AeroPod, PowerPod and Newton

Posted: **Fri Jul 11, 2008 12:24 pm**

This is a GREAT topic of discussion. I, too, have been pretty astounded by the correlation between my Gen II readings and the popular online power calculation tools - in general, as long as winds are calm, my analyzed Gen II results are within a few watts of the on-line calculated values for my weight, CdA, Crr, etc on the training portions of my rides. I'd been in the market for a PowerTap prior to getting the Gen II, but for the time being, I've suspended that search - I'm getting very usable, very stable results from the Gen II and like the feature set of the iBike better anyway (especially the hill profile tracking). All that having been said, I am also getting the sloping green line on every ride (mine goes up). I am pretty convinced this is coming from my riding tilt - I did my CDs and 4MR on my aerobars - that's where I'm at when I'm working hard on training rides (except when climbing), but I dont' ride there 100% of the time, and the iBike seems to be detecting the difference in slope between when I'm on the aerobars and when I'm on the hoods or bar tops, and is resulting in the tilt-derived slope being greater than the barometer-derived slope. My readings on the bike (ie, pre vs post-analysis) seem to get worse over the course of the ride. Once analyzed they're fine, but I'm looking for improved accuracy on the bike. I recognize this is a known limitation of the iBike - you calibrate to a position and go with it, but I'm looking for a mid-ground on setting my riding tilt so that I don't get what seem to be steadily-decreasing accuracy. (most prominent on rollers - hills seem to be pretty accurate even when the slope has diverged a good bit - I suspect because the slope of the actual hill so significantly overshadows the smaller riding tilt difference)

Direct questions for John and/or Travis:

1 - how significant is this problem really? I presume that the tilt-derived slope is directly responsible for on-the-bike (ie, unanalyzed) power readings, and that this divergence of measured vs true slope is skewing the readings over the course of the ride, but I don't want to be chasing ghosts if its not significant or if the problem lies elsewhere

2 - if this is a 'problem', then can it effectively be dealt with by modifying the riding tilt value (and possibly the friction coefficient that iBike uses), such that the readings are still accurate in the different positions? (I'm not looking for accurate watts readings if I'm doing 22mph sitting on the bar tops - I realize that the CdA values are sensitive to speed/position, but if I am in my calibrated position when riding fast, and on the hoods between sections (which is where the slope seems to get skewed) and am on the hoods or bar tops when climbing (ie, CdA doesn't really come into play - its 'right' for the high-speed position, and mostly negligible for the (slow) climbing position), can I expect reasonably accurate readings at speed and when climbing if I adjust the riding tilt and friction params?

3 - if 2 is reasonable, what are guidelines as regards modifying the riding tilt value? I understand there is a relationship between riding tilt and the friction coefficient that the iBike uses. If I choose to manually modify the riding tilt value to reach an acceptable compromise on the reality that I don't ride my whole ride in one position, what other values should I adjust to compensate for that? I have tried playing with the riding tilt value in a copy of my profile and have applied that to rides after the fact, and have gotten nearly-flat green lines, with what appear to be sensible power values, but I don't know enough about the math/physics calculations to know if I should be adjusting other parameters and if so, to what degree/relationship.

Bottom line - the Gen II unit really is compelling - you have my respect and admiration for improving your product so dramatically. The iBike has its limitations, and we all know and accept that (for the most part, at least!

, but there seems to be some potential for working around one of these limitations (with known caveats), and it would be immensely helpful to us if we could get some input from you guys on this.

Thanks!!

Jim

Posted: **Fri Jul 11, 2008 1:17 pm**

Fernando and others have discussed changing the Riding Tilt % value in the Advanced Profile Settings window to get the top green line more level. I tried ireducing the #, for example, going from -.300% to -.270%, and I have also tried raising the number (ex. from -.300% to -.330%). I am not sure which direction is correct.

If you have a constantly downward tilt, then decrease your tilt correction slightly. That means to go from -0.300% to -0.330%. Since you are making the number smaller (further from zero) you are increasing the tilt correction. That will bring your green line closer to zero, however it will also decrease the overall rolling resistance number. So... if I have this correct:

If you are satisfied that your post analysis wattage numbers are correct but you ride numbers are low due to tilt issues, then lower the tilt correction (further from zero) and bring your Crr number back to what it was prior to the changing the tilt correction. That will effectively increase the fric number but make the numbers on your ride look more like your post analysis numbers.

Be careful that in the process you don't create a profile just to make the numbers look good. You might find you are producing numbers that you're not capable of. If you know your FTP or have a history of working with powermeters or even have friends that have powermeters then you'll be OK because you have something to compare to. If you are completely on your own then compare your wattage numbers to some online power calculators, set a profile and stick to it. As long as you improve; I guess that's everyone goal.

