[damo]

When you look at them closely, the Roadster's aerodynamic aren’t great. They do have some things going their way.

Small frontal area
• Almost full undertray (apart from under the engine at the back)
• The Coupe has quite a nice Kammback.
• Front panel gaps are good.

However, there's loads of bad stuff too (in order of importance IMO):
• Huge bulbous front wheel arches (The single worst aero feature)
• Air intakes along the side of the body (2nd worst)
• Round recessed headlights
• The wipers are in an area of high pressure and almost unshielded
• The side mirrors are quite big for the size of the car.
• As standard the wheelarch gaps are quite big (7cm) for a small sporty car.
• The Tridion stands proud of the rest of the bodywork at the sides and top (esp noticeable on the Coupe).
• Large external hinges on the coupe.
• Wind deflectors for softtop. These are pretty much required to reduce the soft top wind noise and buffeting.
• The panel gaps round the doors are terrible (about 8 mm).

The main problem with the bulbous wheel arches, side air intakes and wheel arch gaps together is the unattached airflow along the side of the car.

I have made some mods:
• I put brown P profile weather striping along the side and bottom of the driver's door. Used "Back to Black" to 'paint' the strip black.
• Moved the front number plate up (to partially cover the front grille/ air intake)
• I made polystyrene blocks to fit into the sides of the front grille. Covered them with black insulating tape.
• I made a clear plastic deflector to deflect air past the driver's side-air intake.
• Changed the normal aerial to a shorter stubby.
• Pushed the side mirrors back a little
• Disconnected the Electric power steering
• Upped the tyre pressure to 31/30. Manufactures recommended pressure is 29psi (I have 16 inch spike lines with 205 section tyres), but that's the max I find bareable. I think because they are low profile there's not much gain anyway.

Going to change:
• I've got lowering springs to lower the car by 25mm.
• Maybe remove the passenger wiper.

Possibilities.
• Modify the pass side air intake to channel more (pressurised) air into it.
• Try to get smaller or more aerodynamic door mirrors. The problem is that it's difficult to get mirror bases for the roadster and the standard mirrors have leakage problems anyway.
• Headlight covers from Big performance
• Brabus (or other) sideskirts
• Going to make small aero blocks to fit behind the coupe hinges

Can't do (not worth it ££)
• Steel roadster 15 inch wheels with 175/185 section eco tyres. Also try to fit aerodynamic hub caps.
• Lightweight seats. Standard seats are 20kg each. x 2 is 40kg.

I don’t think Smart ever published a cd figure for the Roadster. I’ve saw 0.41 mentioned for the ordinary roadster, but I don’t know how scientifically that was figured out. I suppose though even with that figure taking into account the frontal area, the cdA probably isn’t as bad as I think it is. I think what’s really interesting is the top speed difference between the Roadster and Coupe ~109 v 112 mph, eventhough the Coupe is 40kg heavier. The Kammback does have a significant effect there.

My car, with the 205 section tyres, probably affects the fuel economy as much as the aero deficiencies put together. But financially (and aesthetically) it doesn’t make enough sense to change them for Roadster steel wheels.