Erratic speed

[jonesy792]

The speed on my iaero has been very erratic lately, I noticed it at first when rolling out with a group ride and seeing a speed of 50+mph (great for self esteem but not helpful otherwise). I replaced the battery in my speed sensor to no effect, when I halved the wheel diameter in the iaero from 2086 to 1043 the speed seemed to come close to normal (based on cadence + gear ratio calculations) but there are still random spikes and dips.

I noticed when listening to the "click" in the sensor when the magnet passes over it that there is in fact a "double-click" every time the magnet passes by, could this be the source of the problem?

[coachboyd]

Try adjusting the distance of the magnet to the sensor. Are you using a very strong magnet? That might cause a double hit.

Was it just spikes where it would read higher, or was it consistent through the ride?

[jonesy792]

The gap is as close as I can get it.
It might be a strong magnet I'll look into that further, mabe go to the bike store and pick up a few other magnets and compare. I don't remember the magnet being new when the problems started though......

It was both consistently higher and spiking higher, I think a photo might be best:


The graph almost never dips below 30 mph for the duration of the ride even though were were doing mabe half that speed

[cjonbike]

You might try pulling the sensor a bit further away from the magnet, if it is a strong magnet the two does not have to be quite as close. Also they seem to work better if the sensor and magnet is further away from the hub. Just a couple thoughts.

[pjboyle]

You might try rotating the magnet 90 degrees on the spoke. In other words, if the magnet is now facing you as you face the wheel, move it so it faces either the front or the rear of the bike. Magnets produce a field that has two poles. You can imagine it as two spheres or an hour glass emanating from the magnet. There are a lot of variables you are already chasing such as strength of magnet and spacing to sensor, but in certain circumstances, the field from both halves of the hourglass pass over the sensor sequentially and it senses two hits. You want the magnet to be oriented so only one of the fields next to a pole (doesn't matter which one) intercepts the sensor. This phenomenon is not just iBike this is can occur in all magnetic sensors. Worth a try.

PJBoyle

[coachboyd]

I am wondering why your wind speed is so high as well.
No matter what the wheel speed says, the wind speed should be reading correct, example: if it was calm outside and you were riding 20mph but your wheel speed said 50mph, the wind should still read 20mph.

I'll bring this up to John, can you email me a file at boyd (at) totalcyclist dot com

Thanks, and we'll get you taken care of.

[coachboyd]

Just an update on my last post for everybody, I just got the file and I think it was a spacing issue between the two sensors. We're going to see if it works better with the sensor placed different.

As with the wind speed, he was using a Quarq paired with an iBike and not doing the wind offset before rides (older firmware). This would explain the wind speed issue, and using wind speed is still a good way to diagnose problems like this.