Is 88mm of seat post in the frame enough?
07-14-15 | 12:09 PM
#1
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Is 88mm of seat post in the frame enough?
I have about 135mm outside and around 85-88mm inside the frame the post is a 27.2mm carbon in alloy frame. It goes about 30mm bellow the bottom of the top tube. To my memory as long as it goes seat post diameter below it should be ok?. Also i weight about 145-150 threw out the year if that matters. Not sure how to post a pic on a moblie will try when i get on the pc
07-14-15 | 12:14 PM
#2
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From: Las Vegas, NV
Bikes: '04 LeMond Buenos Aires, '82 Bianchi Nuova Racing, De Rosa SLX, Bridgestone MB-1, Guerciotti TSX, Torpado Aelle, LeMond Tourmalet 853, Bridgestone Radac
Should be fine. In "the old days", 180mm overall length was a pretty standard size for a road bike seat post. Does the one you are using not have a minimum insertion mark?
Last edited by Eric S.; 07-14-15 at 12:24 PM.
07-14-15 | 12:30 PM
#3
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From: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
There are two conditions that must be met.
The first relates to the local stresses on then post and the tube when the post is loaded sideways. Think about a fencepost in a too shallow hole. If not deep enough, the post or hole distort and it tears out. The rule of thumb for this is about 1-1/2 diameters minimum (note the height of threadless stems which are subject to the same rule). But that's only half the story.
On frames where the seat tube extends above the top tube, as do most frames these days. The post cannot depend on the extension for support, so that 1-1/2 diameters rule applied from the height of the cross braces, and as a rule, you want the post to extend well below the bottom of the intersecting tube, and this is usually the defining issue for adequate insertion.
So, 1/2-1" below the bottom of the cross brace, or 1-1/2 diameters whichever is greater, and you should be fine. (which by your numbers you are).
Lastly, one more condition which applies to heavier riders and on bumpy roads where there'll be plenty of back flex over time. The post must be stiffer than the seat tube, otherwise it won't properly support the tube and the bending stress will lead to cracks at the base of the seat tube extension. This third rule is often overlooked leading to decent numbers of "mystery" frame failures even when the post went deep enough.
The first relates to the local stresses on then post and the tube when the post is loaded sideways. Think about a fencepost in a too shallow hole. If not deep enough, the post or hole distort and it tears out. The rule of thumb for this is about 1-1/2 diameters minimum (note the height of threadless stems which are subject to the same rule). But that's only half the story.
On frames where the seat tube extends above the top tube, as do most frames these days. The post cannot depend on the extension for support, so that 1-1/2 diameters rule applied from the height of the cross braces, and as a rule, you want the post to extend well below the bottom of the intersecting tube, and this is usually the defining issue for adequate insertion.
So, 1/2-1" below the bottom of the cross brace, or 1-1/2 diameters whichever is greater, and you should be fine. (which by your numbers you are).
Lastly, one more condition which applies to heavier riders and on bumpy roads where there'll be plenty of back flex over time. The post must be stiffer than the seat tube, otherwise it won't properly support the tube and the bending stress will lead to cracks at the base of the seat tube extension. This third rule is often overlooked leading to decent numbers of "mystery" frame failures even when the post went deep enough.
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FB
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An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
Last edited by FBinNY; 07-14-15 at 04:43 PM.
07-14-15 | 04:30 PM
#4
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Yea the orginal owner shortended it. And i was given it as a gift just wanted to make sure 30mm under the top tube was enough. On my frame the seat stays are pretty low where they meet i dont need to reach it right?
07-14-15 | 04:38 PM
#5
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From: Bozeman
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There are two conditions that must be met.
The first relates to the local stresses on then post and the tube when the post is loaded sideways. Think about a fencepost in a too shallow hole. If not deep enough, the post or hole distort and it tears out. The rule of thumb for this is about 1-1/2 diameters minimum (note the height of threadless stems which are subject to the same rule). But that's only half the story.
On frames where the seat tube extends above the top tube, as do most frames these frames. The post cannot depend on the extension for support, so that 1-1/2 diameters rule applied from the height of the cross braces, and as a rule, you want the post to extend well below the bottom of the intersecting tube, and this is usually the defining issue for adequate insertion.
So, 1/2-1" below the bottom of the cross brace, or 1-1/2 diameters whichever is greater, and you should be fine. (which by your numbers you are).
Lastly, one more condition which applies to heavier riders and on bumpy roads where there'll be plenty of back flex over time. The post must be stiffer than the seat tube, otherwise it won't properly support the tube and the bending stress will lead to cracks at the base of the seat tube extension. This third rule is often overlooked leading to decent numbers of "mystery" frame failures even when the post went deep enough.
The first relates to the local stresses on then post and the tube when the post is loaded sideways. Think about a fencepost in a too shallow hole. If not deep enough, the post or hole distort and it tears out. The rule of thumb for this is about 1-1/2 diameters minimum (note the height of threadless stems which are subject to the same rule). But that's only half the story.
On frames where the seat tube extends above the top tube, as do most frames these frames. The post cannot depend on the extension for support, so that 1-1/2 diameters rule applied from the height of the cross braces, and as a rule, you want the post to extend well below the bottom of the intersecting tube, and this is usually the defining issue for adequate insertion.
So, 1/2-1" below the bottom of the cross brace, or 1-1/2 diameters whichever is greater, and you should be fine. (which by your numbers you are).
Lastly, one more condition which applies to heavier riders and on bumpy roads where there'll be plenty of back flex over time. The post must be stiffer than the seat tube, otherwise it won't properly support the tube and the bending stress will lead to cracks at the base of the seat tube extension. This third rule is often overlooked leading to decent numbers of "mystery" frame failures even when the post went deep enough.
Seriously good information. You should write a book.
07-14-15 | 04:41 PM
#7
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From: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
I need a ghost writer, editor and illustrator, mainly the illustrator. Any volunteers?
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FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
07-14-15 | 05:14 PM
#8
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Joined: Jun 2015
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Its 30mm below just in your post where it said 1/2 to 1 diameter bellow the cross brace had me confused, if i needed to go below the seat stay joint also. Thanks for your help fbiny and eric. Its nice to be able to ask here as my lbs would tell me i need 100mm for sure in the frame and try sell me a post
07-14-15 | 05:21 PM
#9
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From: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Sorry for that, I don't consider seat stays as a cross brace in typical frame construction, so I was referring to the top tube. I'll keep that in mind and try to be clearer next time around.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
