We report the results of an experiment to increase use of a municipal bike sharing system. Two distinct groups – those who had newly moved close to a bike station (N=3,500) and those who lived in the vicinity of a newly built bike station (N=7,000) – were randomly assigned to receive one of two versions of a postcard offering free rides. In the first version, the offer is framed in terms of what recipients get for “free”. In the second, the same offer is framed in terms of its discount value: $12 off. We find no overall difference in redemption of the offer between postcard frames but for those with a new bike dock in their vicinity the “free” message is significantly more effective. We also note new movers are almost four times as likely to redeem the offer than those with a new dock in their vicinity. As a result of the study the city transportation bureau’s marketing team have used the “free” frame on subsequent materials and continue to target new movers.