Posted: **Fri Jul 11, 2008 1:20 pm**

tommyturbo wrote:How about if we keep it simple at first? If the top green line slopes downard, in which direction do you adjust riding tilt (for ex. would you go from -.300% to -.275%)? If the top green line slopes upward, would you adjust the opposite direction (-.300% to -.325%)?

Is this correct, or would the reverse be correct?

Thank you.

The KISS answer:
If the green line goes downward you need more tilt compensation: from -.300 to -.325
If the green line goes upward you need less tilt compensation: from -.300 to -.275

There are other factors that my previous post addresses.

Posted: **Fri Jul 11, 2008 11:22 pm**

Can one of you guys please describe the process of changing the friction coefficient such that Crr remains constant, in association with changing the riding tilt? From the sound of it, I thought that perhaps changing tilt would automatically induce some change in Crr (in the profile window) or such, but that isn't happening, so I'm guessing there's a little more to it than that!

Appreciate any info/pointers on this!

Posted: **Sat Jul 12, 2008 12:00 am**

Has anyone else found that their Gen II was producing riding tilt #'s quite a bit higher than their previous unit?

I think my iAero (same new technology as the Gen II) is more sensitive to wind and tilt, yet of course less sensitive to road vibration. I don't think you can compare any two units that closely. We're talking very small numbers that can produce substantially different results. Most of the numbers we're dealing with are out to the 3rd decimal place and over the course of a long ride it can add up.

Consistency is the key.

Posted: **Sat Jul 12, 2008 12:20 am**

jim_s wrote:Can one of you guys please describe the process of changing the friction coefficient such that Crr remains constant, in association with changing the riding tilt? From the sound of it, I thought that perhaps changing tilt would automatically induce some change in Crr (in the profile window) or such, but that isn't happening, so I'm guessing there's a little more to it than that!

First, make a copy of your current profile by clicking "copy". Change the Aero and Fric numbers by .001. It doesn't matter which one you change or in which direction. It only matters that they are different for each profile.

Let's look at Fric, tilt correction and Crr. Let's say they are as follows:
Weight 222
Fric 11.360
Tilt -0.461
Crr 0.0069

As a curiosity change the tilt to -0.750
Now you have"
Weight 222
Fric 11.360
Tilt -0.750
Crr 0.0040

Now "fix" the Crr by raising the fric number until the Crr gets back to 0.0069
Weight 222
Fric 14.200
Tilt -0.750
Crr 0.0069

Play with your weight and see the changes that makes. I'm 222# on the bike. If I lowered that to 190# I would have to lower my fric tremendously in order to keep Crr in a somewhat normal range. I wish I could lose the weight as easily as changing it on the screen.

Now... if you want to experiment, click Accept. Take one of your existing rides and switch to this profile after the ride. Then analyze the tilt and you will see changes in the green line and in the power produced. You can make CRAZY numbers either too high or too low. You can fix or screw up tilt error. The one thing you can't fix is a loose or moving mount that bounces around.

The other thing I'm having trouble getting my mind around is how the barometric pressure affects the climbing totals. Sometimes no matter how you try and make a profile work, it won't make it perfect because there are other variables such as barometric pressure and temperature that may come into play.

Please note, I am not suggesting that you constantly change profiles to match rides. That would be absolutely riduculous. I did make minor changes to my best set of CDs to end up with a profile that gives me reliable results on the ride that closely match those after analysis.

Finally, I'm no expert, I just play with the numbers and look at the results. Once I settled on a profile for my iAero, I won't touch it again. It is what it is.

Posted: **Sat Jul 12, 2008 1:20 am**

tommyturbo wrote:It would seem that if your "Before" slope # is -.30%, and your after is 0.00%, it would seem like changing the number from -.300% to -.270% would be logical. Can anyone verify that my thinking here is correct?

Changing it to -.60% is the correct thing to do (I think). iBike2 determined that your tilt was .30% lower than expected... so subtracting .30% from the correction you already have (-.30%) would bring things into line.

IME if you can get your slope to come out +-.1% on a regular basis then you are doing well. This doesn't have a very large effect on the readings... and any errors seem to be handled well in iBike2.

BTW... I'm more annoyed with the wind offset changing with temperature. It is also very important to get the wind scaling thing nailed. Wind (air speed) errors have a big effect on your power, and it isn't so easy to correct for them after the fact.

Posted: **Sat Jul 12, 2008 9:11 pm**

I've gotten little blips in the tilt when I've done a wind offset in the middle of a ride also... no idea why. On the road I've just put two pebbles on the pavement and used them as a reference for doing a tilt cal... but I'm real happy with doing it on the porch before a ride also... it's been +-.1% every ride except for the day when I had a slow leak in my rear tire. Regarding the wind offset, on the ride I did today it was cooler at the start (typical) and by the time I was done the wind offset was ~4. If the VC guys can figure out a way to compensate this for temperature reasonably well, I'll be happy.

