FYI - all of Sherwood's frames are aluminum.I recommend 4" as a minimum insertion on seat posts. This will get you to start of the flared section of the seat tube and will give you maximum strength and stiffness. The minimum insertion point marked on the seat post is for the survival of the post, not the frame. Most people don't take this into account.
On a lot of frames the seattube flares internally after several inches. Therefore, you could insert a post as far as you want, but will only be as sturdy as the unflared section of the seattube allows it to be.FireDog46 said:The seat post must be inserted far enough to pass beneath the bottom of the
seat tube to top tube weld. In the absence of a ST/TT gusset the seat post
should be inserted 10mm to 20mm past this weld. Consider the Ventana
El Fuego vs the El Saltamontes as examples of the differences. One without
a gussest and one with a gusset.