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  1. #1
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    Steel seatpost wall thickness

    Hi everyone,

    I'm in the process of replacing a seatpost on a bike. The original one (aluminum) is rather short (300mm) and I'd like to add a bit of height but I'm having trouble finding a suitable replacement. The size is 28.6mm and I want it long (at least 450mm, but more if possible).

    I don't actually need it that long, but my reasoning is that, the more exposed the seatpost is, the more leverage it exerts on the frame and will cause premature failure. So I'd like the seatpost to go down the seat tube more as well.

    I've been thinking that this size is somewhat of an advantage because you can get regular tubing cut to size and that would allow me to make it as long as I want. I happen to have a mechanical shop (lathe, milling machine and such) so it's not a problem for me to do so.

    My question is: how thick should I order the tube?

    From what I've been investigating 6061 seatposts are about 2.4mm thick on the walls. However, I've been thinking about making it from steel. 4130 if I can get it or regular steel if not.

    My plan is:

    * Get the tube cut to size.
    * Machine the inner top end.
    * Fabricate a reducer interface from 28.6 to 22.2 mm in 6061, so I can use a standard seat clamp.
    * Use dry ice to shrink the interface, push it into the tube and allow it to heat up, for a permanent, weld-less, glue-less fit.

    P. S. I hate those old-style stamped steel clamps, but given that the micro-adjust 6061 ones come integrated in the industrial seatpost, and I don't really want to destroy the old one just to get it (it's in good shape), I can't see how else can I clamp the seat without spending more effort designing and making one myself.

  2. #2
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    Itís unclear if youíre interested in making a seat post because you really want to make one or if you just donít want to buy one/donít think theyíre available. Since it seems youíre looking at making a frankenpost with an existing head, Iím assuming the latter. In which case I think Thomson still makes weirdly sized posts like 28.6 and also long posts. Whether they make 28.6x400ish is another matter. You could also get a reducer from 28.6 down to 27.2
    I make bikes. www.feldybikes.com

  3. #3
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    Quote Originally Posted by Feldybikes View Post
    Itís unclear if youíre interested in making a seat post because you really want to make one or if you just donít want to buy one/donít think theyíre available.
    Frankenpost... hahaha.

    You're right (the later). I'd buy it readily made if it was available, but I've had no luck finding an extra long seat post in 28.6mm. The longest I've been able to find is 450mm but like I said, I'd like it a bit longer (at least 500mm). Not because I really need it that long, but as cheap insurance against the possibility of damaging the frame due to leverage (I'm surprised that some people just raises theirs considerably with no concern about the length that is inserted into the seat tube).

    In case you're curious, I've got an hybrid with a frame on the small side. It has a compact geometry (quite sloped top tube), making the seat tube rather short its size. It doesn't help at all that they decided to equip it with a 300mm post, when most other models came with 350mm.

  4. #4
    pvd
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    Your frsme doesn't fit you. Buy a new frame.

  5. #5
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    Quote Originally Posted by pvd View Post
    Your frsme doesn't fit you. Buy a new frame.
    Standard frame sizing assumes an average rider proportions. I have longer legs than usual so I need a longer stack without increasing the reach.

  6. #6
    pvd
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    Your frame doesn't fit you. Buy a new frame.

  7. #7
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    Quote Originally Posted by pvd View Post
    Your frame doesn't fit you. Buy a new frame.
    See, I know what you mean but it's not that easy.

    I'm 1.82m and, according to the manufacturer charts, I should be riding a 58cm bike. And guess what? I've got a 58cm bike and I don't really like it.

    When I purchased it new it came with 68cm bars and 100mm stem and I felt like I was stretching too much, to the point of feeling hand numbness after riding it for a while. I swapped the bars for a shorter model (62cm) and reduced the stem to 80mm and I can ride it now, but the seat, right on the minimum insertion line, is still 3cm shy of where it should be. So the "proper" frame has too much reach and too little stack at the same time.

    Short of making a custom frame, my only realistic option was to get a smaller stock bike and work from there. Now, be nice and answer my question.

  8. #8
    pvd
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    Your frame doesn't fit you. Buy a new frame.

  9. #9
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    I have to agree that your frame doesn't fit you. It sounds like it may be that it does fit you, but is not what you expect it to be. Hybrid bikes are often not really designed for true cyclist, more for short distance riding beginners or someone wanting to ride with the kids. With the long seat post, what is your drop from saddle to handlebar? That might be what is causing your hand numbness, it's forcing you to put too much weight on your hands. You probably need a completely different bike. Are you sure you even can insert a long post? A lot of times there are bosses for a bottle cage that will block the tube.
    This post is a natural product. Variances in spelling & grammar should be appreciated as part of its character & beauty.

  10. #10
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    27.2 x 600 seatpost + a shim from 28.6 x 0.35 steel as long as you want to make it*

    https://www.flowbikestore.com/ice-re...ilver-seatpost


    * you should make it long enough to extend upwards to support the seatpost to a normal minimum insertion distance and add a second seat post clamp.

