View Single Post
Old 04-10-2009, 08:27 AM   #9 (permalink)
Blue Bomber Man
EcoModding Apprentice
Join Date: Sep 2008
Location: Upstate NY
Posts: 190

GreenMile - '00 Mazda Protege ES
90 day: 34.45 mpg (US)
Thanks: 0
Thanked 5 Times in 3 Posts
Great post Ernie and everything seems spot on except I think I disagree with the stiffness argument.

Now, about stiffness. Remember, I said the weight of the car is held up entirely by the air pressure in the tire? Having stiffer rubber in the tire doesn’t have any practical effect in holding up the car. But, the deflection energy is proportional to stiffness. And a part of that energy is not returned when the rubber rebounds. The effect on mileage is plain to see—tires made of stiff rubber have high Crr, and result in lower miles per gallon.

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.

No kidding, my ADD kicked in early on. Everything ahead of "Tires and Rolling Resistance" can be deleted.

Cutting Crr in half causes—
at 60 mph: 14% better mileage
at 40 mph: 20% better mileage.
Is that true?
Those numbers seem appropriate. According to a governmental study every 10% decrease in rolling resistance will result in in a 1-2% increase in fuel economy. Those numbers seem basically inline with that study. The difference between velocities is due to the proportion of rolling resistance and wind resistance when compared to the total energy. Energy lost to rolling resistance is linear and is related to mass, Crr and miles traveled. Wind resistance is not linear and goes up much faster.
'Blog' on the open source electric motorcycle project.

Please come visit and comment!
  Reply With Quote