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

[DamnedButDetermined]

Okay i have a stock 91 fastback bumper right now, and the car is black. Everytime, I wash my baby and get her looking great, I take it out on the road and ten minutes later I walk around to the rear bumper and there is a shit load of sand and dirt from the road just sitting the the top of the rear bumper. This obviously means that the rear bumper is creating a high level of turbulance which is causing the sand to wip up and settle on the rear bumper.

Question time...

Does anyone else's stcok bumper do this?
If you replaced your's with aftermarket does it still do this?
If it still does it, what brand bumper did you get, and has it gotten worse or better?
If it stopped, what brand kit did you buy.

I doubt this ill look good at all on a fastback, but has anyone photoshopped a C5 corvette style rear bumper onto a fastback?
If not could someone with photoshop skills do it and see what it would look like?

thank you for your time,
DbD

[rainier]

i remember a thread going over all the dust that gets onto our bumpers. iuno about aftermarket bumpers though

[DamnedButDetermined]

any keywords I should search for? I tried dust, rear bumper, sand and dirty

didn't come up with aything

[sykikchimp]

my stock bumper does the same thing. there's definately a low pressure zone right there that allows dust to settle.

I wonder if it's the shape of the rear deck, or the bumper.. it's possible that a rear diffuser or small wing could help.

[mr_240sx]

i noticed that with my stock 89 bumper, i now have a full c-west kit all the way around and i still have the stuff appear on my rear bumper! :wtf:

[mav1178]

Aerodynamics is not fixed by changing a bumper. You have to fix the airflow going over/under the car.

And unfortunately there's no way to fix this problem without spending a huge sum of money on custom stuff on the car.

-alex

[wootwoot]

put a massive spoiler on your car like the porsche gt3 race cars. that'll solve your problem

[MakotoS13]

OR cut holes at the top of the rear bumper to allow air in the pocket the bumper and flat back of the car causes. since our cars are very sleek in the front and boxy in the rear it just sorta drops off and creates drag.

[Annoying Eric]

It gets so dirty because when you wash it the water goes around the trunk and lights and then when you drive, it gets pushed out and gets the rear bumper dirty agian.

[daredevildoug]

It gets so dirty because when you wash it the water goes around the trunk and lights and then when you drive, it gets pushed out and gets the rear bumper dirty agian.



true dat! thats why its best to drive the car for a second to get the loose water from the lights and hatch. then park it somewhere and dry the sucker. see if that solves ur problem

[mav1178]

put a massive spoiler on your car like the porsche gt3 race cars. that'll solve your problem


Ok what part of "fixing airflow going over/under your car" is confusing?

The wing won't solve ANY problem. Proof?

If you ever have had any leaking fluids from the power steering or engine, you'll find that a lot of it gets accumulated on the bumper as you drive freeway speeds. The airflow UNDER the car gets the dust on the bumper, more than anything else.

To fix it you have to remove the gas tank on the S13 and construct a custom rear diffuser that extends past the edge of the rear bumper, fixing this inherent problem with airflow. And the design of the diffuser is the hard part.

-alex

[DamnedButDetermined]

It gets so dirty because when you wash it the water goes around the trunk and lights and then when you drive, it gets pushed out and gets the rear bumper dirty agian.


Thanks for the reply, but this is happening days after the vehicle is washed, and after I haven't wiped off the dust a few times!


I guess there just isn't a fix for this (well cost efficient fix)

[adey]

I'm told that a roof/hatch diffuser will help a with this a little bit. See the EVO MR or MV or whatever it is. (Actually I don't know if it's stock or not, but it was on a red EVO in some magazine like 2 months back) It's got a bunch of little "fins" on top of its rear windshield, which supposedly create vortexes along the rear hatch/windshield/trunk, thus decreasing drag behind the car... or something to that extent.
I think it's supposed to help stability mainly, but might/should help with your dust problem, too.
Just a possibility which I'm not claiming is true... necessarily. :p

[Var]

i've never heard of this dust problem but aerodynamically the s13 is pretty good. Base bodel no spoiler is like .30 drag coefiicient. Adding a wing or body kits will definitely make it less efficient but if you want downforce thats what you pay. And the dust thing...wow. What an excuse to get a body kit :p j/k

[nightwalker]

You guys have to look under the car behind the gas tank. There's a pocket there that traps air going to the back of the car. That's your cause, like Mav has been saying. If someone constructs a nice diffuser or something to block off that area I'm sure it would help. I've also thought about a fiberglass bumper with vents to let out the trapped air.

