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Distance measurement question

SteveRodkey
over 5 years ago

I am using a Garmin speed sensor which is set to automatic wheel size.  My question is when the 830 gives me distance ridden, is this based on the GPS, the speed sensor or a combination of both?  Also, how frequently is the wheel size recalculated?  Wheel circumference obviously changes with tire pressure, tire wear or if you change the tire.  Bottom line is how exactly is the 830 measuring distance and how accurate is it?

  • 0 over 5 years ago

    If you have a speed sensor the distance ridden is calculated using the wheel sensor data.  The distance ridden will be accurate as long as the wheel size is accurate.

  • 0 over 5 years ago

    From my understanding, it actually continuously recalculates wheel size, which in my opinion detracts from the whole purpose of having a wheel speed sensor (which is to have an actual measurement of distance traveled using a fixed measuring device, i.e. your wheel).  

    I have found the auto-calibrated wheel circumference to be inaccurate, so I have measured mine with a ruler and keep the measurement set to manual on my 830.

    Here's a somewhat lengthy discussion of this topic:

    https://forums.garmin.com/sports-fitness/cycling/f/accessories-sensors/157669/re-calibration-of-the-speed-sensor/893841#893841

  • 0 over 5 years ago in reply to lytleric

    I let my 830 auto calibrate my speed sensor on a nice flat straight road. I then turned off the auto setting and used the number from the auto setting so it could not change. That auto generated wheel size was spot on. I have manually measured my wheel.

  • 0 over 5 years ago

    Thanks for the information.  I guess I will need to look at my wheel size more closely.  Anyone have good ideas as to how to measure that accurately?  I know I have had difficulty with that in the past.  I can easily see this is a bigger problem for mountain bikers than road bikers.  For years I rode with and 800 (no sensor) and a cateye on the same bike.  I found the distance was always amazingly close on the two units.  Note this was road riding in reasonably flat terrain. The main reason I added the speed sensor was that the speed on the 800 without a sensor was very noisy, almost useless. With the 830 this seems less of a problem.

  • 0 over 5 years ago in reply to SteveRodkey

    It is easy to measure your wheel size.  It is esier with a helper.  Find a suitable smoothish piece of road.  Using a piece of chalk or some other marker mark a position on the ground. Line up the valve stem of the wheel in question with this mark.  Sitting on the bike roll forward in a straight line until the wheel has made at least one revolution and the valve stem is now at the bottom again. mark the position of the valve stem on the road.  Measure the distance between the marks and this is the size to enter into your Garmin.  The more revolutions of the wheel you do between the marks the better as you will reduce the measurement error..  I would do at least two.

  • 0 over 5 years ago in reply to SteveRodkey

    I use a metal measuring tape (with metric), lay it out on the driveway, and using a mark on the tire (like the label) lined up with the start of the tape, slightly put some weight on the wheel (to compress the tire as if you were riding), then carefully roll along the tape until you reach your tire mark.  Take the measurement from there.  Do it two or three times to make sure of the measurement, and that should be it.