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I rebuilt a kids bike. I had the wheelset and calliper/U-brake replaced. Parts that I used are not cheap parts, lets say its more of mid range parts for BMX, normally made of alloy. I made sure the calliper is installed properly and brake shoe are aligned with the rims.

Still the brake is weak, even on a road that is 15 degrees it won't stop. For me it looks like the brake shoe is slipping and not having a good hold on the rims even if its perfectly aligned. I tried pouring soda on the rims to make it sticky but still no good. Any ideas?

Update: I already did try to degrease the rims and used more expensive brake shoe(Baradine brake pads) still no good. Brakes did improve but not what I wanted. I have to give so much pressure on the brake lever in order for it to stop, when I say so much pressure I feel like the brake lever and the cable wire is about to snap.

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    How old is the bike? I had one which was 10 years in storage, and the brand new rubber brake blocks were hardened like metal. Dreadful stopping power. Replace the brake blocks/pads with new, something like Kool Stop (salmon or salmon and black coloured ones) – Criggie Dec 23 '15 at 4:22
  • the bike is like 3 years old but I gave it an overhaul. So technically its brand new. I agree with your recommendation. I will get more expensive brake shoe later and try. – Jappy Dec 23 '15 at 5:10
  • Just because a pad is new does not mean it's soft. Pads come in a range of hardnesses. Visit your LBS and see what pads they have to choose from -- choose one that's fairly soft. You should be able to press a (temporary) dent into the pad with your fingernail. – Daniel R Hicks Dec 28 '15 at 22:13
  • A 15 degree slope would be nearly 27%. That's an extremely steep hill and I suspect you mean something else. – David Richerby Apr 2 '19 at 9:47
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The easy solution is usually better brake pads. If you're using $2 brake pads, or just old worn ones, you're not going to get very good braking. Spending $10 on a pair of decent pads can make a huge difference.

As well, make sure the brakes are set up properly, especially if you're using two finger (short) levers. With new cables, true wheels and minimum clearance between pads and rim you'll be able to put maximum force on the brakes.

If everything is properly rigid you shouldn't see anything flex when you apply the brakes. If you do, maybe look at a reinforcing plate (called "brake bridges"). Many older U brakes already come with those. It's a U or C shaped plate than goes over the top of the brake to reinforce the pivots. They're also used with V brakes but shouldn't be necessary if the bike is built properly. Old mountain bikes and BMX's often just had U brake or V brake pivots added to a frame designed for calipers, and the tubes would bend or twist when the brakes were applied. That made the brakes no better than calipers in many cases.

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  • Thank you for the input. What I am using at the moment is MOB brake pad. – Jappy Dec 23 '15 at 1:38
  • The Filipino company? They look fairly generic from what I can see, if you can afford it try a pair of KoolStop pads might be worth trying, or Shimano or Avid pads . – Móż Dec 23 '15 at 1:51
  • Yeah! I will drop by a bike shop later and grab a pair of brake pads and see what happens. I will update you guys. – Jappy Dec 23 '15 at 1:55
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    Make sure that the rim is properly degreased. Use a clean cloth with some alcohol to rub them. Also make sure that the pads are toed in, meaning the front end touching the rim first while the rear is still 1mm away. – Carel Dec 23 '15 at 8:54
  • @carel thanks for the suggestion. I actually missed the degreasing part of the rim. I will update you if this fixes the issue. – Jappy Dec 24 '15 at 3:41

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