Posted: **Mon Jul 14, 2008 5:03 pm**

Did you change your riding tilt by -.35%? What did your Crr come out to?

One reason I can see for VC not advocating these adjustments is because it is possible for the uninitiated to screw up their values worse than before. Just because your tilt is fixed doesn't mean that everything is good... your power numbers might be farther off than they were. The correct riding tilt should have been determined when you did your 4mile... it is a mystery why it was off. I'd advise doing your CDs again some day and seeing how they look.

Posted: **Mon Jul 14, 2008 10:30 pm**

Hey tommyturbo, it sounds like you got it right. I also use:
http://www.saris.com/wattscalculator.aspx for reference. Barring wind, it's pretty much in line and gives you another reference.

I'm not sure why sometimes the CDs give these strange results, but I'm satisfied with my numbers at .396 CdA and .0076 Crr

Posted: **Mon Jul 14, 2008 11:07 pm**

tommyturbo wrote:I have been extremely precise in doing my calibrations, and yet I consistently get super low crr values, and widely varying riding tilts.

Seems strange that your riding tilt varies so much, but I'm not so surprised that the Crr comes out low, since it is a relatively small force and may be getting effected by something else. Since the tilt and Crr are combined maybe that is the problem. When you do your CDs, Crr and slope are combined in the Fric constant. If your tilt is different when you do your 4-mile, then this will throw off the Crr. How is your unit mounted? Can you think of a reason why riding tilt would vary?

BTW... using a calculator to calibrate the iBike doesn't make sense to me. The calculator and the iBike use exactly the same equations to compute power, and will give exactly the same power if you put in the same variables. The issue is getting accurate CdA, Crr, air speed, slope, and acceleration, etc... so it gives you the right power value.

Posted: **Tue Jul 15, 2008 11:02 am**

I am failing to grasp how CdA and crr enter into this.

CdA is the result of combining wind and your aero position into one not so easy number.
Crr is the result of combining rolling friction, your weight and tilt into one not so easy number.

I write not so easy because they are dimensionless measurements; you can't touch them, lift them or push them. They are the result of other dimensions you can touch, push and measure.

If using a watts calculator you can either punch in CdA and Crr OR you can think in terms most people understand and punch in your weight, distance, climbing totals, etc. Depending on the calculator you will be asked for different numbers and it will do the best job possible as programmed to estimate watts.

So don't worry about getting your brain around these numbers, most lay people can't and probably shouldn't want to. That's the sweet thing about the iBike, it simplifes the process very nicely. That is until people like you and me get a little more inquisitive.

If you're satisfied that your profile is good, that's all that matters in my book. And that appears to be the case since you are in line with other estimators. Congratulations.

Posted: **Tue Jul 15, 2008 3:57 pm**

racerfern wrote:CdA is the result of combining wind and your aero position into one not so easy number.
Crr is the result of combining rolling friction, your weight and tilt into one not so easy number.

I would explain it differently. CdA is your air drag coefficient (how much drag you have compared to a flat plate of equal frontal area) times your frontal area in m^2. Wind has nothing to do with it... but it is used as a factor in calculating your wind drag force. Since aero drag is by far the greatest force you have to overcome on a flat or downhill road, this is an important quantity to get right. Crr is rolling resistance of your tires and bearings. This is a lesser factor but still important. Tilt and weight have nothing to to with it... but iBike uses tilt and the Fric constant from the CDs to determine what your Crr is.

On a steep climb neither CdA nor Crr are major factors... the biggest thing you have to overcome is gravity (weight, slope, speed)... so it isn't possible to use a climb to determine if these numbers are accurate.

Posted: **Wed Jul 16, 2008 2:11 am**

Ok, I just opened this tread and in a quick scan I can see that I need to chime in, but it is too late for me to tackle it tonight. As you can probably tell I'm still trying to get caught up on the boards--I'll try again tomorrow.

Travis

Posted: **Wed Jul 16, 2008 10:26 am**

rruff wrote:
racerfern wrote:CdA is the result of combining wind and your aero position into one not so easy number.
Crr is the result of combining rolling friction, your weight and tilt into one not so easy number.
I would explain it differently.

@rruff
What I'm saying is that iBike combines these values as an easier way to work with the numbers. I'm not into physiscs or mathematics, I'm just trying to understand and work with the unit and the program to come up with accurate, consistent numbers.

