Rowing Erg Drag Factor: Why It Silently Breaks Your Watt and 2K Comparisons

A rower pulls a 2:01.4 2k in early September, damper on 5, and logs 298 average watts. Six weeks later, same damper setting, same erg, same warm-up, the split comes back at 2:00.1 for 310 watts. On paper that's progress. In the room it felt brutal, worse than the September piece, and the athlete can't explain why a faster, higher-wattage score felt like it cost more. Nobody touched the lever. Nobody changed the plan. The number everyone assumes is locked, the resistance, quietly wasn't.

The damper lever position and the drag factor are not the same thing, and treating them as interchangeable is how a squad ends up comparing 2k scores and wattage numbers that were never pulled against the same resistance in the first place. Drag factor is the number the monitor actually uses to turn flywheel deceleration into a power reading, and it drifts at a fixed lever position for reasons that have nothing to do with fitness: dust in the fan cage, ambient humidity, chain lubrication, a flywheel that's a season older than it was at the last benchmark. This covers what actually separates the two, why a wattage-matched comparison can still represent two very different physical efforts, a protocol for locking and logging the number that matters, and what the handful of real studies on drag factor's effect have and haven't shown.

Damper Setting Is Not Drag Factor

The damper lever, numbered 1 through 10 on a Concept2, controls how far the shutter on the flywheel housing opens, which sets how much air can flow in and interact with the fan blades. That's it. It's a mechanical air-intake setting, not a resistance value. The number the power calculation actually runs on is drag factor, a separate figure the performance monitor computes stroke by stroke from how quickly the flywheel decelerates during the recovery, after accounting for whatever air density and mechanical drag happen to be present in that moment.

Those two numbers usually track together in a loose, predictable band, which is exactly what makes it easy to stop checking the second one. Concept2's own published guidance on damper settings gives a concrete example worth sitting with: a lever set to 5 might produce a drag factor of 125 on one machine and 135 on another, purely from differences in air temperature, elevation, and how much lint has collected in that particular flywheel housing. Two rowers on two ergs, both dialed to 5 on the lever, are not necessarily pulling against the same resistance. Neither is one rower on one erg in a cold morning session compared to a humid afternoon six weeks later.

Why This Breaks Session-to-Session Comparisons

Power on an air-braked erg is roughly proportional to drag factor multiplied by the average flywheel angular velocity cubed. At a higher actual drag factor, hitting a given wattage takes less flywheel speed but more force per stroke. At a lower drag factor, the same wattage number takes more flywheel speed for less force per stroke. So two sessions that both read 300 average watts, pulled at two different real drag factors, are not the same stroke. One is heavier and slower-turnover, the other lighter and faster-turnover, and those two profiles load the posterior chain, the legs, and the finish of the stroke differently across a 2k's worth of fatigue.

That's the trap in the opening scenario. If drag factor crept down between September and the six-week retest, perhaps the fan cage had accumulated more dust, perhaps the chain needed lubricant, the athlete needed genuinely more flywheel speed to register 310 watts than 298 watts would have needed at the earlier, higher drag factor. The wattage number went up. The actual physical bill for producing it went up more than the number alone suggests, which is exactly why it felt harder despite reading faster. Nothing on the display flags this. Only checking the drag factor itself, not the lever, would have shown it.

The Fix: Lock and Log the Actual Drag Factor

None of this requires new equipment. It requires treating drag factor as the variable that actually needs recording, and the damper lever as just the dial you turn to land near it.

