What's Wrong with Trainers?

Trainers don't feel right.

You know this as soon as you get on a trailer. It's especially obvious when you get up to sprint or ride out of the saddle. You simply can't do it on a trainer.

Why? When you stand to accelerate outdoors the pedals "push back harder" than when seated. This increased force is the effect of trying to accelerate the rider and bike.

Trainers model average power at constant speed: they don't respond realistically to accelerations. So when you get up to sprint on a trainer, the pedal "falls out from under you". The acceleration of a trainer is wrong, wrong by as much as several orders of magnitude. After all, the mass of even a large-flywheel trainer is small compared to the mass of a rider.

Acceleration is the root cause of the difference.

The PFS creates forces at the pedal that are a theoretically-sound simulation of outdoor pedaling forces, a much better simulation of real-world pedal forces than today's trainers. It gets acceleration correct. Pedaling feels right because it is right.

This specificity benefits your training.