@tommyturbo
I too get some green lines that stay pretty flat for 4-6 miles then start sloping. I thought some of these had something to do with the ride itself. I generally ride about that distance, stop to meet some other riders, then take off again. I do nothing other than stop and if the unit goes to sleep, I wake it up when it's time to ride. I'm on eggshell smooth roads with no potholes (luckily). I wonder if barometric changes are causing some of this? I guess we'll have to wait for Travis.

Posted: **Wed Jul 16, 2008 12:26 pm**

I've noticed a little movement in the tilt error line, but it is always small. If it varies by +-.1% or so I wouldn't worry about it. The one time it wasn't small was when I spent a lot of time on a semi-flat rear tire because it was losing air and I couldn't fix it. The difference between the tilt cal and your riding tilt will be influenced by tire pressure, stem and fork flex, weight distribution, etc. Going uphill or riding in the drops makes the tilt go up a bit, and going downhill sends it slightly negative.

Barometric pressure is a possibility also. If you start and stop your ride at the same point, you can see if these match up. If the elevation determined by the pressure sensor is different then you know barometric pressure has changed. If it goes up then the iBike thinks you've gone down in elevation, and if it goes down the iBike thinks you've gone up in elevation. IME the barometric pressure doesn't normally change fast enough to cause a significant error, though.

EDIT: One other thing that might be happening. I've been playing around with a brake-bolt mount for my TT bike, and the first try was apparently not stiff enough. I kept getting negative tilt values (big... like -2.5% and variable) when riding, then when I checked the tilt it was still good. I finally determined that vibration was doing it, and a stiffer piece of metal seems to have solved the problem... or most of it at least. I've never seen this with my handlebar mount though.

Posted: **Thu Jul 17, 2008 3:41 am**

tommyturbo wrote: I too get some green lines that stay pretty flat for 4-6 miles then start sloping. I thought some of these had something to do with the ride itself. I generally ride about that distance, stop to meet some other riders, then take off again. I do nothing other than stop and if the unit goes to sleep, I wake it up when it's time to ride. I'm on eggshell smooth roads with no potholes (luckily). I wonder if barometric changes are causing some of this? I guess we'll have to wait for Travis.

I've read through the tread and actually I don't have too much to add. It looks like Fernando and Ron have helped sort out how to edit a profile in order to remove a consistent day-to-day trend in the top green line. (Top green line = the difference line you see when you click the "advanced..." button in the "edit tilt & power..." window.)

Regarding the need to edit riding tilt in your profile

Riding tilt is measured during the 4 mi out & back ride done after the coastdowns. The assumption is that the riding tilt is the same for both the coastdowns and the 4 mi ride, so if anything is happening to break that assumption, then you find yourself in the position that you need to edit the profile to remove a consistent day-to-day trend in the top green line. Some possibilities:

* A biggie: redoing your tilt cal between the time that you your coastdowns and your 4 mi ride. Do one good tilt calibration only. I'm just making sure...

* Another biggie: Removing your iBike from the mount and putting it back on between the time that you do the coastdowns and the 4 mi ride. Again, I'm just checking.

* Unlikely, but listed for completeness: Atmospheric pressure changes significantly during your calibration ride. Normally, this would be accompanied by wind, which you want to avoid in the first place when doing coastdowns. I doubt this is a problem in your case, but there it is.

* Another unlikely item included for completeness: If there were something loose in the front end of your bike (head tube, stem clamp, etc.) and it causes any play, the iBike will detect the tilt changes.

* As I've mentioned a couple of times now, I'm wondering if it is possible that if the steering bearings suddenly change position from their nominal position, it would slightly change the tilt of the stem & bars. Remember that all of the weight & torque of the front end of your bike pass through the bearings. Why would it change between doing your coast-downs and the 4 mi ride? I don't know--I'm just throwing all the possibilities out there.

* A problem that I think is more common than people want to admit: a mount that is not really rock solid. Sometimes it just really takes some patience to get the mount attached to your bike in such a way that it absolutely does not move. (2 part epoxy paste has done wonders for me in this regard since my bars and stem are not nice, round shapes.)

* When you push the center button to do a trip reset before doing the 4 mi ride, the mount could tilt slightly. Even if the mount stays rigid, the iBike could tilt within the mount some. I don't think it would change by much, but the iBike is sensitive to very small tilt changes.

* It might be that you are distributing your weight differently on the bike during the coastdowns than you do during the 4 mi ride. Actually, you know this has to be the case to some degree since you have to pedal during the 4 mi ride but don't pedal during the coastdowns. The force you exert on the pedals bears some of your torso weight, reducing the downward force on the bars from your arms. How much depends on your particular geometry and how hard you go at it during your 4 mi ride. I would say keep your effort during 4 mi ride well below your max effort to keep your weight distribution as close as possible to your weight distribution during the coastdowns.

