I'd like to upgrade my aluminium road racer with a comfortable carbon seat post. But as the seat tube measures 31.6 millimetres and 27.2-millimetre seat posts are more yielding, I'd like to mount such a seat post with shim.
Is this something you can recommend in the case of carbon seat posts?
Reply by Dirk Zedler, TOUR technology expert and bicycle expert
Experience has shown that nothing speaks against it if you use good components.
Mounting a carbon seat post with shim in a frame requires, however, special skills from the person doing the mounting.
The shim must be absolutely free of burrs and at a length of 75 millimetres at least long enough to reach inside the seat tube below the top tube.
The seat post itself must also have the same minimum insertion depth. This is the only way to ensure that it provides a reliable hold in the frame.
The clamping slots must be flush and the combination must match perfectly, i.e. you should feel a little drag while sliding the seat post into the shim. There must be neither play, nor should it be hard to insert the seat post.
In the case of aluminium frames, the seat tube must be lubricated, the inner shim as well as the seat post must however remain grease-free and provided with a thin coat of carbon assembly paste. Use a low torque value which is just high enough to tighten the seat post. Do not exceed the maximum torque value indicated for seat post or frame.
