# How to bend 1.375 x 0.035 tube?



## PornstarRyan (Mar 22, 2018)

A question for you pros out there: What equipment and process do you use to bend seat tubes, specifically un-butted 1.375 x 0.035?

Also, as a follow-up, are there any butted seat tubes that bend well without kinking (is this even possible)?

Any help is appreciated. Thanks!


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## m-gineering (Feb 5, 2012)

PornstarRyan said:


> A question for you pros out there: What equipment and process do you use to bend seat tubes, specifically un-butted 1.375 x 0.035?
> 
> Also, as a follow-up, are there any butted seat tubes that bend well without kinking (is this even possible)?
> 
> Any help is appreciated. Thanks!


Relevant factors here are the OD/wall ratio, yieldstrenght and bending radius. The 'softer' the material, the bigger the wall and the gentler the radius the easier it gets. For a downtube probably need a decent mandrel bender with close fitting dies, and non-heat treated cromoly. With a seattube you might get away with rollers. If it is just the one-off you could bend it hot


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## PornstarRyan (Mar 22, 2018)

I guess I should clarify that this 1.375 x 0.035 tube would be 4130.

Again, I'm curious about the equipment and the actual process steps that you all use for bending.


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## Walt (Jan 23, 2004)

I got sick of dragging them back and forth in my roller die setup, and had Fairing make me a big batch of pre-bent one. 

Anything that big is going to be a wicked PITA to bend with home/DIY gear. If you don't want wrinkles of any kind (basically unavoidable at the curve/straight transition if going DIY) you can get one from me or from Vari-Wall (though theirs is inexplicably so long that it will only work for really big mountain bikes). 

-Walt


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## PornstarRyan (Mar 22, 2018)

Walt said:


> I got sick of dragging them back and forth in my roller die setup, and had Fairing make me a big batch of pre-bent one.
> 
> Anything that big is going to be a wicked PITA to bend with home/DIY gear. If you don't want wrinkles of any kind (basically unavoidable at the curve/straight transition if going DIY) you can get one from me or from Vari-Wall (though theirs is inexplicably so long that it will only work for really big mountain bikes).
> 
> -Walt


Thanks, Walt! That's good info. I always enjoy avoiding purchasing a tool that isn't going to be so useful. What is the part number for Vari-Wall's bent seat tube? I'm curious to see the specs, but their site is nearly impossible to find anything on without clicking on every tube. Also, what is the OD, clamp-end wall thickness, and bend angle of the bent tubes you have?

Thanks again!


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## todwil (Feb 1, 2007)

Walt said:


> I got sick of dragging them back and forth in my roller die setup, and had Fairing make me a big batch of pre-bent one.
> 
> Anything that big is going to be a wicked PITA to bend with home/DIY gear. If you don't want wrinkles of any kind (basically unavoidable at the curve/straight transition if going DIY) you can get one from me or from Vari-Wall (though theirs is inexplicably so long that it will only work for really big mountain bikes).
> 
> -Walt


WUT??? this only takes me a couple minutes in the HB tube roller, but there is a dimple at each end as you said I usually roll it longer and cut inside the dimples then 
add the seat post slug. The down side is limited seat post plunge and droppers are prolly not going to fit


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## Walt (Jan 23, 2004)

Mine are 34.9 x 0.8 straightgauge. You'd use a Paragon topper for whatever size of seatpost you prefer. 20 degree bend at 150mm up. Total length 510mm. You just chop off however much you want (top or bottom), put your seat collar/plug in, and away you go. It's enough bend to do anything I do, and I do *crazy* big wheel/short chainstay stuff regularly.

Vari-Wall part number is CMD34518_C-BST-01. That tube does not require a topper which will be easier for many people to deal with.

-Walt


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## NYrr496 (Sep 10, 2008)

I am far from a professional metal worker and have never built a bike frame but I do some pretty serious hot rod stuff from time to time. Not sure if this would be helpful to you but it works like a charm...
Get some clean, DRY sand. Weld a cap on one end of the pipe, fill with sand, weld a cap on the other end. Make your bend with or without heat. Cut the ends off and save the sand for next time. Haven't collapsed a tube yet. 
I'm actually going to buy a tubing bender soon for a future business venture and I'm wondering if I'll still need the sand so I get bends that look like mandrel quality.


