Minimum Seatpost Insertion
 

I want to raise my saddle about 5mm, but it's currently right at the minimum insertion mark.

Any ideas on how much, ahem, flexibility there is in this matter??

You can get a longer post.

I know. But it's only a few mm!!!

I'd guess there was a reason someone put those marks on there, up to you to see the limits.

You could move it up a couple mm, and it may ride securely forever. Or, it may prang out of the frame and introduce your scranus and your frame directly.

Get a taller post, they're cheap.

It is all a matter of your weight and riding style. But who can figure all that? Are you that determined to try your luck at the safety lottery?

Ok. I will look for a longer post!

Look at it this way:
Is it worth risking the seat post snapping? Sure, because you will have to buy a different post regardless.
Is it worth getting injured when it snaps? Sure, you just call it a rebuilding year.
Is it worth potentially breaking your new Roubaix frame? No, it is not. After the deliberation and story that went in to it, it is MasterCard priceless at this point.

The minimum insertion reduces the load on both the post and the frame, so I wouldn't cheese it.

You need to pay attention to that minimum insertion mark, or you could find out what the maximum insertion could end up to be... ;)

Definitely get the longer post, not one of those things worth risking.

If you were not a heavy rider I wouldn't worry about going 5mm past the mark.

I am a typical road cyclist, weight runs between 135-145 pounds. Not sure if that makes a difference.

A few mm (</= 5) above the minimum insertion line is not gonna hurt at your weight.

I kinda figured that, but was wondering if any structural engineer types can say for sure one way or the other.

I can tell you that with you at 145lb the post and frame won't be as stressed as they would be with a 200lb rider even if you went 10mm past the mark. Add to that a safety factor in the design, you will be fine.

IMHO, it also depends on the frame. If we're talking a conventional tube constructed frame, if the bottom of the post is below the bottom of the top tube, you're likely ok. I rode a steel frame where the minimum insertion mark was just exposed above the frame lug, half the way around ( the lug was curved) for years with no adverse effect.

If the seat tube on your frame sticks up above the top tube, so that you're not getting the seatpost below the top tube, I wouldn't do it. If the seat post still reaches below the tt, you're likely ok.

Realize that the minimum inseertion mark is put on the post to deal with a range of frames. What you actually need for your seatpost, frame interface can varie.

All that said a longer seatpost is cheap.

Oh my. That's rich.

'Round here, that's a typical 8th grader.

This is technically incorrect, but no issue in 20 years:

http://i72.photobucket.com/albums/i1...aramount-3.jpg

Yeah. The min insertion mark provides adequate strength and safety margin at the max design weight and max design road impact. If you're significantly lighter than the max design weight, you can achieve the same safety margin with the post inserted less the min mark. How much less you can get away with depends on a lot of factors. Agree that at 145 lb, you're likely safe with 5mm less than min insertion.

Merlin, you're right about the amount of post that needs to be gripped by the frame. But that is only part of the safety story. There is also the question of how much post is exposed from a post strength and applied torque point of view. The post could be perfectly securely held in the frame and still snap off somewhere along the exposed length, most likely at the clamp. The longer the exposed length of post, the higher the applied torque on the post at the clamp. Just sayin' that minimum insertion varies both with the minimum amount needed for the seat tube to securely grip the post as well as with the total length of the post. and, of course, the post material, wall thickness, and resulting strength play an important part is this determination.

How much is left below the "minimum insertion" mark? That will be the important part. If there is a generous amount, then I don't see 5mm making much difference in normal circumstances.

However, having said that, it's not so much the seat post itself, but the frame material, and whether the seat tube is capable of sustaining the leverage that will become more localised than with a large amount inserted.

If it's steel, probably OK. But if it's aluminium or carbon, there may be a risk. With carbon, if there is a reinforcing sleeve of aluminium bonded into the top of the seat tube, you may also be OK.

Then there is the expansion slot. How far does this extend down the seat tube? If a long way, and your seat post doesn't have much meat below the mark, then you might have an issue.

All said and done, 5mm isn't much for the seat post itself, but you will have to assess the effect on the frame.

I cannot believe you are asking this question.

You are dead to me now.

5mm is this: [ ]

Poser