Re: cutting the Time VXRS seat-post? integrated design...

bjw@mambo.ucolick.org

On Jun 13, 11:50*am, walter <wamann...@gmail.com> wrote:
> so i'm checking out one of those time carbon bikes w/ the integrated
> seatmast.
>
> the included seatpin is about 12"/30cm long and says in big letters:
> DO NOT CUT.
>
> that seems surprising to me...if the benefit of an integrated design
> is supposedly lighter weight, then why cant you cut the length of the
> inner seat-pin to minimize the total weight? *i'd think all you would
> need is 3-4cm of seatpin inserted inside of the mast...but this DO NOT
> CUT limitation means you've got about a foot of seatpin material
> inside of the extended seatmast.
>
> anyone have any info on what the deal is w/ cutting the seatpin w/
> time's integrated seatmast designs?

[crossposted to rbt since the query was also posted there]

Okay, having just dealt out righteous retribution to
Fat Masters and integrated seatmasts elsewhere in this
thread on rbr, I will attempt a serious answer to this question.

For classic diamond frames, the rule of thumb has
usually been that you want the seatpost to extend at
least 1-2cm below the lower toptube-seat tube junction.
This because there is a strong bending moment
exterted by your (my) fat butt and it is best if this is
taken up by a seattube that is braced by the toptube.

For example, some MTB and compact frames have
a seat tube that extends several cm above the top tube.
If you stick a seatpost in there and the end does not
reach all the way down to the junction, your butt is trying
to bend the seat tube _above_ the top tube junction,
where it is unbraced and weaker. A failure here will
damage seatpost, frame, and butt.

I have not seen a Time VXRS DOHC 2.2L in person, but
looking at web pictures, it has a fairly skinny carbon tube
extending way above the top tube junction. My guess is
that this tube is designed to withstand the load of a Fat
Master butt when the mast has a substantial length of
seatpost inside. The seatpost makes the mast stiffer against
the Fat Master bending moment. With a 300mm post
on a road frame, it may run all the way down to the tube
junction or even below it, which would probably be the safest.

Your conclusion from this could be that the integrated seatmast
is partly marketing hooey that doesn't save as much weight
as one might think, due to the extra long post, or the need
to make the mast thick to resist bending. I just can't
imagine how one could think a bicycle marketing department
would stoop that low, though.

Ben