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Old 01-04-2014, 08:54 AM   #1
Gerald81
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Default Custom undertray

I fabricated an undertray for my 2007 STI using this design. What do you guys think? Will it be an improvement over stock in terms of lowering drag, helping airflow out of the engine bay and also getting air into the brakes?



Uploaded with ImageShack.us

Basically what I did was to cut out the frontal shape according to the factory STI splitter profile so it can slot right it, and will have to think of how I want to attach the middle and rear portions to the undercarriage.
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Old 01-04-2014, 10:29 AM   #2
funk32
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what is it made off? looks nice
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Old 01-04-2014, 10:35 AM   #3
Gerald81
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It's made of 5mm coroplast. I'm going to put some aluminium foil tape on the upper surface and am hoping it holds up well to the heat from the extractors.

I originally wanted to use NACA ducts in the area ahead of the front wheels and route hoses to the brakes, but decided it would be too complex to do, so I just made some hollows in the undertray instead to try and route air towards the brakes. Not sure if it works but have seen it done on other undertrays.

Also not sure if having a flat bottom can have the opposite effect of increasing lift? May have to try and setup some rake into the car's stance perhaps to counter that.
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Old 01-04-2014, 06:22 PM   #4
Paul
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looks very nice!!
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Old 01-04-2014, 06:32 PM   #5
vicious_fishes
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well it looks good. go test it!
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Old 01-04-2014, 06:36 PM   #6
Dayglow
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I think that looks pretty awesome. My only concern would be durability. Even my steel ones get a fair bit of damage.
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Old 01-04-2014, 09:31 PM   #7
StrangerDanger2.0
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Very nice belly pan. Should help decrease drag. I would build in an access panel to make oil changes easier.
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Old 01-04-2014, 09:59 PM   #8
Gerald81
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I chose coroplast for its light weight, but yeah I'm not sure how it would hold up to regular usage. What sort of damage would an under belly typically see? My stocker actually looks fine after 7 years.

I have a magnehelic gauge so will try to get some before and after figures. My main concern was actually helping air out of the engine bay by smoothening the airflow underneath, and also helping air flow to the Prova brake "ducts" on the control arms. Next thing I will probably try is to use some of the spare coroplast to duct air through the front of the radiator.
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Old 01-04-2014, 10:18 PM   #9
LIQUIDSK8S
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Looks really good. Where did you purchase the coroplast from? What size sheet? Price?

Last edited by LIQUIDSK8S; 01-04-2014 at 11:07 PM.
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Old 01-04-2014, 11:35 PM   #10
jvsoundwave
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Looks awesome! That's pretty much all that counts
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Old 01-05-2014, 12:51 AM   #11
T-37
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Wow, I really like this. Great job. I've wanted to do something similar for a long time but haven't had the initiative
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Old 01-05-2014, 02:01 AM   #12
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Looking good.

Can you get before and after readings of flow through the tmic if you've still got one.
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Old 01-05-2014, 06:05 AM   #13
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The coroplast was purchased for about S$20 per 4 x 8 feet piece from a plastic sheets shop here in Singapore, so maybe US$15 to US$20 equivalent? I understand you guys can pick these things up for free after elections

Here's a close up of the brake "duct" for those interested - not sure if it would help any if at all though:



Yeah I'm going to try and get before and after readings for airflow through the radiator and TMIC (still using the stock STI one) once everything is done up. I have a HKS Kansai front subframe brace which will provide some handy mounting points for the undertray. The next tricky bit is to drill clean holes into the coroplast to fit screws through. I'm thinking of getting a hollow paper drill bit with an adapter to use it with a handheld drill to do this.
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Old 01-05-2014, 05:19 PM   #14
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Quote:
Originally Posted by Gerald81 View Post
I understand you guys can pick these things up for free after elections

.
I thought it was that material after a quick google. My concern would be it bending since they are kinda fragile/flimsy. Are you going to make a frame on the other side to make it stiffer? How are you attaching those metal brackets to make the ducts??

