Originally Posted by grolby

ARGH. Science lesson time: gyroscopic stability does NOT explain how you balance a bike, whether on the road or on rollers. It's not nearly strong enough to matter. This should be obvious, given that you can balance at very low speeds. This has been demonstrated by the building and successful riding of bicycles designed to cancel out any gyroscopic effect. These bikes are just as rideable as a normal bike, and they even behave the same way when rolled down a hill without a rider. Gyroscopic stability has been discredited both theoretically and practically for some 40 years now. Sheesh.

A bike stays up on rollers the same way it stays up on the road: balance by countersteering.

Originally Posted by Brian Ratliff

^^^
I'm glad someone said it. Grolby is correct. You balance by using very small adjustments in your steering. Lock the headset and you'll soon find that you won't stay balanced regardless of the speed your wheels are turning.