Regarding a "Top Green Line" that suddenly changes tilt during a ride

Many of the items in the list above apply here too. I think in most cases, the tilt of the unit has actually changed. Checking the tilt at the end of a ride doesn't really dismiss this possilibity for a couple of reasons:

* It might be that in braking and jostling the bike at the end of a ride, the mount "pops" back into its original position. Again, I'm just throwing all the possibilities out there.
* The iBike display only show tenths of a percent on tilt, which is too course to disprove mid-ride tilt changes.

Some possibilities other than tilt changes:

* Like Ron mentioned, atmospheric pressure changes.
* Another possibility is that you turned a corner and changed wind direction. The altimeter reading is slightly sensitive to the wind due to the Bernoulli effect on the small hole on the bottom of the unit. Sometimes you can see a jump in the altimeter data at a point in the ride where you stopped suddenly.
* Perhaps as a battery gets low in voltage, it might cross some threshold that causes one of the sensors to change characteristics.

Again, I think the most likely explanation for a top green line changing tilt mid-ride is that iBike device actually changed tilt for some reason.

Tom, I think I've mention most of these ideas to you before in our previous phone calls and emails, but these are the best I can offer--we're not hiding any secret flaws of the iBike that could account for your issues with tilt. As unlikely as each above possible source of error might seem, you have to keep in mind that the bicycle and iBike mount & unit experience very large g forces during the course of a normal ride and it only takes a tiny change in tilt angle or altimeter data to create a "glaring" feature in the "Top Green Line". The green line is a filtered difference between larger signals, so tiny flaws in the data show up as large features in the plot.

Just keep the overall accuracy of the iBike in perspective relative to all of the sensor measurements and calculations that go into calculating the power. We've picked lots of low-hanging fruit in improving the accuracy of the iBike in the last year. In fact, we've picked some higher-hanging too, and we'll continue making improvements as the opportunities are created.

Travis

Posted: **Thu Jul 17, 2008 3:55 am**

With respect to bein' sure that the iBike mount is as rigid as possible, I found that putting Shoe Goo on the inside surfaces of the mount clamp--but not covering the double sided tape--noticeably firmed things up. It was a doddle to remove the excess Shoe Goo and have the mount look ungoo'd.

Posted: **Thu Jul 17, 2008 1:25 pm**

tommyturbo wrote:I have the stem mount on a round stem, and the only play I can detect is in the mount itself, and I have used electrical tape on top of the mount to eliminate as much of that as possible. I'm not sure how I can get it any more solid.

That doesn't sound right. My handlebar mount has no play at all. Any play would allow the unit to vibrate or rattle when riding, and that would surely screw up your results. Exactly where is the slop... between the mount and the iBike, or the mount and the stem?

Yesterday my tilt correction on a 20 mile ride was .00%. iBike2 thought it was .02% but upon closer inspection that was entirely barometric pressure change.

Posted: **Thu Jul 17, 2008 10:15 pm**

For my friend's bike with FSA Plasma stem/bars I put one layer of electrical tape on the stem and one layer on the inside of the iBike wireless mount. I used Wurth two part epoxy because it comes with a spiral mixer that has a very small nozzle opening. While someone held it close to level on the stem, I squirted the epoxy in. BTW this stem is almost square so it needed filler and this worked well. I used the electrical tape simply not to damage the stem in any way. On the bottom half, the regular foam was used. See this topic.

http://www.ibikeforums.com/viewtopic.php?f=6&t=305

It doesn't look nearly as sloppy as the pictures, and the electrical tape will allow removal without affecting the bars or finish.

Posted: **Thu Jul 17, 2008 10:34 pm**

tommyturbo wrote:What can I do to fix this? I use a pretty expensive carbon stem, and I'm not real keen on damaging it.

Do you still have your stock stem? If not, get one off craigslist or ebay and try some epxoy paste.

Posted: **Fri Jul 18, 2008 12:34 am**

Since I haven't seen this definitively addressed, what effect do changes in iBike reported temperature have on key components in the power calculation algorithm? Temperature has been reported to definitely impact wind measurement. I'm pretty sure that it also impacts altimeter performance. Does it play a role in any other key values?

Travis has noted in other threads that temperature changes are much more critical in cooler versus warmer weather. That said, I've noticed recently that my iAero will typically report temperatures in the low 80's when the sun is blocked by clouds and in the low-mid 90's (or higher -- 98 degrees today) when I'm riding in bright sunshine -- all within the same 45-60 minute ride. That seems like quite a big variation. I've seen several of these up-and-down temperature cycles during a number of rides of this same, relatively short duration. This makes me wonder what the true potential is of redoing the wind calibration several miles into a ride on partly cloudy days to allow for "temperature stabilization"??

