Indoor vs outdoor FTP

Your indoor FTP is lower. That is normal and it is not a sign of anything. Here is how your gap compares, and what to check if it is bigger than it should be.

## The gap is normal. Five to ten per cent is normal

If your indoor FTP came out lower than your outdoor one, nothing has gone wrong, you have not lost fitness, and your trainer is probably not lying to you. Most riders test somewhere between five and ten per cent down indoors, and plenty sit at the top of that range.

It catches people out because the obvious assumption is that indoors should be *easier*. No wind, no traffic, no hills, no junctions. In practice almost everything that makes indoor riding convenient also makes it harder to produce power.

## Why it happens, roughly in order of how much it matters

Heat, and it is not close. At 250 watts on the pedals your body is spending around a kilowatt and shedding three quarters of it as heat. Outdoors you are moving through air that removes it as fast as you make it. Indoors you are sitting still in a sealed room, warming it up and then being warmed by it. As core temperature rises, power falls — this is the single biggest factor, and it is also the most fixable.

No freewheeling. On the road, every corner, descent and junction gives you a few seconds off the pedals, and those micro-recoveries add up across an hour. Indoors you pedal continuously. The same average power is simply a harder thing to produce.

One position, no movement. Outdoors you shift about constantly without noticing — standing on a rise, moving on the saddle, changing hand position. On a trainer the bike is bolted still and you tend to sit in one place for the whole session, which loads the same muscles and the same contact points without relief.

Motivation, and it is real rather than a character flaw. There is no road coming towards you, nothing to chase and nothing to get home from. The last two minutes of a hard effort are genuinely harder to hold when the scenery is a wall.

The trainer itself. Different trainers read differently, and not all of them are well calibrated. If yours is a wheel-on model, the tyre pressure and roller tension both change the reading.

## When the gap is too big to ignore

Above about twelve per cent, the usual suspects are worth checking in this order:

1. Airflow. One decent fan aimed at your chest and face is worth more than any other change you can make. Two is better. This alone often recovers several per cent.
2. Room temperature. A cool room, a door open, a window cracked. Garages are excellent in winter for exactly this reason.
3. Calibration. Spin down your trainer properly, warmed up, following the manufacturer's procedure. On a wheel-on trainer, check tyre pressure and set the roller tension the same way every time.
4. Test protocol. If your outdoor number came from a ramp test and your indoor from twenty minutes, you are comparing two different things. Use the same test.
5. When you tested. An indoor test at nine in the evening after a day's work is not comparable with an outdoor test on a Sunday morning.

## Which number should you actually use?

Use the indoor figure for indoor sessions. That is what the calculator produces zones from.

If you set your trainer sessions from an outdoor FTP, every interval will be pitched slightly too hard, you will fail the last one of every set, and you will slowly conclude that you are getting worse. You are not. You are using the wrong denominator.

Keep both numbers, label them, and use whichever matches where you are riding. It is not a sign of weakness to have a lower indoor figure — it is a sign that you have measured two different situations accurately.