What power? What speed?

I’m planning to race my Ironman using a power meter and sitting at 150 watts. But what speed will that end up being?

I went through my last 12 months of rides down at The Spit and grabbed the average power and speed for the 8km out and back section. This road is dead flat but includes a 180 turn halfway through. Because it is on an exposed coastline it can be quite windy, but the out and back should minimise the effects of wind. Here it is:

speed-power.jpg

You can see that the data is quite scattered: 160-170 watts yielding speeds between 26kph and 33kph. I suspect that’s because they are influenced by conditions (wet vs dry), whether I was pushing a steady power or not (called Variability Index or VI in WKO+), and whether I was riding on the aerobars or not. So the data is not precise, but it is a starting point.

Using the trendline, I can guess that riding at 150W is about 28.1kph, 170W is about 30.2kph, and 190W is 31.9.

I tested this out by riding the same route on the aerobars at a steady 170W. The average speed was 31.7kph. That’s a bit faster that the graph predicts (30.2kph) – probably due to the aerobars. I’ll keep adding to this graph using different wattages, all from the aerobars, to get a better picture. I’ll have to especially target lower wattages like the 150W I plan to race at. Once I’ve got enough aerobars-only figures I’ll drop out the unknowns.

Of course this trendline shows Speed vs Power on a flat course. Ironman NZ is a hilly course so I’ll need to dial the expected speeds back a bit. I might compare average bike times from previous years at IMNZ and compare to a flat course (like Ironman West Australia) to work out the course multiplier.

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