    28.6 is used on Raleigh 20s and other folding bikes. It looks like you used to be able to buy 600mm steel seatposts for this application but the supply has dried up.

  11. #11
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    Quote Originally Posted by chazpat View Post
    I have to agree that your frame doesn't fit you.
    It surely doesn't. The question is: is swapping the entire frame the only option?

    When you check the geometry of most bikes, the dimensions of the chain stays rarely change: it's the same all the way from the smallest size to the largest. This means that, regarding the saddle, the only thing a larger frame does is to raise the seat tube (same angle and all), which is equivalent to using a longer seat post.

    Granted, a supported structure is always better than an unsupported one, but that's why I wanted such a large seat post in the first place (so I can insert it well into the seat tube).

    Actually, it may not seem like much difference, but 28.6mm is much stronger at such lengths than 27.2mm. I briefly did some calculations on tube beams and, with all parameters the same except for the diameter, a 27.2mm beam will flex about 20% more with the same load.

    Quote Originally Posted by chazpat View Post
    With the long seat post, what is your drop from saddle to handlebar? That might be what is causing your hand numbness, it's forcing you to put too much weight on your hands.
    On the 58cm bike they are about the same level as the saddle. On the 54cm bike they are higher up (like 8-9cm). So no brutal drops or anything.

    I understand that a larger frame affects other dimensions as well, such as adding head tube length. I didn't mention that, in order to add disc brakes, I swapped the fork and left the steerer tube of the new one deliberately long. I have 50mm of spacers below the 17 degree 130mm stem and the handlebars also raise a little, perhaps 30mm or so. Given that the steerer tube isn't vertical, but slopped backwards, that's an equivalent to a 100-110 mm stem on a larger frame.

    Trust me, on the new bike I have the bars where I want them in both planes.

    Quote Originally Posted by chazpat View Post
    It sounds like it may be that it does fit you, but is not what you expect it to be.
    Actually, the new bike feels really nice. I hadn't given the seat post length much attention before because I tend to ride standing.

    Quote Originally Posted by chazpat View Post
    Hybrid bikes are often not really designed for true cyclist, more for short distance riding beginners or someone wanting to ride with the kids.
    I didn't know I was a "fake cyclist". I thought anyone riding a bike was a cyclist...

    Humor aside, I know enough not to attempt "serious" riding (road riding and such) on a bike that's doesn't fit perfectly. I can injure my knees, my wrists, my back and only God knows what else...

    This is just a leisure bike that I use for short trips and general exercise. I don't commute on it (I have a car). So if playing with the seat post and the stem and such gives me 95% of the fitment I need, I'll call it a day.

    Quote Originally Posted by chazpat View Post
    Are you sure you even can insert a long post? A lot of times there are bosses for a bottle cage that will block the tube.
    That's a pretty good example of thinking ahead. This model is supposed to come with two bosses for water bottles, one in the down tube, and the other in the seat tube. For some reason mine didn't come with the boss in the seat tube, so (fortunately) I can insert the seat post as much as I want (I checked it with a 1-1/8" pipe and I can hit the bottom bracket shell).

    Thank you for your time and comments chazpat.

  12. #12
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    Quote Originally Posted by dr.welby View Post
    28.6 is used on Raleigh 20s and other folding bikes. It looks like you used to be able to buy 600mm steel seatposts for this application but the supply has dried up.
    Yup, I've been searching for weeks without luck. It sucks.

    Quote Originally Posted by dr.welby View Post
    27.2 x 600 seatpost + a shim from 28.6 x 0.35 steel as long as you want to make it*

    https://www.flowbikestore.com/ice-re...ilver-seatpost

    * you should make it long enough to extend upwards to support the seatpost to a normal minimum insertion distance and add a second seat post clamp.
    Awesome find, thanks. *hands this gentleman a virtual beer*

    Honestly, I still hope to find/make one of the exact size and avoid shimming it up, but I'll surely keep that option in mind.

  13. #13
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    I wouldn't overthink building one. Get steel tubing and an off-the shelf seatpost that fits. Cut the seatpost to a stub, epoxy it in, then drill on one side and add a rivet or pin as a mechanical interlock. So if you use 0.049" cromoly (~1.2mm) get a 26.0 cheap seatpost.

    Or do the old IRD seatpost head style. You can find cheap seatposts online that use it, and the only work is to cope the end of the tube and drill a hole at an angle.

    Also one thing to consider is that production seatpost tolerancing is +0mm -0.1mm. Steel tubing might run slightly larger, and if you plan to cover the steel with some kind of finish to prevent rusting that's going to interfere with your tolerance zone.

  14. #14
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    I too have the first reaction that your frame is likely too small - but if you want/need to do this Ebay has lots of seatposts which will fit your requirements.
    "...DirtBaggies may also be the best baggy mountain bike shorts on the market." Velonews - Click to check out other reviews.

