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?
You need to measure the outside diameter of the seat tube where the collar (or clamp) sits. There are several different sizes. The two most common sizes these days are 1 1/4" (31.8mm) and 1 3/8" (35mm).
The seatpost size has nothing to do with seatpost clamp size.