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Originally Posted by Ernie Rogers
Hello, and thanks for the welcome. Pardon my bruskness at times, I don't mean to offend, but--
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No offense taken.
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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.
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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.
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A corollary is that a well-worn tire is more efficient than a new tire--less rubber to bend.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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I don't take things as a lecture. Its more like a debate.
