Mistakes Riders Make With Watts, From FTP Tests to Zone 2

"Watts do not lie." It is the line every new power meter owner hears, and it is half true. The meter measures honestly. What riders do with the number is another story. A twenty-minute test gets typed in as FTP, zone 2 drifts into tempo, a headwind gets blamed on bad legs, and a few hundred dollars of carbon gets bought to fix a problem that was really about position.

This article follows one example rider through the whole chain: a 165 lb rider on a 20 lb road bike, starting with a test, ending with the speed the watts buy on the road. First, how riding by watts came about. Then the mistakes, each with its number. Then the rules that come out of them.

How watts replaced feel

For most of cycling history, training ran on feel, the stopwatch and later the heart rate monitor. Heart rate was a big step, but it lags behind the effort by a minute or more, drifts upward on a long ride, and rises in heat even when the legs do the same work. Power does none of that. A strain gauge in a crank, a pedal or a rear hub measures the force on it many times per revolution, multiplies it by cadence, and reports the work done right now.

The first power meters were laboratory and pro team equipment. Over time they moved into pedals, crank arms and indoor smart trainers, and the question changed from "how do I get power data" to "what do I do with it". The most widely used answer came from coach Hunter Allen and exercise physiologist Andrew Coggan in Training and Racing with a Power Meter. They built everything on one number, functional threshold power, the highest power a rider can hold for about an hour, and hung seven training levels on it as percentages. The same book gave the power profile, a table of watts per kilogram from untrained riders to international pros.

The physics side came from biomechanics. James Martin and colleagues wrote down the power a rider needs on a road as the sum of air drag, tire rolling resistance, bearing friction and gravity, and checked it against power meter data on a real road: the predicted and measured watts agreed with an R² of 0.97. That equation is why a calculator can turn watts into speed and speed into watts.

Three ideas, then: one threshold number, zones as percentages of it, and an equation that links watts to the road. Almost every mistake below breaks one of the three.

The mistakes, with the numbers

1. Typing the twenty-minute average in as FTP

The classic test is twenty minutes as hard as you can hold, and the number at the end looks like an hour's power. It is not. Over twenty minutes a rider can draw on energy that runs out within the hour, so the protocol multiplies the result by 0.95. A 250 W test is an FTP of 238 W.

Twelve watts sounds small until it spreads through the zones. Every zone is a percentage of FTP, so all of them move:

Threshold intervals planned at 263 W instead of 249 W are a common reason riders fail the last repeat of a workout that was supposed to be hard but doable.

2. Mixing test protocols

The second slip is a cousin of the first. Different tests use different factors, and taking the raw number from one protocol into another's formula gives a different FTP every time. Through the right factor, four quite different raw numbers land within 8 W of each other:

The ramp has its own trap: riders with a strong sprint finish it high and get an FTP their real hour cannot hold. Pick one protocol and stick with it, so that a change in FTP means a change in fitness and not a change in test.

3. Comparing watts without the weight

On a climb, a lighter rider with fewer watts can be faster. That is why the power profile is in watts per kilogram. The example rider, at 238 W and 165 lb, sits at 3.17 W/kg, the moderate band of the Allen and Coggan table, in the range of a US Cat 4 racer. The next band starts at 3.56 W/kg: 29 W more at the same weight. On the flat, raw watts matter more, because the air does not care what a rider weighs; on a climb, the ratio decides.

4. Riding zone 2 in the gray zone

Zone 2 is 56 to 75% of FTP. For an FTP of 238 W that is 132-178 W, and in the example rider's position on a flat road that is 17.8 to 20.0 mph. It feels easy, which is the point. The common mistake is to ride the easy days a little harder, at 190 or 200 W, because it feels more productive. That is tempo, zone 3, and it is called the gray zone for a reason: too hard to recover from like an easy ride, too easy to build what the hard days build.

The zones meet without a gap, so any number on the head unit belongs to exactly one zone. A table that rounds 55% and 56% of FTP separately can leave a watt between zone 1 and zone 2 that belongs to neither; at an FTP of 250 W that watt is 139.

5. Expecting double the watts to give double the speed

Air drag grows with the square of speed, and the power to beat it with the cube. On a flat road, with the example rider on the hoods, 150 W holds 18.7 mph. Twice as much, 300 W, holds 24.2 mph: 5.5 mph more for double the effort.

The same table shows the next mistake hiding in plain sight.

6. Buying speed that position gives away

At 200 W, the example rider covers a 40K in 1:11:31 on the hoods and 1:06:28 on aero bars. Five minutes, for the same watts, from position alone. Weight saving is the opposite case. Take 5 lb off the bike and on a flat road at 200 W the speed stays at 20.9 mph. On a 6% climb the same 5 lb lifts the speed from 7.9 to 8.1 mph. Light parts are a climbing upgrade; position is an everywhere upgrade.

7. Blaming the legs for the wind

Holding 18 mph on a still day takes 136 W. Into a 10 mph headwind the same speed takes 288 W, more than twice as much. Hold 200 W instead and the speed falls from 20.9 to 15.1 mph. A ride into the wind that feels like bad legs is usually a ride where the power was fine and the speed simply paid for the air. Judge the day by the watts, not by the speedometer.

Rules that come out of the mistakes

1. Convert every test before you use it. Twenty minutes times 0.95, the better of two 8-minute efforts times 0.90, the best ramp minute times 0.75. Never type a raw test result in as FTP.
2. One protocol, every time. Retest with the same test every few weeks, so changes mean fitness.
3. Recalculate the zones after every test. They are percentages, so a new FTP moves all of them at once.
4. Keep the easy days easy. Zone 2 has a ceiling for a reason; most of the week belongs below it.
5. Use W/kg for climbs and watts for the flat. Compare yourself with the right number for the road you ride.
6. Fix position before buying parts. Minutes come from the air; seconds come from the scale, and only uphill.
7. Trust the watts, not the speed, on a windy day. Speed tells you about the weather; power tells you about you.

Zones for the most common FTP values

The rules only help with the right numbers. Here are zone 2, the sweet spot and threshold for the FTP values riders ask about most. Each row opens the zones calculator with that FTP filled in.

Notice how the sweet spot overlaps the bottom of the threshold zone, from 226 to 233 W at an FTP of 250 W. That is by design: sweet spot is a band across the top of zone 3 and the bottom of zone 4, not an eighth zone.

Power training is not about a better number. It is about using the same honest number the same way every time, from the test that sets it to the zones that spend it and the road that turns it into speed.

Tools discussed in this article

• FTP Calculator
• Power Zones Calculator
• Cycling Power Calculator

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