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Chonk_Master 04-07-2020 10:45 PM

Aero Crosstour- Squeezing blood from a stone
Howdy, all. I'm setting up a build thread for my aero Honda Crosstour project. This post will be a data dump of everything so far and then I'll use it as my primary build log.

Specs, goals, and completed projects will be updated on this first post. All other journal entries will be new posts.

2011 Honda Crosstour Specs:
Currently, MPG is roughly 21 MPG. Regularly updated MPG log is here:

Crosstour curb weight is 3852 lbs. With me and 16.5 gallons of gas weight is calculated to be 4142 lbs.

The Crosstour has Honda's Variable Cylinder Management (VCM) and a Tire Pressure Monitoring System (TPMS).

Goal is an average of 25 MPG (16% increase in MPG)
I expect to have to do more than current prosposed projects to meet this goal. Recommendations welcome.

Proposed projects:
Wheel covers
Lower radiator blocker plate
Foglight covers
Low profile side mirrors
Completed Projects:
Scrum board: Kerika
Image analysis for front surface area cross section:
EcoModders aerodynamics stencil:
This is the image that inspired a possible rear spoiler.


So, first phase is just tooling up and ordering parts to complete the project.

I installed a Harbor Freight drill guide on my old IKEA desk for drilling the lugnuts. To install the guide, I drilled holes slightly smaller than M5-.08 nuts and epoxied them (SteelStik) into the desk for a semi permanent installation and to improve stability. I used a thread tap to clean the still-wet epoxy out while leaving the max amount of epoxy still in place.
Ya'll like Patrick Nagel?

I also designed and 3D printed a simple centering jig for the lugnuts. It's the black thing holding the nut beneath the drill bit. If I let gravity center the bit, it gets pretty darn close to perfect. I'll verify with measurements before drilling.
Side note: I'm considering using steel zip ties vice drilled lugnuts. I'll have to see how centered I can make everything and go from there.

Other than that, I printed some organization tools for the shop and built a scrum sheet for the build using Kerika:
Also printed a drill bit sharpener for a Dremel so everything goes smooth when the time comes.
I'm also planning on fabricating a vacuum molder but I don't have photos for that. This will be how the foglight covers are made from clear Lexan.

First project tackled was wheel covers, since the pizza trays came in first. I wanted to passivate them, as they're pretty cheap and I don't trust the aluminum any more than I can throw it (which is probably pretty far, actually).

Technique was, well....bathtub. I used what was left of the Barkeeper's Friend under the sink and the minimum amount of water needed to cover them.
They soaked for a little over an hour with hot water.
It will be hard to tell how well it worked but I'll definitely be monitoring for corrosion and taking notes.

I'm leaning heavily towards steel ties vs. threaded lugnuts. Utility, ease of install/modification both seem better. Aesthetics won't be any better or worse.

Next big project is the side mirrors. I wanted to do a full elimination with a printed vanity cover but the Crosstour's enormous blind spot makes it a non-starter.

I'll be using ImageJ to try and get an approximation for how much the new mirrors take from the front cross section. I jumped into this project head first and the passenger mirror has already been removed, so I'll cut and paste the driver's side mirror into the image to get a closer estimate.

Here's a side by side to show how much reduction there will be. Not amazing but it's something.
ImageJ will hopefully give some hard numbers on how good it was but, of course, final MPG is king.

Speaking of which, I'm going to intentionally pace project implementation such that I can get a couple of tanks of gas after each mod. maaaaaaybe do A-B-A.

I'll also have to build a plate to cover the old recess that the side mirror was integral to. New side mirrors bolt onto an adapter plate (which is for much older Hondas) and the adapter plate bolts to the car. I'll have to fab an adapter plate from this:
Which really shouldn't be too hard. It's a triangle with no major curvature. Measurements are easy to take with a caliper. The three bolts are what penetrate the unibody to mount the mirror. The plate will be 3D printed and those bolts will be replaced with heat set threaded inserts. I'll threaded a bolt into them from inside the vehicle.
Image of inserts:

Image of the scheme for the mirror mounting hardware:

Next minor project is replacing the front emblem. This is more of an aesthetic thing but it'll hopefully help with some aerodynamic smoothing, since the Honda emblem is pretty big and cavernous.

