# Low Key Montebello hillclimb with SRM & Ibike data



## Francis Cebedo (Aug 1, 2001)

The Low-Key Hillclimb series is here. 7 climbs in 7 weeks

http://www.lowkey.djconnel.com/

It's like a low key race among friends. Some ultra-fast guys and climbers of all speeds and ages.

I joined in the festivities today. This was my first timed climb since my injury so I was happy to be out there. My time today was 37:50. I'm very happy with that since my record is at 35:xx with a lot of training.

There were about 30+ people. The King of the Mountains was Tracy Colwell at 26:+ minutes. Child prodigy, Menso who is sixteen years old and continues to impress at 29:00 minutes. Last year, his best time was 35 minutes.

Anyway, the interesting thing for me was I was actually working. I was product testing the SRM Power meter and the Ibike computer. Both devices measure power but take very different approaches. The SRM uses sensors on the crank. The Ibike measures all outside forces and computes your input powere by adding up all the opposing forces. 

Prices are very different too. The SRM is the gold standard in power measurement tools. It's got gold standard pricing too at $3400. The Ibike is $400. It is a small, self contained unit that's easily moved from bike to bike.

So the Ibike is interesting. It's too good to be true but if it works, it brings power measurement to the hands of many people.

So here's the data. The SRM Graph is on top with power at the green line. The Ibike graph is on the bottom with power at the yellow line. Note that the SRM includes a heart rate monitor (red line). The Ibike includes an altimeter with current altitude denoted by the orange line.

So the power lines are eerily similar. The Ibike seems to work!! Note that this is the core strength of the Ibike... hillclimbs. Wind, draft, drag forces do not interfere too much with the power calculations.

Note that this is my first major ride on the Ibike. I'm hearing it's major issues are:
- rough roads
- riding in a pack/draft
- time trial position
- high wind situations

I'll spend more time with these situations and do the same side-by-side power analysis with the SRM.

francois


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## estone2 (Sep 25, 2005)

What did they say in terms of Avg Watts/Peak Watts etc? Numbers vs Graphs?
I'm not much of a visual person


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## shawndoggy (Feb 3, 2004)

estone2 said:


> What did they say in terms of Avg Watts/Peak Watts etc? Numbers vs Graphs?
> I'm not much of a visual person


'zactly. Can you dump both sets of data into cyclingpeaks and give us your avg/normed power for the durations?

The outputs look to be pretty wildly different in the 14:xx - 21:xx range too... the SRM showing a minimum wattage of about 160-170, and the ibike down to 120ish...


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## djconnel (May 7, 2006)

I extracted data from the posted plot, and plotted the difference between the SRM and iBike, for speed and power, versus the SRM values, and deriving the mean and standard deviation of the difference. I used a 0.25 minute time offset, determined to minimize the rms difference in times.

The plots:
http://www.djconnel.com/cycling/SRM_vs_iBike.pdf

"Francois" claims the SRM wasn't properly calibrated, and was reading high. This is consistent with this, even given a few % of drivetrain power loss, which is included in the SRM data but not, obviously, the iBike measurement. Given this, and given the significant (1 mile) flat portion of the climb, the agreement is striking. Certainly i-Bike has to be considered as a powerful option in gaining the benefits of power metrology for training and fitness assessment.

Dan


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## michaeltop (Feb 2, 2006)

I like that Nokon cable routing , almost makes it look campy like!

can you post another ride comparision?
cheers,
Topo


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## Argentius (Aug 26, 2004)

That was the first thing I noticed, too! I thought "Woah, Shimano ain't quite so ugly with the cables not sticking every whichway." 

I, too, would like to see some basic chart-like numbers of the differnces, for given intervals or whole-ride averages or whatever. I had a bit of a tough time with the graphs.

Thanks for taking this on, Francois.


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## djconnel (May 7, 2006)

*histogram of differences*

I put a histogram here.

BTW, I miss Francis at the climbs: Francis: Bohlman will be a fantastic test-bed for the low-velocity limit of the power comparison... Will you be there?
http://lowkey.djconnel.com/


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## Argentius (Aug 26, 2004)

okay, mister engineer, I haven't had my mathstats class yet.

If I'm remembering this right, this shows iBike averages about 30 watts less than the SRM, with a standard deviation of about 12 watts?


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## wavylines (Jan 27, 2006)

Argentius said:


> okay, mister engineer, I haven't had my mathstats class yet.
> 
> If I'm remembering this right, this shows iBike averages about 30 watts less than the SRM, with a standard deviation of about 12 watts?


