Variable Aero & Gradient %: ??Mounting Position??

[crazymadcyclist]

Hi,

I am a new iPro user and I am having initial setup problems which i'd really appreciate some help or advice on.

Problem 1- High Variabilty in Aero result during CDs :
I have done some 36 successful coast downs across 3 Cal Rides and I seem to be getting very high variability in the Aero number across the CDs. The Aero result ranges from 0.1 through to 1.5+ across a sample of 13 usable coast downs form my latest Cal Ride. (I would have expected the Aero result to surely vary in the second decimal place but not this much). Whilst the final iBike Profile calibration for CdA, Crr, Aero, wind scaling and tilt are all within the expected ranges posted by John (Velocomp on Mon May 03, 2010 3:41 and entitled Re: Profile Ranges), due to the variability in the Coast down results i am very sceptical about the accuracy of the unit at this stage.

Problem 2- High variability in Grad% whilst on flat road sections:
Whilst riding normally i have noticed that the grad % also varies considerably on flat road sections where the road has many bends !!!!?? Noticeable as the bike leans into or out of a corner, or when the bike strikes a sharp dip in the road, or my bike rocks from side to side in a sprint or pedalling hard. Typically I have noticed that the grad % varies outside of 0% (-4% to +4%), causing weird power calculations as if i were powering up a hill or down, but the Garmin track shows a very flat gradient.

As I am at a loss to explain these variances, I am considering if the mounting position can impact the Aero and Grad% measurements.

Mounting Position:
IPro is firmly mounted on a bar extender ahead of the handle bars. Reason for this position is that i am using FSA k-wing bars with a Garmin 705 mounted on the stem (GT setup) where there is not space on the bar either side of the stem due to the shape (profile) of the handlebar. The only logical position would be to mount the iPro in front of the Garmin (inline), and on a handle bar extender ahead of the bars which is usually used for a light mount. The iPro is in clean air in this position with no obstacles. The exact position of the iBike mount bracket centre point is 1'' ahead of handlebar, on an Acor carbon fiber bar extender that is very solidly mounted (no movement or flex), and the unit is parallel to the ground, inline with the Gamin /stem.

Latest (Current) Profile:
CdA 0.35 ; Crr 0.0066 ;tilt -0.6%; Aero 0.506; wind scaling 1.448. I suspect the wind scaling is high due to the forward mounting of the iPro unit (Road surface in New Zealand tends to be rough to coarse chip hence the higher Crr - generally not as good as in USA hence my use of 66 as opposed to the recommended 55)

Weather during current Profile Measurements and Bike Setup:
I live in a rather windy part of New Zealand where we don’t have good quality road surfaces. Wind during this Cal Ride was light at 5km with gusts up to 10km/h at 4pm. (Does not get better then this for Cal Rides in Wellington, NZ). Bike is a cervelo s2 with 50mm carbon rims, and my riding position is aggressive for racing, so i would expect a lower than average frontal area.

Question:
I realise that gradient and Aero are not necessary correlated, but i can't help think that the forward position of the iPro might put it outside of design/test limits for the pressure ports eg the static one under the unit, and this causes the variable Aero result. The forward position may amplify the gradient measurement when the handle bar rocks from side to side or leans into a corner. Am i crazy and the problem is user error, or can a forward mounting position have this type of impact on the Aero and gradient% readings that is causing the high variability in CD results and power calculations? Am i missing something? Happy to provide the Cal Ride and CD data if it is useful.

BTW: I would rather not replace the handlebar with a std round bar but if it is indeed a mounting position, then better to know and downgrade the bar.

Does any iBike user have experience with a forward mounted iBike unit? Has anyone encountered such variability in Aero measurements across different CD’s?

Sorry for long post

[lorduintah]

I have had very good luck mounting the 705 on the top tube of a Trek Madone - zip ties are a very flexible choice for this type of mount and the foam backing on the bracket helps - would suggest placing a strip of rubber on the opposing side of the tube where the zip ties cross. Doing so adds a lot of friction to keep the mount from sliding to a thinner region of the tube.

Yes - tilting will be evident when the bike rocks side to side as well as making turns - where I see a drop in watts even if I am not slowing in any way.

All level roads are not level - much can be optical illusions and some can also be due to the center crest to help drain the road in rains.

Still, you bring up valid concerns and vetting them may be a challenge in some instances.

