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!

[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]

Originally Posted by 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.

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]

Originally Posted by wootwoot

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]

Originally Posted by 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.

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.

[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 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]

Originally Posted by 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.

what the fuck are you talking about? You can't answer for me! Don't you know who I am? 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]

Originally Posted by sykikchimp

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:



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 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]

Originally Posted by 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.

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

[ZK]

Wow, I am actually learning something...

[gogeeta13]

Originally Posted by Rennen

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



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 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]

Originally Posted by gogeeta13

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]

Originally Posted by 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

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.