Tailor

I've got a bike I like that has a stupid 26.8 seatpost. I'm an uber clyde, so periodically, my ass destroys the crappy aluminum seatposts that are available in this size.

I haven't found any company willing to do this, so I figured I'd ask here, as the question is two fold.

#1, is there any seat hardware that does not come attached to a seatpost, which can then be added to an existing tube? If not, I'll just end up speccing a down sizing of the tube at the end and use old style hardware to attach to the seat.

#2 Does anyone know of a small shop that will produce 4130 thickwalled (seriously thick walled, like say, 1/8th to 1/4" wall thickness) 26.8mm OD seat posts in say, 400mm length? I'm sure its a custom job on the lathe, and sadly I don't own a machine lathe.

I care not about weight, but I am sick of seat posts I cannot rely on.

coupster

3/4" steel gas pipe with an old fashioned seat clamp

Andrew R Stewart 

"Classic" seat clamps are 7/8" ID. There are seat pillars (straight w/o clamps) available with this top size for very cheap.


But, yes, there have been one offs made many times in cycling's past. Any agreeable machine shop or tool and die student can do this.


But how do yo ruin your posts? Andy.

FBinNY 

Part of the problem may be the frame size or amount of post exposed above the frame clamp. I don't know any of your details, but consider that the longer the unsupported post, the greater the stress at the fulcrum..

OTOH, depending on how marginal posts are, you might be able to buy yourself some added strength easier than trying to make a post from scratch.

Buy yourself a new post, and a broom handle. Shave down the broomstick to a force fit inside the post. It doesn't have to go all the way up, but must span from the bottom to at least 3-4" above the frames post clamp. That added reinforcement may be all that's necessary.

Before you say that it can't be strong enough, consider that its function isn't to resist bending, but to keep the tubing from ovalizing and buckling, which is why you want a snug force fit.

Tailor

Riding while being a 6'5 "generously proportioned" guy that rides his bike like the bike and him are unbreakable. (Neither are).

I also used to prematurely wear out my aluminum chainrings until I got a non ramped stainless one. I go through bottom brackets faster than anyone I know as well. I just ride every day, ride kind of hard, and am a big guy.

JohnDThompson 

If you're really an "uber-clyde," why are you using aluminum seat posts? Steel seat posts are available that should accommodate you easily even if you gain more weight.

Tailor

Oh there is no doubt thats a big part of the problem. Its a more "modern" frame, which leaves me with a rather long extended seatpost.

Wouldn't have said that. I was thinking in that direction myself some time back, just trying to use a metal sleeve forced into place. I just couldn't come up with a way to accomplish it. Considered filling it up with epoxy for the same reason.

Tailor

If you find me a 28.6 x400 mm seatpost made from steel that is available for purchase*, I will not only buy it, I'll send you a check for 50$.

I do not like Al, and am a big fan of 4130 and similar alloys. I also hate that the bike world has corrupted the word "alloy" to the point where I have met people who think the word means "aluminum".

*IE not a custom order, since thats the avenue I am currently investigating.

Tailor

Its funny you should say that. I had looked at many pipe options, but never realized a 3/4" gas pipe has a perfect (well close) OD. I think I'm going to try this out tomorrow. A nice tight fit from a heavy steel pipe would solve this problem nicely. As long as I can find schedule 80+, the wall thickness should be there as well.

dksix

If you come up with an idea that you think would work but need machining or engineering PM and maybe I can help.

Jeff Wills

Separate seat clamps like this are notorious for not holding their tilt adjustment. Under a rider who destroys standard aluminum seat posts they won't last long.


Have you tried the BMX world? I found this with a quick search on Ebay:
NOS PEREGRINE Bmx Easy Micro Adjust SEATPOST 4130 FULL CR MO 26 8mm | eBay

Grand Bois

It's not just the bike world. Language changes constantly. Don't let it bother you.

https://www.alloywheelsdirect.net/

andr0id

Machinists will make anything you want to give them a drawing and the $$$ for.
But unless you have a friend in a shop, you're not going to like the price. You're probably looking at a $200 seat post for your $200 bicycle.

coupster

Honestly I was being somewhat facetious, but its worth a try. You will also have to use a reduction fitting with a 1/2" nipple. As posted, the typical seat clamp has a narrower internal diameter than the post.

You still haven't told us how the posts fail.

Tailor

I have indeed looked at bmx stuff. The seat posts they have would be strong enough, they are just not long enough.

