FTP Testing and Hammerhead Sharks

Accurate functional threshold power (FTP) testing can be a little tricky. It's rare that you'll get to measure the real number directly -- that can only happen when riding a 1-hour TT effort in competition on a bike with a power meter (my TT bike has none). But you need an accurate number because you use it to set up all of your interval training power ranges. Therefore, you have to use other methods to test for and calculate your FTP.
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Analysis paralysis warning: If you're not really, really interested in FTP, save yourself the pain, stop reading here, and just check out my birthday present from Betty Jean instead. I think I'll call him Clayton -- Jeff seems to enjoy hammering as much as anyone I know.










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Ok, back to the numbers. In converting other testing numbers into an accurate FTP value, you have to consider the racing vs. training factor and also, if using a trainer for testing, a factor for converting your max effort on a trainer to your potential effort on the road. And finally, it's hard to do 1 hour all-out tests. I do three a year. Between those tests, I keep track of FTP by doing 20-minute efforts and using factors as described below. So you have the 20m to 60m conversion factor too.

I ran a 20-minute test last night on the Computrainer to try and establish the wattage relationship between a 20-min computrainer (CT) test and a 20-min test on the road. I started out way, way, too hard, but leveled out eventually and ended up with average 20-min wattage of 268, which is exactly what my last hour-long road FTP test calculated as my FTP. It’s also exactly 95% of my peak 20-min wattage in the Edgar Soto TT a few weeks ago. Also, it’s interesting to note that the CT gave me the exact same average wattage as did my Ergomo, so they matched perfectly.

So there are a couple of things going on here. First, I’ve become very confident that the relationship between 20-minute average wattage and 60-min (FTP) wattage is 0.95. I have multiple tests showing that my maximum 20-minute power output in races is 95% of my calculated FTP. So I’m holding fast to that relationship. I’m also pretty confident in the training-to-racing 103% relationship (adrenaline/motivation in races allows you to generate about 3% higher wattage over 1hr than in training) is accurate. What I have now is a new relationship between CT testing and road testing in training.

So here are some conversions. First, my designations are:
road = training on the road
CT = training on the Computrainer
race = data from a race

FTP has always been defined as 1 hour race average power:
FTP = 1.00 x (60m – race)

In a 1hr training road test, we increase measured power by 3% to account for adrenaline, motivation, et cetera:
FTP = 1.03 x (60m – road)

A 20-minute race TT gives you 5% higher average power than your 60-min race power, so:
FTP = 0.95 x (20m – race)

For a 20-minute non-race test on the road, we have to throw in both the 95% conversion from 60m to 20m and the training/racing adrenaline factor:
FTP = 0.95 x 1.03 x (20m – road) = 0.98 (20m – road)

Ok, here is the new stuff that I've just figured out:
For whatever reason, I’ve found that power output on the Computrainer is about 2% less than non-racing testing on the road for an extended effort (probably due to heat, lack of breeze, inability to move around, mental factors). So we need to account for that extra 2% with a third factor of 1.02, so:
FTP = 0.95 x 1.03 x 1.02 (20m – CT) = 1.00 (20m CT)

So when all is said and done and you get rid of all the numbers, you end up with a very simple relationship for testing FTP:
FTP is your average 20m power on a CT. That's without a doubt the easiest, most consistent way to test.