Using a similar line of thinking, if the altimeter is affected by temperature (real changes in barometric pressure notwithstanding), wouldn't the altimeter plot in the iBike2 software be affected, impacting the "green line?" I, too, have seen my slope and altimeter plots behave in unexpected ways, and I kept thinking it was from an unstable mount or from frame/handlebar flex (i.e., slope measurement issues)...now I'm not so sure (especially since I've really cranked the mount down well -- may still try the epoxy method...).

Is there any truth to this, or am I focusing on a "red herring?" Also, I'm curious: Is the temperature and barometric pressure reported in iBike2 an average for the duration of the ride? Values from the beginning of the ride? The end?

Posted: **Fri Jul 18, 2008 3:27 am**

Temperature readings will always respond slowly since temperature sensors are typically the slowest of all sensors. Air temperature's effect on barometric pressure/altitude readings is insignificant.

The values for temp and barometric pressure are, if you're talking about temp and pressure at any point in the ride, are the temp and pressure at the given recording interval (1 sec or 5 sec)........or at least that's what I'd expect them to be.

Posted: **Fri Jul 18, 2008 12:29 pm**

Ratman wrote:Since I haven't seen this definitively addressed, what effect do changes in iBike reported temperature have on key components in the power calculation algorithm?

The way iBike is designed, temperature does not factor in directly into the power calculation. What the unit directly measures is the stagnation pressure, which is the important parameter for the power calculation and the iBike doesn't need to know the air temperature to measure it.

The wind reading is calculated from the stagnation pressure and the unit does use the measured temperature to calculate the wind. Yes, if the temperature inside the unit is higher than the air temperature then the wind calculation will be slightly off, but it is only a very slight error and, again, it doesn't matter because the calculated wind is not used in the power calculation.

The temperature measurement does indirectly factor into the power calculations because it is used for the temperature compensation of the sensors, but for this you do want the unit to measure the internal temperature of the unit including any extra heating due to the sun. The only temperature-related thing we are asking you to be careful about applies to colder temperatures: do your wind offset cal after the iBike has had time to cool all the way down to your riding temperature. Note that what we call the wind offset cal is actually a stagnation pressure offset cal, so that is why it is important for the power accuracy. The stagnation pressure sensor is temperature-compensated; however, it is not working well at cold temperatures.

Ratman wrote:Using a similar line of thinking, if the altimeter is affected by temperature (real changes in barometric pressure notwithstanding), wouldn't the altimeter plot in the iBike2 software be affected, impacting the "green line?" I, too, have seen my slope and altimeter plots behave in unexpected ways, and I kept thinking it was from an unstable mount or from frame/handlebar flex (i.e., slope measurement issues)...now I'm not so sure (especially since I've really cranked the mount down well -- may still try the epoxy method...).

Is there any truth to this, or am I focusing on a "red herring?"

I'll vote for the red herring hypothesis. The altimeter is temperature compensated. While the elevation data does factor into the calculation of the top green line, any effect from errors in the elevation reading would be slow and small compared to problems with actual tilt of the unit. I would continue to focus on the factors that influence the actual tilt of the unit, including the mount stability.

And keep a sense of perspective about it--if the magnitude of your power corrections when correcting tilt is small, don't worry about it. iBike2 does a really good job dialing in the final bit of accuracy in your power readings when it corrects the tilt. It is just the on-the-road power numbers that are at stake in this thread. If you are using the power reading to pace yourself or for doing intervals, then the effort to debug big problems with the tilt is worth it; however, if you are like me and primarily analyze your power numbers after the ride, then just do so after the iBike2 fixes the tilt.

Ratman wrote:Also, I'm curious: Is the temperature and barometric pressure reported in iBike2 an average for the duration of the ride? Values from the beginning of the ride? The end?

The reported temperature is the average temperature for the ride. The reported barometric pressure is the pressure at the time you set the starting elevation in the iBike setup menu.

Travis

Posted: **Fri Jul 18, 2008 1:55 pm**

travispape wrote:The stagnation pressure sensor is temperature-compensated; however, it is not working well at cold temperatures.

Didin't realize it was already temperature compensated. It seems like my iBike has a consistent tendency report a higher wind offset if it warms up from say 60 to 80 deg and a lower one if it cools down... ~4 Pa in either case. Is this similar to what others experience? If so, then wouldn't it be fairly simple to change the firmware to adjust the offset more accurately... or are there enough variations between units or other random variables to make this impractical?

Posted: **Fri Jul 18, 2008 2:13 pm**

tommyturbo wrote:My iBike does not move when I push the red button. It does not budge when you push hard straight down on the iBike, pushing perpendicular to the mount. It DOES move when you push hard on either end of the iBike, doing a tiny rocking motion as it appears to compress the sticky tape.

