Tire diameter and coast down

[drmiller100]

Heavier tires and wheels will contain more energy as a function of the square of the speed.

When doing coast down testing to determing CDA and CRR, how do you eliminate the effects or rewards of heavier wheels and tires?

[dcb]

Good question, especially when comparing tires. The momentum they store is also a function of their diameter(was in the title, but road speed is the same, so clarifying), i.e. more mass farther out in some tires. Holding the road speed the same may cancel out the effect of diameter, I haven't run any numbers. But even different pressures imply (slightly) different masses.

What Darin did if I recall is to start from a stop, on a hill, so that seems like it would take into account the extra energy needed to spin up different wheels (or wheels with more or less air), and return it on coast out, maybe.

http://ecomodder.com/forum/showthrea...tire-2721.html


probably best to measure the force needed for rolling with some fixture, but for most folks that is probably overkill. Or measure the momentum, torque needed to achieve a certain road speed within a given amount of time on the bench and factor it in somehow.