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View Full Version : how long does a seatpost have to be?


d_douglas
01-27-2008, 04:45 PM
I am looking at buying a 26.6mm Thomson seatpost from a forumite for use on my Gary Fisher Klunker. I dont have a photo to show, so this may not be possible to answer, BUT...

How much post needs to be inserted into the seat tube? The post May be too short, but I am not sure. As the seat sits right now, a 250mm post would leave 100mm inserted. I would be comfortable with that.

The seller says the post is cut to 230mm, leaving 80mm inserted.

Given that this is a rather heavy duty steel frame with a dual top tube (a klunker replica, you see) is 8cm of a Thomson posted sufficient?

I weigh 200lbs, so stress loading needs to be considered. This is an odd sized seatpost, but it does come up on Ebay from time to time - should i take this one, or bide my time for a 250mm or 330mm version that will be unquestionably strong enough...?

Don Ventura
01-27-2008, 04:56 PM
I've always gone by having the bottom of the seatpost at least 2" below the bottom of the top tube when inserted to desired length. I think that guideline was for mtn bikes, but the idea was to have the seatpost supported by more than just the seat tube extension.

Dave
01-27-2008, 05:00 PM
Most carbon posts have a minimum insertion line 70-75mm from the end. The other good rule is to be sure that a least 10mm is below the lower edge of the TT.

stevep
01-27-2008, 05:42 PM
i vote no.
get a kalloy for a few bucks and save yrself some future pain.
not enough for a 200 pound guy.

BumbleBeeDave
01-27-2008, 06:01 PM
If you're putting this on an MTB it's a whole different set of stresses when you come down from a jump or jump over a trail obstacle. The thing could bend or break and you're stuck out in the back end of nowhere with no seatpost. don't know about you, but that would certainly ruin my ride . . . I'm with SteveP. Get yourself a longer Kalloy or something from PricePoint if you can't wait around for a "name" post with the right specs to turn up on eBay.

BBD

Frankwurst
01-27-2008, 06:13 PM
i vote no.
get a kalloy for a few bucks and save yrself some future pain.
not enough for a 200 pound guy.


Good Advice. :beer:

michael white
01-27-2008, 06:46 PM
they say it's not the length but the girth, but what do I know. . .

capybaras
01-27-2008, 07:12 PM
they say it's not the length but the girth, but what do I know. . .

someone had to say it. good job! :banana:

DarrenCT
01-27-2008, 10:15 PM
someone had to say it. good job! :banana:

im very offended by this. pls erase this thread.

:cool:

michael white
01-27-2008, 10:26 PM
I agree, you guys are way wack. . .

capybaras
01-27-2008, 10:29 PM
:banana:

markie
01-27-2008, 10:31 PM
The post definitely needs to go below the bottom of the top tube when inserted. How far below is a matter of debate. 1cm or 2cm might be enough.

I do not like Kalloy 1-bolt posts on mountain bikes. The bolts invariably break and the craddle of the seatposts ends up gently cupping ones delicate areas.

I do not think I have ever seen a broken Thomson. If you run too short a post your frame will most likely break, not the seatpost.

d_douglas
01-28-2008, 03:17 AM
For the record, the Klunker is never offroaded - its a commuter, so the stresses will be significantly less than mentioned.

Also, I am not so worried about breaking a post, but that I might either break a seriously cool and somewhat rare frame or break myself. That would suck - I already had a broken hand from a crash last year!

My instincts tell me that 250mm is the minimum acceptable length...

cheers,

Darren

bnewt07
01-28-2008, 03:59 AM
Thomsons installation instructions state it should be inserted to below the top tube junction.

stevep
01-28-2008, 07:22 AM
I do not like Kalloy 1-bolt posts on mountain bikes. The bolts invariably break and the craddle of the seatposts ends up gently cupping ones delicate areas.
.

markie,
"invariably"?

not true.
i sell many hundreds of these are i have never had one returned defective or busted.
it is simple and effective and will provide long years of use.
it is, in fact, a far better product than a lot of the stuff that costs far, far more.

markie
01-28-2008, 09:22 AM
Main Entry:
in·vari·ably Listen to the pronunciation of invariably
Pronunciation:
\-blē\
Function:
adverb
Date:
1646

: on every occasion : always <invariably late>



Well I imagine your users do not put them on mountain bikes that get ridden hard. I find one bolt posts suck for off-roading. I have broken bolts in two Kalloys and a similar, but nicer titec post.

The moutain bike forums are places to find stories of broken kalloy bolts.

d_douglas
01-28-2008, 10:36 AM
relax - just asking the question. For the record, I don't like single bolt posts on my MTB either. Not that it matters.