PowerPod Readings Jumping Around - Mount?

[ianc]

Hey Guys,

I received my PowerPod yesterday, and took it out for the first time today. On my commute in to work, I followed the calibration instructions, and watched the power count-up to 100 on my Garmin. Things *seemed* to be normal: reasonable wattage on the flats, 0w while descending a big hill. But then about half way in, the wattage went down to 0w.

I decided to take it out over lunch and just do a few uninterrupted laps on flat ground. Initially, the power readings were fluctuating wildly, but seemed to settle in after a mile of riding. But at around mile 5, things dropped back down to 0w again. The .ibr files seems to indicate that the PP believed the bike to be coasting downhill at this point, but I'm pretty sure I was pedaling on flat ground, and would've expected to see 120w or so.

Two things I'm considering:
- The bars on this bike are 26.0mm (not the modern "oversize" bars), so I had to shim the PP mount with a rubber grommet.
- I do not have a cadence sensor on this bike.

Other specifics:
Garmin 520
Garmin speed sensor (hub mount)
FW version 6.10

Anyone have any suggestions?

Thanks!

--ian

[Velocomp]

Yes, I see an abrupt change in watts at minute 19. It takes PowerPod 8 minutes to recover.

My strong suspicion is that you hit a bump that caused your PowerPod to rotate, causing its tilt calibration to be affected.

Make sure your PowerPod is firmly attached to the mount so that it cannot rotate, then all should be well

[ianc]

Hi John,

I realize this is outside your area, but do you guys have any suggestions for a stable 26.0mm mount? Thanks!

-Ian

[Velocomp]

Use some really stiff rubber washer adapters. Your local bike shop probably has some. Make sure your mount is tightly secured so that it cannot rotate.

[ianc]

Hi John,

I went back and used some hard rubber shims, and it looks like things are pretty solid. Definitely not getting any rotation of the unit now. I did a lunchtime group ride to see how the power readings would look. It looks like the wattage numbers are very "spikey". I'm seeing peaks in the 900-1000w range, some dropping way down for no apparent reason, and also seeing the slope numbers showing spiking declines on what was a pretty flat course.

In the attached ibr file, you can seefrom about mile 1.5 to mile 21 has a lot of slope fluctuation, but in reality there isn't much up and down at all. Any suggestions?

Thanks!

--Ian

PS: I'm definitely getting a lot of interest in this device from the folks on the group ride.

[ianc]

Sorry to keep hammering away with the questions, but I pulled up the profile in Isaac w/ "Edit Profile" and I noticed
- the bike weight was 11 lbs (probably closer to 20)
- Wind Scaling is 1.038; I think you mentioned somewhere that handlebar mounted wind scaling should be 1.4, and dual mount 1.7?
- While I have a cadence sensor and HR on my last ride, neither of those is showing up in the Wireless IDs page in Isaac for the profile. Do I need to enter those manually and "enable" them?

Thanks!

--ian

[speedracer]

- Wind Scaling is 1.038; I think you mentioned somewhere that handlebar mounted wind scaling should be 1.4, and dual mount 1.7?

Yes would be nice to have a table with wind scaling values depending on the mountig position.
In my case ISAAC automatically sets always 1.000 - I tried with "Front mount" and "handlebar mount".

I would also have a question why the combo ount has higher wind scaling then handle bar (1.4 vs 1.7)??

- While I have a cadence sensor and HR on my last ride, neither of those is showing up in the Wireless IDs page in Isaac for the profile. Do I need to enter those manually and "enable" them?

At least in my case after pairing and calibration ride it works that the sensor IDs are automatically retrieved by ISAAC when I connected the PP and downloaded the tou data from the alibration ride. Then ISAAC asked whether it should update the profile with the sensor IDs.

[ianc]

Probably obvious to others, but I just discovered the beauty of "scaling" in the ISAAC software. By default it was set to 3 seconds, which is why the power graph was looking like an almost random set of spikes. Setting it to :30 seconds and things look much, much nicer.

[ianc]

I'm starting to wonder if my PP would benefit from a hardware reset too. Went out today to try and do a power-based workout I had uploaded to my Garmin, and it was pretty much impossible given that the power readings were jumping around so much. First of all, I spent the first 8 minutes of the ride on 1000w again. After things finally settled down, I did a few laps on a dead flat course and saw my numbers all over the place in the Garmin on the 3sec display. "90w... no 634w... no 124w... no 400w..." etc...

