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Aerodynamics
Wool Tufting
Want to test your car's aerodynamics but don't have a wind tunnel?
Try wool tufting. The concept is simple, just buy a ball of wool, cut it into strips about 10cm (4") long and tape them all over your car so that the wool is free to flap in the breeze (you only have to do one side since both sides are aerodynamically equal). Now grab a buddy in another car, a camera (better still a video camera) and drive down a dual carriage way side by side, recording what the wool is doing. A good speed for most cars is about 80km/h. Either wait for a still day or drive the car directly into the wind, as cross winds will affect the air flow over the wool.
Now have a good look at the tape or photos. Surfaces where the wool is flat and straight exhibit laminar flow.
Surfaces where the wool is fluttering wildly exhibit laminar separation, or turbulent flow. Areas of turbulence are causing drag. Often changing a small part can yield a CD (co-efficient of drag) reduction of 10 percent. Note these problem areas and then decide the best way of fixing them.
Let's start at the front. Most older cars have a 'brick' front end, that is, the air flow hits the front, has to turn 90 degrees (up and down), then has to make another sharp turn as it reaches the bonnet. Unfortunately, you really can't do much about this without ruining the cars character and appeal. The best solution is that used by door slammers (sectioning and tilting the front end) but that probably isn't practical in many street applications. All you can do is try to soften the transition as much as possible. Bringing the bumper forward and smoothing it into the grill maybe acceptable, depending on whether you value appearance or performance. Another option is to use unobtrusive clear plastic, attached only for competition using quick release fasteners (as on sprint car bonnets). That way you retain originality and may need to drill as few as eight strategically placed holes in your precious metal.
Turbulence around window rubbers. Run your hand over the top of the windscreen and onto the roof of a modern car. Notice that the sealing rubber is virtually flush with the glass. Try this on a pre-aero car and the moulding can protrude something like 15mm from the glass! Check out the XD-XF Falcon or early Commodore. On most cars this can be fixed by flush fitting the glass, but this is hardly convenient and only really suited to a full bare metal resto situation. On the afore-mentioned Falcon the glass is flush fitted from the factory, but Ford ruined it by surrounding it with stupid metal style mouldings, they can be removed, but the car looks awful without them. Still, that doesn't matter on a race track.
Side windows present a problem. If you're an experienced metal fabricator, you could try cutting the tops of the doors off and fabricating flush fitting runners or possibly converting your car to a pillarless vehicle. Hardly worth the effort, unless you really like your car. The only sensible option is to use plastic mouldings similar to weathersheilds on the front doors, though they shouldn't stand out as far. You could also apply wide race tape over the pillars to reduce flow separation.
The Rear of the car is where the greatest gains are to be made. Keep in mind that the perfect aerodynamic shape is a tear drop with the blunt end facing forward. The key is to get the air flow back together without drag. This is where the VW Beetle gets it's shape from, as do many more modern cars such as the Ford Taurus. In a sedan, turbulence is often present on the boot lid. One solution is to change to angle of the rear window so that it is more gentle and feeds laminar air flow onto the bootlid. This is probably not possible for most cars or most people. Look at the Mercedes 190E Evolution II, this had an air deflector fitted to the rear window (similar to those on a station wagon) which pushed air down the rear window. Another solution is to raise the boot lid. Take a look at the latest Mitsubishi Magna, it claims to have the lowest CD of any Australian built car. It also has an exceptionally tall boot lid. The Group A Walkinshaw Commodores used fibreglass to simultaneously raise the boot lid and add a rear spoiler. If none of this is possible/ desirable, you can fit a roof extension spoiler, to make the air separation point as clean as possible. This could be removable if it is fitted using the same method as roof racks. Vehicles without boots (hatches, vans, wagons, utes etc.) can also benefit from having roof extension spoilers. Utes/ Trucks can also benefit from a tonneau cover or possibly a deflector that fits between the tray floor and the tailgate.
