seat post length question.
 

How much of a seat post should normally be inside the frame, in order not to damage one or the other under the load in any way? Now I weigh about 170lb., that's 77 kg. The frame and seat post material is aluminum (27.2 seat post size). The reason I’m asking is that I want to save some weight by cutting off an inch or two.

PS. What happened to everyone’s post count? Did I miss an announcement or a recent discussion about this?

each seat post has a "minimum" mark, which tells you exactly how much of the seatpost should be in the tube. this is different from seatpost to seatpost, depending on material, diameter, weight, etc.

Well, I had 3 bikes myself and none of them had anything close to the "minimum" mark on their seat posts. I' also have never seen those marks on any of the posts in the store nor on any other bikes, I see on the street.

Many seatposts have a minimum insertion line. For my Easton carbon seatposts, it's listed as 2-3/4" but sometimes it's also a byproduct of the frame so you might want to check with the frame manufacturer as well as the seatpost manufacturer. I've been told that as a rule of thumb the post should always go past the bottom of the intersection between the top-tube, seat-tube and seatstay (if applicable).

My seat post is well below the seat-stay and top tube.

Minimum insertion marks aren't always easy to see; they're just etched in and can be obscured by grease, rust, whatever. But since the distance from the marks to the end of the tube varies from one post to the next, it's hard to tell give you a rule of thumb.

I have one cheap aluminum road post, 27.2mm diameter, where the marks are 3.25 inches from the end of the post.

RichC

If it's any help, here is a link to the seat post I have>>> http://www.bontrager.com/posts/detail.asp?id=95

All I know that it's Black Anodized, and I can't find the Part# anywhere:rolleyes:.

you wouldn't be able to see the minimum mark on bikes on the street, since it would be on the portion of the post that's well inside the tube.

i dunno, the seat post manual/notes should indicate it if you don't have a mark on the post... otherwise, i have no idea... i'd say you'll be OK with at least 3 inches? i'd start to worry if you only put 1-1 1/2 inches down the tube, when you have like 7 or 8 sticking out.

Mr. Chuvak........just skip a meal or two before you go riding. You'll save more weight than the weight of the entire seatpost.

The only time I had to trim my seatpost was when I had my MTB built up and discovered that the seatpost at its original length protruded too far through the interrupted seat-tube and could contact my rear shock when fully compressed. Road seatposts come fairly short to begin with. I'm not sure there's much weight savings by trimming it any more.

People please!:rolleyes: I don't want to be known as a weight weenie around this site (I'm really not abscessed with saving weight) Here is my thinking: IF IT'S NOT NESSESARY THEN WHY KEEP IT? If you don’t agree with me then take a look at your own bike. Look at the wheels...... How many spokes do they have? 18, 26, 28, 32, 36? You see where I’m going with this? If you are a 130 lb rider you don’t need a 36 spoke wheel.

If you actually believe that this is a good way to loose weight, you are in trouble my friend.

Mr. Chuvak.......you're absolutely right. That was very bad advice on my part. The last time I skipped an entire meal prior to a long ride, I

Haha, see you didn't have enough strength to finish typing

Just to be safe, keep three inches of insertion or to about an inch and a half below the bottom of the top tube, which ever is longer. You can hack off the rest.
That is odd obout the insertion mark, I heve seen thousands of seat posts and cannot remember one without a minimum insertion line on it.

I got a seatpost with an in-line clamp, but it was very long. I measured out a new max-insertion mark, with 1" extra in cae I need to raise the saddle, then cut off the extra 4" of tubing. You need to round off the bottom carefully, and grind or file a slight bevel or taper at the end

minimum insertion line
 

If I recall correctly, the main factor in determining the minimum insertion line is tube diameter, I don't ride without at least 2 times the seatpost's diameter inserted into the seattube. Inadequate length insertion could lead to a seatpost failure at speed or damage to your frame. "I got a seatpost with an in-line clamp, but it was very long. I measured out a new max-insertion mark, with 1" extra in cae I need to raise the saddle, then cut off the extra 4" of tubing. You need to round off the bottom carefully, and grind or file a slight bevel or taper at the end[/QUOTE]

Your link didn't work for me, so I don't know what model Bontrager you have. But you'll notice the minimum insertion mark on this one.

http://media.trekbikes.com/images/pr...rge/401830.jpg

and this one:

http://media.trekbikes.com/images/pr...arge/86962.jpg

I really doubt Trek's lawyers would allow them to sell a seatpost without a mark. I don't believe I've ever seen a seatpost without one.

The best rule is to be sure that the post extends at least to the lower edge of the TT. A full centimeter below is better. That rule has been around for a very long time. I wouldn't go less than 5cm.

My LOOK 585 frames have a reduced diameter area at the top to reduce the seat tube diameter down to 27.2. The total length is 6cm. Any amount of post lower than that is just hanging in mid air.

+1... it should extend just below the top tube

Dude,

There is not a seatpost on Bontrager's webpage that doens't have a minimum insertion mark visible in the picture.