Powertap virtual cadence (and speed)?

Everything I've read said it is unreliable but I have no proof.  The cadence numbers from mine (note, not a G3) seem high to me but I've never compared them against an actual cadence sensor, it's just not something I care about.  Same goes for speed, I've never compared it against a sensor.  On the trainer, I again don't really care about speed and when I'm outdoors I use GPS.

I would move the GSC-10 to my cyclocross bike, better than GPS speed only.

Makes the roadbike prettier (the weight is neglectable).

In my experience the speed is very reliable on the road. Not sure about the cadence, seems fairly close at least.

Yes- a powertap will give you both cadence and speed.   you don't need the GSC10 if you have a powertap.  This feature is not unique to the G3, but also the previous models.

Speed and cadence seem to be very accurate.  The only time I notice a hickcup in the cadence is when I shift gears.  It will overshoot the change for a revolution or two and then settle down and be accurate again.

Here is a comparison of the speed trace from a bike with a PT (not the G3 -- a PT Pro), an SRM with front wheel speed sensor, and a Polar with rear wheel speed sensor mounted on it. The x-axis is minutes.

Here are the cadence traces from the same ride:

Notice that when the cadence on the SRM or Polar is steady then the PT's cadence is mostly steady but with occasional blips, and when the SRM or Polar's cadence is variable, the PT's cadence is also variable but with occasional blips. This was second-by-second (that is, I'm showing you the full data streams) -- if you average over 3 or 4 seconds the blips almost even out and the PT's cadence looks much smoother and more like the SRM or Polar.

All that said, cadence doesn't much matter anyway.

Thank you for posting the graphs.

This saved me a lot of time as I intended to leave the GSC-10 on the bike and measure with a different ANT+ and then compare to the G3....

While I agree that cadence is not too important, I know that my cadence is rather low and I should work on bringing it up.

Though on the flipside, cadence seems to be an individual thing....

Guess I might need to rethink a bit, as power is the only important and true measure on the bike...

Sort of.  It's actually a combination of power and RPE.  How much power you can put out at the lowest RPE.  It is important to get a feel for different cadences just so your body knows what it's options are.  But as you ride more and more, your body will pretty much figure out for itself what it's ideal cadence is to maximize power and minimize RPE.  And once you figure that out...you really don't need to pay attention to the actual cadence number on your computer. 

I left the GSC10 on my bikes even with the PT.

I will buck the trend a bit on cadence - I think it's important. The actual number compared to others isn't really useful, but after looking at my race and training files, I found that I raced at a higher cadence than I normally trained at. This made for a difference in how I felt during and after a race. After this, I upped my training cadence to my race cadence to better approximate my racing. I could have gone the other way too but I've found during a race I just tend to pedal faster even when I try to lower it.

http://en.wikipedia.org/wiki/Borg_scale

or

http://www.brianmac.co.uk/borgscale.htm

Do you race and train at the same power level on the same (type of) terrain?

I race wherever the race takes place. Sometimes hilly, sometimes flat. I noticed it mostly when training on the trainer or easy group rides - I'd noodle along at an 85 in whatever zones I was targeting, then in a race I'd see I was going 95-105 most of the time (road races/crits, TT's and tris). I would get some strange soreness or cramping in the races because the muscles were being used differently than how I was training.

I tried changing my race cadence, but that only works for TT's and tris. When the pack surges, you either stick with them or your race is over. I don't want to worry about cadence at that time. So I decided it would be best to train how I race.

I would say my experience is similar to yours.  When on easy group rides or noodling on my own, my cadence naturally falls in the 80's.  And when racing (or on hard group rides) my cadence will bump up into the 90's during the hard efforts.

But I think that was to RChung's question...do you race and train at the same power level.  For me the answer is sometimes.  When my power levels are lower than race effort, my cadence feels more comfortable a little lower.  When my power levels are at or above race effort, my cadence tends to pick up.

So *for me*, I don't feel the need to train at a higher cadence persay.  All I need to do is practice my race level efforts, and my cadence naturally follows.  As I mentioned earlier...finding the cadence where power is highest at a given RPE.  I don't need a cadence computer to tell me that.  Just a power meter and the ability to shift gears.  Of course...YMMV.

I would agree. 

This is true. I mostly train on hills or on the trainer (very little flat here). All of my training (indoors and out) is based on power. My point was that training cadence should match racing cadence (or vice versa) when possible. This is also what Allen and Coggan mention when they discuss quadrant analysis in Training and Racing with a Power Meter, and Allen and Cheung in Cutting Edge Cycling. I'm not counting noodling along at low power, etc. but on the trainer I'm always trying to keep my cadence around my racing cadence. There are exceptions. On a 18% grade where I can't spin faster than 50 RPM, I'm just happy to still be moving

I also think terrain should only be taken into account when your gearing no longer lets you spin at your desired cadence. 250W @ 95 RPM is the same on the flat as it is on a 5% climb. The only difference is the gear you are in and how quickly the ground is moving underneath you.

Well, the reason why we train with power rather than by speed only, or by cadence only, is exactly because the latter depend on terrain (and power). And the moral of QA isn't that you need to exactly mimic average race cadence in training -- do you also mimic average race pedal force in training? -- but rather to show you the range of pedal speed and pedal force that is used in your races and in your training rides. As for your example, you can do 250W at 95rpm on either the flat or a 5% climb (assuming you have the right gears), but the crank inertial loads are going to be quite different.