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Old 06-02-2021, 01:50 AM   #1 (permalink)
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I have a goal to increase my average MPG up to 25. Right now we've been averaging between 15.2 and 16.7 depending on the mix of driving. Yes, I realize that's a 50-60% increase which would be insane. But I think I can do it with a few modifications I have in mind. Also, according to my garage, I used to get 19.8 average so I'm already halfway there!

First aero mod is to fix up the belly. A nearly-full belly pan to cover up the engine bay and front half of the vehicle. I have tons of cloroplast from a store liquidation I was a part of years ago that I specifically saved for this project so I'm finally going to get to use it!



The middle seems pretty straight forward. There are two crossmembers that I can attach to spanning the frame (didn't get a good pic of the second one, but you get the idea).


The belly pan will also go full width from side step to side step, wherever possible. The underside of the side steps have little brackets that I think I'll be able to attach to and then the frame on the other side.


Where I'm seeing a problem is how to do a belly pan in the rear. I'm just not seeing anywhere I can attach to between the gas tank on the driver side, the monstrosity of a muffler on the passenger side and the spare tire in the middle behind the rear axle. Not to mention giving space for the suspension to move.



I would also like your guy's advice with the cloroplast distance from the exhaust. If I attach to the cross member, I would only have about 1-1/2 to 2 inches of clearance between the hot stuff and the plastic. In your guy's experience is this enough space or should I leave a cut out around that area?
I know if I ever upgrade to a metal pan, I can just go all the way across.



I've read through many threads and online articles, but any insight, advice, do's and don'ts, don't forget abouts, anything is always helpful and welcome!

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Old 06-02-2021, 02:25 AM   #2 (permalink)
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Use a metal frame across the rear suspension to hold the panelling in place.

My video on bellypans (and be warned, I don't like coroplast):

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Old 06-02-2021, 04:27 AM   #3 (permalink)
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Quote:
I know if I ever upgrade to a metal pan, I can just go all the way across.
So you know about the durability of Coroplast. Okay for cutting templates, but not a lot else.

If you want to use it for test purposes, at least wrap the edges with your favorite tape to seal the corrugations.
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Old 06-02-2021, 06:17 AM   #4 (permalink)
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Consider swapping out the tires for more economy focused ones.
A lower rolling resistance is often underestimated, but it makes a significant difference, especialy in heavier vehicles.

Also:
Holy ****, that underbody looks like an aerodynamic nightmake in stock configuration.

Maybe also consider a different vehicle for daily driving, yours will never be efficient.
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Old 06-02-2021, 06:36 AM   #5 (permalink)
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Quote:
Originally Posted by Autobahnschleicher View Post
Consider swapping out the tires for more economy focused ones.
A lower rolling resistance is often underestimated, but it makes a significant difference, especialy in heavier vehicles.

Also:
Holy ****, that underbody looks like an aerodynamic nightmake in stock configuration.

Maybe also consider a different vehicle for daily driving, yours will never be efficient.
Maybe not *quite* that bad, but yes, the general point is correct.
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Old 06-02-2021, 09:40 AM   #6 (permalink)
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Maybe not *quite* that bad, but yes, the general point is correct.
Let's agree that a flat undertray should be rather effective here.
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Old 06-02-2021, 11:04 AM   #7 (permalink)
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coroplast for belly pans

I have had a coroplast belly pan on my VW Golf for 10 years. I have some things for you to consider, some Dos and Don'ts.

1. I believe coroplast is suitable for a belly pan if it is well supported. It is used seal up the bottom of modern camper trailers here in the U.S. It is cheap and doesn't rust. However, it can sag and is not particularly heat resistant near exhaust systems.

2. As Julian has stated, I recommend building a supporting framework to attach the coroplast to. Build it stronger think you think you need to. Sudden wind gusts at highway speeds can generate a lot of force on your attachment points. Julian has measured downforce on his car from his belly pan. The curved pan is sucking the car down to the road. You don't want this thing coming loose on the highway. If you do not weld you can use aluminum angle and rivets or locking nuts and bolts. Riv-nuts are great for attaching the framework to the vehicle and coroplast to the frame using fender washers and Lock-Tite. You can purchase an inexpensive Riv-nut tool from Harbor F. Design the framework so it can be removed for servicing. A belly pan can accumulate water, snow, mud or dust over time. You may want to have some places for it to drain itself.

