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Old 04-02-2018, 11:32 AM   #1 (permalink)
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Belly Pan Build Thread - Insight Gen 1

I've almost started this project several times. Could not see how to get past a few barriers. Now I'm ready to address it again. I want to have a smooth belly pan, front to rear. As smooth as I can get it, that is.

Much credit is due to Ecky and Natalya who have shared photos and notes on their belly pan builds. And to Scott, who makes and sells the excellent aluminum front belly pan replacement that I and many other Insight Gen 1 owners are using.

My exhaust hangs down a bit below the level of the oem rear belly pans. I'm planning on lengthwise spacer strips on either side of exhaust tunnel, to position the panels about 1.25" lower, for clearance. Details later. Anyway, hopefully no belly pan cutouts unless the exhaust system needs cooling air flow.

The car is now lifted for access to the underside. There's more to this than first meets the eye. Put the front up on jack stands first. Lifting the rear first caused the front to angle down too close to ground. No access to see or get a jack in there. The rear wheels are on ramps to take advantage of the hand brake to prevent rolling off. The ramps point away from car so I have room to slide underneath at the sides. So - you can't drive the car up onto the ramps in this configuration. A jack goes under the rear coil spring support to get the wheels up, then slide the ramps under the wheels.


Last edited by brucepick; 04-02-2018 at 10:56 PM..
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Old 04-02-2018, 11:35 AM   #2 (permalink)
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Really glad to see you back in this, Brucepick. You were a big presence with the HX Civic back when I started on EM seven years ago. My own bellypan dreams foundered too, for similar reasons. And I did not have a lift! But I did build the smooth diffuser.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

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Old 04-02-2018, 11:50 AM   #3 (permalink)
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Cali98, what's a smooth diffuser?

Last edited by brucepick; 04-02-2018 at 10:57 PM..
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Old 04-02-2018, 11:58 AM   #4 (permalink)
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I'm starting at the front. Made some progress today.

My car has one of Scott's aluminum front belly pans. Like the oem panels, it attaches to the oem support bracket at its rear, about even with the front wheels' centers. My first sub-project is to build a section beginning where Scott's panel ends.

Here's a drawing showing a cross section of the rear portion of the aluminum panel (blue) with the oem support bracket (black). I added a 1" x 1/8" x 25" backing strip going across the top side of the panel. That will give the fasteners for the coroplast something more substantial to anchor to.


I considered resting the coroplast on top of the bracket. That is, the horizontal cross member that supports the trailing edge of the forward belly pan. But the up and down features in the aftermarket panel and also the bracket, aren't ideal for aerodynamics - so I plan to hang the new section to the underside of the aluminum instead. So covering up those irregularities.

I will use rubber well nuts to secure the leading edge of the coroplast to the underside of the aluminum panel. They work a bit like expanding wall anchors. Tightening a screw into them forces them to expand, so they won't pull out. The screw is removable and so is the well nut. Mine take #10x32 screws, need a 3/8" hole.

I figured the aluminum belly pan by itself might not be enough for the well nuts so I added the 1/8" backing strip. I used seven 1/8" diameter pop rivets to secure the 1/8" bar stock backing strip above the panel. Rivets inserted from the bottom side.

Here's a drawing showing a compressed well nut with washer and screw, and a pop rivet. Shows the belly pan (blue) and the 1/8" backing piece. I omitted the layer of coroplast that will go between the well nut and the washer, sorry.



Here's a photo of a 3/8" hole with well nut partially inserted. The panel is upside down here. The well nuts and their screws will be inserted from the underside when assembling it.


Here's a photo, panel still upside down, with well nut inserted and a screw + some washers tightened into it. All stainless because I hate when fasteners get rusty. Tightening the screw squishes the well nut so it expands; it won't pull out.

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Old 04-02-2018, 12:47 PM   #5 (permalink)
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Quote:
Originally Posted by brucepick View Post
Cali98, what's a smooth diffuser?
Angled properly but without strakes.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

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Old 04-02-2018, 01:53 PM   #6 (permalink)
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Quote:
Originally Posted by California98Civic View Post
Angled properly but without strakes.
Yes, thanks. I'm aiming for that also. Due to the insight's configuration back there, exoect little upward slope as the lowernaero pan meets the bumper. But I will see when I get that far in the project.

This morning it snowed here, with rain forecast for tomorrow and Wednesday. Work is on hold till thendriveway is dry agin. Hopefully on Thursday..

Last edited by brucepick; 04-02-2018 at 10:58 PM..
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Old 04-02-2018, 05:12 PM   #7 (permalink)
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Black and Green - '98 Honda Civic DX Coupe
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Remember that you don't need much rise at all. Calculate your angle. You might find you have too much rise, axle to rear bumper.

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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

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Old 04-02-2018, 06:25 PM   #8 (permalink)
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It took me a while to figure out that graph. Soooo...

Since my Insight is an aerodynamically shaped hatchback I use curve "a".
If my diffuser has a slope of 7.5 compared to the previous section's slope, the diffuser would have zero effect on overall Cd. If the slope is 10 different, Cd goes up by about .008. Whew.

Is that right?

Last edited by brucepick; 04-02-2018 at 10:58 PM..
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Old 04-02-2018, 06:38 PM   #9 (permalink)
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Join Date: Mar 2011
Location: Coastal Southern California
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Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 59.89 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
Thanks: 1,710
Thanked 1,394 Times in 955 Posts
Quote:
Originally Posted by brucepick View Post
It took me a while to figure out that graph. Soooo...

Since my Insight is an aerodynamically shaped hatchback I use curve "a".
If my diffuser has a slope of 7.5 compared to the previous section's slope, the diffuser would have zero effect on overall Cd. If the slope is 10 different, Cd goes up by about .008. Whew.

Is that right?
Yup. Three degrees is .02 reduction in Cd.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

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Old 04-03-2018, 10:19 AM   #10 (permalink)
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Thanks: 378
Thanked 348 Times in 241 Posts
Thanks for this. Good news.
I did the calcs for this, for my car and I like the results.

I get an angle of 3.69 for the diffuser, using existing available attachment surfaces. That's assuming the panel prior to the diffuser is horizontal, which I believe it is. Pretty close, anyway. The rear bumper lip is 2" higher than the existing support I'll use to support the diffuser's leading edge (which will also support the prior panel's trailing edge). The diffuser is 31" long at the center, so that's an angle of 3.69 per a handy calculater on the web. Allowing for the bumper's curve, if the diffuser length is only 25" at the sides, the angle is still 4.57. According to the chart you posted, either case gives a reduction in Cd. Thanks!

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