nrsmd

Is there a recommended depth for the seat post on a road bike? To gain a little less weight, my seat post is 350 mm and probably 150 mm is below the locking rim. Any reason I cannot cut the post to leave, say, 50 mm extra depth instead of 150 mm depth?

CACycling

The weight savings will be minimal but there is no problem doing it. Easiest way is to measure the distance from the minimum insertion mark to the end of the post and make sure you have at least that amount in the seat tube after cutting (it would be prudent to give yourself a bit of cushion). Would also be a good idea to scribe a new minimum insertion line on the post as a reminder.

davidad

It's a silly reason, but the minimum insertion depth is usually 2&3/4".

cny-bikeman

I would give 2 reasons.

1. You will gain more speed by using the time to ride instead of sawing the seatpost. Saving that little weight will make zero difference (assuming <.1% is close enough to zero)
2. If you resell the bike to someone taller you are going to take a hit for them having to buy a seatpost.

FBinNY 

No problem cutting, but make sure you end up with a post that meets both requirements for seatpost insertion

1- as marked on the post an insertion depth of at least 2 diameters is necessary to prevent the post from camming out. The makers play it safe and usually mark at 2.5 diameters.

2- the post must extend to about 1" below the bottom of the toptube, to ensure no stress on the part of the seat tube extending above. For most frames this is no issue as the first considerations covers it. But for those with extended seat masts of 1-1/2" or more this becomes an important 2nd consideration. I've seen frames buckle because owners diligently inserted a post to the marked depth, and had it ending above the toptube, putting full stress on the cantilevered section of seat tube.

As long as your post meets both requirements, any extra length is only added weight (not much) and possibly higher resale value, but dosn't add any strength.

OTOH-There is an advantage to cutting posts besides weight. Some small frame owners have issues with long posts bottoming on bottle bosses. Also, though it shouldn't happen, if your post should freeze into the frame, extra length makes removal that much harder.

JohnDThompson 

Before the CPSC mandated "minimum insertion" lines on parts, the rule of thumb we used was that insertion depth should be at least 2.5 times the diameter of the part being inserted.

fietsbob

some frames extend the seat tube. above the top of the top tube,
Then, It should be long enough to have the seat post end
below the intersection of the bottom of top tube.