3. Manufacturers usually leave the cat and the muffler uncovered. A full belly pan can trap heat between the pan and the floor of the car. The muffler can be fairly flat. Make your supporting frame flush with the bottom of the muffler and surround it and leave it open to the bottom.

4. There may be places where metal is better than coroplast. On my car I have a rigid aluminum "Panzer Plate" under the engine and radiator as a skid plate. Looking at your photos, covering the front part in this way may be time and money well spent. This will be more durable in the event of an impact with road debris.

Click on my avatar and you can scroll through some photos of mine. I feel that a belly pan can be some of the lowest hanging fruit for aero gains. Also, I just switched to LRR tires and the early results look very promising. If you could afford a second set of lightweight rims with narrower LRR tires, that may help too.
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Old 06-02-2021, 11:16 AM   #8 (permalink)
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here are some pics of w/o undertray

and with undertray. purpose of the undertray was to avoid having the large transverse bar facing the flow, so the udnertray is composed of two parts, one kind of deflector in front, and the bottom diffuser.


my plan for this is to perfom tuft testing to check attached flow on the diffuser. (once i will have the boat tail attached to the end of the car this will be even more important!)
Like this from another thread.

The back end is trickier on rear wheel drive. The fuel tank is pretty smooth and the driveshaft needs some room to move up and down. You may have to stop the front part of the belly pan at the back axle and build another frame and belly pan section behind the axle. Try to seal this section all around the sides to keep air and mud/slush/snow out.
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The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.

Last edited by COcyclist; 06-02-2021 at 11:35 AM..
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Old 06-03-2021, 12:40 AM   #9 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
Use a metal frame across the rear suspension to hold the panelling in place.

My video on bellypans (and be warned, I don't like coroplast):
Quote:
Originally Posted by freebeard View Post
So you know about the durability of Coroplast. Okay for cutting templates, but not a lot else.

If you want to use it for test purposes, at least wrap the edges with your favorite tape to seal the corrugations.
Thanks for the tips guys! Julian, that is an excellent video!
The coroplast is definitely short term for testing, or at least until I can get the funds to use some sort of sheet metal. I'll be sure to seal the sides when I'm done fitting it.

Quote:
Originally Posted by Autobahnschleicher View Post
Consider swapping out the tires for more economy focused ones.
A lower rolling resistance is often underestimated, but it makes a significant difference, especialy in heavier vehicles.

Also:
Holy ****, that underbody looks like an aerodynamic nightmake in stock configuration.

Maybe also consider a different vehicle for daily driving, yours will never be efficient.
I have thought about putting some LRR tires on my old rims and saving these snow tires for... snow. Just haven't pulled the trigger yet. My daily driver is actually my Corolla. My problem, though is that the car is too small for me and is physically hurting me so I need to start driving this, hence the need to improve the MPGs. Back when I was driving hundreds of miles a day, I was using the Corolla, but now that I'm down to about 40 a day I need to bite the economy bullet to save my health!

Quote:
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I have had a coroplast belly pan on my VW Golf for 10 years. I have some things for you to consider, some Dos and Don'ts
Thanks for the tips! At this point getting the proper support is my biggest issue...
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Old 06-03-2021, 01:12 AM   #10 (permalink)
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I got phase 1A done of the front belly pan. It's by no means perfect, but I can work with it as I move along.





The only way I can get it attached using factory mounting points/holes is to use four screws, two in the front and two in the back. I know on the back I could get two more bolts, but that would only be in the middle. You guys said to go more than I think and I think I need more as it is! Plus they are only towards the middle of the pan, the edges are completely unsupported spanning the length. I just don't see how I can attach anything without modification. I don't really want to be using rivets or screwing into the bumper/"air dam". And I would also like to keep the pan frame approximately level with what's already there, I don't want to bring the pan down lower than it has to be.

I'm thinking about cutting the pan in half and covering the whole span so that the bumper section in the corners gets covered and I have a better support throughout. I'm just not sure if I'll be able to get the approach angle to match up with the middle section. More tinkering on that...



But I still need to find a better way to actually attach it. I can screw in through the bottom lip of the air dam, I just don't really want to do that unless there's no other way. I wonder if I could somehow "glue" a nut onto the inside of the air dam lip so it won't be noticeable. Although now that I think of it, that would mean screwing in from the top which I wouldn't be able to access. *thinking cap on*
I was thinking something like this which is how I attached to the front of the pan, just with an inverted bolt.


It's a good thing it's night, I'm going to have to shower and sleep on this. My best thinking times!

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