[gogeeta13]

I would really like a CF or aluminum undertray for my race S13...anybody know of any companies that make them?

[sykikchimp]

Wasnt Adam H. on FA building a diffuser for his race car out of aluminum? He was basically dealing with the same issue of trying to create somoe more downforce back there.

btw guys this issue is about aerodynamics, not a dirty car. The dust on the bumper is merely what prompted him to ask.

[DamnedButDetermined]

Wasnt Adam H. on FA building a diffuser for his race car out of aluminum? He was basically dealing with the same issue of trying to create somoe more downforce back there.

btw guys this issue is about aerodynamics, not a dirty car. The dust on the bumper is merely what prompted him to ask.


what the fuck are you talking about? You can't answer for me! Don't you know who I am? :bowrofl: It's okay just hijack my thread....I really didn't want to talk about dirty bumpers anyways.


On a serious not, can anyone do a real quick photoshop of a C5 Corvette rear bumper on a 240 just for shits and giggles?

[DamnedButDetermined]

My boss and I just bought a 4600 sq/ft warehouse condo that we are going to put a lift in and conduct business out of so when we get all set up I might try and play with a few untray ideas! FYI this will not be until jan/feb 05

[Rennen]

btw guys this issue is about aerodynamics, not a dirty car. The dust on the bumper is merely what prompted him to ask.

yeah, I don't think people are getting the idea. So here is a simple drawing:

http://www.imagestation.com/picture/sraid149/p89dcaf3ee61619d6ffb83de6092c589e/f608543a.jpg

Since the 240 is traveling forward, it punches a hole in the air. the air it displaces moves under and over the car. At the rear of the car (the bumper) there is all of the sudden a hole that air not previously taking up. This creates a low pressure zone. Air quickly flows from the high pressure around the car to the low pressure at the bumper. This means that air that was flowing along the ground under you car(and picking up dirt) suddenly moves upward in a turbulent fashion carrying dirt onto the bumper.


The easiest way to fix this is to delete the 240SX and replace it with a Porsche 911 :D or other "smooth" car.

Or you can keep air from getting under the car(impossible) with an airdam/sideskirts.

Or construct a large cone shape on the rear of the car in order to smooth the flow until the upper and lower flows converge... and if you look at my picture, those flows wont converge until 8 feet behind the car. I am sure you don't want to build that.

-Matt

[Rennen]

I'm told that a roof/hatch diffuser will help a with this a little bit. See the EVO MR or MV or whatever it is. (Actually I don't know if it's stock or not, but it was on a red EVO in some magazine like 2 months back) It's got a bunch of little "fins" on top of its rear windshield, which supposedly create vortexes along the rear hatch/windshield/trunk, thus decreasing drag behind the car... or something to that extent.
I think it's supposed to help stability mainly, but might/should help with your dust problem, too.
Just a possibility which I'm not claiming is true... necessarily. :p

Yeah,

Those vortex generators create small vortices. These small turbulent vortices won't separate from the surface as quickly as smooth flow will, so they "stick" to the rear window. So by creating small vortices of turbulence, they avoid the large vortices created by flow separation and reduce drag. It's really pretty neat, and should work well on a car as tall as a Lancer. If you are riding in an airplane, you can look out at the tops of the wings, and there will be little fences in the middle that do the same thing.

-Matt

[Ian]

^woah


that helps me alot...it's too bad that an 8ft cone wouldnt be stylish though :-\

[TurDz]

ahh fluid dynamics all over again!!

great post with the pic of the airflow. even though i should know all of the engineering lingo, im glad you explained it in laymen's terms. that's about all i can understand anyway :D

[ZK]

Wow, I am actually learning something...

[gogeeta13]

yeah, I don't think people are getting the idea. So here is a simple drawing:

http://www.imagestation.com/picture/sraid149/p89dcaf3ee61619d6ffb83de6092c589e/f608543a.jpg

Since the 240 is traveling forward, it punches a hole in the air. the air it displaces moves under and over the car. At the rear of the car (the bumper) there is all of the sudden a hole that air not previously taking up. This creates a low pressure zone. Air quickly flows from the high pressure around the car to the low pressure at the bumper. This means that air that was flowing along the ground under you car(and picking up dirt) suddenly moves upward in a turbulent fashion carrying dirt onto the bumper.


The easiest way to fix this is to delete the 240SX and replace it with a Porsche 911 :D or other "smooth" car.

