Systemic tilt error

[racerfern]

Alan,

Make a copy of your profile and play with that so you keep the original intact. In the edit profiles advanced screen change the tilt say from -0.400 to -0.500. I 'm just picking numbers, you pick yours. Before you do that look at the Crr number just below and note it down. Now change the tilt and you will see the Crr number drop. Now raise the fric number so that the Crr comes back in line.

Click accept. Now, even though this may be the profile for the ride, you have to "Switch to a profile after the ride". Click that and it will use that profile. Go to the Analyze tilt and power screen and look at the change.

A "typical" negative tilt is about -0.500. I've seen them as high as -0.300 and as low as -0.800 in discussions with other users. Hope this helps.

[alback]

Thanks Fernando,
I will give it a try.

How does this stay as a calibrated change within the IBike/iaero though? His The tilt is calibrated or checked before we ride? I thought the graph that Iodaniell was showing was a raw file downloaded from his iAero? This means his power readings were much more reliable during the ride.

Exactly what I am trying to achieve.

Alan

[racerfern]

You'll need to try a couple of rides. Once you have a profile that you think will work, you can try it on a couple of recent rides by using the "Switch to profile" option.
Then check the tilt (green line). If it's closer to level then you are headed in the right direction.

If you are satisfied with your new profile then "send" it to the iBike and it will become the active one. Ideally, you'll see before and after results that are more in line.

Let me know...

[iodaniell]

You'll need to try a couple of rides. Once you have a profile that you think will work, you can try it on a couple of recent rides by using the "Switch to profile" option.
Then check the tilt (green line). If it's closer to level then you are headed in the right direction.

If you are satisfied with your new profile then "send" it to the iBike and it will become the active one. Ideally, you'll see before and after results that are more in line.

Let me know...

The processes that Fernando outlined are the ones that I used as well... except, I'm a little more daring in that I send my new profile to my iBike Aero right away, ride with the new profile for awhile, and adjust as necessary.

His method is safer. Mine could get you in trouble if you stray too far away from your baseline numbers derived from your CDs and O&B ride.

My legs are feeling fried this week, so my next ride long will be on the flats and half as long--maybe 50 miles or so. I'll post my results, unless something else happens to mess up their validity.

[alback]

Well it took some time but I kept adjusting the tilt and Fric (to return to the original Crr value of .0051).

My starting profile numbers were: and this is my best profile to date.

Original Modified
Aero 0.368 0.368
Fric 6.143 4.455
Wind Scaling 0.855 0.855
Riding Tilt -0.117 0.05
CdA 0.43 0.43
Crr 0.0051 0.0051

My tilt/power analysis always has the tilt rising above the elevation, hence the 30% high average and high watt readings while riding. Using this corrected profile it came close but showed the tilt above and below the elevation. Best yet without using Tilt&Power analysis.

I uploaded the revised profile to my iAero and installed it on the bilke. I checked the tilt at my normal calibration spot, it read 10 where normally it reads 0.3. I did a tilt calibration before heading out. My first reaction was the watts read lower on flat sections and they quickly subsided as I crested hills. It still read upwards to 300 and 400 in the climbs of 10 - 13 slope. I am not that strong in hills so this seems high still.

When I finished my ride, I checked my tilt at its calibrating spot. It still read .03. I uploaded my ride to iBike, and looked at the Tilt and Power analysis. The tilt still rode upwards above the elevation, but not as much as before. Another 10% adjustment was required according to the tilt/power analysis. The new figures would need to be.

Modified New Mod
Aero 0.368 0.368
Fric 4.455 3.7
Wind Scaling 0.855 0.855
Riding Tilt 0.05 0.128
CdA 0.43 0.43
Crr 0.0051 0.0051


So now I am not sure if I adjusted correctly as this worked on yestedays ride but not todays. The new tilt and Fric values now look strange. This does prove that to change the profile it affects how the iAero displays watt values for the next use.

So why is this left to the end user? I know I am guessing at adjustments to the profile numbers. The analysis of the tilt/power in iBike2 has already achieved this? It should let the user know what adjustments were made, and offer the end user to select an update to the profile values, (after saving the previous one first).

I have done 5 calibrations, and looking at my green line after each ride, they are not correct. You guys have started something that will make the iBike/iAero provide more reliable numbers which is great and it is what I have wanted all along.


Alan

[racerfern]

Alan,

You are definitely headed in the right direction. However, I am surprised that you have a positive tilt correction and possibly still needing more. Are you on a TT bike or do you have some really weird setup? All your numbers look like they should work except that your tilt is so positive. Of course your Fric then has to get skewed to keep Crr within reason. How about posting a picture of your bike or a picture of your setup.

