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Old 04-13-2009, 04:21 PM   #21 (permalink)
Ernie Rogers
Ernie Rogers
 
Join Date: Feb 2008
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Quote:
Originally Posted by Blue Bomber Man View Post
. . . I think I disagree with the stiffness argument.

Train wheels are made of steel and thus are exceptionally stiff. They use these tires specifically for this reason, since the majority of a train's losses are due to rolling resistance. This stiffness decreases that internal resistance that you speak of.
Hello, Blue Bomber. You made a good argument. Let's study the question.

Work = force x distance. What we don't want is to put work into the wheel material. For the train wheel, force is a given number, the weight of the train and all its cargo. So, you gain by making the distance as small as you can, and steel wheels don't deflect much at all. (A smart guy willl point out that the rails often deflect a lot, but they spring back and return most of that strain energy.)

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.

Ernie Rogers

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