As for the tilt, if it fails to hold me in place, I'll just grind off the paint and replace those teeth with a 2" tig weld on each side.

Tailor

This is the most recent one. (last month, the new one is already having problems).

They tend to look like this, work hardened then a snap at the stress riser point.





And if by how you're asking what I was doing, for this one, I was riding on a side street and went over a rain gutter (the kind at every intersection around here) without remembering to post up on my pedals. The one before that was taking a ride on a MU path with my kids. The one before that was a ride on the backroads near where I live, normal flat if slightly rougher country road. I forget the one before that.

Tailor

Thanks, I'm not up on machining/engineering drawings, but my thought is that if I can find a shop that has the kind of dies to neck down a piece of 1" tubing down to whatever the size is on the old style bike saddle mounts, then the rest is just getting a slightly oversized peice of heavy walled (3/8ths" wall) 1.15 nominal od 4130 tubing, have it turned down to 1.05", only below the point where I insert my seat posts. I have several aluminum blanks formerly known as broken seat posts showing that length, so thats no problem. Then the seat post clamp will only be helping resist twisting, and the lip of the post itself will hold it up at the height I want.

I'm not even a pretend machinist, but that sounds reasonable to me, for a solution that should last.

Tailor

Ouch.. zinger. Its an 4130 reynolds tubeset bike. It's true that it probably wouldn't sell for much, being that the maker is an unknown, but it fits me really well. Almost as if it were made for my weird tall short legged body type. Something about the overall seat to bottom bracket angles is just perfect for me. I can ride this bike faster for less effort than any of my other bikes.

Alas I don't have a friend in a shop. If I locate someone nearby I might be able to work something out though. Most machinists have at least some need of someone to to weld their pieces together. That part I am pretty good at. And of course, if they don't want to trade work I can just pay them. It's a really simple piece of machining I'm after. I know one guy with a lathe, its just that his is too short, or this would be handled already.

dksix

What I'd suggest is boring out an existing seat post to where it was smooth inside than then turning a steel (or aluminum) to be pressed into it. If you do this to an aluminum tube and heat it a steel or aluminum tube could be pressed in and when cooled the 2 would be practically fused together as one. That would be a simple process using a standard size ream and there would likely be a tube with a standard OD that might not even need turning. That would save having to do anything at the seat mount providing it's not right at the top where you're having problems, this should solve them.

Tailor

That's pretty much what I was thinking when I was looking into putting in a steel sleeve. I just ran into two issues there.

#1, I don't know anyone with that large of a lathe.
#2, The owner of such a lathe might know this answer, but I also don't know how you chuck up a seat post that still has the top parts on it, especially and then get it centered.

dksix

I wouldn't want to hold it in a chuck to hold it but rather hold it in a collet. Thin soft tubing tends to distort in a 3 jaw chuck. I don't know the approximate ID of the tube you're working with but we have a reamer set that goes by 1/16 with over and unders at 1/8 inches (.124, .125, .126 or .749, .750, .751).

Tailor

I'm not familiar with lathe collets, so maybe this is obvious, but it looks like those collets would still need to be gripping the tube from the "goes into bike frame" end, which is also the end you'd want to be applying the tooling to. If this is something that a real lathe user will just understand, I can accept that. It just isn't computing for me.

dksix

A collet would grip the entire circumference and not cause pressure points. Also, collet have to be in a holder, the seat mounting could just stick out the back and turn freely in the spindle bore of the lathe.

fietsbob

26.8 is a common seat post size... An 0.9mm wall 1 1/8" seat tube has a Inner diameter of 26.8.

28.6 - 1.8 (2x0.9) = 26.8


I Own a 4130 ie Chrome Moly steel seat post, chrome plated, sold by Redline, It is 1" and stepped down to 7/8" at the top, for the saddle clip.

There are Machined seat post sizing shims you can Buy * To have the 1" ID and your frame's (as you Say) 26.8mm

Problem solvers /QBP, And Wheels (of Boulder) are sold through Bike shop wholesalers ..

Here is the^^ company site . Problem Solvers

" ST0514... Seatpost Shim [ID] = 25.4mm; [OD] = 26.8mm; [shim tube wall thickness] =1.4mm"




'/,

fietsbob

I also Own a Brompton folding Bike , Brompton penta clip is a premium quality primarily aluminum saddle clip

Pictured here , [there are many Brompton dealers around the World]

Penta clip, Part# QPENTACLIPA – NYCeWheels.com



Multi plate Friction, secures the angle , 1 bolt .



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