Post your "Failure Stories" and lessons learned from unsuccessful modifications

[RedDevil]

Originally Posted by NeilBlanchard

(...) I tried smoothing over the trough along the windshield at the 'A' pillar and it did absolutely nothing.

I've been looking over that Mercedes with its reputed Cw of 0.23 to look for details that are different, as from a distance it looks like nothing special is going on.
On of those details was that the A pillars have a protruding edge, as if the glass has sunk half an inch down.
Then there's the 2015 Jazz, which (as a hybrid) claims to get 80+ mpg. It has that same feature...

So (at least) 2 cars that get better FE than expected have protruding A pillars, or 'deepened' front windows.
What then makes that so good?
You'd expect that it would be good to make the air roll smoothly over the A pillar. These designs, and the trough you covered, all make it less smooth, but they work.

In the Mercedes smoke test you can see the air still rolls over the A pillar, apparently the edge cannot prevent it completely. It makes a sharp radius and gets turbulent. It does not look particularly good, something you want to avoid.
The flow over the roof is very smooth indeed.
I think anything that can lead more air over the roof instead of curling over the A pillar would lower air resistance. Might even add a small ridge to the front of my A pillars one day...

When your mod doesn't do what you expected, it may just be pointing out another direction to follow.

[aardvarcus]

My best “failures”:

Using aluminum sheeting for a belly pan:
I found aluminum is not the best material for a DIY belly pan, especially a first attempt. It is expensive, fairly heavy in 0.060 thicknesses, still needs supports, can develop shakes and rattles, and without specialized tools such as an English Wheel, rollers, brakes, and shears it seriously limits design. With better tools and more experience I think aluminum could be a great choice for a “second version” belly pan material, just take the first version pan and copy it, making sure to adequately dampen rattles.

Belly pan under exhaust:
I had major issues putting belly pan materials under my exhaust, even using metal that could withstand the temperatures and maintaining adequate clearance. It turns the gap between your car and the pan into an oven. If done near your gas tank it can heat your tank up and start boiling off the fuel. Even not near your gas tank the pan trapped hot air under the car and heated the cabin from below. Using aluminum or other bare metal with high reflectivity on the top and no emissivity below makes this situation worse. Cutting out 50+% of the material below the exhaust was not enough to prevent this from happening.

Using expanding foam for removable grill blocks:
Expanding foam will continue changing shape for quite a while even after the outside is hard. It will grow, shrink, warp, etcetera. Note I did not incorporate any internal support in my piece, I have seen others that inserted dowels or similar and claimed this helps.

Grill block when air can bypass radiator:
If the air can just blow under the car or around the radiator it will seriously limit the amount of grill you can block without heating problems. Simply plugging these holes will let you block more grill and still maintain lower temperatures.

[Xist]

First, how many modifications have I actually gotten around to attempting? I tried putting marine wrap on my Forester hubcaps and that seemed doomed. I made a marine wrap grill block and I think that it was too warm. I wish that I had thought of this a week or two ago, driving to the bakery before sunrise it would have been as cold out as 28°F, although I definitely would have needed to remove it for my drive home. I tried cutting coroplast to fit over my fog lights, but just messed up that. I removed my fog lights and sprayed Great Stuff in the hole, not realizing that Del Sol liked having it attached to tin foil. I removed the roof rack cross members, but removing the rails is nearly impossible. I bought a pizza separator, but bought my Civic before tapping out the lug nuts to attach the disc.

I made an air dam, but tried to shrink it to fit better and melted it. I bought pizza separators, but the zip ties attaching them to my wheels started breaking. I pulled the power steering, but I had just driven to the second supplier that was supposed to have the parts to loop my steering the day that my father passed passed away.

Is that all that I have done to Chorizo? That is not much!

[Baltothewolf]

Hmm... Well I made the insight get hot once or twice when testing the limits of the grill, and seeing how much I could block. I also had a prototype belly pan (Coloplast) come lose on the highway. Other then that, nothing I can think of.