That other pic does help a lot please keep us updated with progress and any other pictures would be great

Thanks!!!
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Old 01-05-2014, 10:24 PM   #15
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at the very minimum you should fill those holes.....
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Old 01-05-2014, 10:44 PM   #16
Paul
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with what? Its like cardboard so those holes run thru out
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Old 01-06-2014, 02:24 AM   #17
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Sure it's not terribly rigid, but then again neither is the stock undertray.

I'm not sure about the Impreza but on my Legacy the sides of the undertray are attached inside the wheel wells with plastic rivets. Between those and the 3 bolts (one rear center, two at the front) and the plastic rivets at the front/sides of the bumper, the undertray doesn't need to be super stiff.

Heat would be my biggest worry, but with reflective tape you might be fine. Won't know until you try, I guess. Run it and see how it fares.
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Old 01-06-2014, 08:52 AM   #18
Gerald81
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Quote:
Originally Posted by Paul View Post
I thought it was that material after a quick google. My concern would be it bending since they are kinda fragile/flimsy. Are you going to make a frame on the other side to make it stiffer? How are you attaching those metal brackets to make the ducts??

That other pic does help a lot please keep us updated with progress and any other pictures would be great

Thanks!!!
From what I've read, it's this ability to bend and pop back in shape that can save the coroplast in cases where steel or aluminium sheet would get bent and dented out of shape. I thought of attaching some strips of aluminium to strengthen it but it'll also add weight so we'll see...at this time it feels sturdy enough.

For the ducts, what I did is to make a sort of triangular box out of thinner coroplast sheet (3mm), but you could do it with the 5mm sheet as well. The idea is to fabricate something that looks like a rectangular box sawed in half diagonally. It is best to bend the coroplast along the grain, so try and keep that in mind when designing the triangular duct box.

Then I use some heavy duty double sided tape to attach a couple of L-shaped aluminium brackets to both sides of the duct box. On the main undertray sheet, I cut two parallel slits into where I want to fix the ducts, but I do not cut across to completely remove the material. I then tape the brackets down onto the undertray directly over the slits, and then lift up the "flap" and tape it to the top of the duct box. This ensures that the duct looks nice and smooth from the underside as it rises.

The flap will of course have to extend out a little from the edge of the sheet so that it fits nicely when lifted up to the roof of the duct box, also depending on the geometry of the triangular duct box (just think of it as the hypotenuse of a right-angled triangle). Hopefully this, and the pictures explains it. I'll try get a couple more close ups this weekend if it'll help.
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Old 01-06-2014, 08:54 AM   #19
Gerald81
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Regarding the holes in the edges of the coroplast, I intend to use some black electrical tape to cover most of the edges up. It's not shown in the pictures, but I did tape up the entire frontal edge that fits inside the front lip, to try and prevent damage to the edges when installing and removing the undertray.
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Old 01-07-2014, 09:23 PM   #20
omiotek
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Quote:
Originally Posted by Paul View Post
with what? Its like cardboard so those holes run thru out
you don't need to seal it through. you just need to seal it on the edges to prevent air from going in.

any sort of air that gets between the layers can sooner or later be catastrophic and rip the plys apart.

you could use panel bond or something of that nature to fill the gaps.
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Old 01-08-2014, 08:40 PM   #21
tuner92
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How does this material hold up to heat since you have the header sitting a few inches away. Did you do any heat protection to the top side ?

Last edited by tuner92; 01-09-2014 at 11:09 AM.
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Old 02-03-2014, 04:58 PM   #22
mkvaron
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Very interesting & nice work! I am curious about long term heat exposure as well, but if you are racing alot undertrays see tons of abuse so maybe its a non-issue. I agree that it will need to be rigid enough to hold shape, but from what I have read about coroplast its quite durable and retains shape well. And yes, seal all the cracks & gaps haha, my stock undertray was ripped to pieces once damage was bad enough to make a hole/crack.
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Old 02-03-2014, 06:14 PM   #23
PARANOID56
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yea wondering how this is holding up.
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Old 04-15-2014, 08:40 AM   #24
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Quote:
Originally Posted by PARANOID56 View Post
yea wondering how this is holding up.
.....
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Old 04-22-2014, 05:24 PM   #25
Paul
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bump for updates
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