Confused about seat clamp needed (and rack mount question!)

[jay clock]

I have a Ribble CR3 carbon bike which I want to mount a lightweight rack to for commuting. Weight will not be a lot.

[Summing up I need something which will allow me to fit a rack to my seatpost clamp (or an alternative cunning plan). Any suggestions!]

Details:

It has lower mounts (currently just mudguards but plenty of clearance for rack as disk brakes are inboard) but due to the carbon frame (and square section seat stays) I am looking to mount the upper rack mounts onto the seat clamp

So I looked for seat clamp rack adapters and loads like this https://www.evanscycles.com/fwe-sea...Jdy3P033jLG3L4zD7xHCQ9rIBfTWlxehoCpI0QAvD_BwE which seem to be the sort of thing needed

However the current seat clamp is one of these https://www.ribblecycles.co.uk/ribb...3_1575194970_c275ba0cfd59b8cfc3ce1ec22e5f633e with (apparently) a different bolt for the frame and the seatpost and different sizes quoted 31.8 and 34.9 (one of the comments says "The seat clamp is clever as it clamps to frame and seat post separately")

The seatpost is 31.8 (printed on it)

Summing up I need something which will allow me to fit a rack to my seatpost clamp (or an alternative cunning plan). Any suggestions! And do I need a 31.8 or a 34.9?

 

[Vantage]

You need the 34.9 as that's the one that clamps the seat tube around the seat post.
The 31.8 is to clear the seatpost so not as important.

 

[cyberknight]

You need the 34.9 as that's the one that clamps the seat tube around the seat post.
The 31.8 is to clear the seatpost so not as important.

+1
my old boardman has a 31,8 post and i used a 34.9 clamp