155/80-13 tires (looking for LRR options)

[CapriRacer]

Originally Posted by arcosine

From Sheldon Brown: ..... Tire width and pressure are inextricably linked. It is a serious mistake to consider one independently of the other. Generally, wider tires call for lower pressures, narrower tires call for higher pressures......

That is not only not true for bicycle tires, but it is absolutely not true for passenger car tires and here's proof:



This is a page from The Tire and Rim Association yearbook - the US tire standardizing organization. Notice that regardless of the width of the tire, the table ends at 35 psi (except for Extra Load tires).

.

Originally Posted by arcosine

......The part of the tire that is actually touching the ground at any moment is called the "contact patch." Generally, the area of the contact patch will be directly proportional to the weight load on the tire, and inversely proportional to the inflation pressure..........

Sorry that is not true either. Here's proof:

Barry's Tire Tech

Not everything you find on the interwebz is true.

[arcosine]

Its a law of physics, not a dumb table.

The contact pressure equals the inflation pressure of the tire, period. The load equals the pressure times the contact area.

A narrow tire will had a narrow contact patch, a wide tire will have a wide contact patch. At the same inflation pressure, the narrow tire's belt will deflect more. The energy lost to deflection of the tire is 1/2 times the loss stiffness times the deflection squared. The imaginary stiffness, the part that absorbs energy is related to Tan delta (loss modulus) for the rubber compound and the construction of the belt. Silica reinforced compounds and compounds with tighter cures have lower tan delta that carbon black reinforced NR/BR compounds.

Lets take 2 cases; a 1 inch wide tire and a 2 inch wide tire, same pressure, load,construction and diameter.

The one inch tire deflects twice as much, but the belt is half a stiff, so it has twice the rolling resistance. But the 1 inch tire can be inflated to twice the pressure without over stressing the cords, so now the deflection is the same as the 2 inch tire, but has half the rolling resistance.

Tony Levand

[CapriRacer]

Originally Posted by arcosine

Its a law of physics, not a dumb table. The contact pressure equals the inflation pressure of the tire, period. The load equals the pressure times the contact area..........

I'm sorry, but that is NOT a law of Physics.

That formula calculates the average ground contact pressure of a tire, but the contact pressure is not equal to the inflation pressure - except coincidentally.

Originally Posted by arcosine

......... A narrow tire will had a narrow contact patch, a wide tire will have a wide contact patch. At the same inflation pressure, the narrow tire's belt will deflect more.............

Not only is that not true, you forgot to add ".....if everything else remains the same." - and unfortunately, things do not remain the same between wide tires and narrow tires. For one, every time you go wider, the load carrying capacity goes up - and that has profound effects on RR.

There are quite a few parameters involved in describing a tire and no matter how you slice it, if change only one thing, something else also changes. Let's follow your example and you will see what I mean.

Originally Posted by arcosine

...........Lets take 2 cases; a 1 inch wide tire and a 2 inch wide tire, same pressure, load, construction, and diameter......

What you didn't mention is that the aspect ratio also changed. And what about the load carrying capacity? Did that change as well?

Plus, if we are using a tire with the same construction, was the construction adjusted to compensate for the fact that we have a physically larger perimeter and NEED a stronger cord in order to keep the same relative strength?

Originally Posted by arcosine

...........The one inch tire deflects twice as much....

I'm sorry, that is not true. The load carrying capacity of the wider tire went up, so the deflection went down because the load on the tire is a smaller percentage of the load capacity.

Originally Posted by arcosine

........... but the belt is half as stiff...........

How can a belt be half as stiff if it is the same?

Originally Posted by arcosine

...........But the 1 inch tire can be inflated to twice the pressure without over stressing the cords.................

Not quite. You need to consider the fact that the tire is not a complete chamber. It is tied to the rim at the bead, so the perimeter isn't quite twice - assuming the same construction.

Tony,

While going through an analysis like you did is helpful in understanding the Physics involved, it is important that the end result match what happens in the real world. The Smithers report to the CEC presents some very valuable insight into how passenger car tires operate relative to rolling resistance - and what it says is that wider tires are ever so slightly better for RR than narrower tires. Why?

Well, if we increase the width of a tire - and try to change nothing else - the diameter goes up (remember, we are keeping the same aspect ratio!). The load carrying capacity also goes up - in proportion to the width increase. Therefore the load on the tire is a smaller percentage of the capacity - and therefore there is less deflection for a given load.

Further, while the width of the tire increased, the width of the tread and belts increased a percentage of that change - not 100%. Put a different way, if the tire is wider by 10mm, the width of the tread and belts will be about 7mm wider!

And one last item: There are still only 2 sidewalls - and while sidewalls do not have much impact on RR, they do have some.

But what about inflation pressure? To date, I have not found any studies where the effect tire size has on RR is adjusted for load carrying capacity. Put another way, if we test for a tire's RR at a given load and inflation pressure, then test an otherwise identical, but larger, tire at the same load, but adjust the inflation pressure according to the load tables to get the same load carrying capacity, what would we get? I suspect an ever so slight reduction in RR - one that might not be able to be measured.