Or you can keep air from getting under the car(impossible) with an airdam/sideskirts.

Or construct a large cone shape on the rear of the car in order to smooth the flow until the upper and lower flows converge... and if you look at my picture, those flows wont converge until 8 feet behind the car. I am sure you don't want to build that.

-Matt

Very insignful post. Thank you.

Do you know what a wangan wing for example does to this air flow model you have?

[Zemus]

I would eather add a Wangon style wing, or.. Do it up datsun style and just remove the bumper totaly, 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

[Rennen]

Do you know what a wangan wing for example does to this air flow model you have?


haha, its hardly a model, its a 5 min half-assed drawing with the Photoshop paint brush.

But a wangan wing will make the dirt-on-bumper problem worse I would think. They usually stick out rearward past the taillight surface, this would create an even lower pressure in the area between the wing and hte top of the bumper... and all of the dust you drive over goes there.

In terms of aerodynamic effect, most aftermarket wings are worthless. You won't get any good attributes unless they are about even with the top of the roof, and then they look far too absurd for a street car. So unless you have a wind tunnel to test in, or are buying purely for asthetic reasons, stay away from wings.

-Matt

[Doriftomodachi]

Okay i have a stock 91 fastback bumper right now, and the car is black. Everytime, I wash my baby and get her looking great, I take it out on the road and ten minutes later I walk around to the rear bumper and there is a shit load of sand and dirt from the road just sitting the the top of the rear bumper. This obviously means that the rear bumper is creating a high level of turbulance which is causing the sand to wip up and settle on the rear bumper.

Question time...

Does anyone else's stcok bumper do this?
If you replaced your's with aftermarket does it still do this?
If it still does it, what brand bumper did you get, and has it gotten worse or better?
If it stopped, what brand kit did you buy.

I doubt this ill look good at all on a fastback, but has anyone photoshopped a C5 corvette style rear bumper onto a fastback?
If not could someone with photoshop skills do it and see what it would look like?

thank you for your time,
DbD

EVERY car with a rear bumper that sticks out will gather dust on top of it. That's just how the air flows over and behind a car. I have never seen a bumper that looks like a c5 or c6 on an s13 or s14, not even in all oof my hyper rev and option/option2 mags. The cars usually run with no rear bumpers because they break off when they go drifting. Why would you spend a large sum of money to fabricate a rear bumper that looks like the c5 or c6 corvette just to not have dust gather there?

[driftyour40]

Okay, well I'm not too sure of some of this or most of it, but I do have a pretty good understanding of airflow so I'll try and help.

-Signal auto I think makes a front & rear diffuser. I think (Talk to the driftfactory about it, as this is where I first herd of it.)

-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.

-You’re never going to completely get rid of the dust (w/out an 8' cone that is.)

Yeah that’s all I can think of at the moment. 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 ( Dust + water + wind + heat = hard heavy stuck on dirt) so that’s not your answer for dust. 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 would have to be big holes, and the diffuser would help more. Anyhow I hope this helps.

[driftyour40]

Okay, well I'm not too sure of some of this, but I do have a pretty good understanding of airflow so I'll try and help.

-Signal auto I think makes a front & rear diffuser. (Talk to the driftfactory about it, as this is where I first herd of it. I think.)

-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.

-You’re never going to completely get rid of the dust (w/out an 8' cone that is.)

Yeah that’s all I can think of at the moment.

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.

[mav1178]

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]

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. :fawk2:


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. :fawk:

[Rennen]

Because they are racecars w/ unlimited budgets jackass, and use defuses. :fawk2:


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. :fawk:


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]

Because they are racecars w/ unlimited budgets jackass, and use defuses. :fawk2:


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. :fawk:


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.
http://www.geocities.com/Factory6987/Ferrari550MaranelloGroundEffects.jpg
http://www.thewarfields.com/OldSite/images/Bottom.JPG

[SlideStar]

-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.

http://www.veilside.com/veilsideusa/aero/nissan/32gtsrear.jpg

Here is the Nismo Diffuser on a G35
http://store1.yimg.com/I/ntnsracing_1821_12141166

[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.
http://www.yashiofactory.jp/democar/spf15/rear.jpg

[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 :p

[SlideStar]

can anyone explain rear diffusers in more detail?
http://www.mulsannescorner.com/diffuser.htm
http://www.mulsannescorner.com/diffuser5.jpg

[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]

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]

http://www.thewarfields.com/OldSite/images/Bottom.JPG


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