Question

The gear of bicycle starts making noise after some times ..give reason

Answer 1

It sounds like you may be cross chaining?.. A bike with a double chain ring setup and, lets say, ten rear cogs, or gears, may be called 22 speed but not all those combinations are usable. When on the big chainring, depending on the line the chain takes between the chainrings at the front and the cassette at the rear you can usually only use the smallest rear cog up to perhaps the 4th largest rear cog. If you try to use a bigger cog ( ie, lower or easier gear) the chainline is too extreme and you may hear a rubbing noise as a result. Best to shift to your smaller front chainring. With the small chainring you should be able to use the biggest rear cog down to perhaps the 3rd or 4th smallest rear cog.

Answer 2

Friction causes the wear and tear of parts of a machine. Therefore the friction gets decreased between the gears of the bicycle and it causes noises. It can be prevented by lubricating the parts of bicycle with grease or oil.

Hope this answer helps ☺☺