undercar aerodynamics

I wanted to make my car stick to the road or track, so after much consideration I came up with the following design for an undercar aero package.

The main criteria is that as I have no access to carbon fibre manufacturing or any kind of facilities for laying up fibreglass it had to be made from flat sheet material.

Flat undertrays are commonplace now on normal roadgoing saloons so this was a good starting point as any disturbance to the airflow under the car will slow it down, and we want air going under the car faster than it is travelling over it to give us some down force, or at least to eliminate lift.

looking at the production Nissan skyline R34 GTR it has a flat front and rear unertray. looking at more advanced race cars you can see the flay tray goes the full length of the car, but practically this posed a problem as the car was on axle stands in my garden and working on a car length sheet of metal would have proven impossible.

I settled on the flat front and rear tray. Now on the GTR owners club forum (www.gtr.co.uk) there was a rather long winded thread about undercar aerodynamics. so I gleaned info from this and any other scource that I could.

the basic idea is this.

As the car moves down the road air travels under the front of the car then more air rushes in around the wheels and under the sills generating a high pressure point roughly in the centre of the car, high pressure = lift. this can make a car unstable at high speed and in extreme cases cause accidents.

To stop this happening we can go for low side skirts, not practical on a road car but fine on track.
My car has to be road legal to be allowed on track days. (either mot'd or currently racing in a series with scrutineering) so as my car isn't in a race series, mot is the only way I can get it on a track day.

So really low side skirts are out. The thread on the GTR forum said about using a brush material like you see on the sides of industrial doors to keep draughts out. So I scourced 12 m of brush from a supplier of such things and then proceeded to bolt it to the bottom of the car.

It is worth mentioning here that this type of installation can be extremely dangerous on a road car in high speed conrers, as road are not smooth like race circuits and if the side skirt is raised from the ground too much it will allow air to be sucked in and create lift, exactly when you don't want it this could cause a nasty crash.

there was some basic maths involved for positioning the brushes in the optimum positions this can be found on the aerodynamics thread on the gtr site.





I have done a trackday with the brushes and undertray on the car and I must admit the car performed very well. with coilover suspension maintaining the height of the car even in the tightest of bends body roll was tiny and the car felt really planted to the track.

next I will design some sort of rear diffuser so I can start to generate some proper down force..