Modified shape for Airdam?
 

Hi people,

I have a Suzuki Swift which I converted to electric. I think the Swift body shape is the same as, or similar to, a Metro and the Holden Barina. I have some plastic angle, one side being 50mm wide and the other side being 75mm wide. I have about 3 metres of it.

I have already totally closed off the grill and added a kamback, front wheel spats and deleted the mudguards and radio attenna. I would now like to add a front airdam and I obviously have the choice between a 50mm (2 inch) or 75mm (3 inch) airdam.

Currently the front bumper is roughly 8 to 9 inches (approx 220mm) above the ground and the front suspension appears to be about 2 inches (50mm) lower than that.

I suppose my first question is - does an airdam actually help aerodynamics? I have done considerable research wherein opinions and some tests vary between "Cd can actually get worse with an airdam" to "Airdam gave me a 7% improvement" or "Depends on how big your airdam is."

It appears to me (and I am a real dummy!) that the purpose of an airdam is to get the air flowing around the vehicle rather than underneath. With less air travelling underneath the car there is less pressure and therefore an increase in downforce. An increase in downforce should increase my rolling resistance and therefore negate the benefits of reducing drag(??).

My second question: In an attempt to negate some drag I was wondering if maybe I could shape the airdam in the shape of a "U"? In other words, instead of following the existing bumper right along to its edges and then doing a 90degree turn to follow the sides of the vehicle back to the front wheel wells, maybe I could have the front three feet (one metre) of the airdam follow the bumper bar and then curve it back to the front of the wheel wells. While the front three feet of the airdam will be flush and flat with the bumper, the curve back to the wheels would give the air travelling to the side of the car, less resistance?

I apologise if I have not described this clearly. Any opinions about my idea?

Many thanks in advance
Paul

As I understand it:

A perfectly smooth belly pan and air deflectors around the wheels is likely overall better than an air dam. The air dam trades an increase in area for reduced coefficient of drag be deflecting air from a very aerodynamically dirty area. The pan and deflectors do not add area.

I expect you could curve your air dam like that successfully. You might not want to have it end up completely parallel to the side of the car where it ends, to encourage air to move around the wheel. You'll probably want side skirts too.

Pics?

The general rule seems to be match the airdam to the lowest point of the undercarriage. In your case 2". I would exclude any small protrusions—for instance on my car a tow hook is 1 1/2" lower than anything else.

There's another side to the coin: You'll have about 6" in clearance (measured in metric of course :)). Depending on traffic controls, curb cuts, gravel roads, potholes & etc., you may be able to get by with less. I find 3" is comfortable.

A flat airdam only needs a radius on each end equal to 4% of the width. The further forward of the axle centerline, the more it will drag on approach ramps. The further back from the bumper, the less effective.

I think you picked a good body to work with. Opportunities would include air curtaining the front wheelwells, skirting the rear wheelwells, boat tailing and adding a divergent duct to the lower rear.

airdam
 

You might consider re-positioning the airdam to even with the leading face of the bumper.And then wrap the airdam around the corners of the bumper all the way to the front wheels as you see done with modern cars.
http://i1271.photobucket.com/albums/...Untitled42.jpg
Allowing any air to get under the nose will cause front lift if it can't bleed around the sides.
http://i1271.photobucket.com/albums/...titled1_25.jpg
If you wanted to go deeper,you could create a fascia which would modernize the nose,enclosing more naughty bits.
http://i1271.photobucket.com/albums/...ead2/205-1.jpg

Thanks people,

Thanks Ecky - I follow your logic as far as a belly pan is concerned. We put the car up on a hoist and attachment points were more rare than hen's teeth! As a result a belly pan dropped right down the "to do" list.

Thanks freebeard - I think I need more posts before I can post pics? In Australia, speed bumps, designed to slow cars down, can be anything from barely noticeable to almost requiring you hire a crane to get your car over them! I might stick with a 6" clearance.

Thanks aerohead - the repositioning you suggest was my first thought but I wondered if modifying the shape to a "U" shape might provide less resistance to air trying to get out the side of the car. Of course I forgot all about the "U" shape creating more front end lift which I don't need. I will look into the fascia designs you suggested.

Regards
Paul