Underbody Tray Design - Gas Savers

Bunger · 03-02-2006, 10:15 PM

So I was thinking about my tray designs today at work, and I figure I'll do it in 3 parts; front, middle and rear. I'll start with the rear, as I think it may yield the best results. I basically keeped the same line as the top of the car just flipped it upside-down. Input?

Bunger · 03-02-2006, 10:17 PM

Ohh, and I plan to cut holes, or slots rather, in the rear bumper to "vent" the tray. It should be in the black part of the cover so it wouldn't be terribly visible.

Matt Timion · 03-02-2006, 10:31 PM

Looks good... looks a lot what what I want to do. You realize you could probably sell undertrays to racers and fuel economy guys alike.

Compaq888 · 03-03-2006, 12:28 AM

or make me a quality one. I have one but I took it off because of design flaws. I can't really do much with a dremel, drill and a screwdriver.

krousdb · 03-03-2006, 02:40 AM

Quote:

Originally Posted by Bunger

So I was thinking about my tray designs today at work, and I figure I'll do it in 3 parts; front, middle and rear. I'll start with the rear, as I think it may yield the best results. I basically keeped the same line as the top of the car just flipped it upside-down. Input?

You can sign me up for one. What will you make it out of? You will have cutouts for the muffler? Will there be any interference with the rear suspension?

Bunger · 03-03-2006, 03:34 PM

I have a very large roll of fiberglass, so I was planning to make it from that. I am going to order up some Dzus fasteners to attach it to the underbody and there will be provisions for exhaust outlet, muffler and of course the rear suspension. I may use a piece of thin sheet metal to begin the design and create a mold from.

MetroMPG · 03-03-2006, 04:24 PM

that's an interesting idea. certainly the right concept - tapering the underbody up towards the rear. if i'm not mistaken, the insight does this (without the bumper holes). mimicking insight aero techniques on a crx would never be a bad idea.

but i have no idea whether it's better or worse to taper to the bottom pan to meet the bottom lip of the bumper ( = potentially larger wake but cleaner flow separation), or tucking it up higher and then cutting holes in the back of the bumper.

the best aerodynamic solution would be to cut a higher lip in the bumper. whether the holes help will probably depend on how many and how large you make them.

i believe expelling air into the rear wake area was done on the ford probe v concept. they did it with "used" air from the cooling system (mid-engine layout, so easier to arrange).

Bunger · 03-03-2006, 05:04 PM

Well I crawled under the car today and took a bunch of careful measurements. I've gone and made an image that protrays my car at its current ride height and the new design of the underbody tray.

MetroMPG, before I had even gotten to read your post, I had come to a similar conclusion about the rear bumper. After looking at the amount of space under the car, the new design is the only real practical solution, as the previous one would require that I cut out the spare tire well.

Here is version 1.50:

Sorry its a little big, but its 1/5 the size of the image I was working on. Within 1-2% this imagine is scaled to 1 pixel = 5mm.

SVOboy · 03-03-2006, 05:18 PM

Ah, d00d, totally kewl.

What do you think will have to be done with venting the engine if a full tray goes on? Prop up the hood a little bit?

This is a very neat idea, Matt, can you post the links to the full CRX undertray you showed me before?

Bunger · 03-03-2006, 05:34 PM

As far as venting the engine bay, I had thought about raising the rear of the hood to allow venting over the cowl. This may also help with drag over the wiper arms (if I put them back on... the motor is out too... PITA).

I'd love to see some pictures of a CRX underbody tray! Is it the land speed record CRX in Aussie?

SVOboy · 03-03-2006, 05:40 PM

No, it's a full electric conversion CRX with lots of crazy **** I wanna do.

The guy made his lights all LEDs and the rear lights indicate charge while it's charging and ****, it's awesome.

Bunger · 03-03-2006, 05:43 PM

Nice! Alright Matt, where's the pics???

SVOboy · 03-03-2006, 05:54 PM

Yeah really, he's not around now, so I'll just try and put this d15b7 IM/TB together while I wait, I'm clueless about these vacuum lines.

Bunger · 03-03-2006, 06:14 PM

Quote:

Originally Posted by SVOboy

I'm clueless about these vacuum lines.

Which ones are you having trouble with? I pretty much know them all by heart. =)

MetroMPG · 03-03-2006, 06:21 PM

from center, looking forward...

