Lombard

This thread is the gift that keeps on giving.

tomato coupe

As per the assembly instructions for one of my bikes:
In other words, the minimum insertion for the frame may not be the same as the minimum insertion for the seat post, so you should choose the greater of the two to protect both the seat post and the frame. Depending on the details, the minimum insertion mark of the seat post may not protect the frame, or the minimum insertion of the frame may not protect the seat post.

genejockey 

This is what happens when you make a silly, absolutist assertion which is obviously nonsensical and then find yourself trying to defend it with irrelevant horsehockey. Note that the instructions you just quoted refute your argument that seatpost designers would need to know the specs of EVERY frame, since the frame manufacturer could list THEIR requirements in the instructions, as they have here. The idea that seatpost designers don't concern themselves with not causing the frame to fail is just ridiculous.

tomato coupe

I never stated the seat post manufacturers don't concern themselves with frame failure. I stated that the minimum insertion mark on a seat post is to avoid seat post failure, not frame failure. As in my previous example, the marking on a seat post may not fulfill the frame insertion requirement.

I'm done.

datlas 

LarrySellerz

This idea that seatposts are marked PRIMARILY to protect the frame is not absurd. Hell, in my OP I asked about potential damage to the frame. The reason is simple, a longer seatpost stresses the frame more. Certain frames can take more stress than others, but they just did some math and decided that x is too much force to apply to frames in general, and chose the minimum insertion line based on that.

I'd wager that the distance between the seat and the insertion line is fairly standard as opposed to the distance between the end of the post and the minimum insertion line. If the insertion line is a percentage of the total length, like if 7% needs to be in the bike or something, then that suggests that the minimum insertion line is meant to protect the post. More likely, there is a standard lever arm that the cervelo engineers decided it too large for most bikes and that is why the minimum insertion line is.

This chick at the LBS shop explained a lot of this to me like a year ago but I called her out and thought she was full of it, the frame breaking, really? But honestly it makes sense.

Maelochs

The marks on the seat post are put their by the seat post manufacturer in order to ensure safe and proper use of the seat post. Nothing else. Seems crystal clear. Hilarious that people would argue about it, in a nauseating sort of way.

LarrySellerz

I'm kind of surprised at the "if trimming is required" line in the instructions for your bike. How does one go about trimming a carbon fiber post without compromising the integrity of the layers? It it really just like metal? Maybe those instructions assume the post is not carbon? Is it necessarily inadvisable to take a hacksaw to a carbon post like you could aluminum?

genejockey 

I'm doing single-cell RNA sequencing analysis, and there's a lot of time spent waiting while the cloud computes. So, I got time.

Herzlos

Exactly. The seat post manufacturer is responsible for the seat post. The whole system works because each component can be trusted to be safe in itself, when used properly.

Having a seat post 2mm further out than required might be OK, but what about 10mm? 50mm? You need to provide a cut-off somewhere, as short as possible to withstand the forces applied to the post at the clamp.

Of course, in this case whilst the seatpost is fitted incorrectly, the problem is almost certainly that the clamp was overtightened which damaged the post.

Eric F 

I have used a high tooth-count hacksaw blade for trimming CF steerer tubes for many years. Park Tool makes a blade that is grit-coated to do the job.

LarrySellerz

Its honestly not a contrived argument. The thread is about seatpost failure which could have been really bad, thinking hard about the mechanics of the failure is important, especially if you want to skirt the rules. The line is there so a layperson doesn't hurt themselves, kind of like how boiler code stops people from designing a bomb without them having to understand the math behind the code.

terrymorse 

It's easy. You just need a hacksaw with a thin blade and a saw guide.

Carbon steering tubes are meant to be cut.

LarrySellerz

To use a contrived example, if you imagine a 3 foot long seatpost in a mithril frame, the line that is sufficient to protect the seatpost would crack a normal frame. you would have to shove a lot more into the bike if you have a massive post to avoid cracking the frame. Take that post on a mithril frame and go over the line by 6 inches and youre totally fine because the line is to protect the system as a whole and on the extreme long end of posts the frame is the weak point.

