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Old 04-07-2009, 08:28 PM   #31 (permalink)
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Higher tire pressure works to a point. Going to the max inflation of 44 psi on the Avons on my Mazda ate the tires very quickly. Since I like to slide around a little, I pretty regularly get about half the rated tread life of any tire running between 36-38 psi (IIRC, recommended is 32), but I only got about a quarter tread life with 44 with the center very obviously worn most (and that was only going to 44 towards the end and driving slow when gas went to $4/gal.). 44 vs. 38 didn't make any real difference in mileage—I only averaged about 1 mpg better per tank, but I was also driving slower.

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Old 04-07-2009, 09:09 PM   #32 (permalink)
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Interesting, evolution! Your experience is counter to that of a number of people around here and/or on cleanmpg. In general, running pressures at (or in some cases in excess of!) the max rated pressure for the tire led to longer life over similar driving conditions than driving at the "placard pressures" recommended by the car manufacturers.

And few people there have experienced the center-tread wear from high pressures.

I did to a small extent on my previous tires, but not much of it.

I guess that goes to show that there are no universal rules...

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Old 04-07-2009, 09:53 PM   #33 (permalink)
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Ernie,

First, you need to understand that the language that ANYONE who makes a product speaks is SALES. Speak to them in that language and they will not only pay attention, but the pressures of competition will COMPEL them to take action. But this pressure has not been brought to bear. Here’s an example:

http://www.greenseal.org/resources/r...resistance.pdf

On page 5, that report highlighted the fact that a Bridgestone B381 has an extremely low RRC – over 20% better than the next best tire. This was in March of 2003. You’d think this tire would sell like umbrellas in a rainstorm – and, of course, that Bridgestone will see this and react accordingly. But even though plenty of time has passed, this information has not translated into a whole line of tires with the same properties.

WHY NOT??

Look up this tire on Tire Rack. The average rating for this tire is 6.06 (out of a possible 10).

If you compare that to the average rating of other all season tires in the survey, this tire would be 15th out of 21 – if it had made the list. A pretty poor showing! (This tire doesn’t actually show up on the survey result – AND – the average rating isn’t published as part of the summary – I had to do the math myself.)

The only conclusion I can come up with is that fuel economy doesn’t sell tires. This is consistent with what I’ve heard about what consumers want in a tire. So there is no competitive reason to publish the results of RRC. In other words, why should a tire manufacturer go to all the effort (and expense) of testing a tire for RR when it doesn’t translate into sales?

But I’m a technical guy – and my expertise is not in marketing and sales.

- AND –

The reason I spend so much time posting on the ‘net is to try to educate people on tires and how they work. You’ve asked a very good question:

Quote:
Originally Posted by Ernie Rogers View Post
.......

--I get the feeling at times that the tire engineers themselves don't understand rolling resistance. If they get inconsistent results in different tire sizes, it just means they aren't controlling the right variables. If they will publish the results, we can help them with the problem.
OK, that’s more of a statement.

The problem isn’t consistent results or controlling the right variables. We’ve been at this for over 30 years and we can consistently get repeatable results – except that there is a problem that is best explained by looking at page 31 of this:

http://www.energy.ca.gov/transportat...%20Testing.pdf

Page 31 is the RRC for a range of sizes, but they are all the same tire line. Bruce Lambillotte has paraded different versions of this data around the tire industry and one time he revealed that the tire line is Goodyear Integrity.

This is the problem you alluded to. There is a 30% variance best to worse for the same tire albeit in different sizes. The test is a standard RR test using standard conditions – all tied to the same load table. This is as consistent testing conditions as can be made.

For background, Smithers is a well respected tire testing company. They do work for tire manufacturers, car manufacturers, etc. They know what they are doing and they deliver good results. This data is consistent with what various tire manufacturers have reported with regard to tire size. In other words, an independent lab, contracted by the California Energy Commission (someone who doesn’t have a “dog in the fight”) reports that tire size is a factor in RRC, even though the tires are otherwise the same.

Starting on page 32, Bruce analyzed various factors to see if he could develop some sort of correlation – with the idea that this would lead to an easy way to characterize a tire line. The best correlation was 86% for load carrying capacity. I should point out that statisticians like to see at least a 95% correlation to feel comfortable that they are mathematically describing a phenomenon.

This, in essence, is the problem confronting anyone who tries to deal with RR when comparing tires of different sizes.

Quote:
Originally Posted by tasdrouille View Post
.........

I mean measuring RRC is not rocket science. I don't see why they can't come up with standardized test equipment giving repeatable results. Every manufacturer gets the same test bench and make their own measurements and publicize the results.

.....

