I need to replace the seatpost collar on my 2005 Trek which has a 27.2mm seatpost and I don't know what size collar to get. Do the collar sizes reflect on the outer diamater of the seatpost or the clamping area of the post instead?

For instance. I'm getting a Felt F3c frame which utilizes a 27.2mm seatpost also. Are the collars interchangable or are they frame specific?

Any advice would be appreciated. Thanks.

The clamp diameter is the outer diameter of the seat tube. Best to get a string and measure. If you have a 27.2 seatpost, the clamp will probably be 31.8.

if the frame takes a clamp-on front derailleur, use a seatpost collar of the same size as the clamp

That doesn't always work.

The Bottecchia groupbuy frame is a 35mm FD clamp, 30.0 seatpost clamp.

So don't just go buy something without checking.

That's not always the case. Some seattubes have varying diameters.

Just how do you measure diameter with a string? Circumference, yes.

A 30mm FD clamp might keep the FD from dropping down every time I pull the chain to the big ring

Especially with carbon frames, I would measure with a caliper since they use varying outer wall diameters.