Chain's a bit slack

[librarian]

Ohhhh,

The Grand ol' Duke of YACF, (or should that be AUK? rhymes with York)
He had a fixed wheel bike,
His chain was very very slack,
So he tightened it up again,
And when it was slack, it was slack,
And when it was tight, it was tight,
And when it was neither slack nor tight,
It was about just right.