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Seatpost Minimum Insertion

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Seatpost Minimum Insertion

Old 08-25-20, 05:11 AM
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taylorgeo
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Seatpost Minimum Insertion

Does a shorter seat-tube that requires a seatpost to be extended more have a greater chance of breaking under a heavy rider (350 lbs.)?


Specialized Rockhopper: 400mm Seat-Tube





Kona Dew: 470mm Seat-Tube
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Old 08-25-20, 03:28 PM
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Kind of yes, kind of no. I think the seat stay - top tube junction layout has more to do with how strong things are, and most seat posts are a heavier walled tubing as well. Couple that with most of the flex being in the seat post and things should be fine. Being a heavier rider, I’d be aiming at a longer seatpost so that more sits inside the seat tube, for that bit more support. A longer post you need to cut will be better than one that only just goes in far enough
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Old 08-25-20, 10:56 PM
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I believe the seat post should have a mark indicating the minimum insertion required.
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Old 08-26-20, 03:19 AM
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Make sure that the minimum insertion mark is BELOW THE TOPTUBE. Otherwise you can snap of the portion of the seat tube that's above the toptube. I saw a bike frame that had that happen.

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Old 08-26-20, 04:42 AM
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I go for a longer seat post to minimize the risk.
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Old 08-26-20, 12:23 PM
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Originally Posted by taylorgeo View Post
Does a shorter seat-tube that requires a seatpost to be extended more have a greater chance of breaking under a heavy rider (350 lbs.)?
To answer this question, yes, I think everything on a bike has a better chance of failing under a heavy rider.
I always have the seatpost extend below the bottom of the top tube by a couple of inches on my bikes.
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Old 08-31-20, 10:50 AM
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In general : inside the seat tube with post bottom edge at or below the bottom of the top tube where it joins the seat tube..
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Old 09-01-20, 10:02 PM
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It is easy to be misled by a seat post tube that terminates quite a ways above the top tube/rear stays junction. That bit of seat post tubing above that junction has much less strength to resist the backwards forces that the seatpost puts on it. Everyone is giving you good advice, be sure the seatpost goes below that junction by several inches. I have seen several bike frames broken at this junction by the seatpost forces. I am not a fan of the downward sloping top tube for heavier riders. It can put so much seatpost out from the top tube/seat stay junction that it acts like a big pry bar on that area of the frame. Mountain bike frames are the exception. A high top tube is unsafe for MTB riding.
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Old 09-02-20, 09:14 AM
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Originally Posted by tallbikeman View Post
It is easy to be misled by a seat post tube that terminates quite a ways above the top tube/rear stays junction. That bit of seat post tubing above that junction has much less strength to resist the backwards forces that the seatpost puts on it. Everyone is giving you good advice, be sure the seatpost goes below that junction by several inches. I have seen several bike frames broken at this junction by the seatpost forces. I am not a fan of the downward sloping top tube for heavier riders. It can put so much seatpost out from the top tube/seat stay junction that it acts like a big pry bar on that area of the frame. Mountain bike frames are the exception. A high top tube is unsafe for MTB riding.
This is extremely informative, thanks so much.

So the bottom of the seat post, ideally, would meet the green line (at a minimum) in the pic below:


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Old 09-02-20, 09:57 AM
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Originally Posted by taylorgeo View Post
This is extremely informative, thanks so much.

So the bottom of the seat post, ideally, would meet the green line (at a minimum) in the pic below:


Yes it should be safe but a couple of thoughts. Seek out the bicycle manufacturers recommendation for minimum insertion depth for that model of bicycle. A lot of times that information might be hard to impossible to come by. The aluminum post probably will bend before the seatpost tube but I have seen seatpost tubes break just above the cranks when heavily stressed with very long seatposts and heavy riders. I guess the seatpost was bending the seatpost tube between the crank assembly and the rear stay top tube junction. The seatpost tube eventually cracked straight across as if cut with a hacksaw. I've seen a couple of these types of failure.
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Old 09-04-20, 09:28 AM
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The seat tube, in the above example, may have been bored out to restore the roundness inside after welding distortion..
and you found where the boring stopped

Butted seat tubes reduce the ID towards the bottom, but that's a tapered transition , not a square shoulder..
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Old 09-06-20, 09:33 PM
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As fietsbob notes both of the bicycles that broke above the crank had a brazed on fitting just where the break occured and the bicycles both had a very long chrome moly seatpost that was also inserted quite a ways into the seatpost. Both bicycle frames were steel so both were repaired. I weigh 240lbs and find that my bicycles will show stress on the seatpost/seatstay junction. I run my posts out about 10" but with my weight it can cause issues in this area and I keep a watchful eye on that part of my frames. Every couple of months I will give my bicycles an overall frame/fork inspection and I always look for signs of frame breaks and distortions. My bikes all have steel frames. My wife rides an aluminum Trek 7100 and I inspect it also.
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Old 11-04-20, 06:07 PM
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There's a good change you can actually crack the frame near the Seat tube if nkt inserted far enough
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Old 11-17-20, 11:32 AM
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I'd like to emphasize that this is not any sort of claim of scientific understanding.

Several years ago, a somewhat portly but shorter (I'd guess around 185 lbs., 5-7) rider I knew had his seatpost snap. The jagged end caught in some loose clothing, and as he tumbled to the ground he was pulled towards that jagged end.

The injuries (and years-long ramifications) he suffered were gruesome.
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Old 11-21-20, 07:26 AM
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Originally Posted by Danhedonia View Post
I'd like to emphasize that this is not any sort of claim of scientific understanding.

Several years ago, a somewhat portly but shorter (I'd guess around 185 lbs., 5-7) rider I knew had his seatpost snap. The jagged end caught in some loose clothing, and as he tumbled to the ground he was pulled towards that jagged end.

The injuries (and years-long ramifications) he suffered were gruesome.
Yep, that might take him out of the gene pool!!
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Old 11-22-20, 09:13 AM
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And he was just 185....
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Old 11-25-20, 09:44 AM
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Just asking: are you laughing at / making fun of a person who experienced life-altering injuries?
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Old 11-25-20, 11:40 AM
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Originally Posted by shelbyfv View Post
Yep, that might take him out of the gene pool!!
Originally Posted by Danhedonia View Post
Just asking: are you laughing at / making fun of a person who experienced life-altering injuries?
My thoughts exactly. Says alot about his feeble psychological state (or lack thereof)
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Old 11-25-20, 01:50 PM
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I've had some great interactions here, and some of the info I've gained is wonderful, but there's a definite "middle aged guy who never really got past the HS locker room" vibe among some that's a real turnoff.
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Old 11-28-20, 02:14 PM
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I broke a seat tube that was extended above the top tube. Combination of sloping top tube leading to a longer seatpost extension, and poorly reinforced seat cluster (my own design, sadly) led to an early fatigue failure. I've also had the misfortune of snapping two seatposts, tho I had the good fortune to suffer only bruising to pride and posterior. I think all three breaks were caused by a heavy rider on an extended seatpost; a lot of leverage into a junction that was evolved to support much smaller riders. In my cases the distance of the seat posts into the seat tubes had little bearing on the breaks.

I think if you have a choice, get the biggest frame you can to minimize the amount of seatpost levering against the junction, or maybe a springer saddle to absorb the peak loads on the seatpost.
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Old 11-29-20, 08:47 PM
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when I ordered my Domane, the 52cm frame defaulted with a 155mm shorter seat post. Due to my quirky body frame with longer legs and shorter arms, that would have put me almost on top the minimum insertion point, so I opted to get the 200mm seat post and it was a better option for me as now I have some room to play with if needed.
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