Dissing Min! (seatpost insertion levels).

[uart]

I put this in the retro section because I want to specifically discuss this in relation to old steel bikes, not carbon fibre and any other wonder material.

Just wondering on what people's opinions are on seatpost minimum insertion levels. I realize that one function is to protect the seatpost itself from excessive bending moments and breakage. But here I am interested in the minimum insertion level to safely prevent damage to the frame (specifically the seat tube).

My theory is that if you have a solid lugged steel frame, any insertion that goes below the seat lug (and it's slot), which means it also goes below the where the top tube and also the seat stays connect to the seat tube (conventional frame geometry), then it's going to be strong enough to not risk damaging the frame. I've done this lots of times, and generally it's less than the minimum insertion line on the seat post itself. Just wondering what other people's thoughts are on this?

Well this "theory has always worked for me so far, I've never damaged a seat tube from under insertion. The problem is though, I like to have a "theory" for everything, but sometimes that theory might be nothing more than just a load of old cobblers that I made up. Like this one. :p

[GaryF]

I think the minimum insertion marks on the older Campagnolo Super/Nuovo Record seatposts takes the post to a little below the width of the seat lug. I'd be guessing at 2cm to 3 cm below the seat lug.

I recall an incident I had a number of years ago when a Campagnolo C-Record areo seatpost snapped in half on me whilst I was out on a ride. Back them I was a lean, mean cycling machine (haha) and the post just snapped in half. The seatpost was not extended out of the frame past the minimum insertion line but it snapped all the same. Inspecting the post (I do have a heat treatment of metals qualification) I could not see any physical reason for the seatpost failure and I put it down to metal fatigue. I was lucky I was not impaled in the post. The week before this incident another local rider was not so lucky, he too snapped a post and was impaled on the stub left in the seat tube. He has never been able to use his bowels since that accident.

My choice of seatpost has always been carefully considered ever since. I will not use an aero post for any length of time now; they are purely decorative additions.

Oh yeah, I would not extend a seatpost beyond the safe minimum insertion level.

[LG]

My normal rule of thumb is 2" of seat post minimum insertion. I had to slow down and think about how to word this so it didn't sound rude.

[GaryF]

My normal rule of thumb is 2" of seat post minimum insertion. I had to slow down and think about how to word this so it didn't sound rude.

There's my laugh for the day, thanks LG.

[WyvernRH]

My normal rule of thumb is 2" of seat post minimum insertion. I had to slow down and think about how to word this so it didn't sound rude.

+1
(and nicely phrased...)

I have a suspicion that the 2" rule is something that is written in some Ancient Charter somewhere and was handed down from mechanic to mechanic over the years.
I've not done any calculations but as a mech eng it passes the 'feels right' test but probably has a safety margin built in, you could get away with less maybe? Tho IMHO certainly not on a fillet brazed frame?

Must admit I normally aim for a bit more length just for overkill's sake - maybe circa 3-4".

Richard

[10speedsemiracer]

I think OP has a frame that doesn't fit him...

[find_bruce]

I will not use an aero post for any length of time now; they are purely decorative additions.

I am curious as to your reasons Gary as my latest frame comes from the aero 80s with an aero post like this & I am not seeing how it is potentially weaker than a round post


Although perhaps you were referring to this sort of post with the aero part on top


or this


Min insertion levels don't tend to worry me as I have duck's disease

[uart]

The week before this incident another local rider was not so lucky, he too snapped a post and was impaled on the stub left in the seat tube. He has never been able to use his bowels since that accident.

Ouch Gary. You've got me really frightened of braking a seatpost now.

A local rider (from a group I ride with) broke his carbon seatpost about a week ago. No direct injuries from the post, but it threw him off balance and caused him to crash, giving him a significant concussion and a broken collar bone. It was only an 18 month old bike and he swears the post had never been tightened past 6nm. Happened while he was just sitting and pedaling along a smooth cycleway, and he's certainly no heavyweight either. That's certainly put me off the idea of carbon posts TBH.

[uart]

I think OP has a frame that doesn't fit him...

Yeah it's on the smaller side of what I ride, but it's not a very long seat post either.

I just checked and it's got about 1.75" inches inserted. It goes just a little bit below the lowest point on the seat lug. I'll post a photo in a minute.

[uart]

Here are the photos. The green line on the first one shows where I estimate the post comes to. The second photo just give a perspective of the size of the bike in relation to the seat height.

[GaryF]

Hi Find Bruce, the post I snapped looked like your second photo and the aftermath looked very much like your third photo. It was an early 90's Campy Chorus post. I was peddling hard and sitting on 50kph so I had lots of pressure on the pedals and gripping the handlebar strongly. When the seatpost snapped my body dropped and I nearly went off the back of the bike. I think my effort at the time saved me from impalement. I was pretty lucky.

I thought, at the time, the cross-sectional structure of the post contained less metal in the aero part of the post than the lower, round section. I also thought the aero upper section was only half the width of the round, lower section. My theory but, really, only an observation. Perhaps the riding motion exerts twisting and latitudinal forces on the post as well as longitudinal forces????

uart, your local crash victim sounds like he was caught completely unawares and had no time to protect himself.

From your description and photos, it looks like you have the seatpost in a safe position. My smallest frame that I can ride is 58cm c to c. and my seatpost positions look like your seatpost position.

I've got a round Campy Daytona steel post that I like and a Campy round titanium Record post that I am confident in. Looking back to the 90's, I have bought the round version of the C-Record post. For some strange reason, of which I can't explain, I put my faith and butt into round sectional posts.

[find_bruce]

Fair enough Gary. I suspect my resident metallurgist will say that without inspecting the part under a microscope she can't be sure, but the likely candidates are either inclusions in the alloy or stresses induced by the manufacturing process - my guess is that they started out as a round post and were squashed into the aero shape.

As you say the direction of forces probably also play a part

[WyvernRH]

Here are the photos. The green line on the first one shows where I estimate the post comes to. The second photo just give a perspective of the size of the bike in relation to the seat height.

Well I reckon you are probably OK but me, I'd want another inch of seat post in there (his whole conversation is starting to sound very carefully PC....)

Richard

[bychosis]

I’m not a metallurgist and I would probably over engineer stuff that I make or modify using my understanding of physics and howbstuff works.

I’d be looking for a bit more insertion

[10speedsemiracer]

Here are the photos. The green line on the first one shows where I estimate the post comes to. The second photo just give a perspective of the size of the bike in relation to the seat height.

Yeah, I'd want more post down that tube.

[uart]

Yeah, I'd want more post down that tube.

It does fully cover the seat lug, which looks pretty strong at about 2mm wall thickness. But then it barely goes more than a few mm below the lowest projection on that lug. Anyway, I agree, I'd like a bit more seat post there too.

The 2" rule of thumb is looking pretty good here. Currently the insertion is only about 1.65" (1 5/8"), but with another 3/8" (approx 1cm) of insertion I'd actually be happy enough to leave it at that.

I had originally planned to restore this bike for a family member, but it was a little bit too big for them. It's approx 58 ST (center to top) by 57 TT (center to center), which is just a tad on the small side for me, but still comfortably rideable when set up as pictured above. Right now I just want to give it a good test ride, which I think I will risk with the current seatpost. If I decide to keep this bike however, then I'll probably pick up a slightly longer post. It's a Tange 900 frame which takes 26.6 mm, so not an uncommon size.