Oval carbon saddle rails compatibility

Question

I have a new saddle and I want to buy a compatible seatpost. The saddle rails is CARBON 7x9 as specified by the manifacturer.

I want to understand better the compatibility of this kind of oval rails. Some seatpost manifacturers specify compatibility, here is an example:

Rail Compatibility Round and Square Rails

from http://www.zipp.com/seatposts/sl-speed-seatpost--0-/

However, in most other cases, the compatibility is not specified.

Should I assume 7x9 is compatible by default with any seatpost or not?

Edit Follow-up Note: I happened to notice that some manifacturers offer the options of clamps for different size of rails, this is from the Thomson page, for example:

This seems to suggest that there is some difference between the clamps I have to pick.

Answer 1

7x9 is the size that tends to be used when they're trying to make it compatible with "any" seatpost. Normal metal rails are 7mm, so making it a 9mm oval/rounded rectangle fits in in most cases. It's kind of a hack solution that's become normalized. If you look you could find seatposts where a 7x9 will give either iffy thread engagement or interference/awkwardness with the clamp, although neither are common issues by any means. And of course posts exist that clamp the sides of the rails, and you need to avoid those. What you do want to look for is a post that's got nice abundant contact area with the rails and isn't going to press on them with any sharp edges. You can find posts that are counterexamples of this, mostly older basic ones, and thats what you want to avoid.