  15. #15
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    Quote Originally Posted by Tim-ti View Post
    I too have the first reaction that your frame is likely too small - but if you want/need to do this Ebay has lots of seatposts which will fit your requirements.
    Thanks. At such price range (=cheap) would you trust the grooves/serrations in the head/clamp? I've seen a few of them in person and they don't seem to be machined, just cast, with rather shallow grooves. Like they are going to begin slipping quick.

    I've also noticed that they don't specify the wall thickness. The ones with the old style clamping (7/8 round at the top) list 2.4mm and seem built good enough.

  16. #16
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    Quote Originally Posted by dr.welby View Post
    I wouldn't overthink building one.
    I'm not overthinking it, it's just that making a seat post is deceptively simple. The first concern is to make it thick enough so it won't bend. But if I overbuild it it'll be very stiff, and I won't get some compliance, which is good for both the frame and the rider.

    In theory, I could calculate the thickness myself, but I think it's way better to ask more experienced people about what really works in practice.

    Quote Originally Posted by dr.welby View Post
    0.049" cromoly (~1.2mm)
    1.2mm seems like a good thickness for this diameter. I was reading another post showing one made from 0.9mm 4130 tubing, but it was 31.8mm and the extra diameter adds a lot of strength.

    Quote Originally Posted by dr.welby View Post
    Or do the old IRD seatpost head style. You can find cheap seatposts online that use it, and the only work is to cope the end of the tube and drill a hole at an angle.
    I didn't know about that design. Seems easy enough for me to make in case I can't find one. What I really wanted to avoid is machining angled stuff because the milling machine I have here is manual and it's a real pita to setup that kind of stuff properly.

    However, after studying the design a little, I still don't understand what keeps the long clamp half from pushing down. The end of the tube? Then how you get it to adjust securely at different angles? Only coplanar (parallel) flat surfaces sit securely on top of each other; any tilting and they will only make contact at a single point.

    It seems like a pivot of some sort is in the order to keep both pieces securely attached, with a corresponding mitered (curved) surface on each half. Something like this:

    Steel seatpost wall thickness-poste.png

    What do you think?

    Quote Originally Posted by dr.welby View Post
    Also one thing to consider is that production seatpost tolerancing is +0mm -0.1mm. Steel tubing might run slightly larger, and if you plan to cover the steel with some kind of finish to prevent rusting that's going to interfere with your tolerance zone.
    Good call!

  17. #17
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    why not use an isp topper shimmed onto the tube?
    For a rock steady Gas Tank bag > the DeWidget

    bit.ly/BuyDeWidget

    https://www.instagram.com/drj0n_bagworks/

  18. #18
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    Quote Originally Posted by M1KEMEX View Post
    In theory, I could calculate the thickness myself, but I think it's way better to ask more experienced people about what really works in practice.
    Not very many people make seatposts. It's a simple beam calculation and you have the collective experience of millions of existing seatposts to find starting values from. As long as you stay within current dimensions and normal materials you'll come out with a good answer yourself.



    Quote Originally Posted by M1KEMEX View Post
    I didn't know about that design. Seems easy enough for me to make in case I can't find one. What I really wanted to avoid is machining angled stuff because the milling machine I have here is manual and it's a real pita to setup that kind of stuff properly.
    The lower clamp has a round surface between the cradles that allow for a pivoting motion.

    In some variations, the lower clamp is just a piece of tubing sectioned in half axially with notches for the rails.

    You can buy a nice set of machined "guts" though:

    https://fairwheelbikes.com/kcnc-carb...tpost-adapter/


    While this design is easy to build, the holes for the rod that holds the bolts puts two holes in the worst place possible from a stress standpoint. It seems like existing designs often have extra material in this location. Your design may be conservative enough to never have problems. I have built seatposts with this design that used unreinforced tubing at the top and they were fine for the two years I used it, but I can't stress enough that should it fail there's a lot of things betwixt your legs you don't want to get speared by a jagged piece of metal.

    Bonding in and internal (or external, as dRjOn notes) is much safer, and if it debonds it will tend to spin and give warning. Adding a rivet or pin for mechanical interlock is even safer.

  19. #19
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    From http://www.phred.org/~alex/bikes/bf-vs-swift.html:

    The seatpost is 28.6mm but much much longer than a normal seatpost. The seatpost that came with my bike was made by sticking a normal 25.4mm seatpost into a long piece of 1" ID, 1 1/8" OD nickel plated steel. The inner seatpost is held in position with a pin.

  20. #20
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    Check this:

    https://www.ebay.com/itm/392271494726

    Readily made, proper size, decent length, good price.

    Then... I asked them and they refuse to ship to Mexico. Is it time to cry?

  21. #21
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    Quote Originally Posted by M1KEMEX View Post
    I asked them and they refuse to ship to Mexico. Is it time to cry?
    They won't ship using the Global Shipping Programme? All they have to do is send it to a UK address.

    What about using a freight forwarder: https://www.forward2me.com/

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