I work as an intern at an aerospace firm and they throw away A LOT of carbon fiber. This benefits me immensely.
This is the CAD mock up of the emblem. Pretty easy. I also own a crock pot.
It turns out, the rivets holding in the factory emblem were either missing or sitting in cracked, ineffective housings. So, it popped right out. Anything will be an upgrade after that, I suppose.

And that's it for now. Hopefully, I'll have an image for ImageJ before the end of the week and can have an adapter plate for the mirrors fabbed up in CAD (real CAD, not the cardboard kind. Okay, it'll be TinkerCAD but still). I also want coast down times but the friend who was trying to help is having bad allergies and I may forgo it or do it myself by filming the speedometer.

Let me know what you think. All suggestions are welcome and appreciated!

RedDevil 04-08-2020 03:08 AM

Having a supply of free carbon fiber plates and maybe oddly shaped parts that could just fit somewhere is a real treasure.

Have you considered raising the tire pressure a bit? The first few PSI above OEM tire pressure have most effect on economy. The higher you go the lower rolling resistance will be, but the extra gains will quickly get very minimal. Still, an easy 5% gain.

It is standing fairly high, so lowering it or a front air dam would help. Also, extend the flaps (wheel air dams?) ahead of the front wheels a bit and fabricate them for the rears.

Finally, consider the 65+ mods and 100+ hypermiling tips. Tightening the nut that holds the steering wheel is my favorite ecomod. ;)

Chonk_Master 04-08-2020 04:43 AM

YEAH, some weird stuff comes out of the composites shops, not that I'm complaining. I hope a nice, wide sheet gets thrown away for the rear spoiler but we'll see.

I'm going to be very, very careful with tire pressures. I'd rather have the traction and lose a few %. I don't know where those two things start to rub against each other but it would be a last or, more likely, never resort. That thing is heavy and needs everything it can get.

I'm dying to lower it. I know a few places make well designed springs that keep your steering geometry right while giving you a couple of inches. That's more restricted by a student's budget than anything else, but it's definitely on the radar.

It has a front under tray that goes to about the rear of the front wheels. Is that what you mean by front air dam? I'd like to get into put more trays under the car but that would come after this first round of mods.

And can you point me to an example of extending the front "wheel air dams"? I think I know what you mean but not sure.

And boy, let me tell you about that steering wheel nut. Right now, I'm running supplies around town for people who sew masks for medical workers. My habits are utter garbage but I definitely intend on returning to the "leave earlier, drive slower" routine as soon as I can.

RedDevil 04-08-2020 05:33 AM

By extending the "wheel air dams" I mean extending them down.
It probably is better aerodynamically to shape a cowl to gradually lead the air away instead of just a vertical slab, but much harder to fabricate - and you don't want to damage other cars should it come loose on the highway.
I once lost a tiny piece of bagel box plastic (a 5" wing shaped sleeve around my antenna stalk) translucent and all, and the semi behind me went full on the brakes... An enormous overreaction, but I felt bad about it anyway.

The thing with tire pressure is that each car is different and you need to find out for yourself what works best. Find a place to test cornering and braking, bring a pump.

FWIW, my observations:
I run about 42 PSI in mine (OEM is about 34) and economy went up about 5-8%.
There's much better cornering with less tire squeal. It was rubbing the sidewalls in hard corners at OEM pressure, which I definitely do not want to happen. Higher pressure means less sidewall deformation and less thread warping in corners.

I have one corner with a wide runoff that I use to test cornering speed. That's 52 km/h max at 34 PSI for my OEM Bridgestone summer tires when new, and 57 km under similar conditions at 42 PSI. That's about 9% more lateral force that the tires can handle!

My car's straight line stability is easily affected by side wind, and higher tire pressure all but eliminates that - to the point that I know I need to check the pressure if I notice the effect; especially on the rears.

Braking in a straight line seems unaffected.
Road noise went up, sadly, and going over 42 psi makes the ride less comfortable. That's the same for all 3 sets of tires I have used on my car so far.

redneck 04-08-2020 06:02 AM


Do you have a instantaneous MPG gauge in the vehicle ?

