Garmin Edge + Cadence - a query

fishyweb

I have an Edge 305 with Cadence sensor on my road bike. This seems to be set up fine, in that I get a cadence reading and, also, if I spin my rear wheel while the bike is stationary, I get a speed reading (so the wheel magnet is being detected OK).

However, when I'm out riding, I lose the speed readout when I go under long bridges such as motorway underpasses i.e. when the GPS signal is lost momentarily.

I thought that the wheel sensor was the primary responsible for speed readings when a cadence sensor is present, and that losing the GPS signal should not affect the continuity of the speed track. Can anyone confirm whether I am right about this, or tell me what might be wrong with my setup?

The Edge firmware is at v3.20 IIRC.

jimmcdonnell

I don't think the cadence sensor automatically takes over if you lose the GPS signal? I know when I use my bike on the turbo trainer I have to manually select the 'Use with GPS off' option. After that it's fine, though after powering off the Garmin it will turn back on with the GPS back on by default.

Maybe there's a way to get the sensor to step in automatically if your satellite signal is lost? That'd be handy. Anyone know of such a setting?

rjh299

fishyweb wrote:

I thought that the wheel sensor was the primary responsible for speed readings when a cadence sensor is present, and that losing the GPS signal should not affect the continuity of the speed track. Can anyone confirm whether I am right about this, or tell me what might be wrong with my setup?

I thought this is how it worked aswell. Contact Garmin.

jimmcdonnell

The wheel sensor is definitely secondary. Isn't that the point of forking out £hundreds for a GPS computer instead of a sensor-based one?

When I swap bikes and tell my 705, about 500m down the road it will beep at me to tell me it's used the GPS data plus the wheel revs to compute the wheel circumference. So at any given moment it knows what size wheel I'm riding if it needs to, but I'm sure it only uses the sensor info if you specifically tell it to. Yes, you could contact Garmin or you could RTFM as I'm about to again... If I find I've been riding with false GPS/sensor assumptions, I'll post more.

fishyweb

jimmcdonnell wrote:

The wheel sensor is definitely secondary. Isn't that the point of forking out £hundreds for a GPS computer instead of a sensor-based one?

Hmm, not entirely IMHO. Once the wheel size has been calibrated (via GPS and wheel sensor), then I think the wheel sensor will be capable of providing a more accurate readout of instantaneous speed than GPS alone, due to the inherent margin or error with GPS. If I'm right about this, then I would have hoped that the unit would take advantage of the presence of the sensor to provide that more-accurate real-time data. But I would very much like to hear from others what their understanding of the functionality is, or should be.

sneakybear

It's a (not so well) known issue/feature with the 305 series. They don't 'revert' to the magnet when you loose GPS signal.
That said if you use the wheel sensor as the primary speed sensor you won't have this issue.

Another option is to buy the (much more expensive) 705 which does 'revert'...

Accuracy is also an interesting issue if you scour the forums. GPS is good on flat but as soon as you get hills things change. Not sure which I'm in favour of but once you have calibrated the wheel sensor it will be constant (until you change tyre pressure...)