Zoltan wrote:racerfern wrote:Is the bottom line of this post, "Oops I was wrong"? The iBike is accurate at all grades, even if I can't ride up it. You're a good man for recognizing the problem with your logic.
Yes, that is the bottom line.
The good (?) man is back.
Finally I figured out how to set the tire circumference to 419 mm that is 2095/5, so I mounted 5 spoke magnets evenly spaced on my first wheel . Note: the front wheel has 20 spokes on a paired rim. I could have borrowed more spoke magnets to mount 10, but 5 was enough for testing. And it was ugly enough
I made several short rides, and my findings are the following.
1) With 5 magnets the fluctuation of the slope when accelerating is visibly less as I expected, although Gen3 still needs some time to "zoom" on the true slope in case of a sudden burst when standing out the saddle. I bet adding more magnets would not result in really better accuracy, because the problem is not the acceleration itself, but the moving handlebar and the moving body (Note: my unit on the handlebar, maybe on a stem the effect would be a bit smaller).
2) During my short rides I experienced that the slope is STILL not exactly symmetric when ascending or descending on the same route. It is true with 5 mounted magnets, but also when a single magnet is on.
i) First I thought it was my fault to make not perfect tilt calibration, so I made some more between each phase of the test. But after the calibrations I always tested it on a pretty smooth pavement with putting the bike in one direction and turning the bike with 180 degree. The displayed slopes were either perfectly symmetric to zero or with an error of +/-0.1%. So I am sure that the tilt calibration how I do it is perfect.
ii) The calibration rides I made last week always ended up with a ride tilt of 0.3% and rarely 0.4%. I assume from the beginning that the ride tilt comes from the extra weight of the biker, so it is the extra tilt to be considered after a proper biker-free tilt calibration. But let me ask whether I am correct?
iii) So if my logics are correct and I did the tilt calibration properly and my ride tilt is also OK, there must be something else to explain the differences.
3) Some data.
i) Coasting down on a relatively longer stretch of slight gradient without acceleration gave -2.2 -- -2.3%, while steadily climbing it gave +2.9 --+3.1%
ii) Coasting down on a shorter stretch of substantial gradient with some acceleration (5 magnets on) gave a max of -9.7%, while steadily climbing it gave a max of +10.3%
iii) Going down on a shorter stretch of very steep gradient without substantial acceleration (I used the brakes to try to minimize the acceleration, 5 magnets on) gave a max of -26.7%, while climbing it gave a max of +25.1%
4) Conclusions
i) Even some small acceleration can influence the accuracy of the slope measurement, and in line with the manual the accuracy may be within +/-1%. 5 magnets maybe came it below +/-0.5%, but it is just a speculation from me.
ii) But apart from the acceleration issue there is some asymmetry when ascending or descending. I was very careful with my posture in the test of 3/i) ,so I suppose it did not influence the readings. The difference is more than +/-0.1%. Why? Maybe due to auto tilt, but I have no better guess.
5) As a sidenote I have found that measured wind data is far from symmetric, which is normal, nature is nature, but the extent of asymmetry of wind speed is too much. I also made wind calibrations every time, but my guts feel that something must be with the speed measurement. Again it is a speculation, but the wind profile was so strange on a straight, traffic-free road of 1 km or so. The measured wind was fluctuating between an absolute headwind of 9 km/h and an absolute tailwind of 10 km/h.
6) Other sidenotes
i) the processor of Gen3 seems to be pretty fast and/or the programming logics of the firmware is quite well done. iAero receives and displays the ANT+ information much faster than the other tools of mine
ii) It is time to give up chasing a better slope accuracy than +/-0.5% on ascents and +/-1.0% on descents
iii) it was proved that the slope measurement of Gen3 is not sensitive to changing ambient air pressure coming from wind burst, so it is the most reliable inclinometer that I have ever had
iv) I will keep the 5 magnets on