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Old 10-12-2009, 08:09 PM   #11
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the holy cow,
All the data is out there. Here is a good place to start: http://www.tirerack.com/about/techcenter.jsp

Another good source is common sense. Does anyone think that a vehicle manufacturer for one second would try to get a lessor mpg rating by using a tire that would produce less. Think again. Read it, study it, and understand that their are hard cold facts about tire size, rolling resistance, and MPG...
Jim

ps: This is a much better tire size converter than the miata site example, http://www.wheelsmaster.com/rt_specs.jsp
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Old 10-13-2009, 03:34 AM   #12
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That wheelsmaster tire size calculator is my favorite. I've had it in my sig on another forum for a few months. It gives you a lot more data than others and it's more browser-friendly.

I have plenty of tirerack links in my width and pressure posts. Tirerack doesn't provide data from studies, they just provide opinions and common knowledge...and some of it is conflicting, too.

Manufacturers do not have MPG as their highest priority. Higher priorities are price, liability, noise, longevity, warranty costs, speed, power, comfort, emissions, and general marketing. They meet the minimum MPG requirement goal that they have set for the vehicle and continue to refine the other things.

When trying to guess what their motivation was for a configuration (tire width, exhaust diameter, whatever), you have to guess which of those motivations went into that thing. Tire pressure, for example; they specify the lowest pressure that covers them from liability, so that the car will ride soft. The marketing department sets tire pressure specification after checking with the lawyers, the engineers don't get much say.

As for tire width...I'm not sure which motivations would come into play.
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Old 10-14-2009, 11:59 AM   #13
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Quote:
Originally Posted by theholycow View Post
There is a severe lack of good data and complete studies on rolling resistance.

In my sig, there is a link to my thread on tire width where I placed an excessive quantity of my thoughts and research. In short, I can't prove that either narrower or wider is better for fuel economy.

What I do have is an understanding of some underlying principles that don't get much thought in general.
  • At a given pressure and load, contact patch is expected to be the same regardless of width. What changes is its shape; a narrow tire has a longer, narrower contact patch, which requires more sidewall deformation to make that contact patch. Sidewall deformation for making a contact patch is a major component (possibly the largest contributor) of rolling resistance.
    * I believe that I am correct about all that, but I have doubts. Some data shows that modern automotive tubeless tires' contact patches don't adjust quite that way, unlike (for example) bicycle tires.
  • A tire that holds more volume of air has a higher load capacity at a given pressure. Using less of a tire's laod capacity (again, at a specific pressure) means reduced rolling resistance. A wider tire holds more volume of air.

  • One person's study: http://priuschat.com/forums/prius-te...rc-4-data.html
some of your conculsions are incorrect.

http://www.everytime.cummins.com/***...Whitepaper.pdf
there are sections on tires and other interests that crossover to cars and pickups
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Old 10-14-2009, 12:31 PM   #14
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Quote:
Originally Posted by CA ExhaustCoatings View Post
some of your conculsions are incorrect.

http://www.everytime.cummins.com/***...Whitepaper.pdf
there are sections on tires and other interests that crossover to cars and pickups
Thank you! I'm always happy to get more information. I'll have to give that a thorough reading.
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Old 10-14-2009, 12:46 PM   #15
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Well, there's not much about tires in there, and the only thing I could find that contradicts any of my conclusions is where it says that tread is responsible for 60-70% of rolling resistance, and that worn tread reduces rolling resistance. There's no data to back it up, just like there's no data for much else about rolling resistance.

Tread on big rig tires comes 14/32 to 30/32, according to a quick look at Goodyear's commercial tire specifications. Passenger tires (except snow tires for pickups) rarely exceed 12/32 and tend not to have block/lug tread. However, I'll concede that in brand new tires, tread may often have a more significant effect on rolling resistance than I usually suggest.
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Old 10-14-2009, 01:53 PM   #16
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"What changes is its shape; a narrow tire has a longer, narrower contact patch, which requires more sidewall deformation to make that contact patch."

not true. low air pressure causes that nothing else.
http://www.tirerack.com/tires/tirete...e.jsp?techid=1

a narrower tire increases mpg. the contact patch is changed more by the diameter of a tire. a larger dia has more contact patch, the reason drag cars have taller tires on drive wheels plus wider. more contact area, more friction more traction.
lets reverse the thinking, which would give u shorter braking stopping distances, a narrow tire or a wider tire? the wider tire of course, more contact area.
u made reference to bicycles, do road racers use narrow high pressure tires or fat mountain bike tires to go fast with the least bit of effort?
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Old 10-14-2009, 03:39 PM   #17
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There is no data supporting the contact patch shape not changing with width. Pressure definitely changes it, but at the same pressure, width is bound to change it too.

The theory, against which I've seen precious little data, is that you can calculate contact patch size quite easily from pressure and load. So, if you have 1000 pounds load and 100psi (that's Pounds per Square Inch), you're going to have 10 square inches of contact patch. If you make it narrower, it's going to be longer, but it's going to be 10 square inches.

Road racers use narrow tires for four reasons:
1. High pressure - traditionally, it has not been feasible to put 120psi in a fat tire; the tire would blow off the rim. Regardless of rolling resistance, you need a lot of pressure for safe high-speed cornering.
2. Aerodynamics - Extremely important for road bicycling. You wouldn't think it matters much at human-powered speeds, but it does matter very much.
3. Light weight - Road racing traditionally shaves every gram off the bike that can possibly be removed.
4. Ignorance/blind tradition/myth-following - Everybody assumes that narrower has less rolling resistance, it is in print in many places, so the myth lives on; but reality differs somewhat.

I have done much bicycling, much study of it, and I'm pretty sure there is data showing the issue quite clearly in that context.
http://www.analyticcycling.com/Force...esistance.html
Quote:
Smaller tires flex more than larger ones of the same casing and have greater losses.
That refers to width, not diameter. The data comes from Jobst Brandt, one of the world's foremost experts on bicycle wheels and tires.
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Old 10-14-2009, 04:31 PM   #18
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if u are overloading a tire and/or with low air pressure u would be correct. but it seams if those too variables are not the case then narrow has a smaller contact patch.

http://www.google.com/#hl=en&source=...oq=&fp=1&cad=b
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Old 10-14-2009, 05:53 PM   #19
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There's a large collection of data at http://www.performancesimulations.co...on-tires-1.htm (the first link in that search) showing that contact patch is NOT calculable from pressure and load, taking away the logic about contact patch length and leaving us with unexplained data from the study I posted earlier in this thread...drat, now I don't know why the narrower width in that study had more RR.

I have seen that link before and totally forgot about it. Thank you.

Edit: D'oh! It was already right at the top of my tire width thread, just before the aforementioned unexplained study.
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Old 10-16-2009, 05:23 AM   #20
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http://www.physicsforums.com/archive.../t-330790.html

so it seams we were both half right.

my conclusion is two tires of the same OD and two different widths will basically have the same contact patch. As I had stated earlier, a larger OD will increase the contact patch. So to have the smallest contact patch a tire would have to be wide with a small OD. But according to the white paper posted and as most know aerodynamics reduces mpg the most at 50 mph and over, whereas wide would be bad.
It seams a balancing act is needed for the use of the vehicle and weather conditions one expects to see for the best mpg size tire to use.
Using a larger dia wheel so that the tire side wall is smaller will help reduce tread movement thereby increasing mpg.
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