CF Seatpost Length

[Irretiate]

Unboxed my new 2023 Specialized Tarmac SL7 this afternoon. Noted included CF seat post is their 380mm length item. They have an optional 300mm one ($250) but there was no choice I could see when I ordered the bike a couple of weeks back.

The 23" (58cm frame I think?) steel-framed Motobecane I've been riding for literally decades is prolly too big for me @ my 5'-10' standing height; it's top tube grazes my crotch when I'm straddling it w/ feet flat on the pavement. Measuring seat height from center of crank axle gives 28".

With the SL7's 380mm seatpost inserted as far as it can go (seat tube changes shape close to wheel) I get that same 28" seat height to the crank's axle center. This is on a 56cm frame.

So I'm wondering whether I should try to exchange the as-yet unused 380mm seat post for the optional 300mm item? My fear is that the provided long post won't permit any lowering whatsoever should that be found an attractive geometry once I get to riding this F1-style road bike.

Spending a wad on a 21st century tech package then being confronted with a smallish cockpit-sizing restriction is a bit daunting.

I could (of course) shave the back of the 380mm seat post (I've worked with CF and epoxy before to a limited degree so I'm aware of the particular hazards with this material) so it seats a bit lower (1" would be enough I think) but I'm hesitant to void my warranty or do anything to affect my safety while enjoying this step up in bike tech.

[canalligators]

Don’t base your initial seatpost length on a historic value from a very different kind of bike.  The posture is very different.  I found that configuring a Rans/Phoenix bike to be a long and iterative process.  Changing one dimension or angle usually required tweaking another at tha same time.  For example, little higher might require a seat angle change, which needed a second height adjustment.  It took me about six months and several hundred miles of riding to dial it in.

FYI, my bike is a Sequoia with the stock flat seat used at the time.

You’ll be able to tell in a short time if your post is too long.  If not, I’d leave it full length.  I ended up with the seat higher as I dialed it in