Odometer comes up short after stops

[Russ]

iBike_03_09_2013_1542_3_km_AP_Measure ib8T.ibr

(75.8 KiB) Downloaded 314 times

I recently calibrated my odometer on a measured 1.7km course. I have both a Bontrager bike computer that uses ant+ and the iBike Newton both sharing the same wheel and cadance sensors (separate sensors).
I do this so I can keep the iBike set to show wind and use the Bont. for speed and distance display.

I noticed a divergence between the two after a very careful run through the course to the tune of about one wheel rotation, I thought.

So today I rode non stop one way through the course stopping at the end. Then I picked the bike up and reversed direction. I rode the reverse course stopping once each tenth of a kilometer. The total distance for the return course with all the stops comes up quite short. The round trip total should be 3.40km and Isaac reports that it is 3.35km or you can examine the return trip on its own in the file if you wish.

Here is the ride file, I reset at the start and at the end so it would be dedicated to the two 1.7k runs. Now to be clear, I rode in both directions the full 1.7 k course. In fact, on the non stop run I actually stopped when the iBike popped 1.7k in the display and counted the aprox 1.5 wheel revs back to the 1.7k mark. So there is that amount of slop in the total from the non stop first part measurement (ie one extra wheel rotations past the actual 1.7k point where the iBike measured 1.7k a few meters late.

My wheels are 700-32 pumped to 185psi and circumference set to 2023mm

Thanks,
Russ

[texmurphy]

All magnet triggered odometers require a pass of the magnet over the sensor to assume a wheel revolution.
So how to treat a stopped condition and then a restart?
Many cyclometers simply treat a magnet pass as a magnet pass and add it to the accumulated distance.
Whereas many bike computers instead treat the magnet pass after a stop as a wake up from zero. Still cannot compute speed since they need at least two passes to determine speed. If you are doing a minimum speed to be non-paused (e.g. like a minimum speed setting on a Garmin Edge), then it may take several passes to get to a wake up speed. I don't know what the Newton considers a minimum speed, but it wants to be moving.
So, consider your 2023 circumference wheel. On average you will move 1012 mm before the first magnet over sensor and another 2023mm for the second. You are already 3 meters off and maybe still in a bike computer paused state.

Additionally the Newton rounds any over 0.05 distance value up to the next tenth on display.

Worse case is stopping with the magnet near the sensor! I have had the sensor triggered so my speed measures very high and distance is added when stopped with magnet over the sensor. I always check magnet position when stopped to prevent this. My iAero was very happy to record high stopped speeds. I have not experienced this with the Newton.

[Russ]

All magnet triggered odometers require a pass of the magnet over the sensor to assume a wheel revolution.
So how to treat a stopped condition and then a restart?
Many cyclometers simply treat a magnet pass as a magnet pass and add it to the accumulated distance.
Whereas many bike computers instead treat the magnet pass after a stop as a wake up from zero. Still cannot compute speed since they need at least two passes to determine speed. If you are doing a minimum speed to be non-paused (e.g. like a minimum speed setting on a Garmin Edge), then it may take several passes to get to a wake up speed. I don't know what the Newton considers a minimum speed, but it wants to be moving.
So, consider your 2023 circumference wheel. On average you will move 1012 mm before the first magnet over sensor and another 2023mm for the second. You are already 3 meters off and maybe still in a bike computer paused state.

< regarding above: I don't know about "Many" but the few I have had (Cateye Astral 8 was a favorite) and a couple of Bontragers and one Specialized) have been consistent in proper behavior, from my view point.
My view is that, unless someone walks the bike backwards, they should be able to expect all magnet passes to be counted toward distance accumulation with the first being a zero start and restarts being additions.>

Additionally the Newton rounds any over 0.05 distance value up to the next tenth on display.

< This was certainly NOT the case when I crossed the 1.7k line. If it was then the iBike would have hit 1.7K noticeably early compared to the Bontrager instead of one magnet pass later. >

Worse case is stopping with the magnet near the sensor! I have had the sensor triggered so my speed measures very high and distance is added when stopped with magnet over the sensor. I always check magnet position when stopped to prevent this. My iAero was very happy to record high stopped speeds. I have not experienced this with the Newton.

< I am careful to start the course by backing the bike up two spokes from the start line and then positioning the magnet two spokes before the sensor. After this I do the reset for the start out part. For the return part where I did not do a reset, I did do the same positioning sequence as the start being careful to not pass the magnet over the sensor except the one extra time going out to trigger the 1.7k display on the iBike. Also If in one of my stops I had triggered multiple reads, it would have reduced the observed effect of coming up short! >

So if this is a feature I suppose we may have to live with it but if it Is a feature consider this a feature request to change to count all magnet passes. Else if not a feature then please list this as a bug.

Thanks,
Russ

[Russ]

Please notice my embedded comments to Texmurphy's reply in the quoted section.
Thanks again,
Russ