This was brought up in another post and I didn't want to hijack the thread. Kona brought up the subject of the aerodynamics of leaving the tailgate up or down. I've seen the explanation that leaving the tailgate up is more aerodynamic but I've always wondered if that was a myth. After a quick (read, not exhaustive) google search I found a study done by a mechanical engineering student in which he watertunnel tested a scale model of a dodge pickup. Here are the coefficients of drag that he found:


With the tailgate down, the coefficient of drag decreased as compared to the tailgate up. Interestingly, the cargonets increased the drag. To add some credibility this guy won the 1997 American Society for Mechanical Engineering Young Design Engineer's Paper Competition International Award, whatever that is.

Here's the website.

Yep, and when you consider how much effect wind drag has for normal driving, even highway, you find it just doesn't make much of a difference. Which would explain why I never was able to find any difference in running my truck tailgate up, tailgate down, tonneau covered. It just stayed the same.

At our speeds you'd never know the difference but those tiny improvements can mean alot. For example if you remember the Monte Carlo SS Aero Coupe (built for NASCAR) which basically was the addition of a sloped rear glass instead of the square back typical of the day. This addition ONLY made a .05 improvement in the drag coefficient number. Sounds tiny and was even laughed at but to make that same gain in top speed at the time would have required an additional 50 horsepower. Try coming up with more 50HP within the rules of racing on a motor thats already at limits and all of the sudden that goofy rear window is pretty attractive. <img src="/forums/images/graemlins/smile.gif" alt="" />

OMG!! this is an awsomne post!!@! i was wondering this this morning!! hahaha did you read my mind Robinhood?

What you are seeing is the effect of a huge vortice at the back of the cab. Vortices create big losses due to drag as was exampled by the sloped rear window. (Note: It's no accident the Porshe is shaped the way it is) Since the air is swirling very fast in a clockwise direction if you are looking at the drivers side of the truck, it is basically heading downward in the region of the tail gate. If the wind were directed right at the tail gate it would make a big difference if it were up or down but since is it more parallel with it it does not.

If you were to have a cover coming from the top edge of the cap to the edge of the tail gate while it is down, you'd likely see a large improvement in drag.

Since drag force increases with, amoung other things, the velocity squared, small changes in speed make large changes in drag force. This is why small changes in the drag coefficient make big changes in top speed.

Frank.

I thought that "vortices" was the plural of "vortex". <img src="/forums/images/graemlins/confused.gif" alt="" />

Everything else you stated made perfect sense Frank. <img src="/forums/images/graemlins/bird.gif" alt="" />

My friend would get an extra 30 - 50 miles out of a tank by lowering the tailgate on his 97 ranger.

Hmmmm.... this could be... my teacher used the term vortices... I'm at work but now I'm curious and will check when I get home. <img src="/forums/images/graemlins/smile.gif" alt="" />

Frank.

I used to notice a difference in my '92 Ranger. It stands to reason different trucks with different geometries will see different results.

Frank.

yup
Basic interpretation is closed gate makes a pocket of air in the bed. Higher speed air then flows over the cab and this pocket in the air. Dropping the gate gets rid of the pocket and air that would normally have flowed over the top now get pulled down hard behind cab and exits the rear at deck level, thus more drag with gate down.

Another result of sloped glass is cleaner air over the spoiler and more downforce. Isnt air cool <img src="/forums/images/graemlins/smile.gif" alt="" />