AeroPod, PowerPod and Newton

I've been an iBike user since Gen I and really appreciate the support from the folks at VeloComp and the improvements in each Generation and firmware release. I'm very happy with my Gen III.

I've been using 2 tests to check to see if my iBike is working well: 1) Pedal backwards and see if the iBike goes to zero; 2) Once at 0w, pedal softly to see if the iBike registers watts for very low efforts. Both of these tests have shown that my Gen III is working very well.

I recently had the opportunity to ride a Cannondale SuperSix with an SRM about a dozen times, mostly on my normal ride routes. Interestingly, I found that there were two different scenarios where I still doubted my iBike kind of subconsciously. Specifically: 1) After cresting a climb and starting to sail down the other side while still pedaling, my iBike would sometimes show 250-300w when I felt like I wasn't pedaling very hard; and 2) When downshifting to increase cadence on a climb, the watts would shoot up 50-100w unexpectedly.

Riding the SRM on the same rides that I've been doing with my iBike for a couple of years revealed a fascinating result: when I crested a big climb and pedaled down the other side, the SRM showed 250-300w (like my iBike showed); and when I downshifted in the middle of a climb and increased my cadence, my watts would shoot up 50-100w (like my iBike showed).

I realized that when I saw 250-300w on my iBike on the downhill, I would think "wrong, silly iBike, I'm not pedaling that hard, CdA must be off, I should fix that some day". But the SRM showed the same thing. Apparently after doing 400w up one side of the hill, doing 250-300w down the other side felt like I wasn't pedaling that hard. The experience with the SRM certainly verified that my iBike was correctly reporting/recording my power on the downhills! Same thing with downshifting -- when I downshifted on the Cannondale, the SRM would show an immediate increase of 50-100w just like my iBike does. Though my perceived effort was about the same, clearly I was putting more power into the pedals.

Riding with the SRM has put my mind at ease on these two issues that I wasn't even aware bothered me. They bother me no more!

So, my point is, believe your iBike! And enjoy the ride!

I should also mention that I downloaded the SRM application and data to my PC and used their tools to analyze my rides. Wow, the iBike software is SO MUCH BETTER!! It is hard to understand the SRM graphs.

In addition, the SRM I was using doesn't have altitude, so the elevation profile is non-existant. And, of course, no wind data so I can't see if there was a headwind or tailwind. No braking info, etc. It seems like a lot is missing. It doesn't enable drag-and-stop on the graph to look at subsets of the ride, it doesn't show averages, it is really pretty lame software. Maybe I would like it more if I used it more, but it was not as intuitive and does not show nearly as much useful info as the iBike software.

This is really good advice, believe the iBike! This little device is taking dozens and dozens of measurements every second, and applies a very complicated math formula every second that you ride. It would be impossible for your legs to be calibrated that well to know that you weren't hitting a specific wattage at a specific time.

If you want a good test to see how much your legs can be off by feel, cover up the wattage number on the iBike (or other power meter that you might own). Then try to ride a ten minute interval on varying terrain staying at 70% of your threshold watts the whole time. . .no looking at the wattage numbers. When you download your ride, you'll understand how trying to justify a wattage number based on feel can be way off.

Glad that your mind is at ease now about the iBike!

MultiRider wrote:1) After cresting a climb and starting to sail down the other side while still pedaling, my iBike would sometimes show 250-300w when I felt like I wasn't pedaling very hard; and 2) When downshifting to increase cadence on a climb, the watts would shoot up 50-100w unexpectedly.

Yep, keep in mind that power = force * speed = torque * cadence 2pi. In the first case, increasing your speed for a given propulsion increases your power. In the second, the downshift increased your cadence more than your torque decreased.

Travis

So "watts" is not a measurement of the force that I exert on the pedals? Watts is the force of my effort multiplied by the speed I'm going? So the same force from my legs results in lower watts at lower speeds and higher watts at higher speeds?

Power is proportional to force x cadence.