1. Pull the real drag factor before every benchmark session. On a PM5, that's More Options → Display Drag Factor, or holding the menu button mid-row. Do this before a 2k test, a power test, or anything you intend to compare against a past score, not after.
2. Log the drag factor next to the score, not the damper number. A line reading 2:01.4 2k at DF128 tells you something a line reading 2k at damper 5 doesn't. If your team spreadsheet has a column for damper setting, add one for drag factor and treat it as the one that actually matters.
3. Pick a target drag factor band and hold the block to it. Lighter or newer rowers doing technique and stroke-rate work are often better served in roughly the 100-110 range; heavier or more experienced rowers doing max-power or short sprint work often sit higher. Adjust the lever as needed to land inside that band each session rather than fixating on a specific lever number.
4. Re-check at the start of every session, not once per season. A five-minute row after clearing accumulated dust and lint from the fan cage can shift drag factor by several points at the exact same lever position.
5. For a squad test, equalize the actual drag factor across the fleet before starting, not the lever number. Ergs matched on the same damper setting can still read meaningfully different drag factors machine to machine, and a combine-style test run without checking that produces a leaderboard partly ranking equipment instead of athletes.

How Much Drag Factor Drift Actually Matters

Not every shift in drag factor needs a service call. These bands are a practical way to triage what you find when you actually check the number instead of trusting the lever.

What the Research Actually Shows

Kane, Jensen, Williams, and Watts (2007), in the International Journal of Sports Medicine, put fourteen university club rowers through incremental progressive tests on a Concept2 at drag factor 100 and drag factor 150, tracking heart rate, oxygen uptake, ventilation, blood lactate, and rowing economy across six submaximal stages. At matched submaximal power output, none of those variables differed significantly between the two drag factors. What did show up was a correlation between the drag-factor-driven difference in ventilation and the difference in stroke rate (r = 0.76, p < .01), meaning the two settings pulled the same wattage number out of rowers using measurably different stroke-rate strategies. The physiological engine barely noticed the resistance change at a given power output; the stroke mechanics used to produce that power did. The limitation worth carrying forward: this was a small, single-institution sample tested at submaximal stages, not a maximal 2k effort, so it speaks to the mechanism more than to how much a 2k score itself would move.

Mahony, Donne, and O'Brien (1999), in the Journal of Sports Sciences, compared physiological responses across friction-loaded (Gjessing) and air-braked (Rowperfect) ergometers in ten trained rowers during an incremental protocol to exhaustion. At matched power output, the power value at the lactate anaerobic threshold differed by roughly 40-50 watts between the friction-loaded and air-braked machines (p < .01), while the two air-braked configurations showed no significant difference from each other. That's a comparison across ergometer types and resistance mechanisms rather than a drag-factor change within a single Concept2, so it doesn't transfer directly, but it demonstrates the underlying point at a larger scale: the same displayed wattage number does not guarantee the same physiological cost once the resistance mechanism generating it changes.

Separately, Treff and colleagues (2022), in Frontiers in Sports and Active Living, tested a Concept2's own measurement accuracy against a motorized reference rig rather than human rowers. Steady-state power readings differed from the reference by roughly 2.9-4.3%, and that random error grew as much as eighteen-fold under unsteady, variable-force strokes, the kind an athlete actually rows rather than a machine simulates. Worth knowing before treating any single displayed wattage as an exact number: there's measurement noise baked into the reading before drag factor drift adds any more on top of it.

Mistakes That Quietly Break Erg Comparisons

When the Numbers Still Don't Add Up

If drag factor keeps shifting by more than a handful of points at the same lever setting even after cleaning the fan cage and lubricating the chain, the problem has moved past routine drift. Check the flywheel bearing for play and inspect the fan blades for damage, either of which changes how the flywheel decelerates independent of anything a coach controls. A lever that physically slips out of its detent between sessions produces the same symptom and is worth ruling out before assuming the hardware itself is failing.

For a fleet where machines persistently show different drag factors at an identical lever setting despite being clean and well maintained, stop trying to force every erg to the same lever number. Pick a common target drag factor band instead and let the lever land wherever it needs to on each individual machine to hit it. If that gap between machines keeps widening over a season even with regular maintenance, that's the point to flag specific units for a full service rather than continuing to treat every erg in the room as functionally identical.