by youngs_modulus on Fri Jul 07, 2017 3:47 pm
Carbon seatposts are not sweaters; they won't unravel if you fail to apply nail polish. I mean, there's no harm in doing so, but there's also no need as long as you have a reasonably clean cut. There are places where you'd want to prevent any cutting-induced cracks from propagating, but the ends of seatposts aren't very highly stressed. Most of those stresses are in circumferential compression anyway, and cracks tend to spread in tension, not compression.
People seem to want a little magic to be required when cutting carbon fiber, but many of these same people have no such concerns about cutting wood. They're both composites; a clean cut is a clean cut.
I personally use a dremel tool with a cutoff wheel to cut carbon parts (mostly steerers and seatposts). "Cutoff wheel" is actually a misleading name for those discs...they're just very narrow grinding wheels; they don't actually cut. Of course, that's also true for abrasive hacksaw blades marketed for cutting carbon fiber.
I do agree, however, that pipe cutters are a bad idea for carbon. Even if you just barely tighten them, their wheels make point contact with the post-steerer and the contact pressure is enormous. Using a pipe cutter for the entire cut tends to encourage "greenstick" delamination on the inner circumference.
Besides, if you're a weight weenie, you really want to cut at a ~45-degree angle, with the longer bit in front. Cutting seatposts at an angle is more structurally efficient (which is one reason Ritchey posts are cut this way). Cutting at an angle also allows the tip to deform a bit in contact with the seat tube, spreading the load and reducing the stress concentration in the seat tube.
Apply clear nail polish to the cut if you feel like it. But a clean cut on a post shouldn't require nail polish.
Edit: typos.