cqlink 

Can you chime in why different sized/ flat teeth are reportedly a shifting advantage on a chain ring?

I've a Shimano triple. I noticed two teeth both on opposite sides on the middle ring are flat and somewhat shorter. I've been told it helps shifting.

Please explain.

Thanks!

Andrew R Stewart 

This tooth shaping was called "W Cut". An early attempt to reduce the front shifting effort.

When a chain is told to move to another cog/ring it has to be able to lift up off the current teeth it's on before being able to be prodded laterally to the next set of teeth (cog/ring). If the shifting to cog/ring is larger the chain is readily lifted up off the first teeth but has to climb up to the tops of the next set before settling in on that set. If the next cog is smaller the chain still has to lift up and over the first set of teeth.

When the teeth height is lessened the amount of lifting the chain needs is less. Hense less energy used in the chain's movement.

If you look at the "classic" chainring tooth shapes/profiles (Campy NR, TA, Stronglight, Sugino, Shimano, and others prior to the 1980s) the teeth are all the same height and of a taller amount on average than today's profiles. Give credit to Shimano that they did a lot of engineering work of shifting and how to reduce the effort. The current rings with their lift pins and shaped teeth specific to shifting ease do shift easier than the old classic stuff.

But one can improve their symmetrically shaped teeth chainrings. By filing down pairs of teeth at a few points (I like 5 roughly aligned with the chainring bolts) to about half their OEM height a W Cut like feature can be done. I've done this to a number of customer and personal rings with good results and no drawbacks that we have found later. Andy

cqlink 

Andy, thanks for the response. Greatly appreciated!