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Even with my seatpost down as far is it will go (due to seat tube design) there's still about 6 inches of seatpost on show.
So there's a minimum insertion mark on the seatpost and a minimum insertion inspection hole on the seat tube.
If I cut say 3 inches off the seat tube does that simply theoretically move the minimum insertion mark on the seat post up by three inches or are there other factors to consider such as strength being compromised by a reduction in length?
spot on
I think he means yes to the first part of your Q and I would agree.
Aye I was wondering 😆
Actually it's more like the minimum insertion mark on the seatpost is 30% of the way up the post, cut three inches off and it will still be 30% of the way up the post, so less than 3". Make sense!?
don't think so njee.... i'd scratch the post, marking the minimum the same amount it was before you cut it.
i cut my thomson post because i couldn't drop the post fully due to the bottle cage bolts. fegging long post to begin with (esp in a small frame).
Of course it is, compare a 367mm Thomson to a 410mm Thomson, the minimum insertion mark is not in the same place on both.
It was a bit of a generalisation, and you're certainly not going to go wrong if you just assume the minimum insertion mark goes up the same amount as you've chopped off.
What happens if you get a 400mm post in a road bike and chop 150mm off it? Your minimum mark will be virtually touching the clamp!
Remember its not just the posts min insertion you gotta worry about though, the frame will have a min requirement too.
The min line on the post is a bit misleading as to what your frame might actually need.... My Spesh has a hole in the seat tube defining the min insertion of the seatpost and it has no bearing on the line marked on my seatpost.
That was alluded to in the first post too.
so it was...
min insertion points on frames vary with frame design
min insertion points marked on seat tubes derive at least partially from a british standard [i][b]IIRC[/i][/b], relating to a minimum length related to the diameter of the tube.
*if* that is the case, then chopping 50mm off the bottom would move the min mark up the same amount.
The leverage will be less so it shouldn't make any real difference if you chop a couple of inches off and move the MIM up by that amount.
my rule of thumb, it needs to be inserted to at least the bottom of the seat tube / top tube junction and a bit further below that. That's for the frame's sake; the post itself may have a longer requirement based on the strength of the post / resistance to bending forces, etc.
Of course as you cut a post shorter, it reduces the amount of leverage and i would suggest the insertion depth can also therefore be reduced somewhat but I'd be damned if I'd proscribe a formula for that.