Introducing myself

[Wonderboy]

Hello and welcome, Ernie. This forum is filled with the intelligent, the "crazy", and the advocates of trial and error...and now we have a physicist . Although you'd probably be labeled as all three in the "real world", you've arrived at ecomodder where my initial vote would be to put you in the intelligent category. I've no doubt your voice will be a valuable addition to this forum.

[Ernie Rogers]

Originally Posted by theunchosen

That sounds interesting. Must be using something other than standard motor oil as it breaks-apart and or ignites above 773 K. Yeah I am an ME student, but I used to be a ChE senior, why I asked about the temp(thermo and organic).

Excellent point about thermal stability of motor oil. That's why engine makers are very concerned about keeping cylinder wall temperature cool enough, where the oil film is. I use 453 deg. K for wall temperature in my calculations.

Ernie Rogers

[ConnClark]

Originally Posted by Ernie Rogers

Hello, and thanks for the welcome. Pardon my bruskness at times, I don't mean to offend, but--

No offense taken.

The energy loss in a tire comes from the work it takes to deform the rubber as the tire adjusts to make its contact patch. A hard rubber takes more work to deform it so it loses more energy, and is less efficient.

You are looking at the right relationship in force x distance and what you say would be true if the a harder rubber deformed by the same amount. In this case the force to do the deforming (the weight of the car) is held constant not the deformation. All other things equal, a tire made from a harder compound has a smaller contact patch as well. This is why trains use steel wheels instead of rubber ones.

A corollary is that a well-worn tire is more efficient than a new tire--less rubber to bend.

A well worn tire has a higher rolling resistance because it flexes more. Its side walls are weaker and its belts are less stiff. The same force applied causes more flex and therefore is applied over a greater distance requiring more work. New tires have their least rolling resistance within about 5000 miles. This is because they have had their flashes and tread corners worn off making them more round. This is the reason that the SAE and ASTM standard tests for rolling resistance of tires are conducted over 5000 miles. After 5000 miles the rolling resistance increases due to increased flex.

By the same logic, you should conclude that a low-profile tire is less efficient because more deformation in the wall is required to form the contact patch. This is verified by experiments, and common experience by drivers.

I dispute this for the same reasons listed above. All other things equal a shorter side wall tire will have a smaller contact patch. Most often however short sidewall tires are used in performance/racing applications and are wider and are made from softer compounds to give more grip. This all adds to their rolling resistance which quickly can offset any gains from having a shorter side wall.

High tire pressure improves efficiency because the contact patch is proportionately smaller, and deformation is less.

About high pressure and tire life-- it's my experience that a good tire with a solid radial belt won't wear unevenly in the middle. Many police and highway patrol cars have tire pressures above 50 psi (for improved safety) with no such wear problems.

Police cars receive much more maintenance than a regular car and have their tires checked quite often. Also the abusive driving conditions they go through means the tires get replaced quite often. Due to these factors, tire wear on a police car is not a valid comparison.

A good belt helps, but eventually it wears. As inflation pressure increases the contact patch shape gets rounder and rounder. This increases wear on the center of the tread.

About wheel moment of inertia-- that's mostly an old myth too. Wheels and tires have double the effect on car total inertia because they both spin and move linearly. If you increase the weight of wheels and tires by 15 lb each, the total effect on the car is the same as adding 120 pounds in the trunk, or a small mother-in-law in the back seat. Smile, you will get by just fine.

I agree its not a lot but it does play a roll. It has effects beyond just acceleration as well. Every time you turn a corner it takes energy to over come the gyroscopic forces.

Your comment near the bottom, about larger tires not always working for some cars, is probably true, especially with some automatics. For my car, it's speed for best mileage appears to be somewhere near 50 mph in fifth gear. And, I think you should only make small changes in tire diameter, less than two inches unless somebody has already tried it.

Sorry about the lecture.
Ernie Rogers

I don't take things as a lecture. Its more like a debate.

[tasdrouille]

ConnClark,

It's the other way around. RRC is mostly dependent on how much material you are flexing. When a tire wear, there is less material to flex (lower hysteresis losses), and for a fixed pressure, RR goes down.

The same thing happens when you raise the pressure. The tire is being held by the air pressure inside it, up the pressure and less material is flexing while the tire is rolling.

Same thing again when you are driving on a rough road.

I do not think harder compound reduces RR. Because the material is still going to flex, as air pressure is the predominant factor here, and since it's harder to flex, hysteresis losses are just probably gonna be greater.

[COcyclist]

I'm always glad to see another TDI on Ecomodder. I wonder when one of us is going to get really brave and Basjoos a VW to see what the diesel can really do when optimized for aero with the right gearing.

I do have to side with ConnClark on tire wear. I have been running higher-than-suggested-on-the-sidewall tire pressures for decades. Up to a point you get better treadwear because you don't scrub off the shoulders as much when cornering, especially in a front wheel drive car. However, if you are doing a lot of straight line driving, the belts will bulge from the pressure and the middle wears faster.

[Blue Bomber Man]

Californian Study of Rolling Resistance:

http://www.energy.ca.gov/2006publica...0-2006-001.PDF

Page 27:

Reductions in tread thickness, volume, and mass are among the
means available to reduce rolling resistance,

Page 63 - Has some interesting charts regarding vehicle energy consumption

Page 64:

a 10 percent
reduction in rolling resistance will lead to a 1 to 2 percent increase
in fuel economy and a proportional reduction in fuel consumption.

Page 67:

Reducing a tire’s aspect ratio—
that is, its section height relative to its section width—should reduce
hysteresis if it is accomplished by shortening and stiffening of the sidewalls.

Page 68:

These experiments suggested
that rolling resistance declined by 26 percent over the entire wear life.

I could not find anything in the article regarding the "hardness" of a tire, but it stands to reason that the harder the material the less it flex and thus will experience less hysteresis. The above example of a steel train wheel is a great example of an extremely low rolling resistance wheel.

The article does discuss to a large extent the relationship of the performance of the tire compared to the efficiency (rolling resistance) of the tire. It generally states that their is probably somewhat of an inverse relationship but that tires could be designed to improve their RR without adversely effecting its other properties.

[tasdrouille]

I've read that study before, and I have a problem with the following part:

Reducing a tire’s aspect ratio—
that is, its section height relative to its section width—should reduce
hysteresis if it is accomplished by shortening and stiffening of the sidewalls.

It's a good thing they say "should" because the Lambilotte study does not agree with this, at least for the tire model tested. From the limited data in that slideshow, we can still see the following:

- The lowest RRC regardless of width for Diameter/Aspect ratio are: 13/80, 14/75, 15/70. Above or below those values the RRC usually increases.
- The lowest RRC size for a given tire diameter is usually the width, regardless of aspect ratio, closest to the tire diameter times 14.

[CapriRacer]

Are we really having a discussion about Tire Rolling Resistance in a thread where Ernie introduced himself?

Wouldn't a separate thread be a better forum - particularly considering that folks might be interested - and even join in - if they could identify the thread as such?

[Daox]

Thanks CapriRacer, you're right. For the concerned about tire rolling resistance, start up a new thread and you guys can post up your info there and figure it all out. I can even move over posts for you guys.

Now back to Ernie's intro!

[Ernie Rogers]

Notice: As suggested, I will start a new thread on tire selection. For that purpose, I will post a "lesson" I did some years back on the topic of tire efficiency.

Ernie Rogers