Also, a huge part of why cars don’t rust nearly as much as they used to is that their bodies drain better. It would be tricky to completely seal the bottom of the car reliably, and so the more baffling you add on there the more likely it is that salty snow/water is going to accumulate in the undercarriage. Also, driving in the rain occasionally can keep the bottom of the engine a lot cleaner-- on the trucks I’ve owned with extensive skid plates, the bottom of the engine gets really greasy and dirty over time.
Wouldn’t rain water, washer fluid, tranny fluid, oil drippings, and everything else just end up pooling on the inside of this shield? I don’t know if there are drainage holes…
Or, in other words, Why does my car make a sloshing sound when I round a curve or break too hard?
Sorry. I is slow.
For an every day road car, what’s the use? Limiting to just the OP’s assertion that we’re worried about rust and crud, that’s not a major concern these days. The big concern is weight and cost.
For any modern car (2000+) rust and crud shouldn’t be issues in and of their own. Modern galvanized and galvannealed materials, coupled with modern e-coat processes pretty much guarantee that rust won’t be an issue for the useful life of an automobile. No manufacturer is going to go through the expense and weight (i.e., mileage) penalties for the tiny percentage of people that keep their cars beyond 200,000 miles.
Now there are various examples above of shields that aren’t exactly inline with what the OP proposes, i.e., a complete underbody plate a la Hot Wheels cars. Those are obviously done for various engineering reasons.
Hmm. So rust and crud is not as much if a problem as I thought then? Interesting.
not in general. way back in the day ('40s-'70s) it was a problem for a couple reasons; 1) primer/paint couldn’t cover every surface so when water/salt got to unprotected steel it rusted faster than hell. 2) even when they started things like anti-corrosion dips, the body sheetmetal still had places where water, salt, or wet salty muck could get trapped and sit there and eat away at the body. this was especially bad since these places were usually near weld seams which were already compromised. now, there is keen attention paid towards making sure there’s no chance for entrapment, so the anti-corrosion dips can get to every steel surface and water/salt can drain properly.
put it this way, up until the '80s, if you had a car that was five years old and didn’t have any visible rust, that was an event. now, people gripe if they see rust before the cars fifteen years old. hell, my dad’s '91 Dodge Spirit has one small rust hole on the door from a paint ding, and that car is almost 22 years old.
The air underneath cars is going to very turbulent anyway. Smoothing the surface will create minimal reduction in drag, at a high cost. Some underengine shields will minimize the worst effects. The gaps around the wheels aren’t going away, and the various bumps for strength and access under the rest of the car are a negligible factor. You aren’t going to easily remove turbulence with a smooth surface when two surfaces are passing each other at high speed a few inches apart.
An example of the effect of turbulence on drag can be seen in small planes. Tail-draggers, which have two wheels at the front of the plane, just behind the propellor, do not produce much drag in that turbulent zone. Tri-gear craft, where two wheels are back behind the CG of the aircraft, and are in much less turbulent air, will produce much more drag. Wheel pants may be added to aircraft in both cases, but they will have much more effect on the tri-gear planes.
Note to nit-picky pilots: I know not all tail draggers have the wheels that close to the prop, its an illustration of the principle, not a rule for aircraft design.