I suggest riding at the same pace. That way you can just talk to each other!
Motorola Talkabout T5500AA
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But maybe that isnt the right answer. Take a look at the Motorola Talkabout T5500AA ($50 for two; www.motorola.com), one of the walkie-talkie-type radios that have become so popular in the past decade. The T5500AAs work on both the FRS and GMRS bands, meaning that when used in GMRS mode they have a range of up to eight miles. When used in FRS mode, the power output is stepped down and range drops to about two miles. Use of FRS mode is unrestricted, but the Federal Communications Commission requires a license for GMRS mode ($80 for five years; http://wireless.fcc.gov).
Along with the radios, youll want a pair of the Talkabout Speaker/Microphones ($40 each). These plug into the radio and clip to your jersey, then you stick the radio in your pocket. A button on the side of the microphone activates it so you can talknot quite hands-free, but one-hand. It would work well on a bicycle. Motorola also makes a headset that can be used with a voice-activation feature on the T5500AAs, but those probably wont work with a helmet. Besides, ambient noise will set the thing off.
I tried this radios-on-bikes thing once, when my brother and I were doing a long tour in Oregon. It was a bust. For one thing, even though GMRS radios offer longer range, theyre still line-of-sight devices. Get around a hill and your odds of making contact drop to almost nil. Along with that, battery life (three AAAs for each radio) is about six to eight hours. So you have to carry lots of batteries, or somehow recharge them with a solar charger. The radios ended up being dead weight.
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