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I have a 12 inch wheel girls bike with no seatpost. It turns out a 1 inch diameter copper water pipe fits.

I know I could take some time and find a steel post but in this case does it matter? The seat will never be raised more than an inch or two.

David D
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    25.4mm is a less common seatpost diameter, but they do exist. I see a Kalloy one at Jenson for US$20. You could check your bike store to confirm the diameter and see what they can order/have in stock. – Weiwen Ng Mar 23 '24 at 18:05
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    Check the local bike cooperative - mine had hundreds of old 1" seat posts in stock. – Criggie Mar 23 '24 at 20:01
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    @Criggie I agree, the bike cooperative is the way to go. – David D Mar 23 '24 at 20:17
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    Why are you asking? It seems that you already have the answer, even though it's different than what others have. – ojs Mar 24 '24 at 16:47
  • @ojs I'm asking. It's not wrong to question the answers I have been given. – David D Mar 25 '24 at 13:10

2 Answers2

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Copper has a yield tensile strength of 33 MPa vs 280 MPa for 6061-T6 aluminum or 460 MPa for chromoly steel. Your daughter doesn't weigh 12% of the weight of a typical user of a 1 inch aluminum seat post. You might as well propose using a PVC pipe for the purpose.

Not that seat post height is super relevant for this, but children's bikes seat height can increase suddenly - what if she hits a growth spurt and 16 inch or whatever bikes are backordered, will she stop riding or just raise the seat a bit?

Seatpost failures are very likely to cause injury if the user is pedaling when they occur. Do not do this.

oscu0
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    The only way this could work is if the wall thickness was ~10mm and that much copper is worth more than an entire new kid's bike ! – Criggie Mar 23 '24 at 20:00
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    I agree with the strength analysis. The stock seat has a plastic attachment point (no clamp). The plastic is weaker and more brittle than the copper. The bike comes with a plastic headset. A 40 pound kid could stand on the unsupported end of the copper pipe without bending it. The copper is not as strong as the steel seatpost but given the many weak points of the bike it would not be the weakest point. – David D Mar 23 '24 at 20:16
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    @DavidD The plastic is weaker and more brittle than the copper. Nope. Nylon yield strength is 45-90 MPa. It wouldn't be hard to engineer something nylon that's stronger than a relatively thin-walled copper pipe. Heck, even the polystyrene that's used to make foam cups can have yield strengths up to 100 MPa. – Andrew Henle Mar 23 '24 at 23:53
  • I especially second the final point. Some components on a bike usually fail ‘safe’, but frame components, including seatposts, are not one of those components. – Austin Hemmelgarn Mar 24 '24 at 01:20
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    Your figure is for bulk copper. Copper pipe is work-hardened by its production process to a strength of 200 up to 350 MPa. In other words, it's as strong as aluminum at equal thickness. It also has more elongation to break and isn't prone to crack corrosion failures. So strength is not an issue. – Therac Mar 24 '24 at 08:54
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    @Therac And what's the production process used for a random bit of copper pipe found at your local big-box-pretending-to-be-a-hardware store? it's as strong as aluminum at equal thickness And a thin-walled copper pipe isn't designed to be used as a seat post. How is it going to handle the dynamic point loads that want to both collapse and bend it right at the top of the seat tube? How about when Jr rides off a 8"/20 cm curb and all of his 50 lbs/20 kg slam onto the saddle? Or maybe even does a jump with a 3 ft/1 m drop from its apogee? Like 6- or 7-year-olds are wont to do? – Andrew Henle Mar 24 '24 at 22:38
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    Nevermind work hardening makes metals brittle and more prone to fracturing and quite likely able to absorb less energy before failure. – Andrew Henle Mar 24 '24 at 22:40
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    @AndrewHenle The same process, because it's just how copper pipe is made. Annealed copper pipe (which is still in the 200 MPa range) costs more than regular. Either kind is less brittle than 6061-T6, at 15%-30% elongation versus 8-10% for 6061-T6. – Therac Mar 25 '24 at 03:11
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    @Therac C12000 and C12200 copper pipe/tubing can have yield strength under 60 MPa, so you'd better make sure you know the provenance of that random bit of copper tube you're holding... Nevermind that the wall thickness of 1" type L copper tubing is only 0.050", or barely over 1 mm. And that's for 1" inside diameter pipe. 1" outside diameter pipe is likely to have thinner walls. Type M is even thinner... – Andrew Henle Mar 25 '24 at 11:44
  • @AndrewHenle Your own link shows 50 ksi, which is 340 MPa, for pipes. Even for tubes it's higher than 60 MPa. Are you using the right units? But it's true that aluminum seatposts are usually about 1.5 mm in thickness. – Therac Mar 26 '24 at 05:46
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    @Therac There are at least four listings for "copper tube" on each of those links. Some of which have yield strengths as low as 10 ksi. But hey, let's see if Little Bobby gets a bonus bung hole bored in his taint because daddy grabbed a random piece of copper with no way of knowing what it really is, just because someone posted to the intertubez "it might be as strong as aluminum". Yeah. And it might not. – Andrew Henle Mar 26 '24 at 11:09
  • @AndrewHenle Tube is easily distinguished, it's normally small diameter, and can be bent by hand - that's why they treat it to have very low yield and high elongation. A copper water pipe will always be hard pipe material. If you do in fact find 1" tube, you'll notice it gradually bending with normal use. It will not fail abruptly like aluminum or carbon. – Therac Mar 26 '24 at 13:32
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    I initially read "Your daughter doesn't weigh 12% of the weight of a typical user of a 1 inch aluminum seat post " as saying "your daughter is so light that this is fine" which contradicted the final "Do not do this.". It's on me for not paying close enough attention to the numbers in first sentence on the first reading. "X doesn't weigh 12% of Y" initially led me to "X weighs less than 12% of Y." Might I suggest rewording slightly to highlight the point that "copper is only 12% the strength of steel, and your daughter weighs more than 12% of the typical steel user"? – Joshua Taylor Mar 26 '24 at 15:16
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In addition to structural concerns regarding copper's tensile strength, keep in mind that copper causes galvanic corrosion of both steel and aluminium. Stick a copper seat post in a steel or aluminium alloy bike, and the frame will start to corrode due to the interaction between the metals. This will weaken the seat tube, which could cause the seat post to bend or break.

