cutting holes in your rear bumper

[Sorrow Six Star]

Originally Posted by Tygen1

Sorry, I'm not buying it. I have a slow car, and just a little wheel spin can be a .2 difference. You just can't use a dragstrip to measure aero changes. I could probably list about one hundred things that could happen between two back to back runs that could alter the ET. Have this dude tape up them holes then do some coast down testing, A-B-A, then you can make a believer out of me.

the theory is sound.
if the bumper is filling with air then holes will allow the air to escape.

the better idea is to not let the air get there to start with.


and then there is the few who like it cause it looks cool.

[Tygen1]

I wouldn't know if the theory was sound, I just know that the testing method is silly I drag raced for many years and saw how much missunderstanding there was about going fast and how to test mods to see if they improved performance or not.

[Nerys]

sorrow six. While my knowledge of this is limited and I don't want to be combative My theory logic is the opposite. The bumper will only cause increased drag until it is FILLED with pressurized air. once that happens (micro seconds) it will from then on behave like a solid object. ie think air dam. when you add the holes you are letting this "bubble" escape and now the DRAG of air through the bumper will now persist and not convert to acting like an air dam which my butt sense says is lower drag than the holes.

for an example of this (I have NO tested this and I could be dead wrong on this but my gut says its so) take a CUP aim the open end into an airstream measure the drag.

take the same cup and CAP the end of the cup and do the same thing.

I think you will find the "drag" to be very similar is not slightly lower than the cup without the cap.

now take the open cup and drill holes in the bottom and add STUFF inside the cup (to represent the bolts frame brackets etc.. in the bumper.

now measure the drag. I think you will find the stuff cup with closed bottom will have far less drag than the stuff cup with closed bottom. Because of the closed bottom the stuff in the cup becomes irrelevant to drag. once you allow the passage of air it becomes relevant.

at least thats my gut check take on it.

[thatguitarguy]

How about cutting 1 hole in your bumper to fill your inflatable boattail??

http://ecomodder.com/forum/showthrea...ttail-896.html

[Christ]

Someone please do a test for me:

Turn on a faucet at 6 gallons per hour.

Put a funnel under it that can only flow around 3 gallons per hour.

Measure the "wasted" water that doesn't make it through the funnel.

That will prove that those holes aren't what they're cracked up to be. Keep the air from getting in there to begin with, you'll be better off for it. If you look at those holes, they represent less than half the surface area of the bumper. If the bumper in theory is filled with air, those holes are only relieving some of the pressure build up, causing (probably) more harm than good.

Imagine what happens when you force a fluid through a hole at pressure that isn't tapered, as well. The coanda effect says that the fluid (air) will attempt to adhere to the object it's being forced through, which means that it will expand very quickly as it leaves the hole, possibly causing more drag.

So first, you're pressurizing it, then, you're allowing it to expand. Sounds like a huge waste of energy to me. Why not save your grief, build a small belly pan, or just cut the bumper and add vertical stabilizers?

[Christ]

Originally Posted by thatguitarguy

How about cutting 1 hole in your bumper to fill your inflatable boattail??

http://ecomodder.com/forum/showthrea...ttail-896.html

Now that would probably be a good use of existing drag to shape the larger section of flow.

[gone-ot]

"...No, madam, that is NOT an automotive condom, it's an aerodynamic codpiece!"

[winkosmosis]

Why not cut the bumper bottom edge higher, and combine that with a pan? It would probably create downforce while also reducing drag. How is it different from the boattails everyone loves, unless your vehicle is very low? You guys need to put down the haterade and get off the ecomodder hatewagon.

[thatguitarguy]

Originally Posted by winkosmosis

How is it different from the boattails everyone loves, unless your vehicle is very low? You guys need to put down the haterade and get off the ecomodder hatewagon.

1. There might be confusion as to the purpose of a boattail.
2. What flavors does haterade come in?

[Sean T.]

You forgot:

3. Hatewagons are more aerodynamic usually than hatepickuptrucks, but less aerodynamic than hatehatchback-ecoboxes.