Yep. The 30 watt diff. is (according to djconnel's comment) due to the SRM being out of calibration. So the main story is the 12 watts std. dev., or about 4-5% of the ~250 watt average. IIRC, the SRM claims accuracy to 2%, so a 4-5% difference between them looks good for the iBike. At least on this ride, this bike, this rider, n=1 etc. etc.


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## Guest (Nov 6, 2006)

So this Ibike thing, it gives you a synthetic wattage based upon measured speed, measured gradient, calculated co-efficient of friction, calculated co-efficient of drag, and ride weight?

I mean you can do this post ride with an excel spreadsheet if you have a downloadable computer with an altimeter.

It sounds like a cool idea to be able to see the wattage though real time if you like. What would be a cool idea is to have some kind of wind speed instrumentation on it so it could take into account head wind and drafting. If it did I would get one. A powermeter is too costly for a recreational racer like me.


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## Argentius (Aug 26, 2004)

*Uh, dude?*



the_rydster said:


> So this Ibike thing, it gives you a synthetic wattage ... What would be a cool idea is to have some kind of wind speed instrumentation on it so it could take into account head wind and drafting.


It has a wind gauge. That's why the Ibike is revolutionary -- it actually takes into account all of the forces opposing a rider. It has an altimeter, an inclinometer, the wind gauge, and a wheel sensor. Then you do a coast-down to figure out rolling resistance, etc.

The question is how well it does all of this. So far the answer seems to be "pretty okay in fairly controlled conditions, less well in rapidly changing condtions, not at all on a stationary trainer."


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## Guest (Nov 6, 2006)

Argentius said:


> It has a wind gauge. That's why the Ibike is revolutionary -- it actually takes into account all of the forces opposing a rider. It has an altimeter, an inclinometer, the wind gauge, and a wheel sensor. Then you do a coast-down to figure out rolling resistance, etc.
> 
> The question is how well it does all of this. So far the answer seems to be "pretty okay in fairly controlled conditions, less well in rapidly changing condtions, not at all on a stationary trainer."


Ok I did not realise  !

I just had a look at the website. I see it also contains an accelorometer, not sure what this does though, it says it is x-y?

It looks like you just input an estimated co-efficient for drag. Obviously this will change depending upon if you are in the drops of not. I could live with this I guess, if I am at high speed I am normally in the drops anyway, so I would use that co-efficient, unless I am drafting though, and I guess this tool is probably more useful for individual rides with intervals etc although given the wind speed sensor it should be ok for drafting. 

Changing road surface coefficient of friction should not be a really big deal unless you ride cobbles, your weight will not change significantly over a ride even with a couple of water bottles, they are only a small percentage of your total mass.

Any idea how it copes with buffeting winds? I was thinking about side winds, but in theory it is only the wind resistance/velocity axial to the bike which matters.


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## michaeltop (Feb 2, 2006)

*dash problem solved on ibike*

i tested the firmware fix for the problem that was reported on rough roads. They have fixed that big time. No more dropped calls!

topo


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## djconnel (May 7, 2006)

the_rydster said:


> Changing road surface coefficient of friction should not be a really big deal unless you ride cobbles, your weight will not change significantly over a ride even with a couple of water bottles, they are only a small percentage of your total mass.
> 
> Any idea how it copes with buffeting winds? I was thinking about side winds, but in theory it is only the wind resistance/velocity axial to the bike which matters.


Well, I disagree differences in rolling resistance are insignificant -- I suspect they can easily account for a 10% variation in power on rough roads.

For drinking a bottle -- it doesn't affect overall mass, of course, filling a bottle does. But I'm more worried about ride-to-ride variation in mass, which can easily be 5%. That's a 5% error in climbing power, unless one weighs the bike + body every ride. Variation during a ride is more like 2%, I suspect.

For side winds -- yes, these will cause an error.

Dan


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## b987654 (Aug 18, 2005)

do you have to calibrate the i bike meter every ride? and can the ibike be mounted on the stem? THanks


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## team_sheepshead (Jan 17, 2003)

michaeltop said:


> i tested the firmware fix for the problem that was reported on rough roads. They have fixed that big time. No more dropped calls!
> 
> topo


Please see my recent posts at the bottom of this thread: http://forums.roadbikereview.com/showthread.php?t=75732

To put it briefly, my iBike has recently been showing very high and inconsistent power readings due to road buzz. Controlling for just about every other factor, I've attributed the problem to increased tire pressure. I went from ~105psi to ~115psi and the iBike went haywire. I just downloaded the firmware update and will try it again tomorrow. I hope it works.


