Not your usual aero thread: Questions about rear bumpers and air flow!

[mav1178]

Originally Posted by driftyour40

Oh cutting holes in your bumper wont help w/ the dust it'll get worse and it will also build up on the inside of your bumper. Over time making your car heaver so that’s not your answer for dust. Dust + water + wind + heat = hard heavy stuck mess. If you don’t believe me take your rear bumper off and look at all the crap that’s built up over the years.
(Then clean it, and your car will be lighter, lol.)

On the other hand it will help make your car more stable at high speeds, and also give you a higher top speed because of less drag but the holes would have to be big. The diffuser would help more. Anyhow I hope this helps.

The last part makes NO sense at all...

Holes in bumper = less drag?

HAHA. I think you better check your facts... if so, how come no race cars have "holes" in their bumpers?

-alex

[driftyour40]

Originally Posted by mav1178

The last part makes NO sense at all...

Holes in bumper = less drag?

HAHA. I think you better check your facts... if so, how come no race cars have "holes" in their bumpers?

-alex

Because they are racecars w/ unlimited budgets jackass, and use defuses.


Think real hard for two seconds, it’ll only hurt for a moment.

Think about the way wind travels under your car then think about what the bottom of your rear bumper looks like. It’s like a big car wide scoop that are rushes into then, it gets trapped because it’s got nowhere to go “creating drag”. Ass.

Okay so then you cut/drill holes in your bumper thus creating an escape for air “alleviating drag”. It's not that hard.

[Rennen]

Originally Posted by driftyour40

Because they are racecars w/ unlimited budgets jackass, and use defuses.


Think real hard for two seconds, it’ll only hurt for a moment.

Think about the way wind travels under your car then think about what the bottom of your rear bumper looks like. It’s like a big car wide scoop that are rushes into then, it gets trapped because it’s got nowhere to go “creating drag”. Ass.

Okay so then you cut/drill holes in your bumper thus creating an escape for air “alleviating drag”. It's not that hard.

ummm even though you used a few middle fingers and a badass attitude.... your "speed holes" idea is a bit off.


This massive scoop you refer too will instantly be filled with air once you start driving, creating higher pressure. Since air never flows from Low->High the air under the car will go around this bubble of high pressure air and over the bottom lip of the bumper.

Now if you decide you want your car to be superfast with speed holes in your rear bumper, Air will flow through the area previously occupied by the bubble of high pressure. This air will have to work its way up around the gas tank, along the insides of the bumper and eventually make its way out through the holes. So where before the moving air only saw a surface area of the bottom lip of the rear bumper, it now sees a much larger surface area of Holes+inside of bumper+gas tank.

Now which surface area do you think will make more drag?

-Matt

[mav1178]

Originally Posted by driftyour40

Because they are racecars w/ unlimited budgets jackass, and use defuses.


Think real hard for two seconds, it’ll only hurt for a moment.

Think about the way wind travels under your car then think about what the bottom of your rear bumper looks like. It’s like a big car wide scoop that are rushes into then, it gets trapped because it’s got nowhere to go “creating drag”. Ass.

Okay so then you cut/drill holes in your bumper thus creating an escape for air “alleviating drag”. It's not that hard.

Ok, aside from the fact that I've actually done testing with my friends in aerospace engineering (one guy does aerodynamic testing for a car manufacturer for a living)...

All numbers and formulas aside, drag is not caused by the bumper "scooping" air. Drag, for the most part, is caused by the car's shape and inability to properly MANAGE airflow around the body as it moves in the atmosphere. This is then multiplied by the frontal area of the car for an actual number in drag.

I referenced race cars because they are a prime example of airflow management. Yes they run diffusers, but those and wings are a tradeoff between drag and downforce. They must have the best possible compromise between the two so that you have the least amount of drag with the largest amount of downforce for different track setups.

The point of a diffuser is to manage the airflow so that it is SMOOTH. These "holes" you speak of in the rear bumper will make the airflow turbulent, causing more drag. Fluid dynamics is much more complicated than what you make it out to be.

-alex

[BadMoJo]

owned


Oh and Zemus, post those 240z picts.

[driftyour40]

First off, it's not my "speed holes" idea, and I didn't say it would be the end all be all drag eliminator. Second, my theory is sound whether or not air has to travel through the gas tank and everything else holes at the end would help even if it's only a little.

mav1178 HAHA. I think you better check your facts... if so, how come no race cars have "holes" in their bumpers?

This is a stupid question. Why in gods name would you even think of comparing a streetcar to a racecar? Also if you have done testing, then you wouldn't need to ask stupid questions like that.

If I'm completely wrong that’s fine. But if I am then why is removing the rear bumper so common among Datsun owners.

