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-   -   Max seatpost height (https://www.bikeforums.net/classic-vintage/1245017-max-seatpost-height.html)

shoota 01-11-22 08:00 PM

Max seatpost height
 
How serious do we take the max line on seatposts? Itís just a touch over the line, but the bottom of the post is right at the bottom of the seat lug cluster.
https://cimg9.ibsrv.net/gimg/bikefor...a502ed7e7.jpeg

due ruote 01-11-22 08:07 PM


Originally Posted by shoota (Post 22370529)
How serious do we take the max line on seatposts? Itís just a touch over the line, but the bottom of the post is right at the bottom of the seat lug cluster.
https://cimg9.ibsrv.net/gimg/bikefor...a502ed7e7.jpeg

Thatís higher than I would be comfortable with. I would look for a longer post.

SurferRosa 01-11-22 08:08 PM

The marking doesn't mean much to me. I pay more attention to how much seatpost is below the bottom of the seat lug.

shoota 01-11-22 08:13 PM


Originally Posted by SurferRosa (Post 22370535)
The marking doesn't mean much to me. I pay more attention to how much seatpost is below the bottom of the seat lug.

Me too. How much do you like to see below the lug?

SurferRosa 01-11-22 08:38 PM


Originally Posted by shoota (Post 22370541)
Me too. How much do you like to see below the lug?

The answer might depend on how much is above the seat lug. My tallest seat post is 12cm above the lug. I'd like at least 1.5cm below the lug for that one ... maybe 2cm.

The shortest one is just 10cm above the lug. For it, I'm comfortable with its bottom at 0.5cm below the lug.

I'm only 155 lbs and ride pretty light in the saddle.

gugie 01-11-22 09:51 PM

Just get a longer seatpost. Giving advice to say it's ok to be above the line just doesn't seem like a responsible thing to do.

Biketiger 01-11-22 09:57 PM

I'd be okay with a cm or two but that's well below the line.

P!N20 01-11-22 10:00 PM

A piece of 300 grit should take that bothersome limit line right off. :p

Andy_K 01-11-22 10:00 PM


Originally Posted by shoota (Post 22370541)
Me too. How much do you like to see below the lug?

I think a read somewhere that you should have a length the diameter of the seat tube below the bottom of the top tube.

thinktubes 01-11-22 10:11 PM

This is why I can’t use Campagnolo seat posts

davester 01-11-22 10:16 PM

Do ya think that when they decided to put the minimum insertion line on the seat post they said "well, it's safe to put the seat post in to here, but heck, let's put the minimum insertion line up there just to mess with them"?

P!N20 01-11-22 10:28 PM

If you're not on the limit line your bike is too heavy.

79pmooney 01-11-22 11:04 PM

Two questions. How much is this frame worth to you and what is the seat tube wall thickness.

Basic engineering - your post is trying to rock around the top of the seat lug when you hit a bump. The very end of the seatpost is doing most of the work to hold in in place. The (simplified) load at that end is what you are doing to your seat X the distance, top of seat lug to seat top / the distance, top of seat lug to bottom of seatpost. So, if you have twice as much seatpost buried, the force the end will see will be half. Why does this matter? Because that force is directly against the seat tube wall. Too high and the tube bulges. Probably not catastrophic and if your frame is built like a tank, probably will not happen. But if this is a sweet Columbus SP (say) frame, that bulge would be a deal breaker to a potential buyer and something you see every time you look at the bike.

shoota 01-11-22 11:08 PM


Originally Posted by davester (Post 22370642)
Do ya think that when they decided to put the minimum insertion line on the seat post they said "well, it's safe to put the seat post in to here, but heck, let's put the minimum insertion line up there just to mess with them"?

Honestly, yes, I guarantee you they said ďHere is good enough, but letís move it up a half cm just to be safe.Ē The seatpost isnít the issue here, itís the frame.

ThermionicScott 01-11-22 11:09 PM

Seatposts and stems (and everything else on a bike) aren't omniscient -- the manufacturer had to take a reasonable stab at how much length would be enough to keep from damaging the part or most frames in use.

