GPS speedo accuracy

[CruzLike]

Hi All
I'm cutting my teeth on a new gadget and I'm not sure if this is typical.

I have recently purchased a heart monitor and began training with it. I run the app through my phone. The screen on the phone has all sorts of wonderful data. The speed that is displayed is all over the place. I ride steady at 18.5 on my wheel based computer. The phone app bounces all over the place +/- .25mph.

I plan to purchase a wahoo elemnt to get away from using the phone and use something that is specialized to cycling.

Is this what I should expect from a GPS driven speedo? Is the sensor in the Elemnt or Garmin more accurate/steady than my phone?

 

[ratz]

GPS speed is highly suspect and depends on sky visibility. It's good for a simple start; and is a fine way to go for casual riders. What you see on the Phone depends on the app and how it's written. If you look at the RAW data coming off the GPS it's ALL over the the Place in real time. A good app will smooth the data and actually display your AVERAGE speed on a rolling window of the last 3-5 seconds. That would show you a number that makes a lot more sense than the real time instant readings as it smoothes out the inaccuracies. The free wahoo app is for example better at this than say the strava app; strava tends to show the realtime garbage more even though they record the more smoothed data. Haven't really verified the strava stuff in about 3 years but that was the talk on Slow-twitch at the time. The smoothing stuff I learn about as part of the wahoo app beta testing for the RFLKT units....

Now With something like a Wahoo Elment or other Bike computer; You'll want a wheel sensor in most cases if you are concerned about accuracey. There are magnet based and accelerometer based sensors now. The magnet ones are about 1% more accurate but these really the new magnet-less ones really are the bomb for ease of use and quality data. They are also very Cruzbike friendly. The Wahoo Speed and Garmin are the kills of the hill for that right now. Right now I'd rate the Wahoo Speed the best since it's Bluetooth and ANT+ for $40 US.

Normally you mount these on the rear wheel to get the best data (most wired ones were designed that way) which the coming of wireless you get more options. I'd recommend put them on the front wheel if your bike computer is on the handle bar; that removes the need to try and blast the signals through your body; which as the batteries fade can become a problem.

 

[Mathew Fy]

I've used Map My Ride on the iPhone. It works well only when satellite connection is excellent (no hills, trees, or buildings,) but it saps the power quickly. Now I use a Garmin with good result, but for more reliable readings use the speed sensor (along with cadence, heart rate, power.)
There are opinions to be found here:
http://www.bikeradar.com/forums/viewtopic.php?t=13037377

 

[RojoRacing]

My garmin edge 520 will hold my speed to one or two tenth of a mph for the most part. I've never thought it was lacking in real time accuracy other then massive changes in speed. If I'm accelerating or decelerating really fast it'll always display my speed for one second ago as it's always recalculating. I really only notice this when I coasting down a big hill and I watching my speed skyrocket through the 40s and 50s.

 

[LarryOz]

I use the Garmin speed sensor on my front wheel

 

[CruzLike]

I'll put a speed sensor on my gadget list.

Thanks for your input.

 

[super slim]

On the Garmin Edge 705 the primary speed calculations come from the GPS, then the wheel sensor, with the circumference automatically adjusted using the GPS for distance .
On the Garmin Edge 800 the wheel sensor is the primary sensor, then the GPS, which I assume is done to save battery usage, using local speed, cadence and pressure sensors, and only the high wattage GPS for location at 5? second intervals.

I do not know what the later Edges do, but I assume it is the same at the 800, to save the battery.

 

[Bruce B]

Today, following the advice of Bob and Larry, I moved the Wahoo Speed Rpm sensor to the front wheel of the V. On the rear wheel the sensor would intermittently drop the signal to my iPhone which is mounted on the slider. Installing the sensor on the front hub was difficult as the elastic band is made for a narrower hub.

Following the installation on the front wheel I spun it up and saw that it was out of balance. What to do?

The solution. I found my spool of tin and copper solder which is 1.5mm in diameter. I cut a piece about 3cm long and slipped it under the three bands holding the sensor to the hub. You can just see it, a silver line perpendicular to the circumference of the hub. After sliding it around to find the sweet spot on the hub the wheel is balanced. Two drops of Gorilla Glue should keep it in place.






Edit: 45 mile ride and not a single dropout!