Bicycle Mechanics - How to tell if a chainring is true?

When I was riding the trainer last night, I happened to stare down at the chainring and it looked like it was not always a consistent distance from the FD. Is this an illusion created perhaps by rocking the trainer back and forth (and I am working to improve my pedaling technique), or can a big ring actually warp over time?

What you are seing may be the frame flexing at the bottom bracket in response to pedaling pressure so it will vary depending on which foot is pushing down and where in the stroke you are. That is normal.

Chainrings don't "warp" over time. The will bend if they suffer from some serious impact but they don't warp just from normal use. Spin the crank by hand while you aren't actually on the bike and there is no load. That should show if the rings run true.

Oh, I didn't even think about frame flex! To my knowledge, there had never been any impact directly to the rings, unless it happened in shipping before the crankset was ever installed. In any case, I'll be sure to spin the cranks by hand when I put the bike back on the trainer. Great advice, many thanks!

take it off and see if it sits flat on a table.

When I was riding the trainer last night, I happened to stare down at the chainring and it looked like it was not always a consistent distance from the FD. Is this an illusion created perhaps by rocking the trainer back and forth (and I am working to improve my pedaling technique), or can a big ring actually warp over time?

See: http://sheldonbrown.com/straighten-chw.html

Metal stuff doesn't "warp" over time; if it gets bent it's 'cause something bumped into it.

Sheldon "Planarity" Brown

+----------------------------------------------------+
| If you like baby-boomer rock music, you'll love: |
| http://wolfgangsvault.com |
+----------------------------------------------------+

take it off and see if it sits flat on a table.

After you take it off a peice of flat glass will be flater than your table.