Instructables
Picture of Spreading or adjusting a bicycle frame
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!

 
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Step 1: Preparation and tools

Picture of Preparation and tools
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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).

hmeyer625 months ago

Here is how
we do it over here:

The “tool”
you need is very simple, just a length of thin but strong string with a rubber band
in each end!

Take out
the rear wheel!

Measure the
distance between the rear ends, it should match the width of the rear hub. The
rear fork ends should be parallel.

Wrap the
string around the head tube and route it downwards towards the rear fork ends.
Pass it through the cutouts in the fork ends from the outside and up towards
the rear brake stay. On most frames, you should be able to wrap the rubber bands
around the rear brake, if it is mounted, otherwise pass an allen key or
whatever you have through the hole and hang the rubber bands on it.

The string
should be tight i.e. the rubber bands should be stretched. It is
imperative to check there are no obstructions (gear levers, cables, water bottles
crank arms, whatever) touching the string on its route from the head tube to
the rear fork ends on either side.

What you do
now is, with a ruler (NOT a caliper!), measure the distance from the string to
the seat tube on both sides when the string is at the bottom of the head tube
(Use the bottom headset cone to rest the string on), and repeat this with the
string at the top of the head tube.

A straight
frame will have the same measurements on either side, top and bottom.

It is
important that the measurement on the same side are identical top and bottom. –
If they are not, the wheels will never align and the straightening is complex,
and requires sophisticated tools.

If they are
the same top and bottom on each side (for instance 37 mm top and bottom on the
right side, and 41 on the left side top and bottom) the chain stays need to be
adjusted. They should be the same on both sides, when the distance from fork end
to fork end is correct, determined by the rear hub.

Best way to
accomplish this is to have the frame’s bottom bracket firmly clamped in a vice
(Crank arms off and if possible also the bearings, and then pull/push on the
stays.

Different
rear fork end designs require you to be inventive and come up with a way to route the
string so it is the same on both sides. The "track style rear ends" on the picture here
have no cutouts, but you could just pass the string through the axle cutout
from the outside and then forward towards the rear brake stay for instance.

patdoherty6 months ago

Great instructable, most people (home mechanics especially) dont really think about frame alignment and rear triangle spacing. I will add though that I worked at a shop where we had a frame alignment bar to bent the frame back but we found that a dead blow hammer was much easier and although you wouldnt expect it, it never damaged paint when the frame alignment bar would occasionally. And you nailed it when you said steal frames only!

craftclarity6 months ago

I agree with seamster on this one. Definitely a good subject to tackle. Thanks for sharing it!!

seamster6 months ago

Great info, very nicely done. And I like the current paint job! I wouldn't repaint it, but that's just me!

uersel (author)  seamster6 months ago
To late about the paint... But I got that a lot and I had to tell them all, that the goal was to match the colour of the bike to my trailer ;-) But it looks quite good now. When I'm done with the paintjob there probably will be another instructable about that.
seamster uersel6 months ago
I'm sure it will look good! Can't wait to see it.  

I actually rebuilt a bike and built a matching trailer for it a few years ago. I have a full instructable on the trailer, but here's the main photo.
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