Seat Post : 1cm beyond max limit : SAFE??



jackchoo

New Member
Mar 6, 2006
58
0
0
hi i went through some tough times trying to source for an old shimano 600 seat post and finally found one after foraging into bins of old lbs-es around town.

While setting up I find that in order to fit my height, I need to set it up about 1cm BEYOND the >>> max <<< indicator on the post....The bike is a steel lugged frame, I'm about 195lbs and 6' tall. The post size is 26.8 and the alloy thickness seems to be around 0.5cm (IMHO, quite thick). At this setting the seat post goes about 2" into seat tube and extends slightly beyond the lug itself.

Is there something i should be worry about?
 

alfeng

Well-Known Member
Jul 23, 2005
6,723
254
63
jackchoo said:
hi i went through some tough times trying to source for an old shimano 600 seat post and finally found one after foraging into bins of old lbs-es around town.

While setting up I find that in order to fit my height, I need to set it up about 1cm BEYOND the >>> max <<< indicator on the post....The bike is a steel lugged frame, I'm about 195lbs and 6' tall. The post size is 26.8 and the alloy thickness seems to be around 0.5cm (IMHO, quite thick). At this setting the seat post goes about 2" into seat tube and extends slightly beyond the lug itself.

Is there something i should be worry about?
FWIW. I believe that you are in jeopardy of damaging the frame ... and, in so doing, risking a failure of the seatlug area which could manifest itself as a seeming "failure" of the seatpost.

If cosmetic continuity is important to you (and, it appears to be), Shimano had longer, MTB seatposts in the 26.8 size.

Many of the older/(and, current?) Shimano seatposts were apparently made by Easton.
 
Dec 30, 2007
2,111
8
0
jackchoo said:
hi i went through some tough times trying to source for an old shimano 600 seat post and finally found one after foraging into bins of old lbs-es around town.

While setting up I find that in order to fit my height, I need to set it up about 1cm BEYOND the >>> max <<< indicator on the post....The bike is a steel lugged frame, I'm about 195lbs and 6' tall. The post size is 26.8 and the alloy thickness seems to be around 0.5cm (IMHO, quite thick). At this setting the seat post goes about 2" into seat tube and extends slightly beyond the lug itself.

Is there something i should be worry about?

As mentioned, you probably won't hurt the seatpost but you may damage the frame. The limit line is there for a reason.
 

jackchoo

New Member
Mar 6, 2006
58
0
0
As mentioned, you probably won't hurt the seatpost but you may damage the frame. The limit line is there for a reason.
okay guys, thanks for the vivid picture (really).....I get it! Guess I'll lower it to the limit line for now......

how does it damage the frame though? Is there a guideline other than just the limit line? I have a kalloy post of simiilar vintage which is slightly longer (about 5cm) but the limit line is gone (very used....).
 
Dec 30, 2007
2,111
8
0
jackchoo said:
okay guys, thanks for the vivid picture (really).....I get it! Guess I'll lower it to the limit line for now......

how does it damage the frame though? Is there a guideline other than just the limit line? I have a kalloy post of simiilar vintage which is slightly longer (about 5cm) but the limit line is gone (very used....).

GENERAL rule of thumb, twice the top tube width below the bottom of the top tube for minimum seatpost insertion.
 

531Aussie

Well-Known Member
Apr 11, 2004
12,652
303
83
^ yeah, what he said. The bottom of the post should be at LEAST below the bottom of the top-tube, like this:


cinellieaston2.jpg
 

alienator

Well-Known Member
Jun 10, 2004
12,596
310
0
jackchoo said:
how does it damage the frame though?

It can damage the frame in a couple ways. Without enough seatpost in the seat tube, the lateral loads the seatpost would place on the seat tube might induce strain that would exceed what the seat tube could normally handle, causing either the seat tube to permanently deform or to crack outright.
 

jackchoo

New Member
Mar 6, 2006
58
0
0
ok....so which is it? anywhere below the top tube or 2 x top tube width below the top tube....i attach a pic..thanks!
 

alienator

Well-Known Member
Jun 10, 2004
12,596
310
0
jackchoo said:
ok....so which is it? anywhere below the top tube or 2 x top tube width below the top tube....i attach a pic..thanks!

More like the blue option, but you have to ask yourself if using a too short post is worth the possible frame damage and/or the traumatic creation of new orifices in you perineum. I once had a patient that had a laceration--that opened his abdominal cavity--that ran about 1cm from his nutsack all the way through one side of his anus/rectum. He was exceedingly "uncomfortable." FYI, it's possible for a broken seatpost to do that.
 

jackchoo

New Member
Mar 6, 2006
58
0
0
alienator said:
More like the blue option, but you have to ask yourself if using a too short post is worth the possible frame damage and/or the traumatic creation of new orifices in you perineum. I once had a patient that had a laceration--that opened his abdominal cavity--that ran about 1cm from his nutsack all the way through one side of his anus/rectum. He was exceedingly "uncomfortable." FYI, it's possible for a broken seatpost to do that.
ok ok i got it....reading your vivid reply gave me a really painful thought.....:eek:
 

meb

New Member
Aug 21, 2003
1,219
0
36
jackchoo said:
okay guys, thanks for the vivid picture (really).....I get it! Guess I'll lower it to the limit line for now......

how does it damage the frame though? Is there a guideline other than just the limit line? I have a kalloy post of simiilar vintage which is slightly longer (about 5cm) but the limit line is gone (very used....).

I needed some extra height on the a 400 mm seatpost on a folding bike so went about an inch above the insert line. The seattube extending above the tube (monotube rather than merely a top tube) bent back after about 2 years. I'm about 230. The seatpost had also developed an arcuate bend as well.

I was able to bend back the seat tube top, very easily (this is a steel bike) with my fingers, so obviously it was then very weak and wasn't going to support any more high seatposting. Had trouble finding a seatpost in that size, so elected to get a 600 mm steel pipe from Home Depot. I inserted it all the way to the bottom and got more height than was available from the original seatpost. I sacrificed a couple of pounds, but it was riderable. I had to adjust the rear suspension spring to reduce pogoing with the longer moment arm from the higher and more flexible psuedo-seatpost.

If you can't find a longer seatpost, something else you might consider-you could get a handlebar riser stem extension and mount it on the seatpost as a seatpost extension. Quill insert type might give you a second adjustment point as well.
 

Similar threads