But until we get a study like that, we will have to rely on what informtion is available about what actually happens - and the Smithers report is an excellent source.

[arcosine]

OK, young tire engineer race car driver, I'll ride my road bike hpv with skinny high pressure tires and you ride a mountain bike with big fat low pressure tires and we see who is faster on the road. Of course we don't have to do this because we already know who would win.

So I'll ask your expertize, Which tire would give the best MPG:

185/65R15 at 35 psi
165/80R15 at 44 psi
185R15 at 35 psi

[CapriRacer]

Originally Posted by arcosine

OK, young tire engineer......

I'm sorry, but I do not think anyone I know would call me young.

Originally Posted by arcosine

...... I'll ride my road bike hpv with skinny high pressure tires and you ride a mountain bike with big fat low pressure tires and we see who is faster on the road. Of course we don't have to do this because we already know who would win......

Would you win because my tires are wider, or because my tires are using lower pressure? What would happen if we used the same inflation pressure? (assuming everything else is the same.)

Originally Posted by arcosine

.........So I'll ask your expertize, Which tire would give the best MPG:

185/65R15 at 35 psi
165/80R15 at 44 psi
185R15 at 35 psi

Let's see if I can work this through.

First, the load on the tires isn't specified, but let's assume it is the same - like what would happen if all 3 of these sizes was applied to the same vehicle. I'm going to express this as Rolling Resistance Coefficient (RRC).

According to the Smithers report, a 185/65R15 has an RRC value of ..... well, they didn't test that size (or any of the other sizes), but we can make a reasonable estimate based on the data that is available. If you run a regression analysis of the data, you get a formula for the 3 numbers that constitute a tire size. I did that in the web page I earlier posted. Here is that page again:

Barry's Tire Tech

The Smithers report doesn't say what test method they used. So both the load and the inflation pressure are unknown. But if we assume they used a standard RR test, they would have used some percentage of the rated load and some inflation pressure. The inflation pressure would be the same for each tire regardless of tire size. The load would vary depending on what the rated load would be.

Let's assume that the pressure was the rated 35 psi (a pretty good assumption given that most of RR tests are run at the rated pressure.)

Given all that, the regression equation would give you the following results (the formula is on the web page):

185/65R15 = 0.001124 RRC (35 psi)
165/80R15 = 0.001108 RRC (35 psi) - note: This is specified at 44 psi so we need to recalculate the value - which I will do below.

The only source I know of in translating rolling resistance between different pressures is this: SAE Paper 800087 - The Effect of Inflation Pressure on Bias, Bias-Belted, and Radial Tire Performance - by B. L. Collier and J. T. Warchol, B. F. Goodrich, February, 1980

I summarized parts of this paper here:

Barry's Tire Tech

Unfortunately, I am not allowed to publish all of it - and one of the unpublished parts is the change in rolling resistance as a function of inflation pressure. But I did extract a graph derived from that paper:



This doesn't show the change from 35 psi to 44 psi (+9 psi), but if we use 32 psi to 40 psi (+8 psi), then the RR changes from 113% to 121% of the value at 24 psi (which would have been the rated pressure of the type of tires used in the test). So 121/113 = 107% - a 7% improvement.

So if we decrease the RRC value to 93%, we get 0.001030 RRC.

OK, the next size:

185R15 = 0.001059 The trick here is knowing that the way this tire size is stated implies an aspect ratio of 82.

Summarizing:

185/65R15 = 0.001124 RRC (35 psi) Assigned the value of 100%
165/80R15 = 0.001030 RRC (44 psi) 92%
185R15 = 0.001059 RRC (35 psi) 94%

Caution: The RRC values assume the tires are the same as in the Smithers report - Goodyear Integrity's (my best guess). If you want to compare the effect tire size (and inflation pressure) has, you should use the percentages stated.

Does this say that narrower tires give better RR? No, there's a lot of different variables in the mix. We have different inflation pressures, different aspect ratios, different widths, etc. - all of which are going to affect the results.

If we complicate that further by trying to compare different manufacturer/design combinations (according to Smithers a range of 60%) - well, I think it is easy to see why it is very difficult for an individual consumer to draw conclusions based on a change they might make. Not to mention that worn tires give different RR than new tires (all other things being equal).

When it comes to RR of tires, it gets pretty complicated pretty fast. If the goal is improving fuel economy, it pays to be very careful about comparing apples to apples.

[arcosine]

Age is a perspective.

Cool, the skinnier tire has lower RR and I can say by experience that its gets better MPG and certainly coasts further. The 185/65R15 does handle much nicer, though. I was considering the Federal 185R15 (185/80R15) tire but am concerned over lugging the Saturn engine up hill on the highway (2040 rpm @ 65mph, max torque is at 2200 RPM). My experience with other vehicles has been positive with larger tires sizes, 1968 bug with HR7815 (35 mpg avg, 40 mpg hwy), 1972 Datsun with 195/75R14 rear tires), 1977 Honda Accord with 185/80R13 (50 mpg avg)(155R13 stock). Going from 185/70R13 to 195/75R14 gave 1.5 mpg gain on 1987 Honda Accord.

