Estimating power output (aka: talking myself off the ledge using data)

There are a lot of neat things you can do when you’ve got data from a proper time trial. One of the things I’ve been doing is estimating what my power output is, and what it should be during my race. Here’s an outline of my last time trial as shown in TrainingPeaks.

For comparison here’s a similar time trial I did back in October.

I don’t want to make too many direct comparisons between the two, because there are a lot of different factors. The one in October looks better, since my heart rate is lower, and the whole time trial was over a longer distance. This week the weather was about 10º C lower. I was wearing more clothing, including a jacket that was blowing in the wind. Positioning was different in both, there are different length cranks. For the record, I’m seriously considering switching back to a 165mm.

The main point is that using a 20 or 30 minute average speed on a flat course, you can get a rough estimate of what kind of power you’re putting out by using this calculator.

You enter your weight, bike weight, position and tire type, then adjust the power level slider until you show your average speed showing in the velocity window on the top. In my case it’s showing that I’m putting out about 172 watts. Actually, I didn’t realize that adjusting the temperature would make as much of a difference. Supposedly if the temperature were 22 instead of 7 and I posted the same time, my power output would only have been 164! So pat myself on the back, I’ve probably made progress!

I realize that I’m probably making a big mistake here, since it’s supposed to be a rule that as a triathlete you’re supposed to boast huge numbers, even if you have to make them up.

Using the same adjustment as we did with threshold heart rate, that means that my 1 hour sustainable power output (FTP) would be 169 (172/1.02). There’s something we can work with. I had a look for what percentage of your FTP you should be using for Ironman and half-Ironman distances. Ironman should probably be around 50-60% of your FTP, and a half-Ironman should probably be about 8% higher than that.

In my case, that would put my Ironman power target at 86-103 watts, and my half-Ironman target at 93-111 watts. Double checking that now with my actual half-Ironman bike split, and plugging that into the calculator, I was putting out about 117 watts. Probably a little more when you factor in the hills. This was about 70-75% of my FTP. A little higher than it should have been, but I wasn’t saving anything for my run, since I knew I was walking the whole way anyway.

It’s a nice way of double-checking your work. It looks like that is accurate enough. I should be able to work closer to 60% of my FTP during my Ironman. Again, I’ll be walking most of the run, so I don’t need to worry too much about blowing up on the ride.

The thing that’s been concerning me about the bike course is the climbing. There are some brutal hills on the course. I’ve prepared myself for The Veyo Wall, which is a mile-long hill at about 12% grade by doing a harder hill in Collingwood. I’m okay with that. But what worries me is the average 1.5% grade that lasts 40 km. There are no hills in this area that can compare to that. This will be completely new territory for me.

If I just plug the numbers into the calculator, using my estimated power output, I should be able to climb that 40k at an average speed of 18.8 km/h. Yes, that’s slow. But at least I won’t be surprised, and I will know that I should not be worried by it. As long as my heart rate stays in the right range, and my speed stays close to that, everything should go as according to plan.