CARBON WEEZEL GOES TO MIRA
Summer 2009

Carbon Weezel together with Racecar Engineering magazine took a hillclimb Mini into the MIRA wind tunnel for a hectic but very revealing half-day session to test products and to provide data for a short series of articles in its monthly Aerobytes column, written by former hillclimber Simon McBeath.

The car was provided by Graham and Neal Masters from Southsea, Portsmouth, who are already highly competitive in their class at Gurston.

Here is a brief outline of what was found, for more information please email us.

Produced in association with:
Racecar Engineering Magazine
Simon McBeath

Testing catagories
Airdam
This turned a front lift into modest downforce and reduced the drag on the car.
Splitter
The splitter was tested at 3 intervals 150mm, 90mm, and 30mm and merged into a flat panel extending back under the engine. The splitter 'taps' the high pressure area ahead of the car and, up to a point, the longer the splitter the greater the downforce. Even the 30mm-long protruding splitter worked wonders. The effect was also to reduce the amount of air flowing under the car and to smooth it out under the engine compartment, but also to speed up the flow to reduce pressure underneath. Front grip was increased by up to 8% at 60mph, which would be very useful in a car which might have the tendency to understeer too much.
Diffuser
Diffusers help generate low pressure beneath the car, and our twin tunnel adjustable diffuser did produce rear-biased lift reduction with no drag penalty at 10 degrees. But its performance was hampered by the undulations in the minis floor. The gentle flow of smoke emerging from the diffuser showed the flow was lacking energy, pointing at unwanted disruption forward of the diffuser. Integrating it smoothly with a flat floor that connected to the splitter extension under the engine bay would produce much greater benefit.
Bonnet
The flow at the base of the windscreen is actually reversed, so the racer's trick of raising the rear lip of the bonnet to allow hot air to escape looks like a waste of time. It also caused a further small increase in front lift too.
Sealing engine bay
The car initially featured a large front mounted radiator, and the smoke from the smoke plume could be seen free entering the radiator and then emerging under the car. Which showed that positive lift is being created in this configuration.
Attention to detail on cooling and under-bonnet flows can produce significant drag and lift reductions. Blanking off parts of the grill that did not directly feed the front mounted radiator reduced the CD by 0.021 and knocked 23lb off the front lift at 60mph.
Headlamp intake scoops
In this test we looked at completely blanking off the grill opening and trying a fully ducted cooling system. We used the standard side mounted radiator position and reversed the flow using one of our headlamp scoops. This was then ducted out via slots cut in the bonnet. Air was pulled from a massive area infront of the car by our headlamp scoop and it served to show how beneficial a properly ducted cooling system can be. CD was reduced by 0.022 and produced 41lb of frontal downforce at 60mph compared to the fully-open front radiator layout, which produce front lift.
Headlamp domes
The Mini normally races with flat headlamp infills, but our headlamp domes knocked 0.005 off the drag coeffient and unexpectedly, also produced downforce at the front end! So greater benefits were seen from having our headlamp domes than de-seaming the shell.
Running boards
Horizontal 'running boards' at sill height proved beneficial. For a very small drag increase (CD -0.003) There was a useful increment of rear biased lift reduction. The running boards discouraged air from flowing into the under body region, which helps to preserve the low pressure developed there by the diffuser. If this was integrated with a flat floor arrangement it have increased the benefits much further.
Rear spoiler
Technical regulations in the hillclimbing limit rear spoilers in the 'modprod' classes to half the height of the rear window and, as can be seen, this put the test spoiler into the wake of the car, where it achieved very little. A spoiler or wing at or above roof height, where permitted, would achieve more rear lift reduction or even downforce.
The main section of the spoiler behind the car was mainly redundant, and the section that was producing the lift reduction was the ends that protrude from the side of the car and are in the air flow to the side of the car. We believe that if they are developed further they would produce greater lift reduction to the rear.
De-seaming
Tape was applied to the gutters and seams at the front in the hope that drag might be reduced. And there was a very small reduction of 0.004 on the drag coefficient but also a small increase in positive lift. A similar, possibly greater effect might be expected from de-seaming a shell.