6th Gen Accord Aerodynamics

[GasSavers_brick]

All right, you guys finally did it. After reading about all these aero mods I've decided to build a coroplast belly pan for the Accord. My highway mileage seems to have hit a plateau at 42-44mpg based on technique alone. I have no idea how much of a difference this is going to make, but anybody want to take a guess just for fun?

I should have the material (2 4x8 sheets worth) by some time next week, so in the mean time it's all about figuring out just how this should be done. Cutting and shaping will have to be done as I go, but what about fastening? Are screws the only way to do it right or is there a good alternative that doesn't involve putting holes in my car? I guess I don't mind screwing into plastic parts, but drilling into the steel seems like an invitation for rust.

Any other thoughts from the folks who have done it?

[Mighty Mira]

I'd like to know this too. I'd also love to know how to secure an aluminum undertray on a car without creating a huge sacrificial anode that will fall off the moment the area around the bolts corrodes away.

[MetroMPG]

Brick - I'm with you on the reluctant-to-drill-into-underbody concern, particularly since I live in the rust belt. I'm not sure where you live.

But there's really no other easy way to do it. I'm going to get around to doing the belly pan also, probably later this summer. I think we can take some precautions to minimize the chance that rust will start by oiling or greasing the holes once drilled, prior to fastening the coroplast. Also, you might want to consider stainless steel machine screws, so the heads don't rust (or the threads, inside the holes if they get wet).

The next concern is making sure the belly pan itself doesn't somehow trap water. I've already spotted 2 drain holes in the "frame" members that lead back from my front suspension lower mount points (ie inboard from the rocker panels). I'll have to accomodate those with corresponding holes in the plastic sheeting.

Mira: what about just using plastic washers? Seems like an easy fix to that potential problem.

[Mighty Mira]

Originally Posted by MetroMPG

But there's really no other easy way to do it. I'm going to get around to doing the belly pan also, probably later this summer. I think we can take some precautions to minimize the chance that rust will start by oiling or greasing the holes once drilled, prior to fastening the coroplast. Also, you might want to consider stainless steel machine screws, so the heads don't rust (or the threads, inside the holes if they get wet).

What is the material that you are drilling into? Is it steel? If so, using stainless steel will just act like a battery, eating away at the steel that you have drilled into.

I suspect that using galvanised screws would be the only way, as the zinc would preferentially get eaten away first.

Mira: what about just using plastic washers? Seems like an easy fix to that potential problem.

Hmmm. You would need some way of preventing the tray from moving around. The moment a screw touches the tray, you are going to get corrosion. Any ideas? Perhaps a tiny plastic washer to go inside the hole between tray and screw, and then a larger plastic washer to take the weight of the tray?

[basjoos]

I figure I've be needing to take the underbody panelling off from time to time when I take the car in for major scheduled service, at which time I'll inspect for rust and replace all of the 8-32 screws.

You may consider pressing rubber grommets into the drilled holes on your aluminum underbody panelling to keep it out of direct contact with the steel screws.

[GasSavers_brick]

I finally got around to doing one simple "mod" that may or may not be worth anything: removing the wind deflector from my sunroof. The roof is closed 99% of the time so I figure there's no point in having the extra 2" high x 30" wide piece of plastic on the roof. I think the car looks better without it, anyway.

[MetroMPG]

Brick, FYI:

March 18, 2004
The New A6: Body Structure

In all, the various improvements on the underfloor of the car reduce the drag factor by no less than 0.031 which, related to the car's overall drag coefficient, means an improvement by almost 11 per cent. Travelling at a speed of 180 km/h, this means a reduction in fuel consumption by approximately six percent compared with a comparable vehicle without suitable modifications at the bottom.

You can work the math backwards if you like, and estimate FE gains at hypermiler speeds.

[GasSavers_brick]

Good find! I developed a spreadsheet a few months back that approximates the power requirement to maintain a set speed on the highway, which allows me to plug in different numbers for CD, frontal area, rolling resistance, etc. If I go with the baseline numbers for my Accord (CD=0.33) it tells me that I need 21.03HP at 60mph and 104.0HPto maintain 112mph (~180kph). Reducing the CD by 0.031 to 0.299 results in a power requirement of 19.77HP @60mph and 95.48 @ 112mph, for a net reduction of 6.2% and 8.2% at 60mph and 112mph, respectively.

If you assume that their 11% is correct and just scale my numbers up, (remember that I estimated power, not FE), you get an 8.3% improvement at 60mph.

[GasSavers_brick]

Uh oh. I broke out the wrenches again! I still haven't had time to dedicate to the belly pan, but I made another change that might be worth something. My car came with the factory wing, which I'm pretty much convinced is worthless from an aerodynamic perspective. It's one of those things that's raised up about three inches from the trunk lid and shaped to mimic a down-force producing wing.






Anyway, now she looks like this:





I had to fill five holes where it was mounted, so I used some 1/4 by 1" carriage bolts with cut-down fender washers and the appropriate nuts to seal everything up from the rain. The beauty of it is that I can probably get the wing back on in 15 minutes flat if I decide that the car looks horrid like this (which I don't yet.) The plugged holes aren't exactly the stuff of show cars, but I'm going for function over form here. Now there's room for a functional lip spoiler should I choose to accept the challenge of designing one.

[MetroMPG]

Damn the back of the car looks like an NSX with the spoiler! Particularly in black. Never noticed that look before.

But, um, alright for better aerodynamics!