If the top of the seat tube is weakened and brittle because of corrosion, this will then in turn increase the effective leverage between the seat post and the seat tube, similarly to a seat post that isn't inserted as deep. A seat post that isn't inserted deep enough into the seat tube can fail catastrophically even under normal use.

And even if it doesn't fail, the corrosion can simply cause the seat post to get stuck inside the frame. That's no fun either.

I would not recommend using a copper pipe for a seat post even for a kids bike.

Simon Lundberg
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  • Copper, which is a relatively noble metal, does not usually exhibit a great deal of galvanic corrosion. I'll use an antioxidant paste. – David D Mar 24 '24 at 13:33
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    The copper doesn't corrode; it makes the iron corrode. Just get an aluminium seat post. 25.4 mm posts are a dime a dozen. They sell 'em at Walmart for like ten bucks, and your LBS probably have a few lying around that they'll give you for less than that. If you're not interested in hearing reasonable explanations for why a copper water pipe might be a bad idea, why did you even bother asking in the first place? – Simon Lundberg Mar 24 '24 at 19:32
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    THIS is the✅answer! There are many ways of making a tube much stronger against buckling failure under axial load, very cheap if weight's no issue. One is filling it completely with coarse sand settled under vibration with ~¾"-long end plugs of compatible bulk-curable structural (commonly, epoxy-based) adhesive mixed with sand. Adding a coaxial ~#10 threaded (so it's trapped in the sand) 303 steel rod along the centreline length would take care of lateral loads. But there's no way of making a galvanic-protective coating cheaply at home. The thing may turn steel into loose rust in 2 weeks. – kkm -still wary of SE promises Mar 25 '24 at 22:44
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    @David Come think of it, the retail price of epoxy and the 303 rod comes to the same $16 for an aluminium post at Amazon's. That's assuming you already have clean sand (beach sand must be washed with water to remove salt then n-heptane or like non-polar solvent to remove organics then sifted by grain size), tools and infinite free time and affection with tinkering. Beware that even aluminium posts stick to steel frames because of galvanic corrosion. Remove, wipe and reinstall it on a schedule depending on local climate aridity, roughly every month (coastal) to a year (desert). – kkm -still wary of SE promises Mar 25 '24 at 23:19