This may depend on the seat post and the fixture, but how deep should my seat post be into the frame? I have a bike that came with a post that is pretty long and I cut an inch off the bottom of the post a couple months back so that I could sit lower, now I'm considering cutting two to 4.5 more inches off so I can get further down on the bike during some technical riding and so that I can pump deeper. How far into the bike does the seat post need to go before stability is compromised? Are there any other risks of having your seat post not too deep?
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1Any decent seat post (decent, not expensive) will have a minimum insert line on it showing the minimum depth it should be inserted. If you have a seat that doesn't have one, or you cut yours off, I would recommend buying a new seatpost.– Deleted UserCommented Jan 7, 2016 at 23:12
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1Find the chunk you cut off, and see where the mark is on it. That's the length that must be inserted into the seat tube. Note that there may also be a mark on the seat tube to identify the minimum inserted seatpost length.– Daniel R HicksCommented Jan 8, 2016 at 1:37
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1(But usually there should be no need to cut the post short, as the post can be inserted some distance into the seat tube. The only cases I've seen where cutting might be necessary were (A) where the "seat tube" is only a short length interrupted by the suspension and (B) where the seat tube is bent for some reason (usually minor frame damage).)– Daniel R HicksCommented Jan 8, 2016 at 1:40
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1I've also had problems running into bottle cage bosses and or the mounting screw.– dluCommented Jan 8, 2016 at 4:27
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2 Answers
Most seat posts have a "minimum" insert of 3 to 4 inches. But this varies with the material and the thickness of the post, and the weight of the rider means the minimum might still be too little.
As a tall rider I frequently have my seatposts up to maximum, and have bent several over time, and have fractured one frame. Now I always buy a 450 mm or 500 mm post and almost always reinforce it with an insert tube up the middle. I will not ride a seat post without 5" of frame insertion.
Since you sound like a MTB kind of rider, consider a dropper post rather than chopping ruthlessly through your seat poles with a hacksaw. These are intended to go at a good height for normal efficient riding, but you can turn the interlock and they slide down ready for low-seat technical riding. I understand you can do it one-handed while rolling too.
Once you're done you can raise the seat again without stopping. Downside is they probably cost ten times more than a normal seatpost.
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Yeah, I love dropper posts. I had one on a rental recently. Next time I upgrade I'll definitely get a bike with one. For now, I found the minimum marking on my seatpost, thanks!– James G.Commented Jan 8, 2016 at 5:01
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@JamesG. You found the minimum mark on the seatpost that you already cut short? So the new minimum is above the stamped minimum, by the amount you removed. Please don't ride the bike with only 2 inches inserted, that's not enough.– Criggie ♦Commented Jan 8, 2016 at 6:14
The manufacturer of the seat post would have really answered this with the minimum mark. You should really maintain at least the same length within the frame.
Aside from stability, if the seat post is too short, then you risk damaging the frame of the bike, because there is a higher force within the frame due to the same torque being required, but at a shorter distance.
So to answer your question, I would recommend not cutting your seat post shorter, unless your are sure that you still have the same length of post in the frame that the manufacturer specified as the minimum length.
