My wife uses a Sigma Sport BC 1009 STS wireless on her stoker bars. It was a fairly inexpensive wireless unit that I bought after consulting with a support rep at Sigma (1-630-761-1106). He told me that all of the STS units (such as the current BC 12.12 STS) and all of the ROX units should have enough range to cover the distance from the chain stay to the stoker bars, and we haven't had any trouble.