Duo

Would a bike be unsafe if someone took off 1/16" of the metal on the post?

Road Fan

If you thin the seatpost so the diameter is 1/8 inch smaller than original, very likely the seatpost is now too thin to support your weight, at least there is risk of that. If the place where the thickness changes is a sharp stepped edge, it will be a center of stress risers, and the seat post will break there, if it breaks.

I would not advise doing that.

CardiacKid

I got my Schwinn Varsity when I was 5'8" I continued to raise the saddle and handlebars until I was in college and I was 6'2". The stem was about a half centimeter above the line. It didn't break until May of my senior year, while my roommate was riding it.

red_hook

Post 3 in this thread seemed not too far off the mark.

Citoyen du Monde

Seat tubes are not traditionally "butted" on the top end where the seatpost is inserted. I know this because in the second frame that I built, I fitted the seat tube in upside down, with no butting at the bottom bracket and butting at the top meaning that I could not us teh seatpost diameter that one would have expected with the tubing used.

There are a few exceptions like the shim used in older Cinelli frames, but they are exceptions. BTW, the Cinelli shims insert well below the seat lug area, which would seem to show that they deemed seatpost insertion to be somewhat important.

velo_love

Hey! I know this was 12 years ago, do you have an update? Did this end well?
I'm in a similar situation with my old Peugeot, and a replacement 24mm seat post is very annoying to find.

RobertUI 

Personally, I like to make sure that the min mark on the post is at or below the top tube lug (meaning you couldn't ever see the min mark). If I need longer than that, I will buy a new post. I won't even do a test ride higher than that.... just my $.02

bikingshearer 

To my way of thinking, "very annoying to find" beats the heck out of "ruined frame" or "trip to the Emergency Room." I just don't think the risk is worth it.

dddd

There's a huge variable besides exactly where the limit line sits, and that is the loading conditions.

Whether or not the seatpost and frame hold up to the stress of an extended seatpost has MUCH to do with the saddle's position over the post, as this determines the leverage applied, so the style of post (inline or offset) as well as the rail clamping position and even the seat tube angle can all be important factors.

And there is also the matter of riding conditions and rider weight, as well as the duration of usage, all of which will tend to hugely affect the fatigue life of the frame and the post.

Lastly, the usage history and reputation of both the seatpost and frame will also have a big effect on reliability/safety.

79pmooney

This is a simple beam problem - sophomore engineering. When you hit a hard pump you didn't see (for me, it was a frost heave in a screaming descent of a race in drizzling conditions) you are trying to bent the seatpost back. The seatube and lug are the only things stopping the post from simply rotating back. So now lets look at that seat tube and lug. The seatpost is trying to rotate the seat tube back. Effectively, the post is trying to rotate about the top of the post/lug. So, now we can consider the post as a lever trying to crank on the lug and tube. The top of the seat tube is the fulcrum or pivot. The other end of the lever resisting the rotation is the bottom of the post buried in the seat tube.

Now, a quick lesson in beam theory. A lever applies a "bending moment" at the fulcrum, ie at the top of the tube/lug. To resist this, an equal bending moment must be applied from the other side, ie the buried end of the lever. Bending moment is the force trying to rotate the lever X the distance from the fulcrum to where that force is applied.

Let's apply numbers. Say you are bending the post with a force of 100 pounds at an angle 90 degrees to the post. (At nearly 200 pounds and a hard bump. the load will be far more but all we care about here is that portion at right angles.) Say it is 10 inches from the top of the tube to the seat where your weight is applied. Bending moment is force X distance or 100 lbs X 10" = 1000 in-lbs. Now we need that same 1000 in-lbs acting below the top of the seat tube. The most effective part of the buried post is that farthest from the fulcrum, ie the bottom.

So, to get the resisting force, we take the 1000 in-lbs and divide that by the length of buried tube. 3 inches? 1000 / 3 = 333 lbs. 2"? 1000 / 2 = 500 lbs.

Now, lets look at what steel construction looks like o the frame. The seat tube is a relatively thin tube with a fairly easy job. The top of the tube is not butted or thickened as the bottom it is and the top and down tube ends are. The tube is brazed into the seat lug. The lug provides good support for the tube, but - to braze the two together, real heat must be applied to the lug and tube. Other tube ends are thickened/butted so the loss of strength from the brazing heat is taken into account. Not here. (Butting messes with seatpost support and diameter. Done sometimes but not common,)

And so - if we do not run the seatpost down below the heat affected tubing we 1) have less distance for our lever, hence more force trying to distort the seat tube and weakened metal that will be happy to bulge out with that force. You do not know how careful the builder was when brazing that lug or how far down the heat affected zone is. You do know that the further in the post runs, the smaller the force will be.