The problem ISN’T test equipment or repeatable results. The problem is describing the effect tire size has on RR. If this isn’t overcome, then the tire manufacturers are going to have to test every tire they make in every size they make – and they don’t want to do that. Plus, as I said before, there isn’t enough testing capacity to get this done in a reasonable timeframe.

But this brings me back to the first part of my post. Unless there is an easy to use method to compare tires, then the average consumer is just going to ignore it – which is pretty much what he is doing now.
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Old 04-08-2009, 07:54 AM   #34 (permalink)
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RRC is a tire's property that is important only to few people. You then don't have to test all tires, just the tires someone who cares about RRC would like to buy. Probably even fewer people care about the RRC of a pzero nero than there are ecomodders posting here, but if a company makes a tire with "eco" or "fuel" in its name suggesting that it's a tire with a low RRC, I woud like to know that value. Most manufacturers have what, 1 or 2 model marketed as fuel efficient? That narrows the field quite a lot.

Regarding correlation between RRC and size. For a given model, take a reference size and compare the rrc with other sizes so each size is a percentage of the reference size. You'll get a different percentage for every size and the correlation will not be linear and, like you stated, not completely significant. However, what would be interesting to do is perform the same test on a couple other models and see how the coefficients (percentages vs ref size) for a given size correlates between models. Say 195/50R15 is 1.1 of a 195/65R15 for tire model A, is 195/50R15 also 1.1 of a 195/65R15 for tire model B?
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Last edited by tasdrouille; 04-08-2009 at 07:59 AM..
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Old 04-08-2009, 01:31 PM   #35 (permalink)
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Thanks, CapriRacer,

For being here to help out. That last post was very helpful.

I have been studying the presentation by Lambillotte. Can we see the full test report? The slide show just has too much information missing. And, if the raw data are available, I'm sure we can coax more information out of it. I don't need manufacturers' names, just data by product line.

About the chart on page 31--it clearly shows that larger tire sizes are more efficient. (This is consistent with the widely-known fact that rolling resistance decreases with increasing tire diameter.) But, we need more information about the test conditions such as tire pressure, loading, tread and sidewall thickness, and other variations in the tire construction.

Smithers may be a respected testing company, but it's apparent they don't know what to do with the numbers they have. It's regrettable that this work had so little oversight or peer review.

By the way, I find it almost humorous that the data we have all been asking for has already been collected by Smithers.

Quote:
Originally Posted by CapriRacer View Post
Ernie,

<SNIP>
The problem isn’t consistent results or controlling the right variables. We’ve been at this for over 30 years and we can consistently get repeatable results – except that there is a problem that is best explained by looking at page 31 of this:

http://www.energy.ca.gov/transportat...%20Testing.pdf

Page 31 is the RRC for a range of sizes, but they are all the same tire line. Bruce Lambillotte has paraded different versions of this data around the tire industry and one time he revealed that the tire line is Goodyear Integrity.

This is the problem you alluded to. There is a 30% variance best to worse for the same tire albeit in different sizes. The test is a standard RR test using standard conditions – all tied to the same load table. This is as consistent testing conditions as can be made.

For background, Smithers is a well respected tire testing company.
<SNIP>
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Old 04-08-2009, 01:46 PM   #36 (permalink)
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Further comment on the Lambillotte presentation, and a question--

The lowest aspect ratio (profile) I saw among the tested tires was 60%. It's my impression that most tires sold now are on the low side, 60% or less, down to 40%. I am pretty sure the slides did not include tests on aspect ratio. Is that because everybody already knows the answer there?

Ernie Rogers
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Old 04-08-2009, 03:15 PM   #37 (permalink)
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Great info!! Thanks for the post!
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Old 04-08-2009, 07:09 PM   #38 (permalink)
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I suspect the RMA does have a “gag order” on low RR tires. The reason is simple: Product liability.

If the average Joe knew which tores had the lowest RR, in times of high fuel costs, too many unskilled people would buy low RR (low traction) tires and have accidents and sue the tire manufacturers because they didn’t have warnings against morons driving low RR tires.

Most people here know the trade-offs made to get low rolling resistance and are not lead-foot drivers anyway.

Indeed tire manufacturers know the RR of their tires and they share that data with car manufacturers.

One good surrogate parameter for low RR: Noise. A quiet tire will tend to be one with low RR. Makes sense. Noise requires energy and where would that noise energy come from?
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Old 04-09-2009, 02:42 PM   #39 (permalink)
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Notice: I started a new thread--"Discussion on tire efficiency"-- which is meant to combine two ongoing discussions into one. (This has been one of the two.)

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Old 04-09-2009, 04:10 PM   #40 (permalink)
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Here's the link to the new discussion:

http://ecomodder.com/forum/showthrea...ency-7811.html

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