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IWillTry · 04-20-2008, 04:30 AM

Hi all. fabrio PMed me to come take a look at this thread. Sorry I don't pay more attention to the forums... too busy tinkering

.

Thanks, fabrio for improving my spreadsheet. I agree that changes in air density will have a significant effect on the Cd calculated so it's nice to be able to calculate air density based on values that are measurable (assuming you have a weather station handy).

The moment of inertial of the wheels can be considered too. The force required to decelerate the linear motion of the vehicle is F=MA where M is the mass of vehicle plus driver. The additional force required to slow the rotation of the wheels depends somewhat on wheel geometry but should be close to F=(m/2)A where m is the mass of all wheels. Thus the total force is F=(M+m/2)A. So simply adding half the mass of the wheels to the total mass of vehicle plus occupants may give more accurate results.

There is also the moment of inertia of the drive-train to consider but that is likely small do to smaller diameters of components.

There are also friction and viscous losses in the drive-train. The force of friction should be roughly constant and will result in over-estimating Crr. Viscous force (due to gear oil in diff and tranny) is proportional to velocity and will result in over-estimating Cd.

Note that ignoring friction and viscous losses results in over-estimating Cd and Crr while ignoring the angular inertia of wheels and drivetrain results in under-estimating Cd and Crr, so the two effects may partially cancel.

One thing everyone should keep in mind is that Cd and Crr are only useful for comparing one vehicle to another. If you want to evaluate the effectiveness of a mod, you are way better off simply comparing coast down times. For aero mods look at a high speed range (ex time to drop from 90 to 60 kph). For rolling resistance mods, look at a low speed range (ex time to drop from around 40 to 20 kph). Ideally, do A-B-A tests under identical conditions (preferably just minutes apart). Then air density and other variable factors become irrelevant.

Cheers.

04-20-2008, 04:30 AM	#18 (permalink)
IWillTry Will you? Join Date: Jan 2008 Location: Vancouver, BC, Canada Posts: 7 Thanks: 0 Thanked 1 Time in 1 Post	Hi all. fabrio PMed me to come take a look at this thread. Sorry I don't pay more attention to the forums... too busy tinkering . Thanks, fabrio for improving my spreadsheet. I agree that changes in air density will have a significant effect on the Cd calculated so it's nice to be able to calculate air density based on values that are measurable (assuming you have a weather station handy). The moment of inertial of the wheels can be considered too. The force required to decelerate the linear motion of the vehicle is F=MA where M is the mass of vehicle plus driver. The additional force required to slow the rotation of the wheels depends somewhat on wheel geometry but should be close to F=(m/2)A where m is the mass of all wheels. Thus the total force is F=(M+m/2)A. So simply adding half the mass of the wheels to the total mass of vehicle plus occupants may give more accurate results. There is also the moment of inertia of the drive-train to consider but that is likely small do to smaller diameters of components. There are also friction and viscous losses in the drive-train. The force of friction should be roughly constant and will result in over-estimating Crr. Viscous force (due to gear oil in diff and tranny) is proportional to velocity and will result in over-estimating Cd. Note that ignoring friction and viscous losses results in over-estimating Cd and Crr while ignoring the angular inertia of wheels and drivetrain results in under-estimating Cd and Crr, so the two effects may partially cancel. One thing everyone should keep in mind is that Cd and Crr are only useful for comparing one vehicle to another. If you want to evaluate the effectiveness of a mod, you are way better off simply comparing coast down times. For aero mods look at a high speed range (ex time to drop from 90 to 60 kph). For rolling resistance mods, look at a low speed range (ex time to drop from around 40 to 20 kph). Ideally, do A-B-A tests under identical conditions (preferably just minutes apart). Then air density and other variable factors become irrelevant. Cheers. __________________ www.IWillTry.org - Will you?