Dave B

I ran at the track yesterday and I left a bit disappointed. The conditions were actually pretty good (30.1 baro, 30% humidity, mid 50s), but there was a stiff constant 25mph north headwind with gusts up to 35-40mph. Unfortunately my track faces due north. The gusts were so bad at times I actually got blown backwards out of the staging lights. Anywho, my best of the day was a 14.49@96mph with a 2.30 60' along with a 14.5 and a couple 14.6s. My MPHs were in the 95-96mph range and my 60 foots were high 2.2s and low 2.3s. Wheelhop was really bad today and I think I blew out an exhaust gasket or possibly my flex section

Does a 25mph headwind affect a 14-second car much in the 1/4 mile? I don't see it being much of a factor from 0-60mph, but after that it seems like my 1/8 mile ETs and MPH just weren't where they should be, especially my MPHs. I did some research and I was able to find out a 10mph headwind will suck out about roughtly .10 and 1mph, but I don't about any other wind speeds. Any help would be appreciated.

The good news of the day was that I cut three .50X lights out of my four runs. The other run I cut a .466....whoops.


Dave

deezo

iTrader: (3)

I haven't seen that option when doing 1/4 tests.

Dave B

I don't have the Cartest program, but searching the net reveals that Cartest can/does take into consideration the headwind/tailwind. I dug up a Motor Trend article about a 10-second Hennessy Viper they were testing. In the article they talked about having to run in a 20mph and that they believed it slowed the car down quite a bit. I'm assuming Cartest takes into consideration the CF, whp/wtq, frontal area, etc when determing wind losses/gains. Hopefully Steve will reply with some sort of estimation.


Dave

deezo

iTrader: (3)

I can take a look when I get home then. Ive never noticed the feature when I started messing around with it because i was so tied up in testing the different cars that are loaded in the program.

On the other hand, I know Steve will pop in and have an answer because he uses it more than me.

Quicksilver

iTrader: (1)

Yes, you can add in headwind/tailwind. When you open up a specific car to start testing, just go to the File tab and mouse over the Access General Model Parameters...

Driver weight, fuel weight, track elevation, temp, humidity, roadgrade...it's all there!

Dave B

Thanks.

In case Steve drops in, here were the conditions:

steady 25mph headwind/slight cross wind gusting to 35-40mph
temps in the mid 50s
baro pressure at ~30.10
humidity at 30%
elevation at ~1100'


Dave

SteVTEC

Yup that's all in car specific model parameters, like quicksilver said. Err, yeah I think it's in general model parameters too but you wouldn't want to change it there because that sets it globally.

I'm pretty tied up in the first half of the week, but I'll get to it later this week unless somebody else beats me to it. I know Ari (Nismo87SE) would be able to whip out some numbers too if he sees it.

96sleeper

iTrader: (4)

Using my car's data with your conditions and no headwind netted me a 13.96@100.2mph with a 2.19 60 ft time. Only changing the wind to a 25mph headwind netted me a 14.24@96.56mph with a 2.19 60 ft time. I don't know if cartest accurately calculates wind or it really makes that much difference.

SteVTEC

You beat me.

What CarTest "should" do is calculate the drag force on the body and then subtract that away from the total thrust at the wheels. With a headwind it should just say that the drag force at speed "x" is now "x + 25" and then recalculate acceleration accordingly.

According to my spreadsheet calculations, the drag force on a 4g Maxima body is as follows for these speeds:

(approximate frontal area = 24.36 sq.ft, 0.32 Cd)

70 mph: 97.7 lbs
75 mph: 112.2 lbs
80 mph: 127.6 lbs
85 mph: 144.1 lbs
90 mph: 161.5 lbs
95 mph: 180.0 lbs
100 mph: 199.4 lbs
105 mph: 219.9 lbs
110 mph: 241.3 lbs
115 mph: 263.7 lbs
120 mph: 287.1 lbs
125 mph: 311.6 lbs

So if you're finshing the 1/4 mile pushing 100 mph, there's 100 lbs of extra drag force on your car with a 25 mph headwind.

At 100 mph, a stock 4th gen in 4th gear has roughly 600 lbs of thrust at the wheels moving a 3000 lb car. But 200 lbs of that thrust is being taken away by drag, so at that point 33% of the engine's power is being used to overcome aerodynamic drag. With a 25 mph headwind, now 300 lbs is needed to overcome aero drag and 50% of the engine's power is needed to overcome that. So yes, it can make a significant difference.

600 lbs thrust - 200 lbs drag = 400 lbs total force / 3000 lb car = 0.133g's
600 lbs thrust - 300 lbs drag = 300 lbs total force / 3000 lb car = 0.100g's

0.133g's = 2.92 mph/s acceleration
0.100g's = 2.19 mph/s acceleration

So without the headwind you're gaining about 3 mph per second, but with the headwind you're only gaining just over 2 mph per second. That would definitely be good for a few mph difference in trap speed.

SteVTEC

Using Paul's (Requin6's) NA/VI/ECU dyno and cartest....

14.14 @ 98.79 mph w/2.24 60' (still)
14.24 @ 97.51 mph w/2.24 60' (10 mph headwind)
14.35 @ 96.11 mph w/2.25 60' (20 mph headwind)
14.42 @ 95.34 mph w/2.25 60' (25 mph headwind)
14.48 @ 94.55 mph w/2.26 60' (30 mph headwind)

Drop per 10 mph headwind: 0.113s and 1.413 mph

So roughly a tenth and 1.5 mph for every 10 mph of headwind.


