Your bicycle's chainwheels should run perfectly straight and true, with no side-to-side wobble. This is especially true if you have a newer bicycle with indexed front shifting. With front indexing, it is difficult or impossible to make fine adjustments to the front derailer while riding. While perfection is not attainable in this Vale of Tears, it should be possible to get your chainwheels to run true to within 1 mm or less.
If your chainwheels are not running straight, you could, of course, replace them, but it might be possible to straighten them instead.
There are three things that could be bent, and if you "straighten" the wrong one, you are bending a good part to make it fit a bad part, thus compounding the problem.
The bottom-bracket axle may be bent. This is not very likely, but you should still check it first:
- Use a marker to mark the part of the outer chainwheel which is farthest out.
Remove the right crank from the axle.
- Reinstall the right crank 180 degrees from the way it was previously installed, i.e., facing the same way as the left crank.
- Spin the cranks and see which part is farthest out. If it is the part you marked, then you have eliminated the axle as the source of the problem. If it is opposite the mark, the axle is indeed bent, and you need to replace it.
- Re-install the right crank in its normal orientation.
The spider may be bent. To check this, you need to:
- Remove the chainrings.
Clip a spring-type clothespin to the cage of your front derailer, and use the derailer's control to bring the end of the clothespin close to the spider arms, so that it is almost touching them as they revolve by.
- If the spider is bent, the clothespin indicator will show it quite clearly. The spider can be straightened by bending, using an adjustable wrench on the part of the spider that goes between the two outer rings.
The chainrings may be bent. If steps 1 and 2 come up negative, the chainrings themselves are bent.
I have had the best success straightening them on the bicycle, so I would say to bolt them back on to the spider.
Bent chainrings can sometimes be straightened by using a screwdriver as a prybar. There is also a special bending tool made by Bicycle Research which sometimes will fit, but not usually on modern chainsets.
If you have the guts to try it, a large rubber mallet will often do the trick.
If only the teeth are bent, they can often be straightened with an adjustable wrench, and the edges filed smooth.
If you would like to make a link or bookmark to this page, the URL is:
Last Updated: by Harriet Fell