I then took the PP out McArthur Blvd and rode the relatively flat ride out to Great Falls. The numbers were somewhat more sane after 20 min or so, but still jumping around.. when they did flatten out, it was at a reading that seemed pretty high for my effort. On the way home, at one point, things just went to zero for a while. Super frustrating... it's like a random number generator.

[Velocomp]

Please post a ride file

[ianc]

This one is looking a bit more reasonable, but still the numbers seem abnormally high, even discarding the first 1.5 miles as calibration. One thing I notice if I set the smoothing to 30 sec is that I jumped onto the draft of a big tour bus at around mile 10.5, and for some reason the PP interpreted this as a negative slope instead of a draft. Is it possible it's not getting good slope data?

[Velocomp]

I used the File/Split File at Cursor... command to get rid of the spurious data for the first 8 minutes. Corrected ride file is attached.

When drafting wind speed is substantially less than bike speed.

It looks like you were only behind the bus for 2 minutes, so 30 second averaging is dangerous.

You will see that, behind the bus (where there is drafting) elevation is decreasing...so slope should also be negative, as it is.

You're doing a lot of surging and coasting during your drafting, so while average watts are much lower, Normalized Power is high and so is heart rate. You did a HUGE pull to get behind the bus...

I think everything is working just fine.

[ianc]

Hi John,

Right, but elevation isn't decreasing. This was on flat ground.

[Velocomp]

Both your elevation and your slope sensor readings are consistently showing a slight downhill.

I'd be interested to see you repeat the ride (no drafting allowed!) and see if this repeats itself.

[ianc]

Hi John,

Right, if you look at 10.75 to 11.5 of that ride, it says I'm on about a -5% slope. In real life, that's completely flat though. The corresponding Strava activity for that ibr file is here:

https://www.strava.com/activities/548514974

I attached two snippets of the ibr file, one from 4/16, the other from 4/17. This is the same 1.5 mile stretch of Independence Ave headed eastbound to Capitol Hill. After the two snippets I posted the two full ibr files from yesterday and today.

The 4/17 data file shows a slope of about -3.0-5.0%
The 4/16 data file is better, but still showing a negative slope where there isn't really one.

Thanks!

--Ian

[ianc]

Just adding the last full ride file that corresponds to the 1.5 mile snippet from 4/17. Hopefully this will be helpful...

Thanks!

--Ian

[Velocomp]

Strava gets its data from topographic maps. The data is as good (or as bad) as the maps.

PowerPod gets its data from two separate sets of independent sensors: baro sensor and accelerometer.

You have two separate rides, one on 4/16 and one on 4/17.

The 4/17 ride, the section you highlight shows an elevation decrease of 14 feet over a distance of 0.98 miles (this is nearly level...)

The 4/16 ride, for the same stretch of road, there is an elevation decrease of 18 feet over a distance of 0.94 miles.

If PowerPod is wrong, it sure is consistently so!!!

Note that slope measurements are highly dependent on changes in bike speeds; on the 4/16 ride your speed did not change much (a good thing for slope calculations), but on the 4/17 ride you were drafting behind a bus and had huge changes in speed (causing false changes in slope). I think the 4/16 ride is the better of the two. In either case, both sets of accelerometer data show a negative slope (decreasing elevation).

After having used our products for 12 years, and becoming familiar with the world of opposing forces, I can say that it is nearly impossible to tell what is "flat". I live in Florida, in a neighborhood where there is a virtually flat loop road. Except...in truth the loop is not flat! PowerPod says that there are small changes in slope (+/- 1%) and, by God, when I look carefully, riding the same road over and over, and tune my eyes and body to look for differences...there they are!

[ianc]

Hi John,

Just wanted to let you know I finally got my "real" bike out on the road today, and got the best looking numbers I've seen yet. So I think the issue had something to do with the mount on the 26.0mm bars. Sadly, the unit has stopped talking to the computer, so it looks like I'm having USB port issues like some others here.

[Velocomp]

Sorry to hear about the USB issue. email technicalsupport@velocomp.com and Tom will take care of you.