to brow beat us into buying a tire "speed rated" at 150mph rather than one "only" rated for 120mph...sheesh.

[hummingbird]

Hi,

Newbie here, so sifting through old muck on the site...

How do you think hyperinflation would affect the 'speed ratings' that the company puts on the tyres?

If most energy losses happen due to hysterisis, as heard earlier on the forum, it should be affected severely because now the tyre does not flex much, so hysterisis losses would go down. So, if I get it right, a lower speed rated tyre would hold up to higher speeds (for a comparable energy loss, hence heat buildup) - quite an unintended consequence of hyperinflation...

Maybe I am mixing two issues up, but it certainly would bring clarity to an issue that vexes us each time visiting that tyre shop...

[brucepick]

Makes sense to me.
But if I were going to be driving at 90 or 120 mph instead of max 60-75, I'd want to make darn certain the tire was up to whatever I'm doing to it.

[CapriRacer]

1) In addition to the Green Seal report being quite old (2003), the report also does not account for the different tire sizes. The rolling resistance of otherwise identical tires varies according to tire size - as does rolling resistance coefficient.

The tire size vs rolling resistance (and rolling resistance coefficient) of tires has not been explored enough to be able to predict the RR (or RRC) from a single, but different, tire size.

2) Speed Ratings: There is a test that a speed rated tire has to pass. This is not a "test the tire, then assign the rating" sort of thing. It's a "this tire has this speed rating, so it passed the test" sort of thing.

Among the testing conditions specified are the rated load and the rated inflation pressure. The rated load is the one listed on the sidewall, BUT the rated inflation pressure is 35 or 36 psi for Standard Load (SL) tires and 41 psi for Extra Load (XL) tires. In other words, the rated pressure has nothing to do with what pressure appears on the sidewall. What is written on the sidewall is a MAXIMUM.

However, the load on the tires on a given vehicle and the inflation pressure used for that vehicle are not the same as the test conditions. Therefore the actual speed capability of a tire used in a particular situation may not be the same as the speed rating.

This is an area that is not well explored. While it is known that it is the standing wave that limits the speed capability of a tire (and not the heat being generated), the exact effect of inflation pressure vs load is not well understood - too many variables - except to say that both lower inflation pressures and higher loads reduce the speed capability of a tire.

We also know that there is an upper limit to inflation pressure - the burst pressure. Clearly the burst pressure is dependent on the strength of the materials in the tire's casing. Put another way, the stronger the casing, the higher the burst pressure.

- BUT -

Increased speed also means increased stresses, and therefore a stronger casing will be needed for higher speed ratings. The exact nature of the "stronger casing" is a trade secret for each tire manufacturer - but it is common knowledge that a circumferential overlay on top of the belt greatly enhances the speed capability of a tire (meaning that the sidewall plays very little role in speed ratings)

BTW, the maximum pressure listed on the sidewall of a tire is not a reliable indicator of burst pressure - not to mention that tires degrade over time as does the actual burst pressure. (It has been known for very old tires to burst at normal inflation pressure.)

In addition, overlays of the belt package also decrease a tire footrpint's sensitivity to inflation pressure. In other words, a tire with a cap ply maintains a more uniform pressure distribution over a range of inflation pressures.

[Jim Allen]

When translating the effect of LRR tires into your own mpg readings, it will pay to be up on the topic of speedometer accuracy, or lack thereof, due to tire size differences. Tire Rack (see link below) presented some info that bears consideration, else you make false claims of joyously increased mpg or become suicidal because you lost mpg due to a tire change.

I ran into this some time back checking the FE of my Accord, which, it turned out, has shorter tires than the speedo was calibrated for (namely 205/60R-16). These are factory installed tires ,BTW, but the speedo is calibrated for 205/65R-16 tires. The end result is that I was traveling about 64 mph while indicating 70 and indicating about 110 miles while traveling only 100. Needless to say, I was "Whoo-hooing" my great 31-32 mpg out of a 3.0L V6 while actually getting under 30 mpg.

Well, we're "only" going to get about 70K from the OE Michelins but I will be replacing them with 65 series tires. I contacted Honda, BTW, and they claimed to have no way to recalibrate the car for the shorter tires.. unlike some other mfrs that can reflash the PCM for a variety of tire sizes. The the 65 series tires have a radius about 3/8-in taller and will put the speedo back to a more accurate state, but also drop the rpms by about 75 at 70mph, which could deliver a slight mpg gain.