[arcosine]

Humm, rolling resistance should be inversely related to the square root of inflation pressure.

referencing the unpublished graph of the proceeding post:

24 psi =100
28 psi = 93
32 psi = 87
36 psi = 82
40 psi = 79

100 * sqr(24/28) = 92.5
100 * sqr(24/32) = 86.6
100 * sqr(24/36) = 81.6
100 * sqr(24/40) = 77.5

Not too bad a fit for simple deflection theory. Looks like at 40 psi and above this tire is over inflated and the contact patch doesn't extend across the tread width, but is more in the center, thus the under prediction of rolling resistance due to the higher defections at the center.

How is tread wear related to contact pressure? I know that elastomer fatigue life is related inversely to the fifth power of cyclic strain, but the strain in a tire is very low and would think that abrasion is the chief component of wear.

[roflwaffle]

Originally Posted by CapriRacer

Barry's Tire Tech

This is an analysis of some data submitted to the California Energy Commission. Among the data is a study of tire size. Needless to say, that there are a lot of factors, but if we isolate tire size: Bigger = better - and by bigger, the data seems to point to every parameter in the numbers used to delinate a "size" - wider or taller or larger in rim diameter.

But there is such a huge difference between tires (meaning make and model), your point is well taken.

This is only for a specific brand (Goodyear) and tire type (Integrity), and probably a specific year or years.

I am not sure where I picked this up, but all these tires are Goodyear Integrity's

Someone could say that Goodyear Integrity tires tend to exhibit a lower RRC as tire size increases, but saying the same thing about tires in general is not warranted w/o more data (multiple brands, tire types, sizes, and hopefully years). Even then, saying the bigger=better is incorrect. On page 31 of the pdf, the RRC for 15" tires is better than for larger 16" tires. My WAG is that the tires most commonly used in passenger vehicles where fuel efficiency is a concern, which I'm guessing are 15", are also where Goodyear wants to minimize the RRC.

[arcosine]

Are the low RR tires using steel or kevlar belts or does it matter?

[CapriRacer]

Originally Posted by roflwaffle

This is only for a specific brand (Goodyear) and tire type (Integrity), and probably a specific year or years.

Someone could say that Goodyear Integrity tires tend to exhibit a lower RRC as tire size increases, but saying the same thing about tires in general is not warranted w/o more data (multiple brands, tire types, sizes, and hopefully years). .........

You are right. It would be nice if someone would duplicate the study in another brand/model. That would verify the effect.

But there are laws of physics involved here, and even though I have reservations about the exact tires that were selected to be tested, I do not think there was any attempt to by-pass those laws. In other words, I think the effect demonstrated is real and reproducible. Until such time as we have additional data that contradicts the Smithers report, I don't think there is any reason to doubt the validity of the effect for every tire.

-- An Insider's Insight --

Since the study was presented to a government entity in public view, if there were any dissenting opinions, they could have been expressed at subsequent meetings. To date, there haven't been.

Further, tire manufacturers would have access to a lot of data. They may not have enough to data to confirm the results, but at the very least they would have data to contradict the results. They would not be afraid to publish the results if there was some advantage to do so - and so far they haven't.

There are 2 possibilities:

1) That the effect demonstrated is real
- OR -
2) That the results would have some positive affect on selling tires

I also know that NHTSA has been very active in testing tires for RR. If there was any reason to doubt the data, they would have published their data. In conversations with one of their test engineers, he acts like the effect is accurately described, but disagrees with the tire industry's position about what regulations should be put in place as a result. (THAT is a whole 'nother discussion)

So not only do I think the effect is as described, others must think so as well - and some of those are sure to have access to data that would contradict.

Originally Posted by roflwaffle

......Even then, saying the bigger=better is incorrect. On page 31 of the pdf, the RRC for 15" tires is better than for larger 16" tires. My WAG is that the tires most commonly used in passenger vehicles where fuel efficiency is a concern, which I'm guessing are 15", are also where Goodyear wants to minimize the RRC.

Doesn't this depend on what your definition of "bigger" is?

If 16" tires are larger than 15", then there is a problem - there are only 2 16" data points: 215/60R16 and 225/60R16. To be consistent, then these should be compared to 215/60R15 and 225/60R15 - but that data doesn't exist!

By contrast, the 15" data has 8 data points - ranging in size from 195/65R15 to 235/75R15. There are a number of ways to look at "bigger", but every way I know about would say a 235/75R15 is "bigger" than a 225/60R16 (with the exception of rim diameter).

That's why I did a regression analysis. That would factor out the effcts the other parameters would have on the data and allow one to look purely at the effect one parameter has.

And the regression analysis says that going up in width has a positive affect, going up in aspect ratio has a positive effect, and going up in rim diameter has a positive effect. By any stretch of the imagination, all 3 of these results in "bigger".