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Because you never move. Outdoors you shift position constantly without noticing, which spreads the load. On a trainer the bike is fixed and most people sit far stiller, so the same tissue takes the same pressure for the whole session. A tipped-forward bike and soaked shorts make it worse.
On the road you are constantly making small adjustments: standing over bumps, shifting back on a climb, moving forward on the flat, rolling your hips, coming out of the saddle at a junction. Each one moves the pressure to slightly different tissue and gives the rest a moment's relief.
Clamped to a trainer, the bike does not move under you at all. There are no bumps, no corners, no reason to stand. Most people settle into one position in the first five minutes and stay there.
The pressure is not higher than outdoors. It is just unrelieved, and unrelieved pressure is what causes numbness and soreness.
Level the bike. Most trainers raise the rear axle, tipping you nose-down and sliding your weight onto the front of the saddle, which is the worst place for it. Put a riser block or a solid book under the front wheel until the bike sits roughly level. This is the single most common fix and it costs nothing.
Check your saddle angle once the bike is level. A nose-down saddle makes you slide forward and push back with your arms all session.
Check it is the same fit as your road bike if you use a separate turbo bike. Copy saddle height, setback and bar drop properly rather than eyeballing it.
Some discomfort settling in is normal when you start riding indoors regularly. These are not, and are worth acting on rather than riding through:
A professional bike fit is genuinely worth the money if problems persist, and anything involving broken skin or lasting numbness is worth a GP appointment rather than a forum thread.
Usually not. Try levelling the bike and building in regular standing first, because those fix most cases and cost nothing.
No, roughly level is the aim. You are compensating for the trainer raising the rear, not creating a downhill.
Yes. Forty good minutes beats an hour you spend shifting about in discomfort, and the tissue does adapt over a few weeks.
That is the problem we are actually trying to solve. Turbo Trainer Games turns your watts into the controls of a browser game — your power flies the aircraft, climbs the mountain, pulls the sledge. Nothing to install, and it works with a smart trainer you already own.
It is in closed testing at the moment, so there is nothing to play yet. But you can put your name down.
Ask for early access