TECHNOLOGY – AEROBYTES
Comparing very different racing cars
Rounding up the study of the Arachnid and the Force LM
Simon McBeath offers aerodynamic advisory services under his own brand of SM Aerotechniques – www.
sm-aerotechniques.co.uk. In these pages he uses data from MIRA to discuss common aerodynamic issues faced by racecar engineers
Produced in association with MIRA Ltd
W Arachnid
Tel: +44 (0) 2476 355000 Email:
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e’re going to go back and conclude our recent Aerobytes theme this month
with some summarising remarks about the pair of very different sports racers we have been studying of late. CTR
Developments’ Arachnid closed coupé sports racer has engine and rear wing similarities to the other car here, the Force LM of Force Racing Cars, in that both featured the Suzuki Hayabusa engine (1300cc in the Arachnid, 1600cc in the Force) and both
Table 1 – starting coefficients on the Arachnid and Force LM CD -CL -CLfront -CLrear % front -L/D 0.534 1.084 0.115 0.969 10.6% 2.030
Force LM001 0.676 1.263 0.725 0.539 57.4% 1.868
were fitted with a DJ Engineering Services’ dual-element rear wing. However, the Arachnid is a circuit racer, where drag and cooling featured more prominently on the design agenda, whereas the Force is a pure hillclimber that featured more efforts at achieving outright downforce with less concern over drag. The comparison data made very interesting reading. The Arachnid had typical
sports racing aerodynamic appendages – the upper surface downforce generators being a full width rear wing and a fairly substantial looking splitter at the front. Underneath, the splitter incorporated small diffusers set between the front wheels and the chassis, and the flat underbody continued rearwards into a wide, though fairly conservative, rear diffuser.
The circuit-based Arachnid closed coupé sports racer is essentially a conventional design…
DUAL ELEMENT The Force LM is essentially the Force PC single seater with a widened chassis to meet the hillclimb sports libre regulations that prevailed when it was constructed, plus front wheel pods and sidepods that enclose the rear wheels. It utilises the same dual-element front wing as its single seater progenitor, plus a full width, dual-element rear wing. It also had a lower rear wing element and a wide and fairly shallow diffuser. Although both cars had run
…and produced relatively low drag
fairly extensively on track and there was therefore an amount of subjective information available, this wind tunnel session was the first opportunity to assess objectively the aerodynamic performance of each car, and whether the data confirmed the subjective sense about balance in particular. As ever, we need to keep in mind that the MIRA wind
February 2012 •
www.racecar-engineering.com 41
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