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How does Edge 530 calculate ahead/behind times for Strava Live Segments?

Cankut ESKİN
over 4 years ago

Hi,

Device: Edge 530

Software Version: 7.10.0.0

Case: Testing "Strava Live Segments" feature on a ~1.4km climb with average gradient of 7.9% (a goal finish time of 7 minutes is set and synced to the device)

Segment Link: https://www.strava.com/segments/3840095

Activity Link: https://www.strava.com/activities/5175936601

The climb starts with gradients ranging 6%-9% until half way and goes up to 9%-12% on the last half.

I repeated the climb 5 times and I've noticed that, for the first half of the climb, time ahead/behind shows me that I'm ahead of my goal time by 5-10 seconds.

And for the second half of the climb, the 5-10 seconds advantage starts to drop towards zero, and I start to push harder to keep the time advantage.

If I were to keep my first half effort for the second half, maybe it would tell me that I was behind of my goal or even out at the end.

This ahead/behind time calculation seems a bit confusing to me because I was very confident that since I'm +10 seconds ahead for the first half, I could go easier for the rest of the climb.

My guess is; 

Device calculates an average speed for the whole climb to reach the goal and whenever you ride slower than that pace, it starts to drop your ahead time and vice versa.

A better approach could be taking the changing gradients into account and processing in an algorithmic way.

A few questions for fellow Garmin users:

  • How do you pace your outdoor interval workouts?
  • Do you use Live Segments for interval workouts too?

Thanks in advance.

  • 0 over 4 years ago in reply to cdhargraves

    It should be the same on both.

    Think of a downhill event, either skiing or MTB where they have timing gates along the route. Every time you pass through a gate it updates your time ahead/behind compared the the competitor you are cashing.

    The Edge is doing the same thing with every data point that is in the segment file. The number of data points in the file is a function of the recording rate used by the device that gathered data that was used to create the segment.

    In most cases you are comparing against the pacing used by the person you are chasing. With a goal time that is based on how the site that is creating the goal deicides to build it. They could just use an average speed to get the required time, which might not work that well for segments that get steeper near the end. They might take you PR time and scale it to give the goal time. I don't know how Strava builds their goal times.

    One thing to check is that auto-select is not enable for the segment as this will cause the time to jump around if it changes the target. 

  • 0 over 4 years ago in reply to aweatherall

    If you could post the segment file from your Edge I can see what the data frequency is and how the goal time was constructed.

  • 0 over 4 years ago in reply to Wongataa

    That's interesting because I've been trying to troubleshoot a problem I have with Garmin Segments where I don't get any "live" updates, i.e. ahead/behind times, I only have my PB at the top, a chart, DTG, TTG and I added Grade and 3s Power below.

    I looked through all the settings but can't see a "live" option, not that I was expecting to find one as that's one of the main point of segments. I looked for ahead/behind data fields too but came in empty handed. Am I missing something obvious ? Thanks !

  • 0 over 4 years ago in reply to webvan

    There are two ahead/behind data fields that you can add to the segment page.

    Both data fields are labeled Ahead, but they will dynamically update to show behind if you go behind.

    One is under the Time category and is Time Ahead, the other is under Distance and is Distance Ahead.

  • 0 over 4 years ago in reply to aweatherall

    Thanks, I've added the Time Ahead field in the 4 field display and will see how it goes. I also realized that in the 2 field version the times of the SL and Connection/Self get shown automatically. I had missed that because when you're configuring the DF that part is completely blank of course. I miss the old "simulation" mode of the nüvi systems ;-)

  • 0 over 4 years ago in reply to aweatherall

    So I switched to the 2 field option but it shows a map, not the ahead/behind like in the manual, unless there's a setting I can't see. Choosing the "Time Ahead" field does the trick, with the 4 field option too.

  • 0 over 4 years ago in reply to aweatherall

    Are you sure about that ? It's not the behavior i've seen. When gradient is not regular on climb, time ahead is totally way off ... and can change from 1' to 0 in 3s ...

  • 0 over 4 years ago

    Can someone from Garmin please confirm, is it based on average speed?