I don't know if you ever drive a straight shift car or truck. I say this to set up for an illustration. When you are driving in high gear, with a straight, and you slow down for traffic, if you slow down much, when you press the accelerator you may 'lug' the engine or in other words, you will experience little acceleration for the gas you burn. The engine rpm (cadence) is low and the gearing is high (big ring) in this case and you should normally downshift to gain the gearing advantage. There are multiple reasons race cars operate at very high rpm's but the cadence thing is a piece of it.

Now I am not like a race car, more like a hup mobile but when I hit the hill with the big ring if I don't downshift soon, I will likely come to a stop.... Now I know this is being overly simple but to make a point real obvious... Just move things up a notch to your chosen riding conditions with these above mentioned in mind.

Now ask yourself why Lance Armstrong has developed the capability to ride at 120+ rpm cadence.

Russ

Somewhere in a far corner of my old brain there is something like HP=torque x rpm, or something like that.

IOW you can have all the strength in the world but if you can't spin at high RPMS you're not going anywhere. If I could only follow the theory I would be a much better cyclist.

MultiRider wrote:So "watts" is not a measurement of the force that I exert on the pedals? Watts is the force of my effort multiplied by the speed I'm going? So the same force from my legs results in lower watts at lower speeds and higher watts at higher speeds?

Right, power is different than force because it factors in distance and time. The easier way to think about it is force x speed.

Another interesting ramification of this is that you can increase your power without increasing pedal force by simply being more aerodynamic. Say that you hold your torque and gearing constant riding on a flat stretch of road riding upright on the top of the bars. If you move into a more aerodynamic position without changing the amount of force on the pedals, you will start riding faster and, therefore, increase your power output. But that power boost is not completely free even though you are putting the same force on the pedals. Your exertion will increase (you should see your HR increase) because the force is being applied over greater distance.

Okay, I don't think I understand the force vs power thing yet, but I have a lot of work left to do tonight (working 80 hours this week so I can take 24 hours off next week -- I don't understand that math either) so I'll have to come back to that.

I wanted to add to the intent of the original "believe your iBike" thread. I went for a ride on the Cannondale with the SRM again today and encountered a 3rd scenario where I was doubting my iBike subconsciously -- on a windy day. My iBike seems pretty accurate with head winds and tail winds, but I realized on today's windy ride that I've always thought it read low on cross winds. That was based on perceived effort -- I felt like I was trying really hard, but the iBike showed watts that were much less than how hard I thought I was pedaling. Well guess what happened with the SRM today? During cross-winds, I felt like I was pedaling hard but the SRM showed lower numbers than I expected every time I looked. There are probably multiple factors -- having to control the bike in the wind through constant adjustments to lean and steering, the tension of wanting to stay out of the path of cars, and the ride took longer than anticipated and I didn't take any calories (drink or food) with me because I thought it would be about an hour ride. It turned into a 1:20 ride and the last few miles were on long uphills. In many sections, I felt like I was working really hard, but my watts were 50-75 lower than expected every time I looked down. Just like with the iBike. Riding in cross winds is definitely a harder effort, but not from a watts perspective. And not really from an HR perspective, either, because my HR was lower than my RPE. Pretty weird all the way around -- felt like I was going hard, but watts and HR said "not".

I think of wind a little like the way a sailboat makes progress.

Not many sailboats go anywhere directly into the wind and can go pretty well with a complete tailwind. The key to great sailing is the generation of power in a crosswind or by creating a cross wind instead of sailing directly into a headwind. Always seems that you get a lot of "free" energy by being just a few degrees off of a headwind - you just have to do a lot of tacking - port or starboard to the wind to get some of that energy transferred to the boat.

I feel the same on a windy day riding the bike - I end up feeling like a sail in the right crosswind (and consequently get some help from the wind - even though it it not on my back side.) However - there is one thing I am likely to believe and that is the wind reported by the iBike is probably not as accurate at some angles in a crosswind.

Tom

[quote][/quote] there is one thing I am likely to believe and that is the wind reported by the iBike is probably not as accurate at some angles in a crosswind

I totally agree with you. I have had terrible readings on windy days.

The iBike is also not a weather vane that can swirl around and get the direction and speed in the same way. It still manages to do a pretty good job on average.

Tom