gkamieneski

I've been using a BLE/ANT+ Speed Sensor for quite some time. I started using it to make sure my displayed speed on the head unit did not fluctuate wildly. That, and I found I could get some reasonable data from my roller sessions.

Recently though I started to think about what the sensor is actually doing for me. Is it solely overriding the instantaneous speed that the GPS unit would be showing without it, or does it enter into the calculation for distance traveled and how does it impact the data sent to Strava?

njkayaker

The Garmin will use the wheel sensor for speed and distance over GPS. If the circumference being used is correct, the wheel sensor should be more accurate for speed and distance.

gkamieneski

Are we sure the speed sensor sends data that calculates the distance travelled and that it doesn't just provide a "steady" readout of instantaneous speed? Are we even sure that its data gets involved in the calculation of average speed? Then there is the Strava option to correct for distance and altitude.

njkayaker

Yes, I'm sure.

https://support.garmin.com/en-US/?fa...623Z535geTx2e9

Garmin provides an elevation correction too. It applies it by default for units that don't have barometers (GPS is poor for elevation).

Iride01 

Are you having issues or just worrying?

Wheel sensors are not fool proof, but slightly better than GPS alone.

Seattle Forrest

I'm sure. You're obviously not. I have no idea what it would take to satisfy you that this is the case, so that's not a question I can answer. You could try riding a short distance with the speed sensor in your pocket and expect to see zero for speed and distance when GPS would disagree.

himespau 

My garmin has an option to use GPS to determine wheel size. One of these days I'm going to do the math to figure out how many millimeters off my wheel circumference estimate has to be for it to be more/less accurate than the GPS error over a while divided by the number of wheel rotations for that distance. I keep meaning to and then decided I don't care enough.

gkamieneski

I'm just not sure I get much more than steady instantaneous speed readings because of the sensor. The head unit could be displaying Average Speed from GPS readings rather than the speed sensor, and I probably wouldn't know it.

As I said, where I see the speed sensor definitely producing the data is when I am on the rollers as without it, speed would be 0.

Iride01 

Are you using a Garmin device with your wheel sensor? It might be that you have the default setting of "auto calibrate" set for your wheel sensor. Garmin tries to use the GPS to calibrate the wheel size for a certain time when you start the timer. So if you are indoors on a roller not going anywhere, then who knows what's going to happen. Possibly setting a wheel size in the setup might help.

Speed and distance are meaningless on rollers or a trainer. Garmin many times recommends you disable the GPS when indoors. Some of the newer Garmin's you just select the indoor training profile.

pdlamb

After years of not caring, I've become obsessed with the GPS or wheel sensor difference.

It all started with I changed tires, putting a 35 on instead of a 32. I didn't change the wired computer (solid for a dozen years!) wheel setting, so I expected to be reading a little low relative to the GPS. What I found, instead, was the wired/wheel sensor distance was reading 2-3% higher than the GPS, instead of the <1% difference I've been ignoring all this time. One of these days I'm going to have to do the early weekend downhill leg on the U.S. highway with surveyed mile markers, but I've not got around to it yet.

So I headed out for a short ride; after a mile I checked both devices, and the wired reading was 1.20 miles vs. 1.0 for the GPS. 20%? Holy cow! OCD hit hard, I was watching the distance every half mile. Strangely enough, the delta persisted until I hit a quick right-left. Going into the zigzag, the delta was steady at 0.2 mile; a quarter mile later, it was down to 0.13 mile. Confidence in GPS accuracy dropped 7%, like the stock market on bad news. It was a few miles to go, with a fair few turns around ridges, trees, and houses, but strangely enough when I got home the delta was 0.18 mile.

Conclusion? Pick one:
-- GPS is a flaky on sharp curves and without clear views to the sky.
-- Wired computer might be going bad. Strange that it happened when I changed tires.
-- It just doesn't matter.
-- If it does matter, you need a survey crew.

gkamieneski

Well I have 2 setups. I have a Cateye Padrone Smart that in mirror mode uses my iPhone for the GPS and is also paired with a BLE/ANT+ speed sensor on the rear wheel hub. The other is a Bryton Rider 15 Neo, again paired with a BLE speed sensor on the rear wheel hub.

I have found the means to use both setups on the rollers, measuring speed, time and distance, so obviously in that scenario I can see the benefit of the speed sensor. I may discontinue using the sensors when out on the road as I don't think they make enough of a difference to just using the GPS.

Iride01 

If you are someplace where you get a good gps signal all the time there isn't much need for a wheel sensor. Especially if you won't get bent out of shape if one ride shows a few tenths more or less than another of the same rides.

For the trainer rides, the only numbers that really make sense is time, and if you have it, power.