Quote:
Originally Posted by eagle
... baby steps, that's why I am going for an air dam first 
also, I tried to search what material and thickness I should use
some said marine ply some said pvc , cant find a complete and documented response |
So, baby steps is a fine idea. The problem with a full undertray for new people is that it is a considerably larger undertaking and there is still the matter of tuning the diffuser properly. The problem with an airdam is if it is too big, hanging down well below the lowest hanging suspension, engine, exhaust parts of the car, it will increase effective frontal area and therefore increase total drag, or CdA.
The material you use for your airdam should be hard enough to hold shape at speed and not so rigid that it will break when it scrapes something. I use a product that is intended for lawn edging and I installed it at an arc under my bumper cover that gives it support to hold shape at speed while still flexiing when it scrapes the ground in ditches and such. The material is so cheap that I was able to make several versions in succession and settle on what seemed a best design. Its length, height, arc, and placement under the bumper cover all likely affect its drag effects. We don't have wind tunnels, and our testing techniques cannot detect finer changes in drag, but my experience suggests I settled on a good shape--one I modeled on known good shapes. You can find details on my build thread and garage page (both in my sig file below).
In the end, for low drag, the undertray is better, but you can start this way. You can even just do the airdam if you don't like the hassles for working/inspecting under the car that a tray presents.
Hope that helps.