I have a programmer on my Ford pickup and I measured the tire circumference to the millimeter at my standard pressure, entered that into the programmer and my speedo is deadnutz. As the tires wear, I can remeasure to keep the speedo as accurate as possible. On my other cars, I determine a correction factor as accurately as possible and use that in calculating my true miles traveled.

Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires

[CapriRacer]

Originally Posted by Jim Allen

........... and I measured the tire circumference to the millimeter at my standard pressure, entered that into the programmer ............

A word of caution: The rolling circumference of a pneumatic tire is about 3% less than the measured circumference.

Originally Posted by Jim Allen

........... and my speedo is deadnutz. .........

which may mean the speedometer is calibrated to actual circumference and internally compensates for that 3%. Other vehicles may not do that!

Originally Posted by Jim Allen

............. As the tires wear, I can remeasure to keep the speedo as accurate as possible. On my other cars, I determine a correction factor as accurately as possible and use that in calculating my true miles traveled.

Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires

If you look closely at what Tire Rack did, they took a tire with 12/32nds tread depth and compared that to a tire with 2/32nds. That would be 10/32nds difference in radius = 20/32nds in diameter = 0.63" on a diameter of 24.9" or 2.5%. They only got a 1.5% difference in distance traveled!!!

Tread depth differences don't directly translate into circumference differences!

Also, many of the revolutions per mile (rpm) values are CALCULATED values. And take it from someone who used to do those calculations, the formulae may be old and unverified for many, many years, so their accuracy is suspect. Besides, most folks who would do those calculations would think that within 2% is close enough!!

Also, if you do the calculation on the values that Tire Rack has in the artcle, you'll see the diameter values vary from the rpm by 2½ to 3½%.

So use caution when using published rpm and dimensional values!

[Jim Allen]

Originally Posted by CapriRacer

which may mean the speedometer is calibrated to actual circumference and internally compensates for that 3%. Other vehicles may not do that!

Not quite sure what you mean here, Capri. Would you clarify, please?

How i measured circumference, by the way, is by marking the tire and the pavement, rolling forward one revolution and measuring the distance on the pavement (garage floor, actually). The programmer allows for increments as small as 1mm. I don't know that the programmer or the truck's PCM does any internal compensating, hence my query above, but I happen to know the designer of the programmer, so I can ask.

The other thing is that tires tend to expand a little from centrifugal force at speed. It caries from tire to tire, I guess, but even I'm not THAT pedantic! If there is a rule of thumb, I'd like to know what it is.

[CapriRacer]

Originally Posted by Jim Allen

Not quite sure what you mean here, Capri. Would you clarify, please?

How i measured circumference, by the way, is by marking the tire and the pavement, rolling forward one revolution and measuring the distance on the pavement (garage floor, actually). The programmer allows for increments as small as 1mm. I don't know that the programmer or the truck's PCM does any internal compensating, hence my query above, but I happen to know the designer of the programmer, so I can ask.

The other thing is that tires tend to expand a little from centrifugal force at speed. It caries from tire to tire, I guess, but even I'm not THAT pedantic! If there is a rule of thumb, I'd like to know what it is.

Jim,

That is an interesting way to measure circumference.

Most folks would have used a tape measure around a freestanding tire - and that was what I was referring to when I made my comment about internal calibration. I can think of several ways of dealing with the tire circumference / speedometer issue - This would be just one of them.

Expansion due to centrifugal forces?

Let me put it this way: I would think that load / inflation pressure effects would be greater than the effect caused by centrifugal forces. I know of no rule of thumb for load / inflation pressure nor one for speed.

[Jim Allen]

Originally Posted by CapriRacer

Jim,

That is an interesting way to measure circumference.

Let me put it this way: I would think that load / inflation pressure effects would be greater than the effect caused by centrifugal forces.

Capri: Is that,
"Interesting, that's the dumbest thing I've ever seen!"
or
"Interesting, cool way to do it!" ( : < )

Either way, it's not my idea. It's how the programmer wanted you to find the circumference. Easy to do. I though it was cool. My other method is to measure the loaded radius with the tire inflated to the pressure I will use.

If you have other methods, would love to hear about them!

As to centrifugal forces, a tire engineer told me regarding one particular tire (a big, high profile, 38 inch diameter, 15 inch rim, mudder) that the tire grew in circumference by several mm at high speed (he did not define "high"). I never did get to question him further to see if this was a general phenomenon, or something found just on that tire, or others like it. That's why I asked you about it.