Front Undertray position question
 

Which would be the best mounting location for a front undertray? The Yellow line represents the mounting point at the leading, bottom edge of the bumper and would be 5" higher than the bottom of the air dam. The Red line represents the mounting point at the leading, bottom edge of the air dam tapered back past the motor like the yellow line.
What do you guys think?

The yellow line with the proviso that you also loose the existing air dam.
Look at how baasjoos has treated the wheels on his aerocivic (deflectors at each wheel, channels connecting front to rear & mini-boat tails behind rears).

Then again, what do I know. Mostly replied so I'd get auto-subscribed to this thread so I don't miss what the people with real expertise have to say.

I would keep the air dam in both cases. No baasjoos, just trying to get things started. I got some time and money to make an undertray, I just don't know what position will work best with the air dam.
Thanks for your thoughts...

I suspect it won't make much difference aero-wise since the front of the undertray will be behind the air dam in both cases. But from an engineering point of view the yellow line would be better. That way if you hit a curb with the air dam you won't damage the tray. If the tray is attached to the bottom of the air dam it will be damaged whenever the dam flexes. That dam is way out in front, so it will scrape on slopes and curbs I'm sure.

Conventional wisdom is air dam OR under tray, not both. but who am I to say?

If you look at a lot of current cars you will see that the factory air dams are not a continuous depth from side to side. They are lower infront of the wheels and higher in the centre. I suspect this is to give the benefits of less frontal area in the centre, and also to deflect the air away from the wheels. Your best approach may be to cut out the centre section of your air dam by a few inches and curve your undertray up to match like you show with the yellow line.

Jwc, I like your thoughts on this. I think I'm going to go Instrax thought purely to keep it from getting damaged. The air dam hits all the time. Maybe once I get a scan gauge I can try pulling up the middle to see what sort of gains can be had from that.

i'd go for the yellow line.

here's what my undertray looks like (ive since installed wheel deflectors and experimented with closeing off the gap at the sides)

http://i77.photobucket.com/albums/j5...y-470-0608.jpg

if your car had a perfeclty smooth underside you'd actually want an undertray that angles up in the front and center, but wand to keep some sort of deflector in front of the wheels

the loremo is a good but extreme example of this
http://i77.photobucket.com/albums/j5...ams/loremo.jpg
but as you can see in the pic of my car even there this trick is applied very subtle... and lots of cars have this today

so you might want to considder reduceing the height of the dam in the center.

on the other hand a dam with a higher horizontal surface behind it will couse a little downforce... this has no real benefit for FE, and it won't suck your car down but it can make that difference between an unsteady feel and a solid feel at speeds

so my final advise would be to install a tray that slopes down from the original bumperline to the lowest point of the car, like you seem to have illustrated in your drawing, and that perhaps retain a portion of the dam in front of the wheels and perhaps suspension components

Interesting thought, so leave the air dam to deflect air from around the tires and suspension and eliminate the middle section to make a sort of tunnel, thus reducing frontal area because the air can now travel under the motor undisturbed....
I'll have to draw up some ideas and post them here...

my stock air dam is way to short. im thinking about adding bits of chloroplast to it where its infront of the tires. the chloroplast will be angled following the air dam so the air is deflected out away from the wheels