Oh, that race? I weighed 145 pounds and had plenty of post inside. I still hit hard enough to kick me and bike 2' in the air and bend the seatpost. (A quite literal kick in the a**.) The rest of the bike came out just fine. I finished well enough to win a nice seatpost so that frost heave cost me nothing! Had I less post inside the frame, that could have been very exciting. (I was going 50+. The cop driving the police car, our race pace car, told us after that he had to go 60 between turns to stay ahead of us. We weren't using our brakes.)

thook

if you're trying to find an aluminum post in that diameter, you won't. there's a steel peugeot post out of the UK on ebay right now

https://www.ebay.com/sch/i.html?_fro...ycle+seat+post

Gary Fountain

I have snapped a Campy C-Record post close to the seat cluster while riding. This was back in the mid 90's so the seatpost wasn't all that old. I had it set to a height close to the limit line - about 5mm below the line actually. It just went 'crack' and that was it. It pivoted backwards and I was lucky as I was travelling at about 50kph and had a good hold of the bars. Still, I nearly went off the back of the bike - it was touch and go.

I have metallurgy qualifications and I inspected my snapped seatpost for signs of a developing crack - I didn't find any. I think my seatpost just snapped as I was really putting quite a bit of pressure on the pedals and a fair bit of my force was pushing back in the saddle.

About 5 days before my incident, a mate of mine did the same thing but he was cruising along at about 30kph. He didn't have a good grip of the bars and his Campagnolo (again C-Record) seatpost snapped a fair bit higher up on the post. He fell down on his post and it went fair up his bum. He spent a little over a week in hospital and lost the use of his anal muscles for life. He has to use some sort of bag from then on. I haven't seen this guy for a good 20 years now so I can't give you an update. I never inspected his seatpost.

I have never used those aero (scalloped out) Campagnolo C-Record posts since then. I still use Campagnolo posts but they either have to be round if alloy and preferably Nuovo Record or Campy titanium or steel like the old Daytona posts. I try to ride slightly bigger frames so the limit mark is down a little bit further.

It's up to you but, as suggested, a bigger frame is a good idea or swap to a really hefty seatpost. (I'd save up for a bigger frame). I do trust the Titanium posts (very expensive) and I like the suggestion of the steel post from the reply above.

Colorado Kid

I remember a few years ago a fellow computer raised his seat post way too high. It bent in the strange way and then broke in half. OUCH!! I agree with the others, get a new seat post, You'll thank you're self.

Rage

A neighbor gave me a gt Palomar something like twenty five years ago.
It was way too small for me but I worked it into rotation as a commuter. Of course I had to jack the saddle way high with a lot of seatpost exposed. I was young and stupid and thought, “no biggie. It’ll be fine.”
One day I’m flying down worth st. on my way home from work when I feel the clamp on the seatpost snap!
I, very luckily, jump right off in the middle of bombing down the street, land on my feet and tumble to a crash.
To my utter shock and surprise, I managed to not spear myself on the seatpost as well as survive tumbling into speeding traffic.
I thanked my lucky stars above and vowed NEVER to be so stupid as to do anything like that again.
Imagine if I hadn’t jumped up off of there. I woulda been impaled on that seatpost...

bwilli88 

Be careful have a long seat post hanging out above a seat lug, this one snapped one me. I was lucky having a small file on my leatherman and was able to file the rough edge and put it in for a 10 km ride home.

T-Mar

There was related issue when dual suspension Y-frames were introduced. Parents would insist on buying too big of a frame that their children could "grow into". When the long ATB post was lowered it would protrude out of the bottom of the abbreviated seat tube, limiting the travel of the suspension swingarm. Parents would then request the seat post to be cut. I'd refuse and the parents would often buy the bicycles elsewhere or cut the posts themselves. When the kid's grew, the post be would raised but they'd often forget the post had been cut and thought things were OK because they weren't exceeding the insertion mark. This was the equivalent of raising an uncut post beyond the insertion mark and it resulted in some some injuries and frame damage.

sovende

Based on anecdotal reports of broken seat posts and where the breakage occurred, it seems to me that the issue isn't related to how much of the post remains IN the seat tube but mostly related to how much of the seat post remains OUT of the seat tube!