Head wind vs tail wind-ET impact

cad · 09-11-2012, 01:32 PM

Hi all,
In your experience, what is your experience dialing from a day with a tail wind to a day with a head wind....if DA was similar?

We're running ET finals this weekend at Topeka.
68 Camaro runs around 7.00/96mph 1/8 and 11.10/122mph 1/4 mile.
SBC, glide and 4.88s on a 29x10 tire.

Friday is a NNE 10 mph wind.
Weather forecast says Saturday will be a 7 mph SW wind, and Sunday will be around 12 mph S wind.

Thanks
Clark

Kegracing · 09-11-2012, 05:13 PM

I ran a 68 Camaro for several years, mostly 10.90 stuff. Had the stock spoiler on front and rear. On the T-stop it ran 10.90 at about 125-127 and ran 10.40's off, with most of my motors.

Direct headwind was -.15 in quarter, and tail was about +.10... on my car. Everyone is a little different. Also, the JEGs log books used to have a guide on the first page... not sure if it still does or not. Don't have one handy.

Topeka track runs about due south...

Good luck.

Ken

Tony Curcio · 09-11-2012, 07:40 PM

The Jegs log book said that 8 mph wind = .05 ET, using a 9.90 car as an example.
It doesn't address differences in frontal area.

My experience with a Camaro running a little slower than yours matched the Jegs formula pretty well for a head wind, but is affected a bit less by a tail wind.

cad · 09-11-2012, 07:57 PM

Thanks guys. I appreciate it!

Tom Meyer · 09-12-2012, 08:55 AM

My Chevy II at topeka with a wind change you are talking about was anywhere from 15 20.That place is a dailing nitemare with the wind go luck. Tom

cad · 09-12-2012, 09:21 AM

Thursday looks like rain and Friday looks beautiful, so we'll get the gambler in.
We get 1 time trial Saturday and straight into Race of Champions, so at least we get 1 shot.

gsa612 · 09-12-2012, 12:26 PM

Quote:

Originally Posted by Tom Meyer

My Chevy II at topeka with a wind change you are talking about was anywhere from 15 20.That place is a dailing nitemare with the wind go luck. Tom

X2, the layout of the track at topeka makes it like a wind tunnel if you got a headwind, lost 2 tenths when the wind picked up, the da only changed a couple hundred ft. gsa612

Altronics · 09-25-2012, 06:11 PM

PerformAIRE systems offer wind sensors and wind prediction/correction. You can select between several car type and also features an adjustable "effect" (drag coefficient) to fine tune your wind correction.

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

09-11-2012, 01:32 PM	#1
cad Senior Member Join Date: Apr 2009 Location: Omaha, NE Posts: 697 Likes: 200 Liked 91 Times in 51 Posts	Head wind vs tail wind-ET impact Hi all, In your experience, what is your experience dialing from a day with a tail wind to a day with a head wind....if DA was similar? We're running ET finals this weekend at Topeka. 68 Camaro runs around 7.00/96mph 1/8 and 11.10/122mph 1/4 mile. SBC, glide and 4.88s on a 29x10 tire. Friday is a NNE 10 mph wind. Weather forecast says Saturday will be a 7 mph SW wind, and Sunday will be around 12 mph S wind. Thanks Clark

09-11-2012, 05:13 PM	#2
Kegracing Member Join Date: Jun 2002 Location: Red Oak, TX Posts: 441 Likes: 0 Liked 0 Times in 0 Posts	Re: Head wind vs tail wind-ET impact I ran a 68 Camaro for several years, mostly 10.90 stuff. Had the stock spoiler on front and rear. On the T-stop it ran 10.90 at about 125-127 and ran 10.40's off, with most of my motors. Direct headwind was -.15 in quarter, and tail was about +.10... on my car. Everyone is a little different. Also, the JEGs log books used to have a guide on the first page... not sure if it still does or not. Don't have one handy. Topeka track runs about due south... Good luck. Ken Last edited by Kegracing; 09-11-2012 at 05:18 PM. Reason: spelling

09-11-2012, 07:40 PM	#3
Tony Curcio Member Join Date: Mar 2002 Location: West Chester, PA Posts: 271 Likes: 1 Liked 4 Times in 1 Post	Re: Head wind vs tail wind-ET impact The Jegs log book said that 8 mph wind = .05 ET, using a 9.90 car as an example. It doesn't address differences in frontal area. My experience with a Camaro running a little slower than yours matched the Jegs formula pretty well for a head wind, but is affected a bit less by a tail wind. __________________ Tony Curcio 1860 STK

09-11-2012, 07:57 PM	#4
cad Senior Member Join Date: Apr 2009 Location: Omaha, NE Posts: 697 Likes: 200 Liked 91 Times in 51 Posts	Re: Head wind vs tail wind-ET impact Thanks guys. I appreciate it!

09-12-2012, 08:55 AM	#5
Tom Meyer Senior Member Join Date: Aug 2003 Location: Pierce NE Posts: 933 Likes: 74 Liked 351 Times in 120 Posts	Re: Head wind vs tail wind-ET impact My Chevy II at topeka with a wind change you are talking about was anywhere from 15 20.That place is a dailing nitemare with the wind go luck. Tom __________________ Tom Meyer 5240 SS Stock ???

09-12-2012, 09:21 AM	#6
cad Senior Member Join Date: Apr 2009 Location: Omaha, NE Posts: 697 Likes: 200 Liked 91 Times in 51 Posts	Re: Head wind vs tail wind-ET impact Thursday looks like rain and Friday looks beautiful, so we'll get the gambler in. We get 1 time trial Saturday and straight into Race of Champions, so at least we get 1 shot.

09-25-2012, 06:11 PM	#8
Altronics Sponsor Join Date: Dec 2010 Posts: 92 Likes: 0 Liked 63 Times in 11 Posts	Re: Head wind vs tail wind-ET impact PerformAIRE systems offer wind sensors and wind prediction/correction. You can select between several car type and also features an adjustable "effect" (drag coefficient) to fine tune your wind correction. __________________ Fred Bartoli 847-923-0002

10-30-2012, 05:08 PM	#9
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