Head wind vs tail wind-ET impact

Vic Santos · 10-30-2012, 05:08 PM

Clark, when you think about headwind vs tailwind they should be treated the same when predicting ET. Once the car has any reasonable speed it only "feels" a headwind. Example: if you were sitting on the hood of a car and there was a 20 mph tailwind you would "feel" zero wind when the car goes 20 mph. At 120 mph the the car only "feels" a 100 mph headwind (you would have already fallen off). Conversly with a 20 mph headwind the car "feels" a 140 mph headwind at 120 mph. The car can accelerate more at a 100 mph headwind than at a 140 mph headwind thereby changing the ET and mph .
Each car has its own amount of aero drag and is dependant on the trap speed. It is easy to get the change in ET vs wind speed over a reasonable range, just measure it (at the same DA). The number for my car is 0.0063 sec/ mph of wind and it works for BOTH head and tailwind.

The big problem is to know what the wind speed is where most of the drag occurs i.e. the last half of the race. Very seldom is the wind steady enough so it is the same at the starting line as at the finish AND very seldom is it directly in line with the track!

Good luck. Vic

10-30-2012, 05:08 PM	#1
Vic Santos Junior Member Join Date: Dec 2008 Posts: 27 Likes: 0 Liked 14 Times in 2 Posts	Re: Head wind vs tail wind-ET impact Clark, when you think about headwind vs tailwind they should be treated the same when predicting ET. Once the car has any reasonable speed it only "feels" a headwind. Example: if you were sitting on the hood of a car and there was a 20 mph tailwind you would "feel" zero wind when the car goes 20 mph. At 120 mph the the car only "feels" a 100 mph headwind (you would have already fallen off). Conversly with a 20 mph headwind the car "feels" a 140 mph headwind at 120 mph. The car can accelerate more at a 100 mph headwind than at a 140 mph headwind thereby changing the ET and mph . Each car has its own amount of aero drag and is dependant on the trap speed. It is easy to get the change in ET vs wind speed over a reasonable range, just measure it (at the same DA). The number for my car is 0.0063 sec/ mph of wind and it works for BOTH head and tailwind. The big problem is to know what the wind speed is where most of the drag occurs i.e. the last half of the race. Very seldom is the wind steady enough so it is the same at the starting line as at the finish AND very seldom is it directly in line with the track! Good luck. Vic