If not, you need one... Itís the most important mod... Period... ;)

(Scanguage, UltraGauge, Torque Pro or similar app.)

It allows the steering nut to be adjusted and also allows one to evaluate vehicle modifications quickly and easily.

Increasing tire pressure to the max psi as indicated on the sidewall can easily net gains of up to 10% without losing drive ability and or safety. Use the search function to research, as this topic has been covered many times. A member here (CapriRacer) has written or replied extensively about tire related topics. If I remember correctly he works in the tire industry.

I would take care of the low hanging fruit first before doing suspension mods.



Chonk_Master 04-08-2020 12:18 PM

I'll definitely consider playing with tire pressure, now that you framed it that way. 🤔 I'll definitely look up Capri's post. I'm at 33 out of 44 psi, I'd be flabbergasted if I didn't lose some traction between here and there but, like you said, there could very well be some room for no-cost improvement. If I only I had a skidpad......

*looks at empty Walmart parking lot*

And yeah, springs would come after everything else. I did find some from Megan that aren't expensive and may be worth it to experiment. The tricky part is no one makes just a spring for the Crosstour, explicitly. Everyone I've seen lowering Crosstours gets the Accord spring with the highest spring rate, which I think is for the V6 hatch from the same year. So, I'd need to be able to get spring rate specs on the spring, which shouldn't be too hard.

And I'll look up those instantaneous MPG apps because what the heck. I've been using the VCM eco light as a bellwether for now. 😄

Piotrsko 04-09-2020 09:45 AM

How about the obvious: re arch your existing springs. Places exist that do this for less $ than new unless this is a weekend only thing

M_a_t_t 04-09-2020 10:23 AM

I thought you could only re-arch leaf springs? Never heard of re-arching coil springs.

Chonk_Master 04-09-2020 06:58 PM

Yeah, the Crosstour might look like a van but it is indeed a unibody with coilovers all around. I couldn't find anything refering to re-arching springs. There's always cutting them but I wouldn't do that to the car that drives me to my funeral.

I did, however, find out that Megan Racing makes a set for same-gen Accords that clock in below $150. Just need to check fitment and spring rate and that will probably be the solution.

Chonk_Master 04-09-2020 08:54 PM

Journal Entry 04/09/2020:

So, not much time for progress during the week but got some small stuff done.
Cuts for the vacuum molder were done by a good Samaritan on NextDoor.

It's funny how simple of a plan a vacuum molder is. If you've ever wanted to use one, here are the plans I'm using:Amalgamized: DIY Vacuum Former for Thermoformed Plastic in Your Home Shop

I'm scaling mine up to be a little bigger for the fogs but you can make them just about any size, assuming your vacuum source can do the thing.

Got a tracing of the mounting plate for the side mirrors. Should get me in the ballpark for a prototype. I'll print the prototype at, say 3% infill and just see how it fits. If it's the right size, I'll print the flight model from HTPLA, bake it, and starting mounting hardware.

If I'm lucky (I'm not), I'll be able to use that factory rubber piece to seal the interior up nice and neat. Otherwise, I can get some thin rubber sheet to do it with.

Quick plug for carbon HTPLA. Proto-pasta makes this stuff and it's very neat. It's an HTPLA you can bake in an oven to increase hardness and heat resistance. They claim temperature operating limits of 310 F after baking. I made a shift knob with this stuff for my old Toyota MR-S. I was able to cut a thread in it and leave it in the sun permanently with no deformation.

My limited memory tells me they formulated it to crystallize while baking to achieve the higher strengths and heat tolerances.

I would use some spray-on filler, black spray paint, and polyurethane to finish out the mounting plate for the side views.

This is also where the limitations of TinkerCAD are going to stop me short. TinkerCAD is a VERY rudimentary free CAD program. You can do a lot with it but it's more like putting together pre-assembled blocks that it is true CAD. You can't really make a triangle with specified face lengths. You can make a pyramid and a cone and a triangular prism. But not a triangle with specific length faces.

Anyways, there's plenty of more sophisticated, free CAD software out there I can use.

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