Single pivot, side pull brake - centering issue
 

Hi,

I am trying to fix a kids bike and its rear brake doesn't seem to work properly. (I am not able to post pictures yet). After centering the break, when I apply the brake, both arms clutch the rim, but when I leave the brake the left arm stays close to the rim and right arm moves away from the rim. It appears that the whole brake assembly rotates anti-clockwise around the pivot bolt.

I tried tightening the locking nut on the top and the pivot bolt. If tighten either of them too much, the arms stop moving.

How can I keep the arms loose enough to move but keep the pivot assembly centered?

Thanks

The cable housing may be pushing the brake off center. Those things on kid's bikes should really be called brake shaped objects. Good luck.

To prevent overtightening the caliper arms, you need to have a wrench on both the caliper arm nuts and the mounting bolt on the other side of the crown (front brake) or brake bridge (rear brake), and turn them simultaneously in the same direction. It helps to have thin brake caliper wrenches for caliper arm nuts. Perhaps this will help:

https://www.parktool.com/blog/repair...-brake-service

The caliper spring sits in a groove that is cut into the center bolt's larger diameter "barrel". The spring's center portion is thus held, in rotational aspect, by the center bolt's rotation. Either end of the spring tries to return to it's unsprung position as the spring's center is set to. So by rotating the center bolt, and trapping it from moving by tightening the caliper mounting bolt, the spring's center and the ends also rotate.

John's suggestion is one way to do this spring center rotation. One can also lightly tap the spring's center section on one side of it as it leaves that groove with a punch and hammer to knock the center bolt around. To rotate the center bolt and thus the spring. One can detach one end of the spring and bend it further out to increase it's ability to pull that side's pad away from the rim. IIRC Sheldon Brown's site has some of this better explained and a how to center instructions. Andy

Those nuts aren’t for centering the brake. They serve the same function as the locknut/top race of a threaded headset or the locknut and the cone of a wheel or the lockring and the adjustable cup of a bottom bracket. The inner nut needs to be loose enough so that the caliper arms can move but tight enough so that isn’t any flex in the arms. The outer nut is jammed against the inner nut to prevent it from turning.