Long cal rides do help average out some issues - if you have the freedom to ride a longer out and back. Coast downs can be done both directions if it is just a moderate tailwind in one direction. Try to find a stretch where there is not a lot of cross wind, however, as this is an area where capturing good wind data is tough.

Tom

[coachboyd]

I think the variability of the coastdowns can be directly related to the way that you have your iBike positioned. During a coastdown there are dozens and dozens of measurements happening every second. Any vibration in the unit is going to cause highly variable results. You may think that the extender is solid, but the accelerometer is so precise that any movement can affect it. This can also contribute to the variable readings you are getting out on the road in terms of slope (although hitting the brakes, turning, and accelerating will also vary the slope).

My recommendation would be to mount the iBike on your stem and find another way to mount the Garmin (make sure it doesn't interfere with the airflow into the iBike's wind port).

Looking at your profile numbers, you appear to have a good looking profile in terms of cda and crr. I would have to take a look at a file to know if the wind scaling is correct. You may be able to get away with leaving the iBike mounted in the front as the power readings that get displayed every second aren't as detailed sensitive as the coastdowns.

[rruff]

Need to see your bar extender. A very rigid mount is required for the iBike.

[crazymadcyclist]

thanks lorduintah , coachboyd, rruff, I really appreciate the advice.

hi rruff : Bar extender type that I used ==> http://www.acorsports.com/products_deta ... 1&pageID=2. I made (turned) a cylindrical split sleave of OD 31.8mm and ID 22mm width 1/2" onto which i clamped the iPro to this extender (as the externders tube diameter was 22mm)

To eliminate the mounting position as the cause of my problems I bit the bullet and fitted a std round handlebar and remounted the iPro to the handlebar (as i dont have the stem wireless mount- the stem idea recommended by lorduintah is the way to go with a garmin i think). This made a HUGE difference in reducing the Aero measurement spreads/range across the CDs. Two Longer Cal rides today of 5 -6 kms and avoiding cross winds provided 20 useful CD's to derive a new profile where the standard deviation across the Aero Range was 0.15.

New Profile is:
Windscaling 2.90 (previously 1.44), Aero 1.14 (previously 0.506), tilt -0.1%, CdA 0.392 m2 (seems high?!!). Crr i kept fixed at 0.0066 and Fric of 5.9.
Question: Whilst these all seem inside recommended range, the Cda is on the higher end. I ride a fairly aero bike (cervelo s2 with 50mm carbon rims) and in an aggressive riding position so this surprises me. At my height of 1.79m (71kg) can this be a realistic CdA for a rider my size ???

Variation in Gradient % Problem Remains :
I still however get negative slopes on flat sections at times which results in Zero power calculations, whilst i am definitely putting effort in, or large power calc (600w) when i feel my effort level is low.
Question: Can this be a function of the roughness of the surface?? I have noticed the iPro "looses it" on rougher surfaces (coarse chip) where the power calc is all over the place when i analyse the tract. Is there any way to desensitise the unit to this?

Using a bar extender:
I noticed a considerable improvement in the data recorded and the spreads during CDs when moving the mounting position from the bar extender to the handelbar. I can't really think of a reason other than perhaps there was high frequency resonance on the extender that impacted the iPros measurements. This may be less noticeable with smoother road surfaces. I would recommend using a bar extender with caution as i thought i had a very rigid extender made from alloy and Carbon Firbre which clamped onto the handle bar.

[Russ]

I have a set of off brand aerobars. Early on I built a mount contraption that mounted across between the two bars out front (I ride with them wider than normal for better control).

Problem I had was the bars them selves flexed with various effort and weight loading, with the pads behind the mount point and so forth.

I have a modified extender coming off the stem and though it is actually less than perfectly rigid it works better and usually quite well.

One day I will change to the stem mount and mount the RWS I bought for Christmas

Just another source of problems to consider

Russ

[SHBike]

Rats. I think I'm having the same problems. I've got a ControlTech Carbon Comp "wing" type bar (that I love) and mounted my iBike to a carbon fiber bracket that mounts on the front (attaches to the stem faceplate). Although it's stiff, I know there must be vibration and possibly some deflection. I was hoping that CrazyMadScientist's mount might be stiffer and provide a solution. I'd go ahead and trade my bar mount in for a stem, but I'm using a rather bulky ControlTech stem (the "Unit"). If CMS's mount doesn't cut it, will a Topeak really do the trick?
.

[rruff]

I have noticed the iPro "looses it" on rougher surfaces (coarse chip) where the power calc is all over the place when i analyse the tract. Is there any way to desensitise the unit to this?

