Squealing of rim brakes on a bike My bike for 2 days started to squeal whenever I use my back brakes it has been really annoying and I am trying to fix it. Please give me any suggestion that can help this problem
There are a few things one can do to help stop the noisy rim brake.  The noise is caused by the resonance created when the pads contact the rim and a very fast oscillation of the pads' grip force on the rim occurs. A contaminated pad or braking surface of the rim can contribute to this effect.  Even the presence of water can cause an otherwise quiet, dry rim brake to squeel when wet.   So first and easiest things to try are a good cleaning of the rim's braking surface. Use some Isopropyl alcohol on a rag and use to wipe down the braking surface of the rim. Use plenty of alcohol and change to a clean area of the rag often while rotating the wheel. Make several passes on each side of the rim.  This will remove dirt and oils from the rim. You can do the same with the pads' braking surface.  After cleaning the pads, take either sandpaper or a bastard file and use it to rough up the braking surface of the pad.  The pads rubber compound can harden over time and get a dirty glaze. Using the sandpaper removes the old or contaminated rubber and exposes the cleaner, softer rubber underneath, creating a better braking surface.  A grippier pad on a clean rim changes the resonance frequency of the system which may stop the noise.
One of the best ways to stop the noise requires a little mechanical work on your part. Toeing in the brake pads so that during braking the front, leading edge of the brake pad contacts the rim first, followed closely by the trailing edge. This has the effect of preventing resonance from getting started.  To accomplish the toe-in, loosen the pad fixing bolts. On the back, trailing edge of each pad, loop a rubber band securely around the pad. This allows your hands to be free to manipulate the tool and squeeze the brake lever.  After the bands are in place, line the pads up appropriately to the braking surface of the rim and pull the lever. While continuing to hold the lever secure (you can use some string and tie the lever closed if you need both hands free) and while the pads with rubber bands are held fast against the rim, tighten the pads fixing bolt. Release the lever and remove the rubber bands. Your pads are now toed in since the rubber band held the rear of the pad away from the rim while the front was in direct contact as the brake pad was secured while held in this toe-in position. Using a business card or piece of cardboard paper between the trailing edge of the brake pad and rim  while the pads are loosened and the brakes applied, squeezing the cardboard between pad and rim. This creates the front toe-in which is held fast when the pad fixing bolt is secured back .
Brake pads are made from different rubber compounds which can be softer or harder, depending on manufacturer or use-type the pad is for.  Changing pads entirely to ones with a softer compound may alleviate brake noise.