So I just got a new PowerTap Pro+. My first power meter. At the time I didn’t know the hub did cadence so I also purchased the separate wireless cadence sensor. Couple questions:
How accurate is the hub at telling pedal cadence? Not even sure how that works since the hub doesn’t know what gear I am in. Feels fairly accurate but I’m an engineer…I need precision. Is it some math based on wheel size/speed/torque?
Would the separate cadence sensor actually on the crank arm be more accurate? This is assuming I can get it to work correctly. Right now when I set the computer to listen to cadence from the ‘pedal’ I am seeing numbers way too large. About double the rate I am most likely pedaling. ??? Any ideas?
Is the separate cadence sensor even worth the trouble?
I believe the hub would sense your pedaling dead-spots followed by more power and extrapolate cadence that way. I use the cadence magnet as well since I feel it is more foolproof.
I’ve always felt that the ‘virtual’ cadence reported by the powertap wasn’t always accurate. Cadence it reports usually seems to be on the low side, especially when using smaller gears. Flip side of that is when I download the workout, I have a max cadence of 200-240, which seems pretty unlikely (no, I wasn’t doing superspins, nor could I hit that # even if I did).
For me it’s good enough. I want to know if I’m in my higher range or lower range. I don’t really care if I’m pedaling at 92 or 97, as long as I know it isn’t in the low 80’s. I used mine with another bike computer with cadence for a while and I found it was usually very close, although it takes a little while to catch up with reality.
I used the cadence sensor for awhile until the magnet fell off. But I soon realized the hub numbers were fairly accurate most of the time. Sure, there are some flukey numbers now and then but it’s easily good enough for me since I don’t spend much time focusing on it anyway. If the power numbers did that, I would be concerned. But the cadence numbers, not so much. I just got a brand new harness for a second bike and didn’t even bother to set up the cadence sensor this time.
Did you have to do anything other than just put the magnet on the crank, put the sensor on the chainstay and line them up and then find the sensor with the computer? I did all that and the computer “learned” the sensor but when I was pedaling at maybe 50-60rpm max the computer was showing a cadence of 100+rpms. Tried “re-learning” the cadence sensor and tried adjusting the sensor and magnet and still got the same.
A) if you rae using a seperate cadence sensor you need to go into “settings” and tell it to listen to the cadence sensor over the hub. This is in addition to “learning” it.
B) my experience is that the hub based cadence works about 75% of the time, but other times it gives totally screwy numbers (so i use a seperate wired cadence sensor)
Did you have to do anything other than just put the magnet on the crank, put the sensor on the chainstay and line them up and then find the sensor with the computer? I did all that and the computer “learned” the sensor but when I was pedaling at maybe 50-60rpm max the computer was showing a cadence of 100+rpms. Tried “re-learning” the cadence sensor and tried adjusting the sensor and magnet and still got the same.
I had this happen one time, the unit was reading out exactly 2x my real cadence.
It turns out you can get the magnet located just right so that it trips the sensor twice every time it goes by (magnetic fields having 2 poles & all). It sounds like you already tried the usual fix, which is just moving the magnet or sensor a smidge so this doesn’t happen, but you might play with it again just to confirm. In my case I slid the magnet about 3mm on the crank and it was all good.
Depending on your pedalling style, you may get different results, but I find that the virtual cadence reported by the hub is dead on accurate when ever I happen to look at the head unit. However, when you download the data, there are a few 1 second spikes that seem to occur when you transition from coasting to hard pedaling. This causes the max cadence to be off, and gives you these 200-240 readings. I find that that there are generally no spikes in a steady state situation.
Overall I prefer the hub measured cadence because it just looks cleaner, it’s easier to move between bikes, and I don’t really find max cadence numbers to be all that useful in measuring training. I find average cadence to be more useful, and the spikes are infrequent enough that they don’t seem affect average cadence readings to a significant degree.
RChung did a pretty interesting investigation of the PT hub cadence algorithm. I can’t seem to find the page now but the details were very interesting (especially for an engineer), and the result was that it wasn’t very accurate.
The hub appears to look at the real-time torque data and extract the cadence information by looking for 2 torque peaks per revolution. Whenever you have an atypical force application, the hub can not accurately estimate the cadence.
I rode for a couple weeks before I got my cadence sensor and noticed that most of the time it was good but a pretty large amount of the time (~10%) it came up with ridiculous values.
Since installing the cadence sensor, all the data looks very good now.
Also, RChung is the guy who frequently says that cadence is a red herring.
I don’t install the cadence sensor on my bikes anymore. The difference between the hub and sensor results for many rides is very, very minor (for me). I do steady state intervals.
Is it some math based on wheel size/speed/torque?