How can I transmit music from one Soul Cycle to another for surround sound?
To handle wireless bike to bike surround sound, I purchased a Shure Wireless monitor system, designed for on-stage use by bands. The model I bought was the PSM 200. It came with one receiver. The main disadvantage of this system is the cost: I spent about $500 on the system. I also purchased an additional receiver for about $250. There are less expensive wireless monitor systems available. I wanted to go with the Shure.
On the other hand, the system runs on 12 volts and is fairly simple to integrate into a Soul Cycle system. The quality and range are excellent. We regularly see a range of around 200 feet, as long as there's a line of sight. The system runs on UHF frequencies, which are shared by local television stations. So you'll have to take care to pick a channel that is unused locally, which can be harder to do in a major city like New York or LA. In SF we use channel 6. The 9 volt battery on the receiver lasts at least 3 hours (careful as it turns on accidentally when jostled in a pocket). The other benefit of the Shure wireless system is that you can easily raise and lower the volume from either the receiver or the transmitter.
It would make most sense to wire the transmitter into whomever's bike will be the DJ. It could exist as a standalone module that could be moved from one bike to another. However, this would require that the trasmitter has it's own battery.
Unfortunately it is not possible to use the internal power source of the Soul Cycle Head Unit to power the transmitter. It makes more sense to wire the transmitter semi-permanently into a more powerful Soul Cycle, such as a Soul Cycle Classic, than to move it around. People with a Soul Cycle Head Unit will be receivers. People can 'guest DJ' by preparing a playlist on an MP3 player and handing to the person with the main setup.
The other standard approach to bike-to-bike wireless is to use a low-power FM radio setup. The main benefit here is cost. It's possible to get a transmitter for 150-250, and receivers are ubiquitous. Any FM walkman, in theory, will work. Unfortunately, FM walkmen don't have great receivers as a rule. And whenever the bike doing the broadcast goes around a corner, or out of range, the people receiving the signal hear static. I've heard that using a car audio head unit as the receiver will address this problem. Car audio head units have circuitry that mutes a weak FM signal. However, it's not obvious how to wire or mount a car audio head unit for use on a bike.
The Shure wireless system has mute functionality built in, and it works well. It's a thousand times better for people on your ride to hear silence than static.