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## Clockwork Bikes (Jun 17, 2006)

This guy will bend 1.125" butted seat tubes with no distortion. I've been meaning to get a 1.375" die made but haven't yet.
https://www.flickr.com/photos/clockworkbikes/albums/72157682698322815

I'm offering full shop drawings and CAD files for this in 1.125 and 1.375" for a $50 donation to Planned Parenthood. 5 takers so far.


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## NYrr496 (Sep 10, 2008)

That's a SWEET bender^^^


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## Walt (Jan 23, 2004)

NYrr496 said:


> I am far from a professional metal worker and have never built a bike frame but I do some pretty serious hot rod stuff from time to time. Not sure if this would be helpful to you but it works like a charm...
> Get some clean, DRY sand. Weld a cap on one end of the pipe, fill with sand, weld a cap on the other end. Make your bend with or without heat. Cut the ends off and save the sand for next time. Haven't collapsed a tube yet.
> I'm actually going to buy a tubing bender soon for a future business venture and I'm wondering if I'll still need the sand so I get bends that look like mandrel quality.


In general with thin wall (bike) tubes the sand will only help so much, especially if they are large diameter and you're trying to do a small radius (dropper posts, ugh) to keep most of the tube straight.

Cerrobend will do it but it's nasty, nasty stuff.

I found that I was basically doing the same (laborious) thing every time, so I said screw it, I'm having the tube factory make these.

Joel, your bender looks super cool!

-Walt


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## Damon777 (Jun 11, 2012)

If the dies fit the tube well and you aren't trying to bend too tight, you should be good to go. Our rule of thumb is keep the bend centerline radius 2.5x the tube diameter for mild steel, and we typically don't see any problems. 4130 tends to behave fairly similarly.

I don't build frames, but I do design tooling to bend the tubes.


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## pvd (Jan 4, 2006)

You pay a pro to do it.


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## Erichimedes (Jul 30, 2010)

I built a bender a lot like Joel's, after seeing his design. Mine will do 1.375 x .035 tube at a 16in CLR. I've also bent a Nova .8 wall tube in it without it wrinkling, but that was pushing it. I made a sleeve that fits nicely in the die so I can also bend 1.25 tubing.

I'm happy to bend some tubing to your spec. The CLR usually comes out around 17" with springback, and I can get up to about a 23 degree bend, maybe more.

Walt's tubes are extremely versatile and affordable though.


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## Erichimedes (Jul 30, 2010)

Damon777 said:


> If the dies fit the tube well and you aren't trying to bend too tight, you should be good to go. Our rule of thumb is keep the bend centerline radius 2.5x the tube diameter for mild steel, and we typically don't see any problems. 4130 tends to behave fairly similarly.
> 
> I don't build frames, but I do design tooling to bend the tubes.


You definitely won't get this unless you're using a ball mandrel and wiper die setup. Pretty far beyond most framebuilders, unless you're Peter and order some from a professional bending shop.


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## Walt (Jan 23, 2004)

todwil said:


> The down side is limited seat post plunge and droppers are prolly not going to fit


Yes, this makes it a non-starter for a professional builder. It is extremely rare that I do a bent seat tube that will not have a dropper in it, and many of them require 150+mm of insertion depth these days.

The tubes I had made are 200mm/8" CLR, for what it's worth, and that was the tightest bend they were able to accomplish without problems.

-Walt


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## PornstarRyan (Mar 22, 2018)

Thanks for all the responses, I really appreciate it. Given the mixed results, space (and $$) requirement for a bender, and lack of dropper-post-ability, bending my own tubes is not my first option. That said, I do like that tube bender, Clockwork Bikes.

Walt, I'm going to drop you an email in a week or two about buying some bent seat tubes from you.