I don't think the sticky tape could be a problem... since mine is mounted with it. If anything it will help reduce vibration to the unit. If your tilt is good both before and after rides, but screwy during the ride, then there must be something caused by vibration while riding. Loose headset maybe? Some photos of your mount and unit would be helpful. You also mentioned sticking electric tape in the mount somewhere... Also, what are the magnitudes of tilt error you've been getting lately? You can calculate it for any part of your ride by dividing the the amount of slope error incured (ft) by the number of feet traveled (miles x 5280)

Posted: **Sat Jul 19, 2008 3:21 am**

tommyturbo wrote:I guess the first question is whether this movement is what is causing me a lot of problems?

Here comes the dumb quesion I pose before I tear my mount apart in frustration.

Since vibration is not an issue with a Gen II unit, do you need to use the sticky tape? I could leave it on the bottom part of the clamp, as I don't believe that has anything to do with the tape compression issue. I could use a single layer of electrical tape cut into the shape of the top clamp to protect the finish of my stem. It would even look better without the tape!

Has anybody tried this? Excuse me if it has been posted and I missed it.

Thanks for your help and advice (and counseling!)

In my case, doing away with the double-stick foam tape helped noticeably. Back in May, I used a 2 part epoxy paste to glue my 3 iBike mounts to the bars and stem. I have 3 units mounted: 1 stem mount and 2 on the bars on either side of the stem. One of the challenges that I had that drove me to the epoxy solution is that none of the 3 mounts are at spots that have a nice, friendly circular cross section to clamp onto. My stem has a rounded-square cross section shape and my bars have a diamond cross section shape. So when I used the foamy double-stick tape, it was pinched in some places and loose-fitting in others and did not result in a large foot-pad of contact between the mount and bars or the mount and the stem. The main reason I wanted to try the epoxy paste was mostly so that it would act as a filler. I got it from a local hardware store, but his is the stuff I used: http://www.amazon.com/Protective-Coatin ... 783&sr=8-3. Fernando has had luck with some other paste I believe. Unlike Fernando, I didn't add a layer of tape to facilitate future removal. I'll cross that bridge when I get there, but I figured a little creativity with a dremel tool and a hammer might get the job done.

(One of my mounts on the bars did let go of the epoxy a few weeks back, but the epoxy remained firmly attached to the bar. Since the epoxy paste is the perfect shape for my mount, I left is there and simply glued the mount to molded epoxy using a hot glue gun. That has help up well ever since. I did clean the mount before using the epoxy, but maybe I should have roughed it up a little with sandpaper so it would hold better.)

Ever since epoxy day I have seen a marked improvement in my tilt data. In most of my of my rides, there are no significant changes of tilt in the top green line. In the few rides where there is a change of tilt in the top green line, for at least half of those rides the top green line is the same shape for all 3 units. Further, I can often trace those changes of tilt back to events that happened on the road. For example, a short section of extremely rough road or a high-speed bunny hop over a speed bump. The fact that all 3 units show the same tilt errors in those rides is a very strong indication that the change of tilt was due to something going on with the bike, e.g. maybe the headset bearings or where the stem clamps onto the steering tube, and not due to the mounts.

(Yes, it is possible that atmospheric pressure changes could cause the same slow, small change in the top green line for all 3 units; however, I am talking about sudden changes of tilt that occur at the same time.)

Let me emphasize that even back when I was still using the foamy double-stick tape, I didn't have big problems with tilt errors showing up on the top green line; however, I did see some errors and they were usually different for my 3 units indicating that I had mount issues. Since I glued the mounts on, I have seen an overall reduction in tilt errors and when they do happen it looks like they are due to the front end of the bike actually changing tilt. I still do get a few cases where one unit has a top green line with different tilt error than the others which is why I suspect that the iBike can "rock" in its mount on rare occasions. But ever since epoxy day I have very little trouble with tilt overall.

So why is Ron Ruff getting good results with the foamy double-stick tape? My best guess is because he has a bigger contact pad between his mount and his stem. I think the non-circular cross section of my stems and bars was causing problems. The foam was getting pinched and there was a smaller pad of good contact that made it easier for the mount to rock on the stem or bars. In fact, it might be that the foamy tape does help get a bigger pad of tight contact between the mount and the stem for many people, so I would not make the wholesale claim that everyone should get rid of the foamy double-stick tape.

Regarding multiple layer of electrical tape: I would avoid doing that between the mount and the stem (or bars). I tried something like that in the past (actually, it was for the lower half of the mount) and I noticed that layers of tape can ooze and flow over time on hot days.

Also, I don't know that the foamy double-stick tape does anything to reduce problems with vibration for the original iBike units. One of my 3 units is the original and I haven't noticed any increase in problems with vibration due to the fact that I removed the foamy tape. My guess is that the foamy tape isn't thick enough once you clamp down on the screws to do anything meaningful for reducing vibrations.