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## team_sheepshead (Jan 17, 2003)

b987654 said:


> do you have to calibrate the i bike meter every ride? and can the ibike be mounted on the stem? THanks


No. Although you should be aware of your total bike/rider weight and update that on the unit as necessary. Also, I've been told to zero out something called "offset" before every ride--I think it relates to wind speed. (I just click two or three buttons.)

I don't believe it can be mounted to a stem.


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## VeloPower (Dec 2, 2006)

iBike Pro Review.
I have an iBike and a Powertap and I have been comparing them side by side and here are the conclusions:

The iBike is NOT useful for training it is strictly a gadget and here is why:
The iBike is not at all accurate real time. MOST OF THE TIME the iBike is significantly (often 50-100 Watts) high or low. If you have ever (seriously) trained to power then you know that just a few watts (let's say 10% of your max hour of power; 25-50 Watts unless you are Lance Armstrong prior to 2006 (Sorry Lance!) ) high or low makes a HUGE difference when you are trying to hold a set wattage for a while; and with this much error, you will have a horrible workout with the iBike. Sure, the marketing folks at iBike like to point to average watts and say that it is accurate but this is not relevant to real time training on the bike. The iBike is USELESS as a real time wattage training tool. There are also several other serious problems for example:
1) What about WINTER time ??? YOU CAN'T USE THE iBIKE INDOORS ! You will have to buy 2 power meters anyhow so what good is it ?
2) The iBike is completely inaccurate on the slightest rough road.
3) Turns. The iBike is flat out wrong when going through turns.
4) Drafting. Yes, the marketing team at iBike want you to think that it is more accurate when in bigger packs. BS ! I have tried it. It is WRONG WRONG WRONG in the draft.

Conclusion, If:
1) You just like gadgets and don't care if they really work.
2) You never turn.
3) You never ride in the draft.
4) You don't mind a significantly wrong watts readout while you are training.
5) You are a sucker and believe all of the BS that iBike and those that stand to benefit from iBike sales tell you.
Then go buy an iBike -- There are plenty of them for sale on eBay from the people who have gotten suckered into buying one and can't get a refund.
Otherwise, if you are smart, go buy a PowerTap, SRM, Ergomo etc...


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## djconnel (May 7, 2006)

This was a good review. However, the data I presented, which compared error to power, and a histogram of errors, was not explicitly averaged: it was taking from Francis' posted plots. Could you post data to show the problem in curves and the problem on transitions between smooth and rough roads, for example?

thanks,
Dan

P.S. WRT the trainer problem: this is a software problem only: it would be straightforward for iBike to support trainers, if the trainer speed-load curves for popular, repeatable trainers could be installed. For example, the data for my Kurt Kinetic Road Machine is available at the Kurt Kinetic website.


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## feathers mcgraw (Mar 15, 2002)

Velopower: From what I understand, the iBike smooths its data a bit more than the Powertap. So when you spike the power from 100 to 400, it'll lag behind the PT a bit and report lower wattages. The reverse is true when you go down in power. But it's just a 2-3 second delay (in my experience). Once the power levels off I've found it to be pretty precise.


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## shawndoggy (Feb 3, 2004)

djconnel said:


> P.S. WRT the trainer problem: this is a software problem only: it would be straightforward for iBike to support trainers, if the trainer speed-load curves for popular, repeatable trainers could be installed. For example, the data for my Kurt Kinetic Road Machine is available at the Kurt Kinetic website.


That's not really a "software problem"... it's more that you could rewrite the software so that watts are extrapolated from speed based on a pre-defined curve. Assuming that the trainer power curve is correct (most have a bigger margin of error than a PT/SRM/Ergomo). Which would really mean that you'd have the $400 version of the $50 kurt "power computer" (neither of which would really be telling you how many watts you are generating... rather making an educated guess based on the power curve and speed). The low-budget version of this being to just look at the power curve and write down the corresponding wattages and tape that to your stem. Far from an actual power measuring device for indoor use.


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## djconnel (May 7, 2006)

By "software problem", I mean it could be downloaded and installed without additional hardware.

How do you define a power measuring device? Certainly the iBike does exactly what you describe: an educated guess of power, in this case based on acceleration, measured grade, detected longitudinal wind speed, and measured bike speed. Since bike speed is precisely zero on a trainer (okay, there's still momentum in the wheel and trainer mechanism, so not strictly true during wheel acceleration), the precision should be at least as good using speed alone on a high-quality trainer as it is estimating power on the road.