<img src="http://www.metricmind.com/ac_honda/images/pan13.jpg" width="500">

lots more where this came from:

http://www.metricmind.com/ac_honda/main2.htm

Matt Timion · 03-03-2006, 06:27 PM

Quote:

Originally Posted by MetroMPG

from center, looking forward...

<img src="http://www.metricmind.com/ac_honda/images/pan13.jpg" width="500">

lots more where this came from:

http://www.metricmind.com/ac_honda/main2.htm

Yep... that's the one. I would love an electric car with all of the fancy gadgets like that one.

krousdb · 03-03-2006, 06:32 PM

Way cool! Where can I get the plastic sheets? Home depot sucks for that.

Bunger · 03-03-2006, 07:05 PM

Interesting design! I think I would prefer mine to be a little more rigid and extend out a little more around the suspension.

I had never thought about using plastic sheets like that. That guy spent a GRIP on converting that car! And now that CRX weighs about what a Sherman tank does. The nimbleness is one of my favorite parts about a CRX (I've gone as far as to remove the starter to help lighten up the front end... but got tired of bump starting it everywhere... that thing is HEAVY though).

I am seriously thinking about making a rotisserie for my CRX... so I can have an easier time working on the underside. I'll have to remove the engine again, but without it in, the car weighs less than 1300lbs, so a few modified engine stands should do the trick.

MetroMPG · 03-03-2006, 07:12 PM

Quote:

Originally Posted by krousdb

Way cool! Where can I get the plastic sheets? Home depot sucks for that.

actually, the home depot in my town has corrugated plastic 4x8 sheets called coroplast. they would work. of course, plastic will also catch fire if it gets too hot (not as big a danger on an electric car).

when i tackle the underside of my car this summer, it'll be with a combination of coroplast and aluminum sheet. i'll mock it up with cardboard and duct tape first and do some tests.

the underside of my car is terrible compared to a stock honda. i have NO front undertray at all, my rear bumper might as well be a parachute, and all 4 control arms jut way down into the airflow.

that means i have that much more to gain when i clean it up, i guess

Matt Timion · 03-03-2006, 07:14 PM

Quote:

Originally Posted by Bunger

I am seriously thinking about making a rotisserie for my CRX... so I can have an easier time working on the underside. I'll have to remove the engine again, but without it in, the car weighs less than 1300lbs, so a few modified engine stands should do the trick.

Bunger, I'm sure you've seen this, but for the rest, here is a CRX rotisserie:

With car on:

Animation of the car spinning:

http://www.iconmedia.tv/icon_galleries/Blues_Clues/K_Twirl1.html

Bunger · 03-03-2006, 07:36 PM

I had totally forgotten about those pics! I wonder if that support bar thay have going down the middle is really needed?? I dunno if the bumper support bolt holes can support the entire weight of the car or not. I'll probably just jack the car up about 3-4 feet so I can keep the bumpers on while making the tray.

krousdb · 03-04-2006, 06:20 PM

Quote:

Originally Posted by MetroMPG

actually, the home depot in my town has corrugated plastic 4x8 sheets called coroplast.

I have tried Lowes and Home Depot. When I ask about it they just look at me and say huh? The closest thing I found was pre printed house for sale signe, 24" x 30". Not big enough.

Any other ideas?

SVOboy · 03-04-2006, 06:23 PM

You can use aluminum flashing if you figure out a way to attach the strips together or just do a bit of overlap.

MetroMPG · 03-04-2006, 06:30 PM

try a sign making shop. they often use coroplast (as you found, pre-cut). just beware of pricing.

i first located coroplast locally at a sign shop, and they would have happily sold me as many sheets as i wanted at $40 per 4x8.

fortunately i was just hunting that day, and didn't buy; i found out not long afterwards it's about $20 per sheet at home depot.

krousdb · 03-05-2006, 05:09 AM

While I was walking through Home Depot I was look=king for other potential materials. I found 2x4 ft flouresent light covers in styrene, and in acrylic. Seemed too thin and brittle. I also found a 4x8 ft sheet of plastic pegboard. The holes could act as dimples? Anyway, pretty heavy stuff. Then I saw the vinyl siding. Lightweight, cheap, flexible by rigid when pieced together. A good candidate as long as the seams are aligned in the direction of airflow.

Any thoughts from the aero gurus?