So my best answer to Tom's question about removing the double-stick tape is, it depends. If you can remove it and find a way to get a bigger pad of tight contact between the mount and the stem, I would go for it. Epoxy paste will certainly give you a very good pad of contact, but I'm not sure that you would have good luck with just a single layer of electrical tape. I think you want to avoid pinched contact, so if your stem has a different radius of curvature than your mount, I think you would want to go with either the foamy double stick tape or epoxy. If you have a non-cirular stem cross section, even the foamy tape might not work well and you might want to consider the epoxy or some other creative solution. If Fernando tries the hot glue gun idea, I would be eager to hear about his results. It seems like hot glue would also do a good job acting as a filler for creating a big pad of contact.

Travis

Posted: **Sat Jul 19, 2008 5:32 pm**

I have to reiterate my success with ShoeGoo. I use the stickies, as well as the Goo. There was a significant improvement in tilt behavior after application of the Goo. As a bonus, it cleans up easily. From a physics point of view coupling Goo or epoxy with the stickies may help even more by narrowing the spectrum of vibration modes and frequencies that can exist in the presence of the three different media: clamp, sticky, and Goo/epoxy.
I'm not a plastics guy, but it seems if there were a suitable heat moldable plastic, it could be used in the clamp to allow each owner to custom mold the clamp surface to their stem. I can see complications with that, but I can also see how there might be solutions for those complications. Has Velocomp looked into the heat moldable plastic deal?

Posted: **Wed Jul 30, 2008 8:59 am**

7/19: Less mount rock is good--sound like that was at least part of the issue.

7/23: Personally, I can't vouch for shoe goo just because I have no experience with it. I'll take your word for it that at this point we have removed mount stability as a source of tilt problems.

The 7/24 ride looks like what you would get if you got a slow leak in your front tire starting about mile 26. I've had this parabola shape happen to me before due to a deflating tire.

The 7/25 ride after the calibration rides looks just like what you would get if you changed ride position from the drops to the hoods at the beginning of the climb at mile 3. Since the slope is straight before and after the cusp at mile 3, something changed suddenly like that. The other possibility is that you took a hit on the road and the bike or mount shifted in tilt.

7/27: Actually, the tilt error in this ride isn't that bad. The fact that tilt is off from the beginning suggests that somehow the tilt cal was off or that your bike immediatly "creaked" into a new tilt when you put your weight on the bike.

--------------------------------------------
Patient: It hurts when I do this.
Doctor: Well, don't do that.
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Please don't take this wrong, but is there any chance that you are trying things while your are riding just for the sake of seeing how they affect your tilt? I'm only asking just in case--I have friends who like to test the boundaries of gadgets just for curiousity's sake--there's nothing wrong with that. There are lots of things you could do to shift your weight or take uncessary hits just to see what the effect is. I'm not accusing you of anything and I doubt that you are doing this--I'm only bringing it up as one idea because I'm struggling to reconcile why you seem to be having more difficulty with tilt. In case you aren't doing this already, bring a new mentality to the game and try doing everything you can during the ride to eliminate anything that might be affecting the tilt of the unit.

It seems like the iBike is going nose-down on you most often. Maybe another debugging exercise you could try would be to use packing tape after you have mounted your iBike and put a strip around the iBike and stem to keep it from going nose down. I'm talking about a loop of clear packing tape around the side of the iBike towards you--maybe even covering the lower 3rd of the display since the tape is clear--and under and around the stem. The goal is just to see if you have a loose-fitting iBike/mount interface and to remove that play. Try it for a few rides before you jump to conclusions.

Also, I have to bring up the possibility that there is something weird going on with the bike. Is there any chance that you could bring yourself to ride a different bike for a couple of weeks? It would be a very good debugging exercise at this point. If you are frustrated enough making things work on this bike, it is worth trying.

That's it--that's everything I can think of right now--plus all the other stuff I've already written in this thread.

Travis

Posted: **Wed Jul 30, 2008 1:34 pm**

I don't sit on my bike before doing a tilt cal, and it is fine. Just thinking... on many bikes there is ~.5% tilt offset that is caused by weight distribution and flex of various components... so it wouldn't surprise me to see +-.1% or so variation (maybe more) between uphills, downhills etc. But IME this cancels out pretty well on a ride that ends where it starts, and it isn't a large error anyway. If your tilt just changes, then it seems likely that some part of your bike is shifting to a new position... and the headset comes to mind. You could try pushing up and down on the bars and rocking the bike fore-aft with the front brake locked and see if you can get the cal to shift. At least then you might have an indication of the cause, and there might be a way to fix it.

AeroPod, PowerPod and Newton

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