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Re-Calibration of the Speed Sensor

As you know, the Speed Sensor is really useful on MTB twisty single track trails. It provides accurate distance. The GPS distance (and therefore speed) is often up to 10% too low. You can easily lose 4-5 miles over a 50 mile ride using GPS for distance!

So clearly, we can't count on the auto-calibration to work on one of these rides... since that uses GPS, which can be 10% too low in that use case.

So, when I get a new speed sensor I ride on a clear/straight section of trail or road, and that sets it up accurately.

Many in this forum suggest the speed sensor continually auto-calibrates based on GPS. Wrong. If so, after a couple miles on single-track, the calibration would be bad, based on bad GPS distances on those twisty single track trails. And I don't see that at all. My speed sensor constantly gives me accurate distance, that is 6-10% more than the GPS track.

My question is: can we tell the Garmin computer to re-calibrate a new wheel or tire automatically? If we know we'll be on a nice clear/straight section of trail or road for the next mile or two?

I know I can manually set the tire circumference, but it would be so much easier to tap a button on the Garmin to re-calibrate. I just can't find that option.

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0 aweatherall over 5 years ago

Nearly all the newer units do constantly compute the wheel size if it i set to auto. Some of the older units only did it the once. What unit are you using?

0 DaveBrillhart over 5 years ago

Hey - I am using the new one that does not require a magnet. So I guess it does somehow update all the time? Strange because it doesn't seem to slowly resolve to the same distance that GPS registers, which it should if it constantly uses GPS to re-calibrate. Anyway, I will set it to manual so I get it right and it stays right. Thanks!

0 aweatherall over 5 years ago

The wheel size calculation is performed by the head unit. The speed sensor just detects rotations. The head unit then converts the number of rotations to a distance. This is done either using a static value if you set a manual size for the wheel circumference or via a dynamic value that is adjusted using GPS data that is run through a slow moving average to eliminate a bad GPS position value.

Older head unit like the Edge 800 only performed the wheel size calculation once. Newer units re-compute the value all the time an activity is being recorded.

0 biker128pedal over 5 years ago

I never auto calibrate. I found it not accurate. I lay a tape measure down and roll the bike with my weight on it to measure the wheel circumference. Millimeters tapes are easier to find know. I got a small 12 footer at Home Depot for less than $10. Anyway I eye a sidewalk mark near the valve stem. Tread lug on the mountain bike. Or mark. Roll and measure. Then write it down in a note on my phone with tire and air pressure. Then I think in sensors you can enter the circumference in millimeters. If you measured inches multiply by 25.4 to get millimeters. Hope this helps.

0 theBuggane over 5 years ago in reply to aweatherall

Would it make sense for the unit to be constantly recallibrating? The whole point should be that it is calibrated on a flat run of a km or two and then the speed sensor is the reference for places where GPS might be wrong (steep hills, patchy reception, etc.).

If constantly calibrating (effectively constantly changing wheel size with the terrain), the speed sensor only fills in when the GPS reception is lost. If you have just climbed/descended a steep hill, that calibration is likely to be wrong.

Is there a reference anywhere to how this works? I would rather have a "calibrate now" button to trigger the km or two of callibration when I know i'm on a flat straight bit!

I'm an Edge 130 owner...

0 aweatherall over 5 years ago in reply to theBuggane

The reason it constantly computes the wheel size unless you set it manual is that it has no way of knowing if you have moved the sensor on to a different bike or wheel set that have different sized wheels from the ones it originally calibrated on. The data for the recalculation process still needs to meet the data requirements of the original calibration,

0 winfij over 5 years ago

Since it's done by the head unit from gps, one would like to imagine they're smart enough to only use flat, straight sections with solid gps signal to do the auto calibration from, not trying to do it all the time on twists and slopes. I've no idea if that's the case, but it should be

0 bigHUN over 5 years ago

I have one bike with two wheelsets (one for gravel and one for asphalt) and a magnetless sensor on each (front) hub. Manually set both sensors in my head unit (VA3) for both wheels circumference. btw a difference between two wheels circumference is 42 mm.

On 50 km ride with friends we are usually within 300 meters range we compared four garmin gps units and two cateyes. We were riding waves average elevation difference about 350 meters. My city is in a smaller-shallower valley, no matter which way we start we need to climb up first.

Bottom line is that I am happy with this accuracy from my manual setup.

0 lytleric over 5 years ago

I actually just checked the circumference value auto-calculated by my Edge 830 last night on a road bike and found that it was short by 60 mm. So that means that the Garmin thought I was riding at a slower speed and shorter distance than I actually was. Not sure what 60 mm equates to for an average speed difference, but I will admit that it felt like I had been riding faster than what the average was. I manually overrode it with a measured value and will keep it that way. Need to update all my other speed sensors also (I have four).

0 ewarnerusa over 1 year ago in reply to aweatherall

I have an Edge 530 paired with a Garmin Speed Sensor 2 and use auto calibration. I recently got a new bike with new wheelset that I moved the speed sensor over to. I had this exact thought of how will the Edge/sensor know it is on a new wheelset? I also wondered if I can force a recalibration?