When your FTP isn’t your FTP

Installing a wheel with a Powertap hub is simplicity itself. Just change the wheel and pair the Powertap to your Garmin or other head unit, ride and start collecting data.

The trick is what to do with that data. Any book, coach or article will say the first step is to set your Functional Threshold Power (FTP). That is the highest power you can sustain for an hour. It is the basis for almost everything in power based training. There are several ways to establish your FTP. Here are three common ones:

1.     Ride a flat 40km Time Trial. Most people take an hour or so to ride that distance so their average power over that distance is a good estimate for FTP. Hills cause your power to fluctuate so flat is best.

2.     Just ride your bike hard. Then look at your Power Curve at the 20 minutes point and take 95% of that as your FTP. Strava Premium has a nice feature for this and can show your estimated FTP for the last 6 weeks, 6 months, a year or a custom time period. It also compares this year to last year which is nice if it has gone up! Here is my Strava Power Curve:

No doubt Power Agent, Training Peaks, Golden Cheetah or WKO+ can produce something very similar.

3.     Turbo Trainer. Warm up properly for at least 20 minutes. Structured warm ups are best. Then ride 20minutes at the highest power you can sustain. Your FTP is estimated at 95% of that 20min average power.

In all cases it is important to be relatively fresh before taking on an FTP test. Most people I know use the Turbo approach and use that to set their training zones. Ideally one should retest every 6-8 weeks to measure what effect training is having and make appropriate adjustments.

Our club did an FTP test in our turbo session last week. My average for 20 minutes was 212W 95% of that is 201W. That’s 33W less than Strava calculated based on data collected from the road this year. Leaving aside that neither value is high - remember I am racing in the middle of the 55-60 AG pack and FTP/kg drops with age – a subject for another day. I’ve talked to many power users, with different equipment, to coaches and conversed on a number of forums. There is a clear consensus that Turbo FTP is typically 10-15% lower than Road FTP. So which should you use ?

The answer depends what kind of person you are. If you are happy with “close enough” then pick whichever you like and just use that. Most will either just do one test, or take the highest number they can and could consequently end up with power zones on the turbo which are actually too high or zones on the road which are too low.

After a lot of discussion, thought and research, I use a “horses for courses” approach. I use my Turbo FTP on the turbo and my Road FTP on the road. As a result I have different training zones for each. The following table compares the two:

	Turbo (W)	Road (W)
FTP	201	232
Recovery (Z1) <55%	111	127
Endurance (Z2) 75%	151	174
Tempo (Z3) 90%	181	209
Threshold (Z4) 105%	211	243
VO2 Max (Z5) 120%	242	278

As you can see there is quite a difference. It is really important for pacing in races. At Challenge Vichy in 2013 I used my Turbo Zones and raced at the top of Endurance. I was 20W lower than I should have been. At Challenge Almere this year I fixed that and set a Bike PB by over an hour! Granted I was fitter and the route was flatter, but I’m convinced better power management was a lot to do with it.

Having two FTP numbers means that I have to do different calculations for things like Normalized Power, Intensity Factor and Training Stress Score depending if the ride was on the road or on the turbo. That makes my training log a bit more complicated and there are some manual steps, but I am happier that it is more accurate.

There is a good argument that I need 3 or more FTP numbers. My road FTP was set on a good length climb in Mallorca. I was seated holding onto the tops of the bars most of the way. I have yet to produce that power in the full aero position on the flat. But I have been pretty close. Probably the most accurate approach is to train in the position I am going to race. That means I should set my Turbo and Road FTPs in the aero position and base my zones from there. More work to do there.

The more you look at it, the more you realize that an FTP is not some magic number, it is a snapshot of a performance in particular conditions on a particular day. Do it again a week later in a different place and you might get a slightly different result. Even so it is extremely valuable and is the basis for some really helpful sports science. It is a much better basis for training zones than Heart Rate or Rate of Perceived Exertion, both of which are subject to all kinds of variations due to heat, mood and goodness knows what else.

Normalized Power, Intensity Factor and Training Stress Score are all Registered Trademarks of Training Peaks.