Problem(s) solved?

[smhagger707]

Sorry for the lengthy post, but I think this post (assuming I'm correct in what I'm witnessing) may be useful to other folks out there...

I have had some difficulty since receiving my iBike (first iSport, then iPro) getting a good calibration / profile and I think I may have finally figured out why (read on...).

When I first received my iSport, I proceeded to install the unit as near 'perfectly level' as I could measure (using 'clinometer' app for my iPhone). I had various problems which I think (now) may be related to the wind port not getting the best 'view' of the oncoming air. Long story, details omitted, I installed a new iPro late last year and still seemed to struggle with getting power readings that seemed too affected by the wind / speed (causing watts to drop off to suddenly while the effort remained the same and grade wasn't changing that quickly). I most recently had my iPro installed on a pair of 38 CM width bars.

Due to a recent 'Teres' injury (muscle in your back, between your lats and delts), I was advised by my cyclist / Chiro friend (after some examination of what position was aggrevating my muscle) to go back to a wider bar in effort to keep my elbows 'out' a little more than they had been. Thus, last night I re-installed my 40 CM bars-but before doing so, I was careful to measure exact position-including the existing angle of my iPro. The bike was being held very still on a known 'level' spot on my basement floor, also held level laterally by my wife while I measured the position. The result: the iPro was installed at a -1.4 degree angle (nose down).

Having seen several pictures of the new mount, and reading comments about angling the iPro slightly upwards where it made the display easier to read, I decided to install it on the 40 CM bars with a +3.5 degree tilt (nose up). So far, after the cal ride, the unit has performed exemplary and has not exibited sudden drops in power, etc like it had been doing on my previous install. The perceived effort seems to line up very well with the wattages shown now, and seems very stable over bumps, cross winds, and various efforts. I'm now actually VERY pleased (I hope it remains this way). Today I did a 41 mile ride and it seemed to work great for me.

The calibration numbers were also significantly different WRT my wind scaling numbers: @ -1.4 deg, the WS # = 1.395; @ +3.5 deg, WS # = 1.182. The aero number went from 0.437 (-1.4 deg) to 0.372 (+3.5). The CdA was nearly the same (.313 vs. .315), and may be different since I entered a slightly heavier body weight in the fast start setup.

I would be interested to hear comments on how the WS # change and the aero number change work together, with the knowledge that the unit's tilt was changed.

Anyway, after this experience, I'm starting to conclude that the iPro unit really does need an upward tilt to get a clean flow of nearly laminar air into the wind port. I think the slight downward angle may have tended to deflect some airflow down away from the port and/or created some low pressure from turbulence that artifically caused the unit to reduce the power readings as a result of the quickly changing behavior of the wind 'signal'. This quick drop off would drive me nuts as my power would fluctuate when trying to hold constant power during intervals. Thoughts? Comments?

So, bottom line: if you're having trouble with readings that seem to drop and or bounce high when holding a relatively constant effort, perhaps you should look at how the unit is mounted (nose up vs. nose down vs. level) and op for re-mounting the unit with a slight nose up position. This may (or may not) eliminate the erratic behavior for you (like it seems to be doing for me-but time will tell...).

[rruff]

That's great! But... it didn't seem to help when I tried that awhile back.

[smhagger707]

Time will tell. My friend loaned me his PowerTap wheel while he's out of town for the week and I was able to do a direct comparison between the two-they seem VERY close right now. The conditions were calm and I was only able to do a quick 12 mile route (solo) so I don't know how the wind, group riding, variety of road surfaces, etc. will impact the readings (going to do a group ride tonight). But so far, I'm pleased with how the calibration seems to be working out.

I can post more details of the comparison if anyone cares for more details...


Sean

[smhagger707]

Ok, so my excitement was premature! Power seems to match a borrowed PT for the most part on flats and climbs, into wind or calm conditions. However, I was riding today and we had strong winds out of the NW and, when I was riding south, downhill, I was nothing but irritated: the PT was showing me riding at or near my threshold power of 225W, while the iPro was nearly pegged at '0W', but then did eventually bump up to about 20W (that's correct: twenty watts). Boy was I upset...

So a little later on into the ride I decided to stop and reset the file on both units, rezero the wind (which was now starting to show a -5.1 in the wind cal screen-yes, I let the unit acclimatize for almost an hour pre-ride), and arbitrarily bump up my Crr from 0.060 to 0.065 to see if that helped. It did help bumping up the Crr and rezeroing the wind meter. My watts were still low under that one condition (wind out of the NW, me riding south), but now they were at about 50% - 60% of what the PT was reading. As a result of my bumping up the Crr, the iPro was now a little high on the climbing power readings compared to the PT (10W - 20W at times).

So my point: I would REALLY like to know which parameter affects this performance and how to tweak it:is my CdA low, my wind scaling off, or my Aero off. Which param do I tweak and in which direction? I was thinking about upgrading to the iAero and doing a calibration against it, but I'm really not sure I want to invest ANY more money into this-I'm nearly 70% invested into just buying a powertap built into a wheel already (from a custome builder).

[rruff]

Increasing your Crr was probably not the right idea. The wind cal of -5 could easily account for the error. On climbs the tilt is the most important thing to get correct, but if the climb was short it could be off a bit. Was it a long climb?

If you got good data on a flat road with a headwind or no wind, then your calibration is probably good.

[smhagger707]

Increasing your Crr was probably not the right idea. The wind cal of -5 could easily account for the error. On climbs the tilt is the most important thing to get correct, but if the climb was short it could be off a bit. Was it a long climb?

If you got good data on a flat road with a headwind or no wind, then your calibration is probably good.

Thanks for the comments. I live in the midwest so most of the 'climbs' are more like small rollers (100' or less), but some get very steep (13+% grade). The 'hill' I was on was very small and not very steep either (maybe -5% tops at the peak steepness on the downhill portion).

The thing that frustrates me is that I suspect this is tied to the unit changing temperature during the ride due to being wind-calibrated inside my garage (with the garage door open, sitting in the ambient temp for nearly an hour before the ride) and then being exposed to the sunshine when I started to ride. I think this is what throws the 'temp' of the unit off, messing up the wind cal. So do I need to recal my wind sensor 10-15 minutes into my ride, after letting it sit for an hour before the ride? It's not too big of a deal if I'm riding solo, but when I'm with a group its a deal-breaker as they don't want to (or flat out won't) stop for me to recal the unit.

Alternatively if I let the unit sit in the sunshine outside pre-ride, then the temp will be artificially 'warm' at the ride start and will come down several degrees after the first 10 minutes or so. On a day like yesterday, when the winds are in the neighborhood 20-25+ mph, it's hard to get a spot that's calm enough to do a wind cal accurately.

[rruff]

Yes, it seems to be a common issue. One thing you might try is to see if it is truly tied to temperature in a consistent way. Zero it, then but it in the frig, then put it in the sun or under a lamp and see how much the wind cal changes. Then repeat. I think it would be a good idea to allow a firmware adjustment if it's consistent. My Gen3 screws up some in the winter cold but is pretty good now.