On a super short post, the post becomes the weak point and primary consideration. Dang now that I typed it out it seems obvious

Lombard

Maybe that's why there are no 3 foot long seatposts. Now that seems obvious.

veganbikes

If they had stopped Elvis that would be great. Big Mama Thornton deserves way more praise. Also would have happy to have seen the 10+ others who did covers of it. before Elvis decided he had to be so original and do another cover, get more praise. I do like El Vez though. Mexico did Elvis way better.

I will agree that Larry is unique but unique is not always a great thing.

veganbikes

Ok never mind, I tried to be nice and care a little, I guess you just are trolling us or something. Ugh!

In case someone thinks this is a real thread. USE A TORQUE WRENCH ALWAYS, INSTALL THE SEATPOST WELL BELOW THE MINIMUM INSERTION LINE AND BE TRUTHFUL AND HONEST.

Camilo

As are most of the arguments taken beyond a couple of exchanges. A lot of people just have to be "right" and don't worry about looking like they need something meaningful to fill some void in their life.

Camilo

cut off the end with a hack saw and dress the inner and outer edges with a file or such. Just like a steer tube, except that while a steer tube needs to be cut fairly square, a seat post doesn't have to be anywhere near square cut.

As for why? I once had to cut a seat post for my wife's bike because we somehow got a very long one and it wouldn't go low enough before hitting on the a bottle holder insert in the seat tube. I cut the post, re-marked a minimum insertion line correspondingly, and made sure it still inserted into the seat tube far enough, both in terms of the new minimum insertion line, but also far enough in the seat tube. I can't remember what spec I used for that, I think I was satisfied that it still went in several inches past the TT-ST junction, the "rule of thumb" I recall was a couple inches beyond the lower edge of the top tube at the junction.

LarrySellerz

So I should trim and clean up the jagged edges on the seatpost? It fit in just fine I think it’s okay as is

Herzlos

For a simpler example, put a metal pipe into a vice/clamp. Leave 1" sticking out and try to bend it - virtually impossible. Try again with 6", 12", etc and you'll eventually find a point where it's easy to bend. Then you can mark a line just above where it transitions from hard to bend to easy to bend and that's your minimum insertion point.

The seat post should bend before the frame is damaged - it's thinner and weaker, so I'm not sure why a seat post manufacturer would be worried about damaging the frame.

Herzlos

Yeah, why not. The point where you broke it is likely to be the only damage and you're not planning on actually sitting on it anyway.

I'd carefully cut a few inches above the crack, file it smooth and then mark a new minimum insertion line based on how much was cut off.

Trakhak

Just in case that post doesn't spark enough controversy:

Why not miter the bottom of the seat post? Wouldn't that serve to distribute the bending force from an insufficiently deeply inserted post, protecting the frame better than a square-cut post? That's how head tube lugs are designed, after all.

Lombard

No, you should get out your crowbar and buy a new seatpost of the correct length - unless you have very good dental insurance, than go for it!!

Then again, if he's not planning on sitting at all, just leave the saddle and post out. Just think of the weight savings!

Maelochs

With a metal seat post, if the clamp is loose and the seat post is not precisely milled, allowing a tiny bit of motion, the seat post could ovalize the frame. I know steel frames used to have a slit at the back of the top of the seat tube, and rocking back against this (if the clamp were a little loose) could conceivably spread the two sides.

However ... I do Not believe the people making seat posts bother to calculate how each seat post might affect each frame. Too many different frames made by too many different manufacturers out of too many different materials ... and what about older frames? Do people designing seat posts consider every frame made in the past two, three five, ten years? Obvious nonsense.

Manufacturers of seat posts calculate bending forces on the posts they make and determine how much of the post needs to be supported for the post to function safely under normal use. Frames are not considered.

How could the mark be the same for a CF frame, a steel frame, a butted or double- or triple-butted frame ...... How about a Y-frame (remember the old Y-frame MTBS?) a Y-frame style of bike might only have two inches of "seat tube."

The designers of the post have to assume that the post will be inserted in a stable, rigid frame tube. Beyond that it would all be guesswork.

The mark on the seat post is all about the seat post.

Anyway, I have done my part to keep this monstrosity alive one more day. Not sure if I will be damned or praised for it, but .... "It lives!"