Just wondering, I have a 456 I am building up and it looks like the seatpost is going to be nearly at the minimum insertion point.

My question is this: does the seatpost need to be a minimum length in the frame in order for the frame not to fail? If so, how far in?

Thanks people. I would have thought 350mm would be long enough (5'10", 33" inside leg, on an 18" frame)

at least an inch or two below the top tube/seat tube join would be my view.

Brant's (and the general view) is 1cm below the bottom of the seat tube/top tube weld.

The ally insert factory-fitted into my Lynskey would mirror that, it's about 60mm long and about 10mm past the top-tube weld.

Ok… I will go home and check tonight. I have a feeling my next question will be about 400mm seatposts… Never thought of myself as long-legged before.

Huh isn't this why seatposts have a minimum insertion mark? Or am I missing something?

The minimum insertion mark on the seatpost is for the seatpost – the frame may require more insertion depending on design

you miss the point that frames vary in the amount of seat tube extension above the top tube, seat post markers are about seat post strength only. an inch or two below the top tube / seat stay lower weld is a good guide to avoid frame damage, perhaps a bit less if the fit is really snug, but always below the lower welds.

Ditto TJ (ugh!)

Just buy a 400mm post, end of discussion.

5'10" and a 33" inside leg – don't tell me you've never noticed you buy trousers in LONG, or are they always at half-mast round your ankles?

Well, I wear 32" trousers normally. I only measured my 'real' inside leg when fitting for a road bike – the shoving a ruler up against your inner thigh method. I was quite surprised to find I am actually 33". Maybe I just wear my jeans too low.