Maybe Topeak will make a stylish air-horn that uses 16 gram CO2 cartridges and attaches conveniently to the rear top-tube for easy access by the stoker and sells for 19.95? I'd buy that...
Cheers!
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2008 Red Co-Motion Speedster Co-pilot (Redster)
2009 Surly LHT (captain's commuter)
2009 Surly Crosscheck (stoker's road bike)
2007 Giant FCR2W (stoker's commuter)
1980's NOS Legnano (stoker's toy)
1970's Stella rebuilt as fixed-gear (captain's toy)