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Old 05-01-2009, 01:03 AM   #31 (permalink)
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With no easy way to quantify r.r. (no ratings) I hope it is more than a marketing ploy.

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Old 05-01-2009, 02:33 AM   #32 (permalink)
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Hi Ernie,

Quote:
Originally Posted by Ernie Rogers View Post
Has anyone heard about this?

I bought two tires today, from Sears. (Michelin Energy MXV4 Plus-- they were on clearance because they are a discontinued tire.)

Oh, that's not the news The manager said they had new "energy" tires coming from Goodyear, called "Goodyear Assurance Fuel Max" tires. So far, they have to be special ordered but should be in stock soon.
Yes, I posted about them a little while ago -- they are also listed in the new TireRack.com catalog; along with a new Michelin HydroEdge Green-X, and Bridgestone Ecopia EP100 Grand Touring Summer LRR tires.

http://ecomodder.com/forum/showthrea...ires-7844.html

Here's a link to the TireRack page for the Goodyears:

http://www.tirerack.com/tires/tires....rance+Fuel+Max
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Old 05-01-2009, 06:24 PM   #33 (permalink)
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We need a normalized tire rating

Hello, Frank,

You have put your thumb on the big issue the tire makers have to find an answer to.

Here is an idea. Any tire makers listening? Test all tires at the same (standard) spring rate, by adjusting pressure. This will help to simplify the situation.

The next possible simplification could be to all agree to publish the rolling resistance coefficient for certain specified sizes that are common-- for example, one size for compacts, another size for SUVs.

What do you think?

Ernie Rogers

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With no easy way to quantify r.r. (no ratings) I hope it is more than a marketing ploy.
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Old 05-01-2009, 10:39 PM   #34 (permalink)
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Quote:
Originally Posted by Ernie Rogers View Post
<SNIP>
Okay, now for the car tire. In this case, it's the distance (related to the contact patch) that is fixed, based on the air pressure in the tire. So, now you want to make the force to flex the rubber as small as possible, and that means to choose material that flexes very easily.

In both cases, materials with low internal friction are the best to use.
If there was any truth to this then why do steel belted radial tires have a lower rolling resistance than than an ordinary radial tire?

Also note that the soft or hardness of a rubber has nothing to do with its internal friction.
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Old 05-01-2009, 11:42 PM   #35 (permalink)
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Steel can be flexible when it is thin. #0000 steel wool feels soft, and burns in air if a spark gets on it.
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Old 05-02-2009, 12:41 AM   #36 (permalink)
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Quote:
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Steel can be flexible when it is thin. #0000 steel wool feels soft, and burns in air if a spark gets on it.
The steel strands used in a steel belted tire is considerably thicker than that used in steel wool. Steel belts are used as stiffeners in tires to make the tread inflexible. This lowers the rolling resistance.
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Old 05-02-2009, 01:02 AM   #37 (permalink)
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Actually, the main benefit is not stiffness per se, but the directions of stiffness and flexibility. The belted tire tread squirms less as it conforms to the road.
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Old 05-02-2009, 01:25 AM   #38 (permalink)
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Quote:
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Actually, the main benefit is not stiffness per se, but the directions of stiffness and flexibility. The belted tire tread squirms less as it conforms to the road.
The stiffness however contributes to the low rolling resistance.
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Old 05-02-2009, 02:36 PM   #39 (permalink)
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I said that soft rubber gives lower rolling resistance.

Then, Clark said,

Quote:
Originally Posted by ConnClark View Post
If there was any truth to this then why do steel belted radial tires have a lower rolling resistance than than an ordinary radial tire?

Also note that the soft or hardness of a rubber has nothing to do with its internal friction.
Well, I think I will start with a disclaimer-- While I have designed many structures over the years, I have never designed a tire. I will be giving my impressions, based on engineering principles and sometimes I might get it wrong. Now, here goes.

In my view, the pneumatic tire is one of the greatest inventions of all time. (Along with bar soap.) Here's why an air-filled tire is such a great invention. First, having some give in the tire is essential to protecting other car parts from jarring loads. Essentially, we want the tire to act as a spring. There are lots of springy materials a person could use in a tire, and they all have a weakness, except one. The weakness is that when you flex a solid material, you never get all the energy back when you let it unflex. Air is the wonderful exception. When you compress air in a short time (adiabatically-no heat loss), and then let it go back, no energy is lost. So, except for whatever energy is lost in the tire shell, a pneumatic tire has perfect efficiency.

Now, let's deal with the shell. The ideal shell material holds the air pressure but has the least involvement in the spring action. It needs to have good flexibiliy, no stretch in the tread area (but flexible), and the right give in the side walls (with minimum stretch) to form the contact patch.

Bending is the desired type of shell deformation. When a sheet of material bends, energy is absorbed as strain energy, by a formula like this--

S = a K T^4

Which says that the strain energy S is proportional to the stiffness K times the thinkness of the material raised to the fourth power. Each time the material flexes, energy is lost. The energy returned is proportional to the coefficient of restitution, let's call it R, and for the energy lost, it's 1-R:

Energy lost = (1-R) S

You should notice that the energy lost is proportional to the stiffness, K. In some ways at least, the thickness of the shell is set by the strength needed to contain the pressure. Steel wire in the tire provides great strength, allowing the shell to be much thinner and thereby reduce the strain energy in the rubber, which is where the energy loss occurs. The steel cord has very low internal friction (1-R) and lies on the neutral axis in bending, so almost no energy is lost in the steel.

So, contrary to intuition, steel cord has the effect of improving flexibility in the rubber layers and reducing stretch, and thus improves efficiency.

Now, as to whether or not hardness (stiffness) of the rubber is related to the internal friction, I admit there is some uncertainty there. But, the formula directly above suggests that the energy lost is proportional to the strain energy (for a given value of 1-R) and that in turn was proportional to the stiffness. It seems that silica is often added to the rubber to lower internal friction, and I'm sure that doesn't make the rubber softer.

Ernie Rogers
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Old 05-02-2009, 03:31 PM   #40 (permalink)
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Nope, I'm gonna have to go with liquid soap being greater.

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