Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Coastdown testing question: does starting speed matter for consistency?

aerohead · 03-13-2009, 04:40 PM

It probably doesn't effect consistency,but the higher initial speed allows the air drag to "emerge"dramatically from the velocity/time coordinates of the coastdown curve.Since the air drag force varies as the square of the velocity,if you take the car above 100 km/h and monitor the deceleration,the air drag is really significant and Newton's F=MA relationship will shine light on aero vs RR at these higher velocities.Down around 32 km/h Reynolds Numbers are falling apart and the RR portion from the becomes dominant,helping you to identify it from the aero.SAE recommends starting from around 112 km/h,begin taking your trace at 100,and continue down to 32,back-to-back in each opposing direction for a minimum ten runs.

03-13-2009, 04:40 PM	#8 (permalink)
aerohead Master EcoModder Join Date: Jan 2008 Location: Sanger,Texas,U.S.A. Posts: 15,895 Thanks: 23,972 Thanked 7,223 Times in 4,650 Posts	consistency It probably doesn't effect consistency,but the higher initial speed allows the air drag to "emerge"dramatically from the velocity/time coordinates of the coastdown curve.Since the air drag force varies as the square of the velocity,if you take the car above 100 km/h and monitor the deceleration,the air drag is really significant and Newton's F=MA relationship will shine light on aero vs RR at these higher velocities.Down around 32 km/h Reynolds Numbers are falling apart and the RR portion from the becomes dominant,helping you to identify it from the aero.SAE recommends starting from around 112 km/h,begin taking your trace at 100,and continue down to 32,back-to-back in each opposing direction for a minimum ten runs.