# Seatpost depth: is this too little? Will it break?



## .je (Aug 25, 2012)

Is this seat post inserted too little? It's 3-4mm above the 'minimum' line, and looks like it could be just above the lower edge of the TT. I don't want it or the frame to be broken.

It's a 22.5cm carbon seatpost adjusted to my regular height (tho I'd likely want 5mm even more). The bottom of the post barely, if actually, reaches the fillet at the bottom of the TT, and there is 75mm plunge depth where the 'minimum' line seems to be at 80mm. I wonder if I should return the new bike to the LBS for a new post (it seemed to fit well at first).

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## BlueMasi1 (Oct 9, 2002)

Your fine. The minimum insertion line was established by lawyers not cyclists.


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## looigi (Nov 24, 2010)

Hmmm...I would have suspected that it was engineers who determined the minimum depth of insertion based on the strength of materials, the structure, expected max dynamic loads, etc... I didn't think lawyers were good at that sort of stuff.

I'm not a lawyer but if the bottom of the seat post is supported inside the seat tube (not in some void created by the junction with the top tube) I suspect you'd be OK as long as your are significantly less than the max design rider weight.


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## mikerp (Jul 24, 2011)

I agree there is a bit of margin with the max height as long as you are not at the max weight (my guess is you are not near the max weight).

I missed the part in regards to wanting another 5mm of height.
Get a longer post.


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## .je (Aug 25, 2012)

The user manual says this:


> Make sure that the minimum insertion mark, also called Limit Insert height mark (Fig.4.5) remains inside the frame. A minimum of 2 inches (64 mm) of seat post must
> remain in the frame. Riding with the seat raised above this height may cause loss of control resulting in personal injury or damage to your bicycle


I just looked at the frame, and it's a continuous seat tube, so the full 3" (7.5cm) depth of plunged seat post is supported, which is a bit of a relief. I guess the only concern is the bending moment in the exposed seat post (it's 3mm wall thickness, carbon), but I suspect that's not really going to be a problem with my 175-180lb? I could go to the store where I bought it for their opinion.


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## danl1 (Jul 23, 2005)

I never thought "minimum" was such a confusing word.


But yeah, there is some margin, and was defined based on not being able to know all the possibilities about frame design, rider weight, and use. So, you are eating into your margin. The question is: How far, and how willing are you to deal with the consequences if you guess wrongly? Is that risk (however great / small you perceive it to be) worth the relatively minimal hassle of returning it and getting the right thing? 

And what about the endless hassles of dealing with following riders with the 'hey, did you know..." 's ?


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## 2:01 (May 10, 2010)

I've always made it a point to be 1/2" to 1" below the top tube. I've seen too many failures from using the "min insert" line (albeit on mountain bikes).


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## jpz (Jan 19, 2007)

I would tend to agree with danl1. I wouldn't want to be worried about what *could* happen. I would just buy a longer seatpost.

JimPz


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## MR_GRUMPY (Aug 21, 2002)

You're gonna die.

Goodby frame.

If you never ride "on the rivet" or slide way back, or go over bumps....You'll be fine.
.
.


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## brucew (Jun 3, 2006)

danl1 said:


> I never thought "minimum" was such a confusing word.


Yeah, it's right up there with "maximum" which some people define as "inflate to at least".


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## Roland44 (Mar 21, 2013)

2:01 said:


> I've always made it a point to be 1/2" to 1" below the top tube. I've seen too many failures from using the "min insert" line (albeit on mountain bikes).


Yeah same here, a friend of mine was in wheelchair for 2 months because of this mistake. Don't take that chance..


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## Kerry Irons (Feb 25, 2002)

.je said:


> The user manual says this:
> 
> I just looked at the frame, and it's a continuous seat tube, so the full 3" (7.5cm) depth of plunged seat post is supported, which is a bit of a relief. I guess the only concern is the bending moment in the exposed seat post (it's 3mm wall thickness, carbon), but I suspect that's not really going to be a problem with my 175-180lb? I could go to the store where I bought it for their opinion.