WRT the Kinetic computer: well, that requires swapping out the computer, which is a pain unless the mount happens to be compatible, assuming one doesn't want to ride around with an extra computer mount.

Dan


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## shawndoggy (Feb 3, 2004)

djconnel said:


> By "software problem", I mean it could be downloaded and installed without additional hardware.
> 
> How do you define a power measuring device? Certainly the iBike does exactly what you describe: an educated guess of power, in this case based on acceleration, measured grade, detected longitudinal wind speed, and measured bike speed. Since bike speed is precisely zero on a trainer (okay, there's still momentum in the wheel and trainer mechanism, so not strictly true during wheel acceleration), the precision should be at least as good using speed alone on a high-quality trainer as it is estimating power on the road.
> 
> ...


PT / Ergomo / SRM: while bike is on the trainer will actually measure the power generated by the rider (ergo, "power measuring device").

using ibike to extrapolate power from wheel speed based on trainer's power curve -- not truly "measuring" power. Those power curves have relatively big margins of error too. Greater than five percent would not be uncommon. At 300W, plus or minus 15W feels a lot different.


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## wavylines (Jan 27, 2006)

shawndoggy said:


> PT / Ergomo / SRM: while bike is on the trainer will actually measure the power generated by the rider (ergo, "power measuring device").


No power meter directly "measures" power. They all measure a secondary effect of power and then estimate/extrapolate/whatever the power based on that measurement. PTs and SRMs measure voltage across a thin metal strip embedded in the structure. Ergomos measure the difference between two optic sensors in the BB. You can't really say powermeter X "measures" while powermeter Y "extrapolates." They're all just different ways of instrumenting a mechanical process, and each has its own characteristics and error bars.

If the trainer technique has an error bar of 5%, that would put it in line with an SRM Amateur or ergomo. Lots of people seem to be using those devices for pretty effective power training.


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## shawndoggy (Feb 3, 2004)

wavylines said:


> If the trainer technique has an error bar of 5%, that would put it in line with an SRM Amateur or ergomo. Lots of people seem to be using those devices for pretty effective power training.


So you are saying that you think that using a preset power curve (which, remember, doesn't account for the tire you are using, or the tire pressure, or the amount of resistence put on the tire by the user of the trainer) is going to be just as accurate as an SRM amateur? I just have to think that's wrong.

For example, let's look at Saris' electronic trainer (which it looks like they've discontinued). As opposed to their own powertap, which has a margin of error of 1.5%, their own _computer controlled_ trainer had a much more significant 8% margin of error. I'm doubtful that the Kurt or any other trainer can possibly be any better. Same goes for Kreitler's "linear power curve" for its rollers -- see Squint's observation in the roller thread that different tires more than doubled the power required at a given speed.


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## wavylines (Jan 27, 2006)

shawndoggy said:


> So you are saying that you think that using a preset power curve [snip] is going to be just as accurate as an SRM amateur? I just have to think that's wrong.


Sorry, I should have been clearer. I was using the 5% figure from your post. I've never seen accuracy numbers for trainers' power curves in a real world setting, but I would imagine that the 5% number you picked is close to accurate. You've got two sources of resistance in a trainer, rolling resistance between the tire and drum and the drag of the resistance unit. Rolling resistance, however, is fairly small, as it's proportional to the force of the tire on the drum, and on most trainers, that's low, maybe 20 lbs. So that leaves differences in the resistance unit, either due to manufacturing variability or thermal changes (heating up). I would imagine that, once the unit is heated up, power could be reproducible within 5%.


> For example, let's look at Saris' electronic trainer (which it looks like they've discontinued). As opposed to their own powertap, which has a margin of error of 1.5%, their own _computer controlled_ trainer had a much more significant 8% margin of error. I'm doubtful that the Kurt or any other trainer can possibly be any better.


I'm not familiar with the Saris trainer, but if it was using the computer to vary resistance, then I would expect a product like the Kurt Kinetic to do much better. Variable resistance just throws in another source of error. It doesn't matter whether it's a computer or a rider pushing the resistance lever back and forth.


> Same goes for Kreitler's "linear power curve" for its rollers -- see Squint's observation in the roller thread that different tires more than doubled the power required at a given speed.


Entirely true. Resistance on a set of rollers is almost entirely determined by rolling resistance between the tires and drum. Depending on rider weight, tire selection and psi, this can vary by about two times. But as I mentioned above, trainers don't have this problem, since rolling resistance is such a small part of total resistance.


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