Regaining frame aligment I bought a belt driven bike which had had the rear triangle of the frame moderately bent (i.e. bent but no tubes collapsed or cracked) – it was a gamble as to whether it’s fixable….  It has a Nuvinci 330 hub with disc brakes which requires a large 135mm dropout spacing.

The frame is made of steel and I’ve been able to bend it to near alignment.  However, as I don’t have a reference bike to compare measurements from, I’m struggling to figure out whether I’ve aligned it properly or not.

Firstly I aligned it based on the string test (Sheldon web page and also a Youtube video) so that the distance between the string and the seat tube frame member was equal both sides.  I got this right to within 1mm and was happy with myself.

But then I dropped in the rear hub and tried to pedal it.  It turned but it was making a noise which wasn’t very happy and you could tell the hub wasn’t aligned with the pedal wheel, with the belt rubbing as it seated itself on the gearwheels.

So then I bent the frame some more and now turning the pedals works really well and looks like it’s all fitting nicely, however if I do the string test there’s around a 6mm difference in the distance between the string and the frame between the two sides.  However if I measure distance from each rear axle end to the same point on the headset it’s the same distance within measurement error, so I think the axle is sitting perpendicularly to the line of the frame.

So the question is this.   Does all of the above mean my frame is bent in some way where I just won’t be able to fix it?  The bottom bracket is one of those split types so that you can tension the belt – it looks horrifically weak structurally to my eye and it may well be that the bottom bracket axis is no longer perpendicular to the frame plane which I doubt I could fix.  Or maybe a 6mm difference in the string test is not enough to worry about?  Or are some frames deliberately made so that the string test should not return the same measurement which would mean that the way I have it now is perfect?

Thanks in advance for any help.

PS.  I am waiting for a rim before I can restring the rear wheel and this is taking a while to arrive – hence I’ve not been able to ride it to see how it feels.
A couple things here that could be tripping you up:


Both belt drive and Nuvinci hubs can be noisy all by themselves. Belts frequently also make noise in the stand that vanishes when riding and vice versa. Getting belt tension just right is a whole other step and noise will frequently be the result when it's not dialed. Lubricating it is another piece. (I always use silicone cleaning spray, and yes this is a Gates-sanctioned thing). If the rear cog was visibly out of line with the front, then perhaps this isn't likely to be the problem, but worth mentioning.
When you bent the frame around the second time and the noise went away, it could have affected the belt tension.
If somehow the chainline (beltline) was incorrect with the parts that are on there, it could create the situation where it made noise when the alignment was right and didn't when it was wrong.
Doing what you're attempting without dropout alignment tools or an improvised substitute is iffy. What they do is integral to the whole process of aligning a frame after it's been severely misaligned.


You mention that you're seeing some torsional misalignment between the BB and the front wheel. A way you could go about trying to fix that is taking the fork out, clamping the BB in a vise with some old steel cups, removing the fork and headset, and getting a long steel pipe that mates closely with the inside of the head tube to use as a leverage bar. Then to check the alignment (which you should do first) you can hold the frame up against a light source and eyeball the parallelism between the seatube and headtube. This isn't likely to have any bearing on rear torsional alignment though. You need dropout tools for that. 

Something you can do is take a long, straight piece of square aluminum extrusion, like 80/20, and use it as a straight edge referenced off the bottm bracket. For example, clamping them together in a vise, measuring with a caliper between the seat and the straight edge all the way down their lengths, repeating for the down tube, then removing the straight edge, re-clamping the bb, and reefing on the tubes. If there is shell misalignment, something like this is how you'd fix it.