Altair 4

My wife picked a child on our church's "giving tree" who asked for a bike. The child is about 6 or 7. We've picked up a bike a Target (yeah, I know) and I've gone over it to make sure everything was okay, tires inflated, etc. Everything seems fine except for the rear rim brake. Why this bike even has a rim brake is beyond me, considering it has a coaster brake, too.

When the brake is applied, both halves move to the rim. But only the half marked "A" retracts when the brake is released. The "B" half stays in contact with the wheel. I need to deliver this bike to the church this weekend. I've tried loosening and realigning the whole piece, but this hasn't helped. The front brake works perfectly fine, with both sides retracting.

Is this fixable? What am I missing? Any quick help would be appreciated. Thank you!!

Here's a photo of it:

hokiefyd 

Friction somewhere is preventing B from retracting. You can put small dabs of grease where the return spring hooks slide against the brake arms (near where you have A and B labeled in the photograph) to help mitigate friction there. It could also be, or is more likely due to, a poor pivot design up at single pivot bolt and this part needs disassembly and grease/filing/fitting, etc. It may be that this specific brake caliper will never work right due to design, but reducing friction points is a good place to start.

ClydeClydeson

First, there's really nothing wrong with kids' bikes from box stores, as long as a knowledgeable person takes the time to set it up correctly. Kids bikes are generally massively overbuilt in most regards, and they will be outgrown in a year or two, usually long before the bike is worn out or otherwise ruined.

When installing a single pivot caliper brake, you need to ensure it is as close to perfectly centered as possible when not being activated. There is a 10mm nut on the back of the mounting bolt that attaches the caliper to the frame, and you can loosen this nut, pivot the whole caliper in the direction opposite to that in which it sticks, then tighten the bolt. It's not rocket surgery, but it might take a few attempts before you get it right. Also make sure the brake cable isn't positioned such that it is pulling the brake excessively as this can make centering more complicated.

Another trick for kids' bikes is to slightly bend the spring in on both sides to reduce the preload and make the brake easier to actuate for small hands.
There may also be a tiny screw on the brake lever that pre-sets the lever blade inward to allow easier reach for small hands. THese won't make the brake caliper imbalance better (might actually aggravate it) but once everything is set up, make for a more pleasant ride for the young person.

Here is a sketch of the caliper showing the location of the nut for centering (red arrow, pointing to the nut on the BACK of the caliper against the brake bridge of the frame) and of the bending that can be done to the spring. You likely need to 'unhook' the ends of the spring from the arms of the caliper to bend it, and it's a bit tricky... it's also not 100% necessary, but worth a try, in my opinion.

FBinNY 

Assuming you centered the open brake at the pivot bolt, you need to consider friction. This could be at the pivot bolt itself, but more usually it's where the springs bear on the arms. As the brake is used, there's movement there. As noted earlier, lubing that area will help, but IME it's often not enough. The best solution is to slide a bit of Teflon or other plastic tubing onto the spring to form a sleeve where the arm and spring slide. If you can't find tubing that works, disconnect the springs and polish the arms where they rub making sure to work along the arm, not across. Then lube.

BTW- often folks have trouble properly centering the brake because the center bolt turns while tightening, even if you hold the arms. If this is happening purchase a lock washer and use it between the brake and frame to prevent that.

Andrew R Stewart 

As usual, good advice. One can rotate the caliper/center bolt unit a bit past center so that when the backing nut is fully tight and the center bolt did it's slight spin thing it ends up on center. Andy

icemilkcoffee 

Just throw these garbage caliper brakes in the trash where they belong. The coaster brake is perfectly fine. My kids had Specialized Hotrock bikes when they were little, and I threw away the caliper brakes (and kick stands) to save weight. The coaster brakes were totally adequate.

Altair 4

Thanks for all of the advice. I disassembled, lubed, and reassembled and it retracts now. If this were for my kid, I'd take them both off. That's what I did on my daughter's bike on her first bike. It was even worse than this bike - the rim brake was just on the front wheel. I could just imagine her going over the bar. Took the caliper off and pitched it.

smd4

You really thought your daughter would be going fast enough on, what, a 16 inch bike, and could squeeze the lever hard enough on a toy rim brake to lock the front wheel and launch her? Hoo boy…