...instead of just being ostentatious. That's Cd as in coefficient of drag; reducing this will reduce the amount of power required to maintain a certain speed and thus the amount of fuel consumed by the car at that speed, coincidentally, it will also increase the car's top speed. The bits required for each car would vary but I'm fairly sure that a full or partial undertray (a panel covering the underside of the car) would feature in most of the kits (I'm only aware of these being fitted to BMWs as standard) as the underbody area is usually pretty rough, in the case of my Micra the cavernous void around the engine must disturb the airflow quite a bit.

Provisions would have to be made for venting the hot air from the engine bay, I guess it normally flows out under the car so vents could be placed in the undertray and shaped so as to draw the hot air out.

This idea has already been done in production with the Vauxhall Astra ECO 4 (see link) but the idea is that these can be retrofitted to old cars as an improvement. The most obvious disadvantage to this is that the kit will add weight, reducing the car's rate of acceleration.
-- squigbobble, Aug 28 2005

Vauxhall Astra ECO 4 http://vauxhall.co....e.jhtml?indexNo=183
Vauxhall news item [squigbobble, Aug 28 2005]

Someone with the same idea http://www.max-mpg.com/html/tech/main.htm
VW Beetle spoiler [squigbobble, Sep 06 2005]

Most exotic supercars these days have full undertrays, but naturally that's an exception to nromal driving.

At any rate, I'm a major proponent of undertrays--the Cd advantage far outweighs the amount of weight it adds to the car, and it constantly boggles me why more cars don't do it.
-- 5th Earth, Aug 29 2005


The undercoating could be simply made of aluminum foil. To do maintenance, just replace it for about $10 (and you can recycle the old foil). It would cut easily to any shape car.
-- sophocles, Aug 29 2005


[sophocles], I can imagine that Al would not be the best material, it'll flap about all over the place increasing drag and making a hell of a racket. It'll also get perforated by just about everything that gets thrown up by the tyres, probably including water spray. However, thin Al sheeting (like aircraft skin) would work and it could have a rib structure on the unexposed side so that it can be made thinner and lighter.

[Pa've], it's meant to be removable, screwed on or in slots (like the side of a puter case) or somet' like that so u can take it off if u drop your spanner into the engine bay. As far as maintaning the undertray itself goes, it would require the same maintenance as the underbody of a normal car.
-- squigbobble, Aug 31 2005


Aerodynamic bodykits for lorries can be obtained (and frankly should be compulsory, because the fuel saving is something like 20-30%).

[Pa've]'s got a point, though. Whatever they make it out of has got to withstand quite a beating. I can see the headlines now: 'Speed cameras baffled by drifts of aluminium foil'...
-- moomintroll, Sep 01 2005


Foil is a bad example, but as mentioned just thin plate Al or steel is fine. Mount it is multiple sections so you don't have to remove the entire bottom as one piece, and it's really not a big deal at all.

It need not be any thicker or stronger than, say, the material your exhaust pipe is made out of.
-- 5th Earth, Sep 01 2005


I think whatever guage steel the exhuast pipe is made from is probably overspec (and weight). Fibreglass might be adequate (and cheaper for low volume production), it seems to work ok on the topside of a car :D
-- squigbobble, Sep 01 2005


Once you're doing 30mph or so, aerodynamic drag is definitely the primary cause of friction on the car, and keep in mind that friction is the only thing keeping us from infinite gas milage. (The rolling resistance is negligible at this speed.) A savings of 20% of the drag, assuming everything is in check (that the motor can run lean enough or that the torque converter is efficient enough, etc.), would result in a 20% gas savings.

In other words, the extra weight (and therefore extra rolling resistance) encountered by the plates is negligible when you're cruising. Besides... extra weight on the BOTTOM of the car will only IMPROVE handling. For a whole bottom of a car, I can't imagine the plates--even of a cheap steel--weighing any more than a backpack full of books.

In the rear of the car you're going to want a pointed, tear-drop-like shape. I wonder how this would work considering license plate location restrictions, though. Maybe there should be a plexiglass portion shaped so there's a cone surrounding the plate?

One thing I also propose is a very low rubber body kit capable of withstanding a hit from road debris, a parking curb, or a speed bump. This would be good for downforce if that ever interests you.
-- kevinthenerd, May 31 2006



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