This happens to mine also with a handlebar mount, or with a custom brake bolt mount on the TT bike. When the road is rough, the grade tends to read lower than actual (low power). After 5 minutes it will sort of correct, but you will still have large fluctuations. A very rigid mount is necessary, but if you have it on the bars, then that is about as good as you will get. They changed the processing of the Gen2 and 3s to minimize this (the Gen1s were really bad) but it still goes wacky on some roads. I doubt this can be easily rectified.

[lorduintah]

Anything you mount the iBike on that removes it from the stem/handlebar by any distance will magnify the vibrations. If this also happens to be a thin rod and the like you will get even more flexion and thus more vibrations interfering.

This system has fairly sensitive accelerometers and so every wiggle it senses adds up to watt issues. If they increased the vibration damping or integration, there would be those who use these that would complain about lack of sensitivity and lost accuracy.

The best I can suggest in my opinion is to use the 5 sec recording intervals and stay away from the 1 sec - especially on the rougher roads.

Tom

[crazymadcyclist]

Thanks Tom, i will change the recording time on the next ride when the weather improves.

By way of example -- Here is a screen dump of a 16 min section of a ride where the road was flat (no steep climbs/descents) and relatively smooth-- note the fluctionations in slope. I placed some arrows at an extreme position where slope reading as much as 10% plus with a power spike. This is extreme point i realise, but if you follow the slop% line you'll note quite a significant variation in slope and therefore power on what is otherwise a flattish road. In some places the power is zero despite lots of effort going in

Will try your advice at 5 sec recording and hopefully this works.

Kevin

[crazymadcyclist]

I just dont get it... why does the slope vary so much. It is driving me nuts as i believe i am getting inaccurate power readings. Attached is of a simple 38km ride. Note the slope line is all over the place- high slopes on flat gradients. Is there a problem with the unit ??As above the recording interval is now 5sec, which accroding to the manual is an average over 5 second intervals. I have done 7 Cal ride and over 150 CDs (as per above post), so i don't think it is calibration (tilt 0.3%, Aero 1.08, scaling 3, CdA 0.34m2, Crr 0.0064 => all within the expected range for a handel bar mount position.) The Gramin which i am using does not measure such a wide variation in slope measurements (i know it is less senstive but the reading does appear more reliable).

Can any one please help me explain this one or suggest what i am doing incorrectly? Maybe this is just the way it should be as ruff suggests above. I am getting to the point where ebay is looking like an option for the iPro as it just does not seem believeable, which is rather dissappointing as i sooo wanted this to work.


rgds Kevin

[Velocomp]

Overall your iBike is working very nicely.

On the flats, by definition slope hovers around zero and one might think that the iBike slope data would always be close to zero. This is not the case.

First, and very importantly, there is no such thing as a flat road. There are minor variations in road grade even on the flattest of roads. Even here in south Florida, next to the ocean, there are minor variations in the road grade. The iBike is extremely sensitive to even the smallest changes in acceleration (gravity is a form of acceleration) and will pick up the gravitational nuances caused by undulating roads.

The iBike measures the total acceleration of your bike. Total acceleration includes hill slope and inertia (bike acceleration). The iBike has an accelerometer that measures total acceleration 16 times per second, with very high precision, to about 0.05%.

To derive hill slope we use the accelerometer output and wheel speed measurements. That is, roughly speaking, hill slope = total acceleration - bike acceleration

Deriving hill slope from total acceleration is less precise than other iBike measurements. The accelerometer measures total acceleration. Bike acceleration is, by definition, changes in bike speed. Bike speed is measured each time the wheel speed magnet goes by the speed sensor (time is measured between revolutions; speed = distance/time). At 20 mph speeds your wheel rotates about 4 times per second, so there are 3 measurements of acceleration per second. Bike speed is resolved to 0.1 mph, which at 20 mph is 0.5%. Total acceleration is resolved with accuracy of 0.05%, or 10 times better.

So, it's easy to have discrepancies between total acceleration (which is measured very precisely, 16 times per second) and bike acceleration, which is measured with less precision and only 3 times a second.

Also, there are even more issues involved related to the processing of the data signals. To do the correct "subtraction" the signals need to line-up properly in time. This is easier said than done.

The net of all of this is that there can be short-term variations in reported hill slope.

In practical terms this means that cadence changes, gear changes, stomping on the bike, brake application, backing off on pace, even gusts of wind, can show up as short-term hill slope changes. Even when on the flats you'll see spikes in hill slope due to these many factors.

