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Bent seat post.

Old 06-01-22, 01:52 PM
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PimpMan
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Bent seat post.

Uh oh aftermarket Chinese seat post so soft it broken in two days of use.

I weight 200 lb, was riding city only, did not avoid potholes to test suspension, did not jump off the curb.






Could it be because i installed the seat mount facing front of the seat rather than facing towards rear of the seat?


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Old 06-01-22, 01:58 PM
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Even if u did not exceed the minimum insertion depth, having that much leverage on a seat post is probably not a good idea. You are too heavy for that configuration basically. Find a bike more suited to your size.
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Old 06-01-22, 02:24 PM
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That looks like way too much seatpost exposed above the collar, but it's also pretty much the lowest-common-denominator style of seatpost. Get a 400mm+ seatpost that's made out of aluminum and has a head and fixing bolts attached.

And don't lean your bike up against poison ivy.
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Old 06-01-22, 02:50 PM
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Seatposts that require the 14mm nut seat clamps are made with thin cheapie steel that is very soft.

Modern aluminum seatposts with the seat mounts will be stronger because it'll be a lot thicker.

Now is your chance to get a really long one like 450mm if you can find, so it will sink well into the frame.
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Old 06-02-22, 12:38 AM
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Originally Posted by ign1te View Post
Even if u did not exceed the minimum insertion depth, having that much leverage on a seat post is probably not a good idea. You are too heavy for that configuration basically. Find a bike more suited to your size.
Forgot to mention this seat post cost $3 int he store and its made of metal, but weights no more than aluminum seat post, so too thin.

I have even smaller folding bike i want to use, frame size should not be the limit.


Originally Posted by Rolla View Post
That looks like way too much seatpost exposed above the collar, but it's also pretty much the lowest-common-denominator style of seatpost. Get a 400mm+ seatpost that's made out of aluminum and has a head and fixing bolts attached.

And don't lean your bike up against poison ivy.
Its 400mm seat post, exposed only 240mm min insertion point marked at 90mm so its all within specs.

I had to raise it that much so my knees don't bend when pedals in the lowest position.

Because rear shock gets compressed when i sit down i had to raise seat more.

Originally Posted by soyabean View Post
Seatposts that require the 14mm nut seat clamps are made with thin cheapie steel that is very soft.

Modern aluminum seatposts with the seat mounts will be stronger because it'll be a lot thicker.

Now is your chance to get a really long one like 450mm if you can find, so it will sink well into the frame.
Thanks for advice, why i need 450mm though? 400mm seat post was filling 95% of the frame when adjusted for my height, if its any longer it will be sticking out of the bottom potentially hitting the rear shock when its fully compressed.

Last edited by PimpMan; 06-02-22 at 12:46 AM.
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Old 06-02-22, 01:54 AM
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My frame is 25.4mm can i use 25.2mm seat post?

Reason i want this because currently 25.4 seat post moves too tight and scratches off the paint when adjusting.

Or perhaps it moves too tight because 25.4 + paint on top actually measures 25.5 or so if i buy silver post without paint it measure exactly 25.4 and be easier to adjust?

Last edited by PimpMan; 06-02-22 at 02:36 AM.
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Old 06-02-22, 04:55 AM
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You need to use the right diameter seatpost for the frame. I agree with others that you should get a higher quality aluminum post. But your frame is also too small and you may continue to bend seatposts. Your folding bike hopefully has a seatpost with a much larger diameter to help avoid this issue.
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Old 06-02-22, 05:03 AM
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The cheapest way to buy something is to buy it once. Sometimes a deal, isn't a deal. You got a valuable lesson without having to go to the hospital. Remember it when it's time to buy other components that support your weight via contact points.
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Old 06-02-22, 07:51 AM
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Originally Posted by PimpMan View Post
I had to raise it that much so my knees don't bend when pedals in the lowest position.
If that is the case, that bike (or whatever that thing is) is too small for you.
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Old 06-02-22, 08:01 AM
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" frame size should not be the limit." PM

And why should this be the case? Were it a truth we could all be riding around on the same size bike that a 4 year old fits, and thus take advantage of the bike's lesser weight. If true than why do better bike brands make the same model of bike in many size frames? Think of the cost savings, remember it's money that makes the world go round:

