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Do bumps affect the magnetless speed sensor?

A little long but bare with me!!
I had really had my distance under-reported on my favorite mountain bike trails. I bought the garmin speed sensor. I chalked the wheel and measured it on my driveway with a tape measure. It came out to be exactly 91 inches. I entered the wheel size in my 5x+ as 2311.

I measured a partial loop and a full lap with riding.
And the partial loop with running.

Here is what it looks like (this was 2 laps recorded with 5x+ only). It was really shorted on distance.
https://connect.garmin.com/modern/activity/2927775698

Here is what is crazy

Full Lap:

5x+ with Speed Sensor full lap = 8.52 miles
705 GPS full Lap = 7.65 miles
830 GPS full Lap 7.5 miles.

Partial Lap

5x+ with Speed Sensor partial loop = 4.6 miles
Run: 5x+ with Stryd partial loop = 4.25 miles
705 GPS partial loop = 4.18 miles
Run: 5x+ GPS partial loop = 3.88 miles

So the thing that shocked me was the 8.52 miles. The trail is a lot of small climb and drop (25-30 feet up and downs).
Would that small of elevation change account for almost an extra mile? Doesn't seem like it.
Did I calculate it right? I am positive I measured the wheel right.
So it dawned on me that it may be hitting roots is registering a wheel rotation.

0 Former Member over 6 years ago

I think yes. I've got a cheap magnetless speed sensor and I got erratic velocity when I hit bumps at speed.
I'm doing some research to find out if there's some magnetless sensor which doesn't suffer this, but it seems that also Garmin sensors are affected.

0 aweatherall over 6 years ago

The Garmin units does not use accelerometers to determine speed. The accelerometer in the Garmin speed sensor is used just to wake it up. The Garmin unit determines speed by detecting the movement of the sensor through the earths magnet field.

It can be negatively influenced by part of the drive chain being magnetized. This can be tested for by moving the sensor to the front wheel. If the issues go away then then part of the drive chain is magnetized.

0 indymtb over 6 years ago

The Garmin units does not use accelerometers to determine speed. The accelerometer in the Garmin speed sensor is used just to wake it up. The Garmin unit determines speed by detecting the movement of the sensor through the earths magnet field.

It can be negatively influenced by part of the drive chain being magnetized. This can be tested for by moving the sensor to the front wheel. If the issues go away then then part of the drive chain is magnetized.

This has been bugging me so I did an experiment by purchasing a Cateye speed sensor with a magnetic sensor. The distance of the trail measured 8.4 (Catey) vs. 8.42 (Garmin). I will stick with the Garmin as I like the magnetless sensor.

The reason I thought it was off is because the 705 I have reported 7.5 miles. The Fenix 3HR reported 6.8, My brothers 830 reported 7.5. It is tight single-track and the GPS cannot accumulate data accurately with out the speed sensor.