Looking carefully at the hill slope variations in your ride, there are many changes in speed, cadence, and wind while in the (reasonably) flat areas. All things that will affect bike acceleration and, therefore, reported hill slope.

When I'm climbing a hill and want to know its slope I get on the hill and hold an even pace for a few seconds (an even pace means no bike acceleration). The hill slope number I see then is really good.

In the iBike software I highlight hills to see how steep they are. For example, at km 19 you had a short hill of 3.2% slope. Your big hill at km 32 is about the same slope.

But since I live in South Florida, where the only hills are from bridges, I don't even display the slope data in my graphs.

It is incorrect to conclude that hill slope variations mean something is "wrong" and that your watts data is inaccurate. As explained, we don't use hill slope in any way to determine watts.

[crazymadcyclist]

Thanks John for the Detailed explanation. It certainly helped my understanding. I think i need to ignore topic now and get out and ride and get more experience with the unit.

[rruff]

If your roads really are as rough as you say, a simple thing you can try is to lowering your tire pressure. Believe it or not, if you have bad chip-seal a pressure in the 85-90psi range will probably be fastest... and certainly smoother than running a higher pressure. Good quality tires (generally "open tubular" style) will also help. And if you are still convinced that vibration is giving you incorrect readings, you might be a candidate for one of those damping gizmos that fits in the stem... Velocomp used to sell them for the Gen1 models. It might not even help the Gen 2s and 3s though.

If you search the forums, you'll find that very few people complain of vibration issues with the Gen 2 or 3.

[Zoltan]

Deriving hill slope from total acceleration is less precise than other iBike measurements. The accelerometer measures total acceleration. Bike acceleration is, by definition, changes in bike speed. Bike speed is measured each time the wheel speed magnet goes by the speed sensor (time is measured between revolutions; speed = distance/time). At 20 mph speeds your wheel rotates about 4 times per second, so there are 3 measurements of acceleration per second. Bike speed is resolved to 0.1 mph, which at 20 mph is 0.5%. Total acceleration is resolved with accuracy of 0.05%, or 10 times better.

So, it's easy to have discrepancies between total acceleration (which is measured very precisely, 16 times per second) and bike acceleration, which is measured with less precision and only 3 times a second.

Just a non-conformist idea. Have you ever thought about putting 4 wheel speed magnets on a wheel and certainly modify the speed calculation?

It just came to my mind out of the blue that while all the traditional cycle computers are "happy" with one magnet per wheel, it would be easy to double it or maybe quadruple it. The limitation can be spokes, I mean I am not an expert at spokes, that is I am not sure whether there are spoke number of only 16, 20, 24, 28, 32, 36 or there are others not dividable by 4. But there can be some setting for 1, 2 or 4 magnets in iBike.
And my caveat is that even in this case the magnets should be precisely positioned in order to get the best outcome.

I know it may sound crazy, but the hardware part of the story is a cheap adventure, the weight part is not a deal breaker neither, the software part is the question, at least I deem so.

Just my two Hungarian forints.

[Russ]

I think I did that a few years ago with gen 1.

All you have to do (I did two magnets) is lower your circumference accordingly. I am sure the calculations need no tweeks in the iBike.

So two magnets, half the circumference and so on, not sure what the minimum value allowed is though

Russ

[Zoltan]

You got the point, simple but clever idea, but there may be some other consequences. I mean there may be some built-in threshold in iBike for magnet signals per second to indicate any malfunction of the sensor.

OK, i started to really think about it. Yesterday it was just an idea, but there are more problems with installing 4 magnets. Rims can be even and uneven, they can be laced radial and with X, 2X etc.

So not in all cases it is manageble to install 4 magnets in an even way. I have a DT rim of 20 spokes not even, but paired. No way for 4 magnets, since there are 10 pairs, the offset between the pairs is 36•.

[Zoltan]

Yes, as I forecasted the software part is the dealbreaker of the idea for the time being. 700mm is the minimum in case of Gen3 without getting error, so 2 magnets are OK, 4 are not.

[Zoltan]

Finally I figured out how to set the tire circumference to 419 mm that is 2095/5, so I mounted 5 spoke magnets on my first wheel evenly spaced. Note: the front wheel has 20 spokes. I could have borrowed more spoke magnets to mount 10, but 5 was enough for testing.

My findings in another thread: http://ibikeforum.com/viewtopic.php?f=6&t=2546