Thursday, May 01, 2008

Vindication (Ergomo vs. PowerTap)

My long held (well, at least since March) suspicions have been confirmed. Under certain but fairly common conditions, the power readings from my Ergomo tend to drift downwards during a ride. It's to do with the bottom bracket shell cooling down over time, because the Ergomo measures deflections in the bottom bracket axle to calculate power.

You have to zero it before each ride, but as I've found, a temperature change during the ride can throw it out by ~20 watts. This will do your head in if you're trying to use it for pacing a time trial effort. Your average power continues to drop even when you know you're putting in more, or are at least pacing yourself evenly. And then you start to Doubt, which is not being good for the time trialling. Ja.

Today I was able to compare the Ergomo with readings from a PowerTap SL 2.4, which I understand is not subject to these sorts of variations. On a pootle up the bike path, I recorded several intervals, and found that the early ones were consistent across the PT and the Ergomo - to a few watts. But the later ones showed that 20 watt difference, with the PowerTap always giving the higher readout.

This doesn't always happen with the Ergomo, but it does seem to show in races more than training. Maybe the higher speeds are cooling down the bottom bracket? And today the path was wet, which wouldn't have helped. I'm not sure.

I'll use both in the Bath 10 on Saturday. I've going to ride it at 90% effort (aiming for 290-300 watts) and the PT should give me a reliable rolling average.

What I like about the Ergomo is that it's crank based, so you don't need different wheels with PT hubs in order to collect data.

On the other hand, if you have different wheels built from PT hubs, then you can easily use the PT on different bikes.

I like the display of the Ergomo too, as it gives you five readings instead of three on the PT. So you can display current power, average power, cadence (or speed), heart rate, and time. On the PT, you can only display current/average power, heart rate/speed and time/distance/cadence unless you want to fiddle around navigating the buttons while you're racing.

But I love the simplicity of the PowerTap. All it is is a computer and a hub. No sensor, no cables, nothing. The Ergomo has a wire from the bottom bracket to the rather large computer. If I had to pick, I'd probably go for the PT 'cos it's cheaper and seems to give more reliable data.


Laura W said...

Can you do an analysis of "reported" average watts for the Ergomo, Powertap and SRM? I would hazard a guess that the higher the cost of the tool, the higher the average watts reported on riders' blogs will be - directly proportional to the ego of the rider as well, I imagine. I've seen some pretty outrageous numbers from masters fatties out there! Present company excluded ;-)

My early model Powertap always measures about 10% less than riders of my same weight who use SRM's while climbing the same hill at the same pace. Maybe I'm just soooooo aero? Or maybe they're just lying about their weight...

Jeff Jones said...

That would be an interesting project, I think. We could call it the time trialist's cost-wattage-ego trichotomy. Catchy, eh?

10% is probably within the confidence limits of this thought experiment, especially as your early model PT would have depreciated more than that. Maybe you should be expecting 20% lower power? That could be accounted for by a combination of the ego and weight factors, whichever is bigger.