Automobile Engineering in ME
The function of a second ring is that it is a

Back up oil control ring which will work in place of the bottom ring when it is broken
Cushion ring which prevents piston slap and knock
Compression ring and aids the top ring in sealing and cooling
Back up compression ring which will work in place of the top ring when it is broken

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Automobile Engineering in ME
In a ventilated disc brake

A duct directs air towards the caliper for cooling while the vehicle is moving
Disc contains many small holes for optimum cooling performance
Caliper is covered with cooling fins
Disc contains radial vanes between its rubbing surfaces for optimum cooling performance

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