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!
