[andylaurence]

Originally Posted by 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)

The front wings are bolt-on, so I've thought about taking a vacuum-formed mould of the current wings and laminating the inside of the mould to reduce the size of the bulge before making a vacuum-formed wing from the reduced size mould. Time is my main barrier to doing this!

Originally Posted by damo

• Air intakes along the side of the body (2nd worst)

Are they that bad? They could easily be covered with some vacuum-formed plastic as they're nearly all decorative.

Originally Posted by damo

• Round recessed headlights

Big Performance do some Perspex headlight covers that should help with the lighting aero, as you've discovered.

Originally Posted by damo

• The wipers are in an area of high pressure and almost unshielded

I've fitted some Bosch aero wipers, which helps a little. I've thought of fitting a cover to the bonnet that send the air over the top but that'd stop the bonnet opening!

Originally Posted by damo

• The side mirrors are quite big for the size of the car.

I've bought a rear-view camera, which I'm going to fit to remove the need for the wing mirrors. You can't realistically fit mirrors inside the car, simply because the shape of the car would prevent you seeing anything with them!

Originally Posted by damo

• As standard the wheelarch gaps are quite big (7cm) for a small sporty car.

You're right, they're huge! Making new front wings would help here, I think. Alternatively, some plastic sheeting could cover the rear arches entirely and most of the front arches to make the gap smaller.

Originally Posted by damo

• The Tridion stands proud of the rest of the bodywork at the sides and top (esp noticeable on the Coupe).

Really? I've never noticed that. I'll take a peek later.

Originally Posted by damo

• Large external hinges on the coupe.
• Wind deflectors for softtop. These are pretty much required to reduce the soft top wind noise and buffeting.

Have you tried removing them? It's on my todo list, once I work out how they're attached. You can hear the wind noise they make, so it must be pretty bad!

Originally Posted by damo

• The panel gaps round the doors are terrible (about 8 mm).

All the panel gaps are huge. The bonnet and the boot are also pretty bad.

Originally Posted by damo

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've noticed (in the rain) that it reattaches at several places, the windows for example. The constant separation and attachment is really bad for the Cd.

Originally Posted by damo

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.

I don't quite understand that. Do you have photos?

Originally Posted by damo

• Moved the front number plate up (to partially cover the front grille/ air intake)

Cunning! I've noticed that temperatures are never high on the Roadster, even at 35 degrees or more. In fact, it's only heat soak that makes the temperature go up when you stop. I've seen 100 degrees on the gauge!

Originally Posted by damo

• 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

Do you not find the steering a little heavy?

Originally Posted by damo

• 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.

I've been looking at the wheels from a ForTwo. They're the same diameter as mine (15") but not so wide. They fit 145 section tyres, so there's gains to be had there. The front arches can then be correspondingly smaller.

Originally Posted by damo

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

I've also been thinking about a rear diffuser. That should minimise the wake of the car. In addition, adding creases to the trailing edges of bodywork should help separation. The round shape of all the panels can't be helping at the moment.

Originally Posted by damo

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

Good list! Some to add to mine there.

Originally Posted by damo

Can't do (not worth it ££)
• Steel roadster 15 inch wheels with 175/185 section eco tyres. Also try to fit aerodynamic hub caps.

I saw a noticable difference by going to Michelin Energy E3A @ 40psi. From memory, it was about 5%. Flat hub caps are on my list too. I was thinking about a sheet of plastic cable-tied to the wheel spokes.

Originally Posted by damo

• Lightweight seats. Standard seats are 20kg each. x 2 is 40kg.

I had the idea of using polyurethane expanding foam and a plastic bag to make a lightweight seat. It's only needed for padding. However, you'll need to fabricate some seat belt mounts as they mount to the seats as standard. Also, you'll lose the ability to move the seat, which is fine for me, as the only driver and at 6'3", I have the seat far back anyway!

Originally Posted by damo

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.

Certainly. Although, take the top speed with a pinch of salt. Last month, I did 197kph (according to GPS) on a German autobahn in my notchback.

Originally Posted by damo

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.

You're getting better mileage than me!