Aerodynamic drag of mountain bike tyres

This paper proposes the concept of a front wheel air deflector for downhill mountain bikes. Downhill mountain bike tyres are becoming larger with more aggressive tread patterns in order to cope with the increasingly severe conditions of modern downhill courses, which in turn leads to increased aerodynamic drag forces caused by air flow, and consequently a reduction in speed over a downhill course. A front wheel air deflector was designed and constructed from fibreglass that, when fitted to the bike, could be adjusted to give different clearance settings between the tyre and deflector inner surface to cope with varying mud conditions. Various designs of downhill bike tyres, a cross-country tyre and a slick racing tyre were selected and tested in a wind tunnel in the range 0–72 km h^–1 to determine their benchmark aerodynamic drag characteristics with no deflector fitted. Further wind tunnel tests were then conducted for one of the tyres with the deflector in various clearance positions. The trend of increasing aerodynamic drag with current downhill tyre development was verified, while the drag tests indicated that the power input to the wheel can be reduced by up to 50% with the deflector fitted in its smallest first clearance setting. This represents a drag reduction of up to 10%, and a reduced drag coefficient of almost 40%.