Seat post - how long is too long?

roy_batty

I was wondering what the minimum length of seat tube that should be in the frame?
i.e. what's the shortest length that the frame needs to clamp.

Bit worried, as I just bought a new bike (well, new to me!) and I've only got 2 inches of tube in the frame.

Cheers

redddraggon

There's normally a minimum insertion mark, but you should aim to have the seatpost inserted to below where the top tube meets the seattube, this is normally about 6 or 7cm.

roy_batty

Thanks - will have a look at the bike tonight.
The minimum mark has long worn off the tube. :oops:

King Donut

Stupid question, but is the bike big enough?

roy_batty

Ah...... there is that... :?

pieinthesky

On many steel and alloy frames (and probably titanium) the ends of the frame tubes are butted (thicker) to give more strength at the joint.

Often, but not always the top end of the seat tube is not butted ie is thin. This is done because the close fitting seatpost reinforces the top of the seat tube effectively doing the same job as a butted tube.

By having insufficient seat post inserted into your frame you might be compromising the strength of the frame. So dont risk it, especialy considering that having alot of seatpost showing will be increasing the leverage and hence the stress on the top of the seat tube as well.

I would suggest 3 to 4 inches insertion minimum but you could check with the frame builder to be sure.

I dont know how this applies to carbon frames but would err on the side of caution.

If you found that a bit complicated, what I was trying to say is - go and buy a longer seatpost!

roy_batty

cheers