Edge 305...do I need the speed sensor one?
Looking at buying the 305 for my mountain and road bike.
It seems like the speed sensor backs up the gps signal for speed when it losses the signal?
I'm guessing that happens a lot while mountain biking?
Is it worth the extra bucks?
Not exactly. I'm told the speed sensor is useful because it is also a cadence sensor and because it allows you to use the edge on a bike attached to an indoor trainer. Do you want that or not? Simple question, really.
I suppose it would help if you lost signal, but ever since I bought my 76CSx, I haven't lost a signal in the woods once.
I think the 305 supports 2 sensors: a heartrate monitor and a cadence sensor. The cadence sensor measures pedaling cadence, not bike speed. I think this is more commonly used by road riders than mountain bikers, but you might find it useful.
The Garmin sensor is a speed / cadence sensor, as Nate says, and mounts on the rear chain stay with a magnet on the crank arm and another on the rear wheel. The GPS head unit will pick up cadence if you have the sensor pickup enabled, but only will monitor speed if the GPS signal is completely lost, or if you manually turn off the GPS to use it on an stationary trainer, for example.
Loss of signal is very rare on the 305, since it has the SiRF Star antenna that is high gain. Dense tree cover on the north side of a very steep slope that blocks satellite reception to the south sky will do it, but I have never lost signal for more than 5 seconds, and that is very very rare.
I'd say get the cadence / speed sensor only if you really want to monitor cadence or will use the bike on a stationary trainer.
Thanks for the advice...
I will just pick up the 305 with HR and if I feel I need the speed / cadance then I will pick up the sensor.