As far as being left to the end user, I think that most users here have taken this to the next level of thinking. The iBike and the software have improved dramatically since the first version. Somewhere down the pike the conversations and suggestions that run through here will make it into the next software version.

[travispape]

You are definitely headed in the right direction. However, I am surprised that you have a positive tilt correction and possibly still needing more.

I have seen a few people with positive riding tilts, but normally riding tilt is negative for sure. A couple of things that would tend to cause the unit to go nose-up would be the fact that there is more weight on the back tire normally and if it squishes more than the front the nose will go up. What size tires do you use? The other thing is if the headset is loose, putting your weight on the bike might make the whole fork/tube/stem assembly rotate nose-up a little. Combine that with a stiff stem and a light rider and maybe you end up with a positive riding tilt.

Whatever the case, it is what it is and by watching the trend on that top green line you can certainly dial-in your riding tilt as you are doing. Just make sure you don't generalize too quickly on a couple of rides. There is always a chance of outlier events and if you get a couple of them in a row and assume they are the trend you can get frustrated.

Also, make sure you a are doing good tilt calibrations, rotating the bike 180 deg getting the tires on the same spots on the ground and without using any wedges or anything under the front wheel.

Travis

[alback]

Thanks for the replies,

My bike is a simple roadie, with a 7 degree upward stem. My iAero mount is a high angle and physically tilts slightly upward.
The tires are 700-25 on the rear, and 700-23 on the front. This is about a 1/8" difference in diameter. I checked my tilt/power analysis files for rides before the tire change, they were upward direction also.

I tend to slide slightly forward in my seat as I ride. Sight of the front axle is forward of my handlebars and I keep my tire pressure at 110.

I have attached the bike photo.

Trek photo

iBike Trek photo 01.jpg (180.26 KiB) Viewed 12424 times

Here are the 2 screen shots,
pre tilt adjustment,

Tilt Pre adjustment

July 19 tilt analysis.jpg (435.01 KiB) Viewed 12420 times

tilt adjustment .05,

tilt adjustment .05

July 26 tilt analysis tilt .05.jpg (473.1 KiB) Viewed 12421 times

I have a screen shot of the adjustment using .328 tilt, which came very close to the tilt/power analysis, but I have hit the limit on uploading pictures.

Following the last adjustment the power&tilt analysis came really close, however the tilt line doesn't match the elevation. I felt the watt values during the ride had improved.

Alan

[racerfern]

Wow. Your bike looks perfectly normal. You're going to have to increase the tilt correction a bit more. Try .1 degree more then you can fine tune. Again, don't forget to keep the Crr in line.

Perhaps some Velocomp people have a better idea.

[alback]

I have already run the tilt at .328, on my latest ride, it seems to level out the tilt/Power analysis. The average watts is very close between bike data and analysis results. I just think the Fric number of 1.80 is strange and there are times when the tilt gets below the elevation line. I will post the picture when I get to my computer later today.

There were some things I should address that Travis brought up.

1. Unless my stem is too short (90mm), and that I never see the front axle from behind the handlebars, I believe I ride forward on the bike.
I will attempt to do a tilt while sitting on the bike in my normal riding position to see the outcome.
2. I am not experienced enough to know if my head set is loose, however if a loose head set makes noise or allows movement, I haven't heard or felt any.
3. I set up a specific area for doing my tilt calibration, there are marks on the floor for both wheels, when placed on these marks the bike is vertical and I can rest it against the wall to hold perfectly still. I wait for the unit to settle before pressing the button to finish the 180 turn.
4. I do tend to stand up on the pedals early in the ride, perhaps the swaying side to side has an affect on starting the tilt in the wrong direction?

thank you Fernando, and Travis for your input.

Alan

[travispape]

If you are getting consistent results with your tilt correction, then there is nothing to worry about with your positive riding tilt. It is what it is. The smaller fric number is merely a reflection of the positive riding tilt--nothing to worry about on that front either.

Just a little concern about your tilt cal procedure. It is probably fine to lean the bike against the wall for stability, but just make sure the bike is very close to verticle. A little lean on the bike will affect the tilt measured by the iBike.

Travis

[alback]

Here is my latest ride with the tilt pushed up to +.328. The Crr is maintained at .0051.

Analysis using Original Profile Values

July 27 tilt analysis org profile tilt.jpg (419.45 KiB) Viewed 12352 times

Analysis using tilt +.328

July 27 tilt analysis tilt .328 2.jpg (450.74 KiB) Viewed 12353 times

You can see that although it meets at the end, the slope and elevation only meet at the ends. Still this is the best I have to get the iAero to post more realistic power and other values as I ride.

I hope the software makes this correction to our profile in the next release.


