In early summer 2013 I bought a beat down bicycle, made by NSU (germany). According to the frame number it was built around 1952. The idea was (and still is) to create a bike worthy and capable of towing my red kids-trailer.
This instructable is about straightening the frame of this bike. There will come some more ibles about other parts of the (still ongoing) project.
When I had the bike in a rideable state, I quickly discovered, that something was wrong. It was nearly impossible to ride it freehanded. So I checked and found out that the back wheel was not in center of the frame. So what to do now. A nice guy in a forum pointed me to a method first described by Sheldon Brown and that's what this instructable is about.
A safety advice in advance:
Perform this method only on steel frames! Aluminum or carbon will break! And even with steel frames be very careful. It's better to get close to the desired result in many small steps than in one big attempt!
Step 1: Preparation and Tools
To get it right you need the proper tools: A crate, box, chair or ladder as a support point, a piece of lumber as a lever and a measuring device. The first two are fairly easy to get, the last is easy made.
I took a aluminum rail, a screw and some nuts. Then I drilled a hole into one end of the rail and fixed the screw through it with one nut. The other nut sits lose on the screw and is used to compare left and right side of the back fork by pressing the rail firmly to headtube and seattube of the frame in a way that the screw points trough the axle eyelets. Like this you can find out which side to bend in (rigth side in my case) and which side to bend out (left side here).
Step 2: Bending the Frame
For the bending part it is not necessary to completly strip the frame, but it is much easier if you do so.
Take the crate or something similar (ladder, chair) as a supporting point and the piece of lumber. Stick the lumber through the frame according to the desired result: to bend out go trough the middle to the seattube, to bend in go from outside to the seattube (check pictures for easy reference) in a way that the longer end of the lumber rests on the crate and the other on the seattube and the headtube of the bike rests on the floor. Now apply pressure on the seattube, best in a firm short push.
Now measure the width of the frame and check symmetry.
Repeat until you're done and be careful not to overbend.
Step 3: Results and Variations
After the treatment it is now possible to ride the bike freehanded. It still tilts a bit to one side, but it's much better than before. The method takes time and there is a lot of trial and error involved.
It is possible to do it in a more basic setup as you see in the second picture of this step. I didn't strip the bike completly and used a piece of lumber and a ruler to compare sides. It was my first attempt on this method. It worked, but the result wasn't as good as with the aluminum rail for comparison of sides. And you need clamps or a second set of hands...
For now the bike stays like this in it's rat-look. Next will be the fenders and the paint job...