Bicycle Mechanics - When to replace tire?

How can I determine it's time to replace a tire? Is there a rule of thumb or? I have a rear tire that the tread has worn flat in the middle. My immediate response was to replace it, but they are so expensive, I don't want to do it before I have to. Thanks

You're riding a mountin' bike fulltime on the pavement aren't you? If so switch to a slick.

A good time to change the tires is in 4 possibilities.

1)Knobs are loose and or Falling off; for road: You see the casing.

2)You're not getting good traction, id dry warm conditions, it will seem like your sprinting on a solid sheet of ice.

3)The tires are so old, they're showing cracks on the sidewall.

4)There is a flat spot on the tire.

Situation #4 is your issue. So go out there and buy a new tire.

For road bike tires, the rear tire will always develop a flattish profile on the rolling surface. Assuming other factors are okay, you can ride a road tire until you start seeing patches of thread on the rolling surface. After that, you're on borrowed time.

I occasionally rotate my road tires. Some of the roads on my regular ride have a pretty steep crown to them, and my tires wear assymetrically. I flip them around every few hundred miles! Anyone else?

L8R