Alan

[racerfern]

Alan,

I think you need to check a couple of things. Even though when you get back after your ride the tilt shows the same, I am concerned about movement. The main reason is in the first set of graphs there are quite a few high vibration areas. I have never seen that with an iAero unless you are EXTREMELY rough roads. So assuming you are on regular roads there might be movement in the stem or the mount. Something is not right with your setup is my inclination.

Also I agree with Travis that leaning against a wall might not work so well. I merely hold my bike as steady as possible in both directions. But I think your focus needs to be on your mount or maybe your stem.

[coachboyd]

The tilt does look a lot better on that. I have found I get the "hammock" type difference in my rides sometimes. . .where it slightly goes down in the start and then rises later. I think this could be to stuff settling in the ride and the ibike slightly tilting upward. Notice how it only goes down to 30 feet difference and then takes a long way to come back. Changes these small are unlikely to give you noticeable differences from actual wattage on the bike.

The one thing I would be concerned about is how different your aero and cda measurements are. Based on your numbers, it looks like your wind scaling is .856. I remember Richard saying you want that to be as close to 1.0 as possible. I don't know if you did this ride with another power meter, but adjusting the wind scaling would lower your cda and lower the average watts for the ride

[rruff]

I hope the software makes this correction to our profile in the next release.

I don't think that is such a great idea. It is easy enough to tweak the tilt value if necessary... but the mystery still remains as to why it is off to start with. The CDs and 4mile ride do a good job of calibration unless something is wrong. If you can't get a good cal using the procedure, then it would be good to find out why.

[alback]

Thanks to everyone, good points.

Fernando:
The earlier graphs - July 19 would have been using the iBikePro as the replacement IAero hadn't arrived. The recent graph (July 25) was using the iAero. Our roads vary from poor to good chipset and about 20% on smooth pavement, on the July 19 ride I had some dirt/gravel sections too.

I will take a close look at the mount, but the unit would show a movement at the end of a ride. I sat on the bike and checked the tilt. It measured 0.3% empty, and 0.1% with me in a normal ride position.

As to the tilt calibration, when I marked the floor, I used a home laser to make sure the bike is vertical. The tilt is affected by side to side movement, it has to be a better calibration to hold it rigid in each direction.

CoachBoyd:
If I adjust the wind scaling from .855 to .856 is that a significant difference? Or is this the start to tweak that hammock effect, and it may require more? I tried the .856 and no other values changed but I didn't do a ride to see a result. I will look at that.

Ruff:

We may be able to tweak the numbers ourselves, but our adjustments are guesses. This product is the result of extensive formulas in the unit (firmware) and the software which brings out the best reliability to the numbers. I am looking for, and others have said the same is the same reliable numbers as I ride. Seeing the elevation and slope arrive at similar values at the end, provides me with more reliable values as I ride. The software can perform these tweaks much better than me.

The key is our profile, and if you look at the values below, my profiles are all over the map. These were using the iBike and last set using the IAero.

IBike Profiles 2008

Profiles 2008.jpg (287.43 KiB) Viewed 12303 times

Looking at these numbers it is difficult to believe this is the same bike, same rider and the only difference is the roads, the mount and unit. These profiles were taken with equal care, more experience in the latter, and the last set with a cadence sensor. These were derived from 5 actual calibration rides, where extreme coasts being included or deleted.

On a given ride, each profile can affect my average power 30% and more, which means the power reading on my unit would display those same higher values. If the software could adjust the iAero head unit to read closer to the true values, then that is where I want it to go and I don't see why you wouldn't either.

In essence, the software can adjust the profiles so the unit reads closer to our true performance. This excercise started by Iodaniel and fernando, is in the right direction.


Alan

[coachboyd]

CoachBoyd:
If I adjust the wind scaling from .855 to .856 is that a significant difference? Or is this the start to tweak that hammock effect, and it may require more? I tried the .856 and no other values changed but I didn't do a ride to see a result. I will look at that.

Alan

The wind scaling won't do anything to correct your tilt, it's just making sure that you are getting consistent correct power numbers. If you changed it from .855 to .955, you will see what it does to your cda measurement. Now if you apply that to a ride you can see the difference in watts it makes. Of course, I have no clue what the right wind scaling number is for you, but I do know it should be as close to 1.0 as possible, and if it was off it might be a sign that you didn't have everything calibrated at the start of your coastdowns and 4 mile ride.

I think people look over the wind scaling number and it can be the most important number. If the wind is off during your 4 mile ride, that will directly influence your cda values, which has the biggest influence on wattage on flat terrain.

