Friday, July 21, 2006

Behold, the Power

Okay, so it's been quite awhile since I've totally geeked out, so this will be interesting I imagine only to me. So.

I have this new fandagled piece of technology on my bike, called an iBike. It was actually supposed to be here in April or May, but they're a tiny company just starting out and had all kinds of production issues or something. So I finally got it a few weeks ago, and was able to actually do something useful with it yesterday in an hour long ride that culminated with a 20 minute time trial.

The iBike is a Power Meter, and it uses Newton's 3rd law to analyze the forces pushing against me to determine how much force I'm using to push the bike. (There are other, far far more expensive power meters out there that are devices actually in the cranks or the wheel to measure power in different ways.) Blah blah blah, but here's the gist of it (flashback to 8th grade Physics...who knew I'd one day give a damn...) - when you work, you can measure your efforts in watts. The higher the watts, the harder you're working. Right, there's a lot more details about that having to do with Joules, Kilojoules, Calories and calories (the small and capital "C" do represent different things...), but we're sticking to the basics here.

So in using watts to measure power, one can get an immediate and completely objective measure of his effort. Remember that I also use heart rate to measure effort, to some degree, as well as speed, to a lesser degree. But that's very variable - if it's a hot day, my heart rate will be higher. If I'm in a head wind I may be going slowly but pushing harder, with a higher heart rate. If a dog barks at me and scares the bejeezus out of me, it might spike. Heart rate is a metric, but is best used in combination with other things for a complete picture. Enter the Power Meter. How much power I'm using to push the bike - watts - is totally objective. Doesn't matter if it's a headwind or a tailwind or hot or cold, power is power. It has nothing to do with my environment, only how much force I'm using to push the bike, regardless of speed or heart rate or the time of day or the price of beer. That objectivity is useful in a lot of ways. For instance, if I'm only using speed and heart rate for metrics, what happens in a headwind? I push the pedals harder to try and sustain some speed, and my heart rate will probably climb. That's fine...but what happens 5 hours from now with all that effort I've been throwing behind the pedals in a headwind? Or if I'm climbing a hill (of which there are several major ones at IMWI), I'm pushing the bike with greater force than on a flat, or certainly descending, when gravity is at work and I'm not pushing the bike at all.

So the trick is to try and devise a sensible relationship between power, speed, heart rate, and cadence. If all 4 are in a happy place, then I'll be in a happy place 5 hours from now. But if I'm pushing the bike too hard early in the ride, when I'm fresh, then I'll run out of steam later in the ride...and so will have to push harder to sustain the same speeds I was...which will elevate my heart rate...which will affect digestion...see how complicated it all becomes? You really can't, in an Ironman, just get out there and ride. It's just too long of a race, and there is too much at stake having to prepare for a marathon run off the bike. You have to have something to give you feedback so you can plan ahead. So for me, the scientist I am, having all the data possible helps.

So you have this thing called Functional Threshold Power (FTP). That's the number of watts you can push over a sustained 60 minutes of effort. Kind of "as hard as you can go for 60 minutes". But to measure it, you only do a 20 minute time trial - after some warm up stuff to fatigue you a bit - and then do some calculations to calculate the 60 minutes. You also coordinate that effort with your heart rate - an average heart rate over those 20 minutes of hard effort gives you another baseline. So yesterday I tested these things and came up with an FTP of 285 - that means for one hour at hard, race-like effort, I could push 285 watts consistently. More than an hour, and that would start to fall off. Less than an hour, and I could push more watts. My anaerobic threshold - that point, measured in heart rate, when my muscles start to create lactic acid and I'm burning more sugar than fat - is about 151bpm.

So with that data in hand - FTP of 285 and LTHR (Lactate Threshold Heart Rate - hey, I told you this was geeky and interesting only to me) of 151, I can chart scenarios for a 112 mile Ironman ride that is designed to keep me eating and hydrating, avoid blowing my legs up, consistent in my effort, no crazy spikes of fatigue, and somewhat fresh for the marathon. Here's my plan, which I'll execute for the first time in whole on Sunday:

I generally want to keep my watts between 68 and 78% of my FTP for an Ironman distance ride. Lower than that is okay, too, for certain sections of the ride - like the first 15 miles. But for the bulk of my Ironman paced ride, I'll try and stay between 194-222 watts. So I'm pushing the pedals with power between those numbers. Higher than 222 and I'm working my legs to faster fatigue - 250 watts would be nothing in mile 5, but I'll pay for it in mile 95. Slower, and I'm sacrificing power where I don't want to be. This is a more critical number than speed or heart rate, because it directly ties to my leg fatigue. This means if I'm in a headwind, I can't speed up. I have to stay with the watts in this range. It also means early, when my legs are fresh, I need to back off so I don't blow everything in the first 2 hours of the ride.

My heart rate - and this is a surprise - should stay below 128. I do know that's optimal for me for digestion, but I'm usually close to 135 or even sometimes 140 on my rides. Heart rate is a bit variable, as I mentioned, but this is the general place I want to be. So I'm going to need to be disciplined to keep comfortable and my heart rate low.

So I have no idea how it'll translate, but it may mean an overall slower ride than I'm used to on Saturday, or a slower time than I've been anticipating for Ironman. If that's the case, then I'll need to just get over it and hope that it translates to fresh legs for the run (I have a 30 minute run following Sunday's ride). If I work myself into a frenzy on the bike and finish with a "good time" - whatever that is - only to blow up and lose 2 hours walking in the marathon...well that would be just stupid. Typical, but stupid.

So there you go! The Science of XT4. We'll have a pop quiz later today. And no chewing gum.

2 comments:

Kylie said...

great way to figure out what is right for you!

So I read all of it... but one part was missing: are you satisfied with the iBike? Easy to use/read/etc? I've been playing with the idea of some kind of power thing, and haven't found a great review on those.

xt4 said...

Hey Kylie - yeah, I didn't really "review" it did I - it's okay. I don't have anything to compare it to (PowerTap, etc.), but I've read that it's pretty consistent with them, and anyway it seemed consistent with itself and me, which is all I need. The setup has a hassle factor of 10...it needs to know the weight of you and your bike, and a loaded down long ride bike weighs differently than a time-trial ready bike...and then you have to do this calibration thing that involved a few minutes of getting up to 20mph and then coasting down to 8mph. So if you input a new weight, you have to calibrate again. And sometimes again, if it seems moody. But it does have a lot of great features, and it's made by a small company owned by two guys where are trying to buck the several-thousand-dollar-powermeter-system, so I applaud that. Anyway...I recommend it if you think the information would be valuable to you and you're looking for a low cost alternative to the big boys that maybe requires a little more manual labor than you'd prefer. If set it and forget it is more your thing and you can afford it, one of the others might be a better bet. I can say that this assisted me a great deal in strategy for my long rides lately, and I've seen great results from those rides. Good luck!

By the way - no opinion on the handlebar tape? We're dying to know what you think! Check the blog for the vote...wheeeeeee!