Cars that are trapping lower would be less affected since aerodynamics are less significant at lower speeds. Maybe I'll do the same experiment on the model I have with a stock dyno later. But for now, back to sk0ol work - homework due tomorrow (2.75 GHz voltage controlled oscillator design and layout, yippe!)

deezo

iTrader: (3)

I'm going to mess around with this. Fun, fun, fun!

Dave B

Thanks guys. I really appreciate the effort. I had never run in wind like that and my car just didn't seem to want to pull as hard once I approached the 1/8 mile. It's good to know that it wasn't all in my head and that I don't have to really worry if there was something wrong with my car. My BS-dyno guess was that the wind might have been accounting for around .2 and 2-3mph. It looks like I was fairly close. Yesterday was also the first time I've run without the UDP.

I have friend with a Contour SVT in my Fast4Doors group who's giving me crap because I didn't do so well yesterday. He's swearing up and down that his Contour SVT with the 3.0 motor swap is going to smoke me if all I can muster is 14.4s. I told him talk was cheap seeing that his 2.5 liter motor with every bolt-on could only do high 14.6s@95mph with a 2.2 60'. I know the 3.0 swap he has is a bit stronger (mostly torque) than the orginal 2.5, but he keeps forgetting seeing my car is probably still making more HP and TQ plus my car is 100lbs lighter. On my next outing we'll race. I'll have a new B-pipe, lightweight battery, and most importantly drag radials. Hopefully I can shut him up

Thanks again guys


Dave

SteVTEC

OMG Dave you got DR's

heheh...13's baby....13's.

deezo

iTrader: (3)

Not to go too off topic but I regenerated a configuration for my 15.6 stock run the software is showing that I needed to make over 200 crank HP to achieve that time. I found that interesting being that we talk about factory freaks an such.

SteVTEC

- Standard 4g Auto:
- 190hp @ 5300 rpm (changed from 5600rpm since this is where it's actually centered)
- 205tq @ 3400 rpm (changed from 4000rpm since it actually makes it at 3400)
- Coefficient of Kinetic Friction = 1.0 (set to 0.85 by default but gives horrible launches)

15.82 @ 86.78 mph w/2.47 60'<----

Gotta crank up the traction because they launch better. Switch this to 1.2 or something and play with it until it matches your 60' times.



Now click on reoptimize launch method so that it recalculates the the optimal launch RPM with the higher traction taken into consideration. With that setting at "1.2" you now get....

15.62 @ 86.86 mph w/2.33 60'<----


Never touched the power figures aside from the RPM's. The 60' times are what you really have to pay attention to in CarTest and I usually have to tweak it on a per-car basis when I'm trying to model something. To model ga2000's 14.6 time in his stock 04SE/auto I had to crank it up a lot higher than that, but he was pulling some kickazz 2.1-2.2 60' times.


BTW, you can see where to play with all of the headwind and grade stuff there too. Pretty much all of the custom parameters to play with are all in that panel.

deezo

iTrader: (3)

Gotcha!!!!!

Nealoc187

iTrader: (23)

Dave are you sure that the wind was really 25mph gusting to 40? I'm sure there was a strong wind, but often people overestimate wind speed. As a pilot, I have a bunch of experience with this. I often hear people estimating wind velocites at the track to be on the order of 25mph when in reality they are more like 12-13mph winds. 40mph gusts are MORE than halfway to that of a category 1 hurricane. 40mph gusts make it very VERY difficult to even walk into the wind. Was it really that difficult to walk? To put this further into perspective, a 40mph gust is ALMOST enough so that a Cessna 172 could TAKE FLIGHT without the engine on. That is it could be sitting there on the runway with no one in it, and the 40mph velocity would be almost enough to create the lift needed to lift the plane into the air. 40mph winds are ungodly strong.

That said, headwind does affect trapspeeds significantly, ET not so much, though there is obviously an effect. In changing from a 10mph tailwind to a 10mph headwind, I lose about .2 seconds and about 2-3mph.

Dave B

I totally agree, people do overestimate the conditions they run in. The constant 25mph headwind was no overestimation according to an onsite weather station a superpro dragster owner brought out and put near the staging lanes. His concern was obviously lift so he was adjusting his spoiler and the oriention of his chutes accordingly. As for walking, it wasn't hard to walk in the wind, but there were times where it would unbalance you when a gust kicked up. My big gripe was the constant debris blasting (sand, dirt, trash) The wind in Kansas can sometimes be pretty brutal. 50-60mph winds during thunderstorms are very common and open areas on the plains can see 70mph+ winds on perfectly sunny days when a high pressure system is moving in. I've seen a handful of tractor trailors tipped over in open areas of the Kansas thanks to strong north cross winds.

Here are the conditions according to WeatherUnderground for the day (I did my four runs between 1230pm to 130pm which was apparently the worst part of the day):

http://www.wunderground.com/history/...LLALMANAC=KOJC

It appears the offsite weather station was showing ~22-24mph speeds. According to onsite weather station, the highest gust I saw while there was 39mph, but according to Wunderground the highest gust recorded in the area was ~31mph.



Dave

Nealoc187

iTrader: (23)

That sucks that you had that sort of a head wind. Too bad it wasn't a tailwind hehe.

Dave B

A 20mph tailwind would have been really nice because all the other conditions were quite favorable. There's always next time. At least I know it was probably the wind slowing me down and not my car running like crap. My first slip was a 14.61@95mph and I was just about ready to leave the track that moment.


Dave