What is your riding style? Some people "ride light" by lifting off the saddle a bit every time they see some rough stuff coming and riding in the smoothest part of the road whenever possible. Others "ride heavy" by sitting dead on the saddle and not paying much attention to upcoming bumps. Riding light vs. riding heavy is like a 30-40 lb (15-20 kg) weight difference.


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## Peter P. (Dec 30, 2006)

The seatpost needs to extend BELOW the top tube otherwise maximum stress will be imparted onto the intersection of the seat tube/top tube/seatstay junction.

The post needs to be extended below the top tube a distance equal to the diameter of the seat tube. In your case it looks like 3cm more than it currently is. The extra 3cm is because the greatest stresses will be imparted onto the strongest, straight section of the seat tube, and not the intersection mentioned above.


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## velodog (Sep 26, 2007)

As long as you're wearing a helmet nothing bad will happen to you when it breaks.


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## .je (Aug 25, 2012)

I'm not sure why the manufacturer recommends a minimum distance of 2", then puts that depth chart on the post that recommends a distance of 3-1/4". Also not sure if the strongest part of the seat tube maybe isnt the part that has bracing on either side? Id be curious to hear from a mechanical design POV if this is important (it's my background so I am curious). A depth 1" below the top tube is a bit much to demand, if something like half the user group of this bike is excluded from using it? 

The shop it came from suggests that some customers ride like this, but would be happy to take a look at it, so I'll try to do that. I don't really know if they sell carbon seat posts... seriously, I'm tempted to return the whole thing and get that R3 or Tarmac SL3 I really liked that I thought were $1000 too expensive.


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## AJ88V (Nov 14, 2012)

2:01 said:


> I've always made it a point to be 1/2" to 1" below the top tube. I've seen too many failures from using the "min insert" line (albeit on mountain bikes).


^^ +1 ^^
The seat post, even a great seat post, is the cheap part. Is it really worth risking your frame. It might be ok with an aluminum frame which has some ductility and yields before failure.


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## wim (Feb 28, 2005)

.je said:


> I'm tempted to return the whole thing and get that R3 or Tarmac SL3 I really liked that I thought were $1000 too expensive.


Might not be a bad idea. Since you apparently plan to jack the post up even higher than you have it now, there's more than an even chance that the frame into which you're sticking this post is too small for you. On the other hand, the longer post (as others have suggested) would be way cheaper.


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## .je (Aug 25, 2012)

Just to wrap up, I'm going to get a new post because the original one is too short, like I was thinking (a 250mm post! I only need 25mm more), and was concerned about using it. No to decide whether to get one from China (3-4 weeks) or the US for double the price...

Even though the user's manual needs 2.5" of insertion, I asked the company if the post was OK (they're probably reading all this), and they came back that this could cause damage to the frame eventually and void the warranty, and suggest to use a longer seat post to get the proper insertion into the frame. To be honest, some of the advice on stress and strength are pretty comical and don't jive with anything I've learned or done in years of engineering and mechanical product design. I know you mean well, and are giving the right answer, but the basis of argument are fanciful and imagined, at best, and just wrong, at worst. I, like you, and any other person, don't like or deserve to be directly or insidiously called an idiot. Thanks for trying, we agree on the answers, anyway.

This post was helpful too, http://forums.roadbikereview.com/sa...ndations-looking-bargain-not-best-301629.html, now I just have to order one. Thanks all yous, from me and the cat I also consulted for this.

--

I picked up this post, and it's 100mm longer, and 4" (100mm, no surprise) below the top tube. It's also 60g lighter than the post that came with the bike (I weighed both, the new one is 193g vs 253g) which is a nice bonus, and if I really wanted I could take 2", even 3" off the end to save another 10g - 15g, my bike would probably _actually be buoyant in air_. It looks to use 2.5mm thick material instead of 3mm (the frame is 3mm all around) so I hope it doesn't break even though it's a safe and secure 4" below the top tube. 
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