Your statement flies against over 100 years of millions of people making and riding bikes that came before you. If you can't understand and believe this yet you will be spending more of your money and time on poorly fitting bikes till you do. Andy
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Old 06-02-22, 11:00 AM
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Get your butt off the seat when riding through potholes!
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Old 06-02-22, 11:20 AM
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Cool story, OP.
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Old 06-02-22, 12:27 PM
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If that is indeed supposed to be a 25.4 post, then a long enough and strong enough one is easy to find. Only challenge - finding a seatclamp that will fit on to a 25.4 tube. I'm sure they exist. 1" steel tubes have been around longer than bicycles.

Quality steel is FAR stronger than aluminum. With the right wall thickness, it will hold up under your weight forever. Go on-line and see if you can find a seatclamp to fit a 25.4mm / 1" tube. If you find it, come back here and I'll go through my steel booklets and tell you what tube thickness and alloy to get. Quote me so i see the email. The outfit Metals Supermarket can cut you the exact length. (Now, it will be steel and rust so it needs paint or something else to protect it. I'd coat it with grease for inside the frame, ride it, dial in the seat height, then take a brush or spray can and paint what sticks out. Or I can tell you what stainless steel to get but that will cost yo a lot more.)

There is a 150 year tradition of making bikes out of available steel tubing. This is nothing new!

Edit: And fun story. My racing days long ago. I was riding my Fuji Professional down Smuggler's Notch in Vermont. One rider up out of sight ahead with the field likewise out of site behind me. Incentive to go fast! It was drizzling so with my glasses, I could barely see more than the centerline stripe. At the last second I saw a big frost heave. I had just enough time to lock my handgrip like a vise. Then I was kicked into the air by my seat! A foot? 2 feet? Bike came down straight and everything felt just fine so I continued on (and caught the rider ahead). Finished the race and won a beautiful Zeus seatpost. (Like the classic Campy NR; same clamp style, same overhead bolts, requiring the same "Z" shaped wrench and a bunch lighter). Post was 27.2mm so I could not put it on since I had a Japanese 26.8mm bike.

Later. looked at my bike and the post was bent! Like the OPs but far less. (The place Fuji saved money. It was a Hupel Rider of soft aluminum. I didn't get around to going to the shop that had offered the post as a prize that week so I rode the State Championship on that slight bend. Mistake! The later portions of the race were unbelievably painful! My mom, watching her first race, got to see me finish, then ride onto the first lawn with no curb, ease on the brakes and keel over, still strapped in but off that seat! I was at the shop swapping posts before I rode that bike again!

Seatposts are pretty darn simple. They have to work and they have to be strong enough. Everything else is gravy.

Last edited by 79pmooney; 06-02-22 at 12:47 PM.
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Old 06-02-22, 02:50 PM
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Originally Posted by Andrew R Stewart View Post
" frame size should not be the limit." PM

And why should this be the case? Were it a truth we could all be riding around on the same size bike that a 4 year old fits, and thus take advantage of the bike's lesser weight. If true than why do better bike brands make the same model of bike in many size frames? Think of the cost savings, remember it's money that makes the world go round:

Your statement flies against over 100 years of millions of people making and riding bikes that came before you. If you can't understand and believe this yet you will be spending more of your money and time on poorly fitting bikes till you do. Andy
How come small bikes have longer seat post and don't bent...



As i said problem is that i used cheapest seat post i could find i already said its weights close to nothing wall thickness is 1mm made of soft Chinese metal.




Originally Posted by 79pmooney View Post
If that is indeed supposed to be a 25.4 post, then a long enough and strong enough one is easy to find. Only challenge - finding a seatclamp that will fit on to a 25.4 tube. I'm sure they exist. 1" steel tubes have been around longer than bicycles.

Quality steel is FAR stronger than aluminum. With the right wall thickness, it will hold up under your weight forever. Go on-line and see if you can find a seatclamp to fit a 25.4mm / 1" tube. If you find it, come back here and I'll go through my steel booklets and tell you what tube thickness and alloy to get.
by seat clamp you mean this part that will fit on 25.4mm diameter tube?




You want me to use solid metal bar that is 24.4mm thick, and 350mm long, that will add ton of weight to my bike isn't it?