Thanks again for taking the time to respond!


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## GrayJay (May 16, 2011)

Not exactly the question that was asked, but for my latest amateur hobby garage frame build I wanted wanted to see if I could successfully bend the 1.125" diameter, 0.7mm wall seattube and toptube that I was using (TT Versus non heat treated CrMo).

I don't have a roller mandrel bender but I have successfully bent my own fork legs and chainstays using a simple bending form so wanted to try similar process for the seattube. Bending larger diameter, thinner walled tubing is definitely more of a challenge but I can report it is viable option for an amateur builder with access to more time than money for tooling.

I started with watching the "pithy" youtube video in which he builds a fairly intricate bending press that subsequently does not work without crushing the tubing. 





Lesson I learned from this video is that a V shaped form is not sufficient support, you need the form to exactly match and support the tubing diameter. I also did not see the need for the mechanical screw-press that be built into his form, just securing one end of the tubing to the form with a tightly wrapped fabric strap and pulling/pushing down the other end works fine, especially with a bit more leverage from a cheater pipe stuck inside the tube being bent.

I built my form using two pieces of 3/4" plywood that I first cut out (using a 29" rim as the arc template) and then glued and screwed together. I then ran the wood arc through a router table to form the support groove. I didnt have a 1.125" router bit but used a smaller round router bit and slowly formed the groove with several successive passes through the router. For the last step of fabricating the form, I used a coarse 1/2 round file to carefully smooth out the imperfections left by the router and fit the groove to tightly support the tube I intended to bend.

















I plugged one end of the tube, filled with water and let it freeze to ice before making the bend. I put a bit too much bend in the downtube and got some minor wrinkling on the inside radius of the seattube but the slightly more gentle bend on the toptube worked fine. This frame will be a winter fatbike, I am trying to get short CS and low TT standover height.

















Admittedly, it is a slow process that a pro builder would likely avoid but for a hobbyist it is a viable DIY option. Doubtful it would work acceptably with larger diameter or thinner walled tubing.

If I were to try again and build another form, I would make the radius of the form a bit more gradual, making bends without wrinkles would be easier and I really did not need the radius as tight as I made it in order to provide good clearance for the tire, a slightly more gradual bend would have worked fine for tire clearance (no dropper post considerations on this snow bike).


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## mickuk (Jul 6, 2007)

I'm doing it in almost the same way - plywood former and webbing ratchet strap to hold the tube. I cut the circular profile using part of a holesaw tube notcher and a bedplate at work - plywood former roughly shaped and then rotating about it's centre past the holesaw (like a crude milling machine / router).

Filled with sand, I found that leaning on the tube just made it kink. I now use the lever shown with an old (washing machine...) ballrace running on the back of an aluminium former / slider.

The one in the photo kinked, but all others have gone OK. I've got a couple more to do this week which I'm going to try with ice (got some cold chambers at work).

One thing I did find was a massive difference with different tubes. The original trial tube was cut from an old Peugeot racer (hi-ten steel, quite thick wall) which was really easy. Going up just one grade to basic Columbus cro-mo needed about 3 times the leverage!

Not had any problems with getting enough straight sections with dropper post and large (ish) radius bend.


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## OddBurnsey (Sep 6, 2017)

PornstarRyan said:


> A question for you pros out there: What equipment and process do you use to bend seat tubes, specifically un-butted 1.375 x 0.035?
> 
> Any help is appreciated. Thanks!


I can make these for you, mainly because of your pornstar name - OR you can give me a call and I can point you in the right direction if you're wanting to invest in the tooling to do it yourself.

816-812-8598

Cheers,
Burnsey


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## Tim-ti (Jul 27, 2005)

I'm trying and failing to find a pro to bend a tube for me. Do you have any recommendations (I’m in SoCal)?

I need two equal radius bends in a 7005 T4 tube with 2"OD, 1.4/1.1/1.5t. The bends will be in the thick parts of the tube. I only need one tube and I can adjust my design (within reason) to accommodate mandrels the shop already has.


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