Bent seat tubes

My next frame is a Knard-sized tire 29er for myself. I don’t have the tires yet, so I’m un-patiently waiting for them to arrive like many other people. They’re late getting here from…wherever they’re made. In the meantime I’m drawing up a frame to build. The Knard, as i posted about a few weeks ago, is a different ballgame – 29 in diameter x 3″ wide. Going to be hard to fit it with traditional chainstays and a 73mm bottom bracket shell.

So I’m going to try 435mm Chainstays with the sliding dropouts all the way forward, 1cm less than the Surly Krampus. The big bend chainstays will help a lot, but I’m also going to bend the seat tube for the first time on one of my bikes. Yes, I’ve entered the bending world! It opened up a can of framebuilder whoopass on me though. Everything changes with bending tubes.

I primarily use BikeCAD to design and build my bikes. It really helps me visualize everything all at once, and the interactions of how one thing affects the others. Being a visual learner, this helps a ton. I also draw life-size stuff out when i know it’s going to be close, like with this bike’s rear triangle, but if you learn to use and trust the program you don’t really need to.

Part of the difficulty is just my inexperience in bending tubes, but part of it was not seeing how I can combine my bending technique (tube roller) with what was given in the software. The tube roller I have is the Harbor Freight roller with a few sets of the SWAG off-road dies for 1.125, 1.25, and 5/8″ diameter tubes. When rolling on this roller, you just kinda roll until you get the needed tire clearance (checked by a drawing or measuring offset from the old centerline of the tube). But there’s no way of measuring the radius of the bend while rolling. If someone’s figured it out, let me know please! It’s such a big constant radius bend that results, I’m sure it’s something like a 3ft center line radius or more…?!

So anyways…BikeCAD gives you three parameters to enter for bending the seat tube.

1) You get “A” which is the distance from the center of the BB to the center of the bend. This doesn’t exist when using a roller. For my first try, I just used the middle of the bend in the tube for “A”. Since I left a straight section at the top (part seat collar, part tubing) it is not just the middle of the tube but that number minus the distance I left straight.

2) You also get “B” which is the offset of the newly bent tube from the old centerline of the tube. There’s probably a way of measuring this after rolling, but instead I took Walt’s advice and measured the change in angle from vertical. Then I adjusted the “B” value to match that change in angle from vertical to the bent seat tube. For example, I rolled the tube randomly to be 6 degrees angled backwards from vertical, so I futzed with the B-value until i got 6 degrees of angle from vertical. This turned out to be a value to 15mm of offset from tube center (the centerline from BB center to top of ST). Next time I’ll get more bend but you don’t need much for good tire clearance.

3) Then you get “R” which is the radius of the bend in the tube. This is pretty easy to set if you have a bender like the JD-squared or other such mandrel benders. But for tubes this large in diameter (1.25″) and with a wall so thin (0.035″ or even double butted 1/7/1) those types of benders aren’t as easy to use as a roller. Some builders don’t divulge this information (probably because it was a PITA to figure out how to do), but it seems to me that most are using rollers of different styles to create their bent seat tubes.

In the new version of BikeCAD (v.9) there is a new ‘checkbox’ that measures the angle from the BB to the top of the seat tube. It’s found under Dimensions/Jig Setup. This angle is neither the “effective” seat tube angle (STA) – which is measured from the center of the BB to the top of the center of the seat, nor the regular STA – which is the angle from the center of the BB up through the center axis of a straight seat tube. With bent seat tubes, this all changes, and if you don’t be careful, your saddle setback will change significantly. The actual angle of the ST means almost nothing other than where it puts your saddle and possibly some added soft-tail effect when it’s super slack (the seat post acting as suspension). But for builders it’s necessary to get the angle right to set up the frame fixture right and the new version of BikeCAD offers this up thankfully. You can of course use math and Excel to calculate it yourself if you’re so inclined.

The last thing — if you are a more experienced builder and tube bender (or a geometry nerd) go to this site and help me/us figure out what equation/s would be great to use for bending tubes for bike frames. Brent of BikeCAD sent me this link, so he’s interested in improving the capabilities of BikeCAD with regards to bent tubes but needs builder’s input!

4 Responses

  1. You mentioned having trouble measuring your tube rolling radius while on the bender. I’ve of one of these radius finders from Trick Tools.
    Pricey, but the small one (2.5 to 30 inch radius) covers anything I do.

    Otherwise, contour guages, with the stack of thin wire rods or plastis sliders are good for checking against drawings. Smaller cheaper ones for tiling etc from hardware shops.

    Hope that helps…loving the blogs.

    • Thanks Dan! Great tip! I had no clue those existed. I officially need one.

      Maybe one way to get a system going would be to roll a sacrificial tube and count revolutions of the wrench/driver bolt (Walt’s idea there) and removing the tube every 1/4 turn to get a radius datapoint, then enter it into a spreadsheet to get a regression (of questionable accuracy).

      But then you could work forward from BikeCAD to the bender instead of the other way around. the way I do it now is to bend to a good enough bend for what I’m looking for, then measure the new effective seat tube angle on paper/calc the new seat tube angle to set the fixture, and THEN enter into BikeCAD and futz around to adjust the geometry accordingly for miter lengths (get the right setback, etc.).

      • I don’t use Bike Cad (still on full scale paper) so can’t help much on that side, but I think if you’re looking for a curved lower section followed by a straight top section, you’ll need some different tooling.

        Fred at has a set of hardwood large radius formers, maybe 6 different radii, to gently curve his tubes to the required radius. These would be a tighter radius that the finished tube radius because of the considerable spring back of the tubing. I mostly use 6061 alu tubing, and find that with 1″ x 0.058 a 275mm radius former will spring out to approx a 325mm finished tube. I think Fred only uses this with PG 4130, rather than the real thin stuff.

        I have used built up wheels as gentle radius formers. Clamp it horizontally so it can’t rotate, clamp a section of tube to the rim with a g-clamp (might need reinforcing plug inside). Fit a tommy bar in the end and bend the tube around the rim in a single smooth move.

        Then there’s the tight little return bend to get the top section closer to vertical again. This would need a guide/ former bender (JMR, Baileigh, Mittler etc).

        Using a sacrificial tube to generate a guideline is ok, but I find that different batches of tubing (even top quality seamless stuff with mill certificates etc) often react very differently, so it will always be just a guideline.

        All the best,

      • Those are great words of advice, thanks again!

        I saw Fred’s bender rack in a picture on his blog, very custom and nice! I’d not know yet what radii to make but it’d likely turn out better than what I’m doing now! And thanks for the springback info and experience, something I hadn’t considered.
        For my method I leave the dent that the roller makes at the stopping point so that there’s 4-5″ total (including collar) of straight tube above the bend. It’s not as pretty as not having the dent but I doubt it does anything structurally at that point in the seat tube. I may fill it with brass and smooth if it’s a really rough rider’s bike.

        I just can’t imagine how hard it’d be to bend a 35mm tube, especially a heat-treated 1/7/1 butted tube, on a wood mandrel even with a bar extension! Straight 4130 would be easier than bike tubing but…he must be strong or I must be naive.

        I would like to know how other builders do this in addition to Fred since lots do but few share their methods. Seems like most use heavier straight gauge instead of butted, and have custom benders. JD and the other large radius tube benders don’t seem to do it without failure as far as I’ve seen. Using a wheel is a cool idea, to match that radius and you already have the “bender”. Ever collapsed the rim when bending?

        Anyways, all good notes to think about. May be time to make some hardwood mandrels and test them out.


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