I am from Europe its tough to find same parts as available in US.

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Old 06-02-22, 03:00 PM
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I think the suggestion was to get a tube (hollow) with a 25.4mm outside diameter, but thicker walls than the cheapie seatpost. Despite what was written above about steel vs aluminium, the strongest seatposts you can get today are aluminum and ones made of steel are generally of the cheaper and weaker type. There may be some good quality steel seatposts for sale somewhere but I haven't seen one for years.

As to why the folding bike with more seatpost extension did not suffer the same fate, probably because that seatpost was designed to have lots of extension. Not sure what the purpose of your 3$ seatpost was if not to raise the seat, but we have good evidence that it was not sufficient for your purpose.

Also, that department store full suspension bike is too small for you if you need an extra long seatpost to use it properly. This is common with low-cost bikes that are only sold in one size. Being too small isn't the worst problem, though.
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Old 06-02-22, 03:11 PM
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Different bike designs have different trade offs, some designs are about lowest cost, some about smallest packaging. Smartly speced bikes use parts that work better then those on lowest cost ones. I think we are in agreement in that the cheap post on the MtB is not up to the task of supporting your weight/riding style. Certainly that it what the seat post is saying by bending. I speculate that you don't ride the small wheeled bike (unsure of the brand, doesn't look to be a folder or take apart type) in the same manor as you do/did the MtB. So not sure how comparable the two examples are.

Way back in the early 1980s I worked at a LBS that was a pretty service focused shop. The bosses use to tell a story of the CEO of Huffy (a very low cost bike brand sold in big box and discount stores, remember this is long before the interweb became the product purchasing force it is today) speaking at a share holder's meeting that their bikes were ridden an average of 76 miles before no longer used (due to whatever reason). When a broken Huffy would come in for service we would joke (not to our customer though) that this Huffy must have been ridden 77 miles or more Andy
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Old 06-02-22, 03:24 PM
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Originally Posted by PimpMan View Post
How come small bikes have longer seat post and don't bent...



.
Run it into a pot hole.
Report back on what bends, IF you are able.
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Old 06-02-22, 03:48 PM
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For your next seatpost upgrade attempt, you might try inserting a second steel tube inside the new post for reinforcement. A steel tube of 150 cm or so in length would likely work, since the unreinforced section of the seatpost would be high enough above the frame to lessen the likelihood of bending. If you're able to find a correctly tight-fitting tube, I suggest epoxying it in place inside the seatpost.

By the way, the folding bike you posted above looks as if it has a seatpost that's larger in diameter than 25.4 mm. Other things equal, and oversimplifying a bit, the strength of a tube increases as the cube of the diameter, so even a small increase in diameter makes a big difference in strength.

[Edit] Forgot to mention: if inserting the 25.4 seatpost resulted in scratches in the finish of the seatpost, that means that the top of the frame tube into which the seatpost inserts has been damaged. Not uncommon. If the frame is steel, you can probably safely pry the binding ears apart to where they were originally if you're careful enough.

If the frame is aluminum, might be safer to file the frame opening until it no longer abrades the seatpost.

If you're not sure whether the frame is steel or aluminum, see whether a magnet, such as a refrigerator magnet, sticks. If it does stick to the frame, it's steel. If not, it's aluminum.

Last edited by Trakhak; 06-02-22 at 03:59 PM.
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Old 06-02-22, 05:27 PM
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Originally Posted by PimpMan View Post
How come small bikes have longer seat post and don't bent...



As i said problem is that i used cheapest seat post i could find i already said its weights close to nothing wall thickness is 1mm made of soft Chinese metal.





by seat clamp you mean this part that will fit on 25.4mm diameter tube?




You want me to use solid metal bar that is 24.4mm thick, and 350mm long, that will add ton of weight to my bike isn't it?

I am from Europe its tough to find same parts as available in US.
Yes, a clamp like that in 25.4mm / 1" but it will need a cap or ledge to hook over the post top to prevent it from sliding down and getting you impaled with the post through the seat.

And no! not a solid rod. A tube with the appropriate wall thickness out of strong, quality steel. In the US, you can get 1" tubing with a choice of about a dozen wall thicknesses. There might be bike standard 1" tubing (top tubes of frames maybe?) If so, that would be available in Europe though quite likely not to the consumer.