Those of us who like to work it out for ourselves will want the seatpost to extend completely below the junction with the top tube, and having one seatpost diameter beyond that should be plenty. Some frames have a fair amount of seat tube extension above the top tube, so IMO it's good to think about this even if you weren't trying to cut anything close!

shoota 01-11-22 11:11 PM


Originally Posted by 79pmooney (Post 22370655)
Two questions. How much is this frame worth to you and what is the seat tube wall thickness.

Basic engineering - your post is trying to rock around the top of the seat lug when you hit a bump. The very end of the seatpost is doing most of the work to hold in in place. The (simplified) load at that end is what you are doing to your seat X the distance, top of seat lug to seat top / the distance, top of seat lug to bottom of seatpost. So, if you have twice as much seatpost buried, the force the end will see will be half. Why does this matter? Because that force is directly against the seat tube wall. Too high and the tube bulges. Probably not catastrophic and if your frame is built like a tank, probably will not happen. But if this is a sweet Columbus SP (say) frame, that bulge would be a deal breaker to a potential buyer and something you see every time you look at the bike.

I canít argue this, it makes total sense. Itís a Centurion Ironman, so while not super special I donít still donít want to screw it up. The Centurion website said it should be a 27.0 seatpost but itís clearly labeled 27.2. That makes sourcing a longer one much easier, and is the route I will go.

Miele Man 01-12-22 12:56 AM

I like to have two inches below the top of a seatlug like yours. Ona bike with more seattube above the toptube I want two inches of seatpost below the top of the toptube.

I seen bicycles that had broken seatubes above the toptube because the seatpost was too high and acted as a big lever until t he seattube finally failed.

Cheers

verktyg 01-12-22 01:16 AM

Rule of thumb for both seatposts and stems: Minimum insertion depth 75mm to 80mm (around 3")...

You don't want to be a "hey, watch this" kinda guy do you?

verktyg :50:

jdawginsc 01-12-22 03:54 AM

Save that post for a high collar Cannondale and watch the insertion mark disappear under the rim of the seat tube! Safe!

i have one if you need it...

oneclick 01-12-22 04:03 AM


Originally Posted by 79pmooney (Post 22370655)
So, if you have twice as much seatpost buried, the force the end will see will be half.

No, it'll be even less than that because what you add to the bottom you take away from the top; the pivot remaining as the top of the seat lug.

As to the question I'm with the get-a-longer-one camp.

bwilli88 01-12-22 06:56 AM

My Touch is a whole lot lower than yours. That looks like 1/2 inch or more, a touch would be 1/8 or so.

shoota 01-12-22 09:55 AM


Originally Posted by bwilli88 (Post 22370820)
My Touch is a whole lot lower than yours. That looks like 1/2 inch or more, a touch would be 1/8 or so.

Have to agree, I thought the line was lower when looking at it in my dimly lit garage. Longer post it is.

jdawginsc 01-12-22 10:00 AM


Originally Posted by shoota (Post 22371037)
Have to agree, I thought the line was lower when looking at it in my dimly lit garage. Longer post it is.

That seatpost needs a nice Cannondale to partner with it...

Andy_K 01-12-22 10:08 AM


Originally Posted by jdawginsc (Post 22371045)
That seatpost needs a nice Cannondale to partner with it...

I'm not convinced the Cannondale high collar really helps much. The pivot point is still the top tube. I think the high collar just offers a false sense of security. Or takes away the manufacturers "room for error" if you prefer.

I'm picturing the forces kind of like this:

https://ak.picdn.net/offset/photos/5...set_572504.jpg

And the collar above the top tube would be like having another kid on the other side of the see saw trying to hold it up from the bottom.

SJX426 01-12-22 10:39 AM

The Colnago is on the small side with keeping the components consistent, hence the Camp seat post. The saddle is a bit too short but.....I used a Brooks Pro that gives a little more leeway with the post insertion due to its higher rail to sit height. I feel I am at the absolute max with this arrangement. The line is 55mm from the end of the post.
https://live.staticflickr.com/65535/...5cc3641a_b.jpgP1030606 on Flickr
This is the shorter version of the post. I am sure the longer version is a needle in a haystack.


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