Zemus I would eather add a Wangon style wing, or.. Do it up datsun style and just remove the bumper totally, and make smooth out where the bumper was so that their is no problem in flow, and make it smooth and go under the car, if you need pics, I can supply them what it look like on a 240z

[Colorado S14]

Do Luck makes a universal CF rear diffuser that you cut to fit, SPL Parts shows it on their site under the Aero section.

What do you guys think about getting some sheet plastic and custom fitting it to the bottom of the car to create a smooth path for flow, Ferarri does it on all of their cars.

[SlideStar]

Originally Posted by driftyour40

-Signal auto I think makes a front & rear diffuser.

-Lowering your car will help. Will help a lot more if the front is lower than the rear. The idea is the lower your car is the more air will flow over the top and less under it. Air will chose the path of less resistance.

He's right. Nismo also makes a rear diffuser for the 180sx. This is the best solution to the air flow problem. Adding a wing will create downforce, and more resistance. As was mentioned earlier, the wing is a trade off because the rear diffuser reduces whatever tubulence the wing might add. Removing the rear bumper all together would cause a larger space to trap tubulent airflow. ...although there wouldn't be anywhere for the dirt to settle.

[nightwalker]

links or pics of the Nismo diffuser please.

[SlideStar]

Here is one of a veilside skyline. The Nismo diffuser looks similar but it's just and add-on not the whole bumper. I'm try and find a picture of it.



Here is the Nismo Diffuser on a G35

[s14falcon]

Man, i meant to post yesterday. Anyways, i knew i had seen a big diffuser before. yashio factory makes a big one for the s15. I dont know much abou these aerodynamics, but i'd imagine it is meant to address this problem.

[TurDz]

can anyone explain rear diffusers in more detail?

anyway, I found a great website. it's very helpful and I'm eager to learn more about auto aerodynamics.

http://www.up22.com/Aerodynamics.htm

btw...the "fastback" design is a good design according to the website.

edit: Rennen, according to the website above (which sounds like a credible source), is correct again. If you create holes in the bumper, some air will obviously flow through those holes, but the drag created by many of the components of the underside of the car will negate or worsen the affect compared to just a rear bumper without holes.

I agree with the people above who've said that rear diffusers aid in smoothening flow to converge, i.e. that 8ft cone behind the car. I guess I answered my own question

[SlideStar]

Originally Posted by TurDz

can anyone explain rear diffusers in more detail?

http://www.mulsannescorner.com/diffuser.htm

[Rennen]

Except for maybe the Yashiro Factory piece, none of those are long enough to anything but look stupid.


A diffuser is a simple channel that goes from a narrow width to a wide width and works like a reverse venturi. This slows down the air flowing exiting under the car, but more importantly manages the flow underneath to keep everything smooth. With the airflow under the car smooth, and the air speed at the rear matched to the air around the car, the air under the car can move with a high velocity, and create a low pressure zone. This lower pressure can "suck" the car to the ground if speeds are high enough and the car is low enough.


This design principle needs the front bumper, under body, and rear diffuser to work together to manage the flow.... So all these aftermarket bumper slap ons that claim to be diffusers, really do nothing at all.

Here is a link that explains it a bit better than I can: http://www.atlasf1.com/2000/feb16/gray.html
-Matt

[mav1178]

Originally Posted by driftyour40

First off, it's not my "speed holes" idea, and I didn't say it would be the end all be all drag eliminator. Second, my theory is sound whether or not air has to travel through the gas tank and everything else holes at the end would help even if it's only a little.

Ok your theory is sound, we'll all listen to you.

This is a stupid question. Why in gods name would you even think of comparing a streetcar to a racecar? Also if you have done testing, then you wouldn't need to ask stupid questions like that.

DId you not read what I wrote? Let me recap what I wrote two posts above yours...

I referenced race cars because they are a prime example of airflow management. Yes they run diffusers, but those and wings are a tradeoff between drag and downforce. They must have the best possible compromise between the two so that you have the least amount of drag with the largest amount of downforce for different track setups.

The point of a diffuser is to manage the airflow so that it is SMOOTH. These "holes" you speak of in the rear bumper will make the airflow turbulent, causing more drag. Fluid dynamics is much more complicated than what you make it out to be.

If I'm completely wrong that’s fine. But if I am then why is removing the rear bumper so common among Datsun owners.

Because perhaps the car was built in the 60's and 70's when aerodynamic testing wasn't very advanced? When everyone had round metal bumpers that didn't match the shape of the body and is not aerodynamically efficient?

Anyhow, you can take my comments for what it is worth. If it is a crock of shit to you, it's a crock of shit to you no matter what I say, so we'll just keep it at that.

-alex

[DamnedButDetermined]

Originally Posted by Colorado S14

This is exactly what I am going to experiment with when we get our shop up and running!...Thanks for the pic.


ZEMUS...Please just for fun post those picts of the 240zx