I'm not suggesting that anyone should go out and do this. If I were in your shoes, I might. Quirky engineering challenges fascinate me. You would need to weigh the cost - both money and time against just how much this bike is worth to you. A much simpler answer might land in your lap.

And - brain ticking away - a solution to keeping the clamp from slipping - put it on and tighten it. Now drill a hole clear through the clamp, the seatpost and the other side of the clamp. Run a bolt through, add lockwasher and nut. Done.
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Old 06-04-22, 08:06 AM
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Originally Posted by PimpMan View Post
How come small bikes have longer seat post and don't bend?

That folding bike probably has a seatpost that's 34mm in diameter... much more rigid than the narrower one on your bike, and *necessary* because of the length.
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Old 06-04-22, 08:33 AM
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So the Post Bent instead of breaking. That's a good thing. Also the rest of the bike looks intact. I suspect you might have been doing something you should not have to cause this failure, if it is a failure. Most bikes are rated to about 100 Kg/220Lbs. Things might have been worse if other parts of the bike had bent or broke instead of the post... Just saying...
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Old 06-04-22, 08:57 AM
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This is aluminum seat for $16, it comes out about 220mm out of 400mm.




They sell solid aluminum seat post for $25 free shipping from China, should have ordered this i guess.



Originally Posted by Trakhak View Post
By the way, the folding bike you posted above looks as if it has a seatpost that's larger in diameter than 25.4 mm. Other things equal, and oversimplifying a bit, the strength of a tube increases as the cube of the diameter, so even a small increase in diameter makes a big difference in strength.
Originally Posted by sweeks View Post
That folding bike probably has a seatpost that's 34mm in diameter... much more rigid than the narrower one on your bike, and *necessary* because of the length.
Every seat post regardless of diameter has marking min insertion point showing how far out of the frame it can go as long as min insertion point is inside frame its within spec seat post was designed for, even if raise 300mm. Or whats then the reason they have this markings?

If seat post not intended to raise too far out of the frame why manufacturers make them so long, for instance i bought 550mm aluminum seat post for folding mini bike, why seat post made so long if rider only supposed use 200mm max?


Originally Posted by zandoval View Post
So the Post Bent instead of breaking. That's a good thing. Also the rest of the bike looks intact. I suspect you might have been doing something you should not have to cause this failure, if it is a failure. Most bikes are rated to about 100 Kg/220Lbs. Things might have been worse if other parts of the bike had bent or broke instead of the post... Just saying...
From my experience metal never breaks even if it folds 90 degrees, you'd have to bend it back and forth few times for it to actually break off.

I had piece of fishing rod made of aluminum i broken before throwing it away, it bent quite a bit before breaking, hope aluminum seat post will also not break instantly and start to bend first so i can notice it going out before failure.

Last edited by PimpMan; 06-04-22 at 09:48 AM.
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Old 06-04-22, 12:17 PM
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Originally Posted by PimpMan View Post

Every seat post regardless of diameter has marking min insertion point showing how far out of the frame it can go as long as min insertion point is inside frame its within spec seat post was designed for, even if raise 300mm. Or whats then the reason they have this markings?
.
The purpose of the minimum insertion mark is mainly to safeguard the integrity of the FRAME.
Too little insertion and the post will lever itself out of the seat tube, or simply breaking the top of the seat tube off.
How much exposed seat post to use becomes a multi-dependent judgement call based on quality of post, rider weight and type of riding.
Unless the manufacturer has published any recommendations.
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Old 06-04-22, 03:35 PM
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Originally Posted by zandoval View Post
So the Post Bent instead of breaking. That's a good thing.
You ain't just whistling "Dixie"!
A few years ago, I was riding my folder to the train station on the way home and suddenly the seat felt weird. I pulled over and got off... and the saddle fell off!
The "Thudbuster" suspension had fractured off the top of the seatpost. I rode the last mile standing up, but I made my train!


The saddle and suspension resting on the rack on the train.



Cyclic fatigue failure of the aluminum post where it was pressed into the suspension unit.



Another view, minus the Lizard Skins "Crud Buster" cover.
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