I've browsed some sites looking for Crr (rolling resistance coefficient).

Wouldn't it be nice, after making changes, to measure your car's rolling friction or determine how aerodynamic your car is?

I found the "coast down test" is a common thing designers do for cars. It pretty much is the opposite of a drag race. One starts fast and ends up at zero, the time it takes to get there, and the time at various speeds along the way, tells you the power - drag - friction - loss for the vehicle.

It all seems like some fun, and the basic data is easily gathered with a stop watch and a speedometer. level road, get rolling at speed, shift into neutral and coast to a stop. note time and speed along the way to the stop.

The tricky part is making sense of the clipboard data from the test. How to separate the various components of loss from the data, aerodynamic losses, rolling friction losses, drivetrain friction losses. human power web sites and others have spread sheets for plotting the data and so calculating the rolling friction vs aerodynamic drag.

Here's a dump of some of the threads I found discussing this http://www.ihpva.org/tools/TCoastF1.xls http://www.ihpva.org/tools/TCoastF3.xls

http://www.recumbents.com/MARS/pages/proj/tetz/other/Crr.html

One older engineer on "engineering tips" website said he used to test rolling friction by hooking a spring scale onto a lawn tractor and pulling a vehicle at 2mph. Now that's easy enough to do. His point was that rolling friction doesn't increase with speed, so you just have to measure how hard it is to pull the car behind you like a trailer.

I was planning a similar kind of testing for determining the outcome of any aerodynamic mods, or to help tweak them. Rolling friction tests you'd have to do at low speed to take out the aerodynamics and aero tests you'd have to do on a drop between say 65 and 50 to leave out as much rolling resistance influence as possible.