vvaffle wrote:I would say not very stiff at all. The weight of the plates should do a fairly good job of keeping them down. I would try it without springs first and then if that's not good enough, put some soft ones in there and keep putting in stiffer ones until you're happy with it.
These are interesting because the faster you go the more air you get anyway. If you're just wanting good air flow then mounting the plate off center is going to be the way to go because around town and at slow speeds the plate should be visable to the local authorities. While at the same time low speed cooling is a concern in some climates. In such climates upgrading/fixing the cooling system is going to be better for the car as opposed to rigging it up.
They are really only good for hiding your plate number from cops going the other direction and traffic cameras on high speed sections.
I would say that the plate needs to fold under at around 50 mph or maybe even 75. Depends on preferance and driving habits I would assume.