[travispape]

Actually, a wind scaling of 0.855 is very typical. Yes, you should get a WindScaling close to 1, but were talking a range of maybe 0.7 to 1.6. I've had a unit that went from 1.2+ on the bars to something in the 0.8's just by moving it to a stem mount. It is all about the geometry of the system.

[travispape]

Looking at these numbers it is difficult to believe this is the same bike, same rider and the only difference is the roads, the mount and unit. These profiles were taken with equal care, more experience in the latter, and the last set with a cadence sensor.

Do keep in mind that according to your notes that there are several things that varied that should cause differences in your calibration results.

Crr will be affected by the new tire, the roughness of the road, the tire pressure each day, and other factors. It is good to choose a typical road for your riding.

CdA will be affected by the clothing (jacket one day), the backpack, and drops vs. hoods. In fact, these are all pretty big factors.

WindScaling will vary unit to unit and you would also expect big changes depending on mount location. Riding drops vs. hoods even makes some difference since your body position affects the stagnation pressure at the nose of the unit. Another important factor is the quality of your wind offset and whether you did it at riding temperature.

RidingTilt depends on mount location, drops vs. hoods, weight, the quality of your mount, size and pressure of tires, and dozens of mechanical details of your bike.

(I can't help but think that something changed with your bike to cause your riding tilt to become a positive number. I don't really have any advice on how to address that, but as long as you do good tilt calibrations, you shouldn't be seeing as much variability as your spreadsheet shows. However, the key is repeatability. If it continues to be a positive riding tilt from day to day, don't worry about it.)

In sum, there are several reasons that could account for the variability of your calibration results. Another "randomizer" in all of this is the wind. Gusts are your enemy when doing coast downs.

And then finally, we get down to the learning curve. Some of the varibility in your results is just you learning how to do things,just don't attribute all of the variability to the learning curve. Yes, it is desirable to automate all of this more in the future and we are working to that end.

These were derived from 5 actual calibration rides, where extreme coasts being included or deleted.

In general you don't have to worry about deleting extremes. Coastdowns are weighted according to their quality in the analysis. If you know something went wrong with a particular coast-down, go ahead and delete it if you can keep track of which one it was.

Travis

[rruff]

On a given ride, each profile can affect my average power 30% and more, which means the power reading on my unit would display those same higher values. If the software could adjust the iAero head unit to read closer to the true values, then that is where I want it to go and I don't see why you wouldn't either.

You are talking about tweaking the riding tilt number... yes? If your tilt is off by a *consistent* amount, then it is very easy to adjust this manually. If it is not consistent, then you need to find out what is wrong. If it differs a lot from your calibration values, then you also need to find out what is wrong. The software can't do this.

Like coachboyd, IMO the wind scaling, wind offset, and CdA are most important for getting good power numbers in most riding situations. The riding tilt has gotten a lot of attention because we have a very good check on this (ie we can tell when it is off), but there is no way we can look at a ride file and tell if the aero stuff is good.

One way I've found to check my wind scaling is to do repeat back and forth runs over a course that is fairly open, light wind, little or no traffic, and allows me to ride in the same spot on the road in both directions. Make sure the wind offset is zeroed... temperature changes will effect it... in fact it is best to do this when it is cloudy, but at any rate make sure it stays zeroed. When you analyze the wind in iBike2 it should be zero or very close to it. The wind scaling is determined during the 4 mile, but things like traffic, buildings, variable wind, and temperture can effect the results.

Again if it is way off, then it means your cal is no good, and it would be best to figure out why or at least redo it.

[Russ]

I know you guys don't like a dashboard full of wind dragging instruments on your bikes (I do .

But... I find that my catseye showing speed and the iBike displaying wind speed give me a good
eyeball on the quality of my wind scaling. With no wind, the wind or 'air speed' should match the
bikes speed.

Another benefit is being able to tell when I have a head or tail wind condition.

If the iBike made it easy to display both simultaneously , I would can the catseye.

Russ

[rruff]

Unfortunately, a day without wind very rarely happens around here...

I like your suggestion... but I'd like having the ability to display *relative* wind even better.

[Russ]

I live in a windy place too

With my catseye showing speed and my iBike showing wind spd (air speed really);
Some wind calibration accuracy checks I use:

If some flag or hanging cloth streamers are still, no wind so check for a match.

If my preestablished downhill coasting speed is normal for no wind conditions, check for a match
(this can be post checked in iBike software view and in the max wind/bike speed values if only one hill)

My prodesign aero bars whistle at 30KH no wind condition. If they whistle when my iBike shows
30KH air speed, things are looking good for that speed (unless a cross wind is disturbing the whistle)

I think looking for both a mid and high speed match for air/bike speed's is necessary to be sure of
good wind scaling value.

Russ