Why indoor power and outdoor power don’t match

They are not the same ride. Stop treating the numbers like they are.

A 250 W threshold block on the trainer is not automatically a 250 W climb. Cooling is different. The meter is often different. The bike does not move. Cadence and torque are smoother. That stack is why the file lies when you copy zones from one world into the other.

The problem

Most riders use two clocks.

Outside: pedals or crank. Inside: the trainer at the cassette.

Those sensors sit in different places. Drivetrain loss sits between them. Each device has its own tolerance, temperature drift, and calibration habit. Dual-record the same session and a 2–8% offset is normal. Some trainers are worse.

Then physiology piles on.

Indoors you lose the 20–30 mph air you get for free on the road. Blood goes to the skin. Heart rate drifts. Repeatable power drops even when the target watts look identical. Short sprints can fall off more than a 20-minute hold because you cannot change position or recruit the same way on a locked trainer.

So you get two false stories:

• Indoor FTP looks high, outdoor intervals die
• Outdoor FTP looks high, trainer sessions feel like a furnace and you “fail” quality work

Neither file is fake. They are measuring different conditions.

The rule

Pick one source of truth per week for the sessions that matter.

Key intervals: use the same meter you will race on, or pair the trainer to that meter (power match). Do not mix trainer watts and pedal watts in the same block.

Endurance rides: the offset is noise if RPE and heart rate sit where they should.

Do not raise outdoor zones off a well-cooled ERG test.

Do not cut indoor targets 20 W “because trainers lie” if the only problem is one desk fan.

If you must use two meters, measure the offset once. Same bike, same pedals on the trainer, 10 minutes steady at Z2 and 10 minutes at threshold. Write the delta down. Apply that delta. Do not keep guessing.

What actually moves the number

Two sensors. Pedal/crank vs trainer. Fix this first.

Heat. One weak fan is not outdoor airflow. Two strong fans on torso and face close most of the durable-power gap. If indoor heart rate at the same watts is higher than outdoor, cool the room before you touch zones.

Position. Front wheel too low, saddle slammed, different bike. Indoor “threshold” that you cannot hold in the drops outside is a fit problem.

ERG smoothness. The trainer holds the watt. Outside you pulse. Mean power can match while the work feels easier or harder. Judge the session by whether the target duration holds, not by a 3 W average.

One-sided pedals. A left-only meter with a real left/right imbalance will not match a trainer. That is not indoor vs outdoor. That is the meter.

What to do this month

Calibrate both devices. Every session if the meter asks.

Dual-record one workout. Note trainer vs pedal average at steady Z2 and at threshold.

Train key work to one number. Prefer the outdoor meter if the event is outside.

Build indoor cooling like it is equipment, not a luxury. If you still cook, drop volume of the interval, not the prescribed watt — same rule as heat training.

Keep Z3+ inside the 3-hour / 20% cap. A “higher indoor FTP” does not buy extra hard minutes.

The last check

If outdoor threshold intervals die and indoor ones look perfect, trust the road for race work and treat the trainer as a controlled stimulus with its own offset.

If indoor work dies and outdoor work is fine, fix fans and hydration before you rewrite zones.

One curve. Two environments. Do not let the second environment quietly inflate the first.