q factor

[LittleWheelsandBig]

Prefer whatever you wish but even a USA-ian pedal manufacturer refers to tread in preference to Quack-factor. They also produce interchangeable pedal axles in multiple lengths.
http://www.speedplay.com/index.cfm?fuseaction=home.chooselong

[jsabine]

That Speedplay page differentiates between tread and q-factor, saying that tread is measured between pedal centres while q-factor is between the inside faces of the cranks.

I'd always understood q-factor to be between the outside faces of the cranks - Sheldon Brown's glossary confirms this, and also gives tread as a synonym rather than agreeing with the Speedplay view.

Oh, semantics and pedantry. What fun.

[LittleWheelsandBig]

I'll use the true pedantry of Quack-factor, rather than the more popular shortened version.